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Tectonics of the West Antarctic Rift System: New Light on the History and Dynamics of Distributed Intracontinental Extension

Cooper, A. K., P. J. Barrett, H. Stagg, B. Storey, E. Stump, W. Wise, and the 10th ISAES editorial team, eds. (2008). Antarctica: A Keystone in a Changing World. Proceedings of the 10th International Symposium on Antarctic Earth Sciences. Washington, DC: The National Academies Press.

C. S. Siddoway1

ABSTRACT

The West Antarctic rift system (WARS) is the product of multiple stages of intracontinental deformation from Jurassic to Present. The Cretaceous rifting phase accomplished >100 percent extension across the Ross Sea and central West Antarctica, and is widely perceived as a product of pure shear extension orthogonal to the Transantarctic Mountains that led to breakup and opening of the Southern Ocean between West Antarctica and New Zealand. New structural, petrological, and geochronological data from Marie Byrd Land reveal aspects of the kinematics, thermal history, and chronology of the Cretaceous intracontinental extension phase that cannot be readily explained by a single progressive event. Elevated temperatures in “Lachlan-type” crust caused extensive crustal melting and mid-crustal flow within a dextral transcurrent strain environment, leading to rapid extension and locally to exhumation and rapid cooling of a migmatite dome and detachment footwall structures. Peak metamorphism and onset of crustal flow that brought about WARS extension between 105 Ma and 90 Ma is kinematically, temporally, and spatially linked to the active convergent margin system of East Gondwana. West Antarctica-New Zealand breakup is distinguished as a separate event at 83-70 Ma, from the standpoint of kinematics and thermal evolution.

INTRODUCTION

Heightened interest in West Antarctica (WANT) and the West Antarctic rift system (WARS) comes from new determinations of the mantle thermal profile (Lawrence et al., 2006) and the context for active volcanism (Behrendt et al., 1994, 1996) arising at a time of instability of the West Antarctic ice sheet, when information is sought about the influence of underlying crustal structures on glaciological and glacial-marine systems (e.g., Holt et al., 2006; Lowe and Anderson, 2002; Vaughan et al., 2006). The question of heat flux arising from warm mantle beneath thinned crust is of obvious consequence for ice-sheet dynamics (Maule et al., 2005). The area of thin crust corresponding to the WARS (Figure 1) includes the Ross Sea and Ross Ice Shelf, the West Antarctic ice sheet (WAIS), and Marie Byrd Land (Behrendt et al., 1991; Storey et al., 1999; Fitzgerald, 2002; Siddoway et al., 2005).

In the geological record the WARS has distinctive but differing expressions in both Cenozoic and Mesozoic time. By far the better-known rift phase is the mid-Cenozoic to Present interval. Widespread basaltic volcanism (Behrendt et al., 1994, 1996; Finn et al., 2005; Rocchi et al., 2005), slow mantle seismic velocities (Danesi and Morelli, 2001; Ritzwoller et al., 2001; Sieminski et al., 2003), and anomalous elevation of thinned continental crust (LeMasurier and Landis, 1996; LeMasurier, 2008) are the hallmarks of the Cenozoic rift. The Victoria Land Basin and Terror rift, on the western limit of the WARS, record modest extension on the order of 150 km in Eocene-Oligocene time (Stock and Cande, 2002; Davey and DeSantis, 2006). The dramatic relief of the Transantarctic Mountains (TAM) developed in the Eocene (ten Brink et al., 1997; Fitzgerald, 2002), and voluminous basin sedimentation commenced (Hamilton et al., 1998; Cape Roberts Science Team, 2000; Luyendyk et al., 2001; Karner et al., 2005), considerably later than major extension in the WARS. Not surprising in light of the dominantly Eocene activation of the TAM boundary (ten Brink et al., 1997; Fitzgerald, 2002), onland structures attributable to preceding Cretaceous events are few in the TAM (Wilson, 1992).

1

Department of Geology Colorado College, Colorado Springs, CO 80903, USA (csiddoway@coloradocollege.edu).



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Cooper, A. K., P. J. Barrett, H. Stagg, B. Storey, E. Stump, W. Wise, and the 10th ISAES editorial team, eds. (2008). Antarctica: A Keystone in a Changing World. Proceedings of the 10th International Symposium on Antarctic Earth Sciences. Washington, DC: The National Academies Press. Tectonics of the West Antarctic Rift System: New Light on the History and Dynamics of Distributed Intracontinental Extension C. S. Siddoway1 ABSTRACT and the context for active volcanism (Behrendt et al., 1994, 1996) arising at a time of instability of the West Antarctic The West Antarctic rift system (WARS) is the product of ice sheet, when information is sought about the influence of multiple stages of intracontinental deformation from Juras- underlying crustal structures on glaciological and glacial- sic to Present. The Cretaceous rifting phase accomplished marine systems (e.g., Holt et al., 2006; Lowe and Anderson, >100 percent extension across the Ross Sea and central West 2002; Vaughan et al., 2006). The question of heat flux arising Antarctica, and is widely perceived as a product of pure shear from warm mantle beneath thinned crust is of obvious conse- extension orthogonal to the Transantarctic Mountains that led quence for ice-sheet dynamics (Maule et al., 2005). The area to breakup and opening of the Southern Ocean between West of thin crust corresponding to the WARS (Figure 1) includes Antarctica and New Zealand. New structural, petrological, the Ross Sea and Ross Ice Shelf, the West Antarctic ice sheet and geochronological data from Marie Byrd Land reveal (WAIS), and Marie Byrd Land (Behrendt et al., 1991; Storey aspects of the kinematics, thermal history, and chronology of et al., 1999; Fitzgerald, 2002; Siddoway et al., 2005). the Cretaceous intracontinental extension phase that cannot In the geological record the WARS has distinctive but be readily explained by a single progressive event. Elevated differing expressions in both Cenozoic and Mesozoic time. By temperatures in “Lachlan-type” crust caused extensive far the better-known rift phase is the mid-Cenozoic to Pres- crustal melting and mid-crustal flow within a dextral trans- ent interval. Widespread basaltic volcanism (Behrendt et al., current strain environment, leading to rapid extension and 1994, 1996; Finn et al., 2005; Rocchi et al., 2005), slow mantle locally to exhumation and rapid cooling of a migmatite dome seismic velocities (Danesi and Morelli, 2001; Ritzwoller et and detachment footwall structures. Peak metamorphism and al., 2001; Sieminski et al., 2003), and anomalous elevation onset of crustal flow that brought about WARS extension of thinned continental crust (LeMasurier and Landis, 1996; between 105 Ma and 90 Ma is kinematically, temporally, LeMasurier, 2008) are the hallmarks of the Cenozoic rift. The and spatially linked to the active convergent margin system Victoria Land Basin and Terror rift, on the western limit of the of East Gondwana. West Antarctica-New Zealand breakup WARS, record modest extension on the order of 150 km in is distinguished as a separate event at 83-70 Ma, from the Eocene-Oligocene time (Stock and Cande, 2002; Davey and standpoint of kinematics and thermal evolution. DeSantis, 2006). The dramatic relief of the Transantarctic Mountains (TAM) developed in the Eocene (ten Brink et al., INTRODUCTION 1997; Fitzgerald, 2002), and voluminous basin sedimentation commenced (Hamilton et al., 1998; Cape Roberts Science Heightened interest in West Antarctica (WANT) and the West Team, 2000; Luyendyk et al., 2001; Karner et al., 2005), Antarctic rift system (WARS) comes from new determina- considerably later than major extension in the WARS. Not tions of the mantle thermal profile (Lawrence et al., 2006) surprising in light of the dominantly Eocene activation of the TAM boundary (ten Brink et al., 1997; Fitzgerald, 2002), on- land structures attributable to preceding Cretaceous events are 1 Department of Geology Colorado College, Colorado Springs, CO 80903, few in the TAM (Wilson, 1992). USA (csiddoway@coloradocollege.edu). 91

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92 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD ing regional unconformity RU6 evidently was delayed until the Eocene to Miocene (Hamilton et al., 1998; Wilson et al., 1998; Cape Roberts Science Team, 2000; Luyendyk et al., 2001; Karner et al., 2005). This is despite the rapidity of the large magnitude extension, on the order of 600 km in the south, up to >1000 km in the north achieved in as little as 20 m.y. (DiVenere et al., 1996; Luyendyk et al., 1996). A second paradox is that breakup between WANT and New Zealand (NZ) did not exploit rift structures but rather cut at a high angle across basement highs and basins of the Ross Sea (Tessensohn and Wörner, 1991; Lawver and Gahagan, 1994; örner, rner, Sutherland, 1999). Wrench deformation and the presence of strike slip transfer systems was postulated (Grindley and Davey, 1982) but not substantiated from exposures on land. New perspective on intracontinental extension in the WARS comes from geological and geophysical research that investigates the exposed bedrock of WANT, NZ, and the Tasman Sea region (Figure 1). WANT, NZ, and subma- rine plateaus formed a contiguous segment of the conver- gent margin of East Gondwana in Early Cretaceous time, with arc magmatism recorded in Marie Byrd Land-NZ. Transtension–extension occurred in a back arc to inboard setting, forming the intracontinental West Antarctic rift sys- tem and Great South Basin-Campbell Plateau extensional FIGURE 1 The Cretaceous West Antarctic rift system at ca. 90 Ma, illustrating the positions of Marie Byrd Land and New province (Figure 1). Zealand/Campbell Plateau along the East Gondwana margin. The Since 1990, data acquired from geological investigations rifted margin corresponds to the –1500 m contours (dashed-line on land and from airborne and marine geophysical surveys in pattern). The tight fit of the reconstruction, the linear to curvilinear the region of Marie Byrd Land have dramatically increased continental margin, and the pronounced depth increase suggest the understanding of the eastern WARS, with consequences fault control and steep fault geometry. The diagram is based on for our conception of the Cretaceous to Present multistage the reconstructions of Lawver and Gahagan (1994) and Sutherland evolution of the West Antarctic rift system as a whole. The (1999). The present-day position of the Transantarctic Mountains, aim of this paper is to summarize the tectonic evolution of as labeled, corresponds to the western tectonic boundary of the West western Marie Byrd Land (MBL) (Figure 2) and of neighbor- Antarctic Rift System. FM = Fosdick Mountains; EP = Edward VII ing segments of the proto-Pacific margin of East Gondwana Peninsula; 270 = DSDP site 270. (Figure 3). Little affected by Cenozoic events (cf. Fitzgerald, 2002; Stock and Cande, 2002), the eastern Ross Sea region preserves a clear record of the kinematics, magmatism, and The lesser-known phase of extension and lithospheric thermal history of the Early Cretaceous large-scale opening thinning that brought about formation of the vast rift system of the WARS. (~1.2 106 km2) did not occur in Cenozoic but in Mesozoic Knowledge of the Cretaceous tectonic evolution of time (Tessensohn and Wörner, 1991; Lawver and Gahagan, örner, rner, the WARS-NZ-Tasman Sea region provides an important 1994; Luyendyk, 1995). Although the origins of the WARS foundation for contemporary research in WANT, including may be linked to Weddell Sea opening and Ferrar magmatism studies of the Cretaceous to present landscape evolution in the Jurassic (Grunow et al., 1991; Wilson, 1993; Jokat et (LeMasurier and Landis, 1996; LeMasurier, 2008) involving al., 2003; Elliot and Fleming, 2004), dramatic intracontinen- a postulated orogenic plateau (Bialas et al., 2007; Huerta, tal extension occurred in Cretaceous time. Much of the basis 2007), the origins of the Southern Ocean’s diffuse alkaline of knowledge about the Ross Sea sector of the rift comes magmatism (Finn et al., 2005; Rocchi et al., 2005), the causes from ocean bottom seismograph, multichannel seismic for Cenozoic structural reactivation (e.g., Salvini et al., 1997; reflection, and gravity surveys that revealed a N-S structure Rossetti et al., 2003a,b) and seismicity (Winberry and Anan- of elongate basins marked by a positive gravity anomaly dakrishnan, 2004), and the affects of inherited structures and high seismic velocities in the lower crust and 1-4 km upon ice-bedrock interactions of the dynamic WAIS (Lowe of inferred Mesozoic sedimentary fill (Cooper and Davey, and Anderson, 2002; Holt et al., 2006; Vaughan et al., 2006; 1985; Cooper et al., 1997; Trey et al., 1997). Paradoxically, Sorlien et al., 2007). major sedimentary infilling of basins with material postdat-

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93 SIDDOWAY Extent of the West Antarctic Rift System (WARS) and These are intruded by Cretaceous alkalic plutonic rocks Character of WARS Crust (Figure 4) that are genetically linked to the WARS (Weaver et al., 1992, 1994). Lower Paleozoic Swanson Formation The region corresponding to the Cretaceous WARS includes represents one of the packages of voluminous quartz-rich the Ross Sea and Ross Ice Shelf, the area of the WAIS, and turbidites deposited in regionally extensive clastic fans shed Marie Byrd Land (Behrendt, 1991, 1999; Behrendt et al., from the Ross-Delamerian Orogen (Fergusson and Coney, 1991; Cooper et al., 1991a,b; Storey et al., 1999; Trey et 1992) or distant Transgondwana orogen (Squire et al., 2006). al., 1999). Measured orthogonal to the TAM, the WARS The rock assemblages that were contiguous along the East widens from 600 km in the south (Storey et al., 1999) to Gondwana margin (Figure 3) include the Swanson Forma- 1200 km across the northern Ross Sea (Luyendyk et al., tion in MBL (Bradshaw et al., 1983), the western Lachlan 2003). Thickness of continental crust ranges from 17-19 km Belt in Australia (Glen, 2005), the Robertson Bay group in for the Central and Eastern Basins of the Ross Sea to 23-24 north Victoria Land (NVL) (Rossetti et al., 2006), and the km beneath basement highs (Cooper et al., 1991b, 1997; Greenland Group in NZ (Cooper and Tulloch, 1992; Adams Davey and Brancolini, 1995; Luyendyk et al., 2001). There et al., 1995, 2005; Gibson and Ireland, 1996; Adams, 2004; is a similar range beneath central West Antarctica (Behrendt Bradshaw, 2007). et al., 1994; Bell et al., 1998). The crust underlying western The Swanson Formation was deformed and metamor- MBL is ca. 23 km thick, based on airborne geophysics (Fig- phosed to low greenschist grade (Adams, 1986) prior to ure 2) (Ferraccioli et al., 2002; Luyendyk et al., 2003). This emplacement of latest Devonian to Carboniferous calc- provides evidence that the region of western MBL that is alkaline plutons of the Ford Granodiorite (Figure 3) (Adams, above sea level is part of the WARS province. 1987; Weaver et al., 1991). Ford Granodiorite represents The western margin of the WARS extensional province the first in a succession of convergent margin arcs devel- coincides with the TAM, at the long-standing tectonic bound- oped upon the East Gondwana margin from Ordovician ary of the East Antarctica (EANT) craton that initiated as a through Early Cretaceous time (Pankhurst et al., 1998), and Neoproterozoic rift margin (Dalziel, 1997), underwent con- it has correlatives in New Zealand (Muir et al., 1996). Both vergence during the Ross Orogeny (Stump, 1995), and was Swanson Formation and Ford Granodiorite were affected by reactivated during the initial two-plate phase of Gondwana high-temperature (HT) metamorphism and their high-grade breakup in the Jurassic Era (Dalziel et al., 1987). Tholeiitic equivalents are exposed in the Fosdick Mountains migma- Ferrar magmatism (Elliot et al., 1999; Elliot and Fleming, tite gneiss dome (Siddoway et al., 2004b; Saito et al., 2007) 2004) and modest extension to transtension initiated in the (Figure 4). Temperatures in excess of 800°C, sufficient to WARS at this time (Storey, 1991, 1992; Wilson, 1993). The cause voluminous melting, were attained two to three times Cretaceous WARS has been inferred to be an asymmetrical in the history of the dome (Siddoway et al., 2006; Korhonen extensional system with the TAM forming the structural et al., 2007a,b). The most recent migmatization phase coin- upper plate and the WARS, the lower plate (e.g., Fitzgerald et cided with alkalic plutonism in MBL marked by Byrd Coast al., 1986; Stern and ten Brink, 1989; Fitzgerald and Baldwin, granite and mafic dikes (Weaver et al., 1992, 1994; Adams 1997; compare Lister et al., 1991). et al. 1995; Siddoway et al., 2005). Beneath the Ross Sea, gravity and marine seismic data Pankhurst et al. (1998) introduced the term “Ross delineate a crustal structure of N-S grabens, marked by high- Province” for the Swanson-Ford association in western density material in the axial regions, separated by basement MBL (Figure 2). Correlatives of the Ross Province exist highs. A large positive gravity anomaly in the basin axes is throughout the former Gondwana margin, including a interpreted as mafic igneous material emplaced into the lower number of culminations of HT metamorphic rocks derived crust (Cooper et al., 1997; Trey et al., 1997). Sedimentary from Paleozoic protoliths (Tulloch and Kimbrough, 1989; fill in the deep basins is cut by faults and overlapped by Morand, 1990; Ireland and Gibson, 1998; Vernon and John- a regional unconformity, RU6, that predates thick glacial son, 2000; Richards and Collins, 2002; Hollis et al., 2004) sediments. The thickness of Mesozoic to early Tertiary sedi- (Figure 4). It is probable that Ross-Delamerian orogenic ments is comparatively modest, reaching 4 km in the Eastern sediments and the intermediate plutonic rocks that intrude Basin, diminishing toward the coast of MBL (Luyendyk et them constitute the majority of the crust within the Ross al., 2001). Sea sector of the WARS (Bradshaw, 2007), its continua- In a region of thinned crust but exposed above sea tion into New Zealand (e.g., Cook et al., 1999), and into level, the Ford Ranges and Edward VII Peninsula (Figure the submerged extended continental crust bordering the 2) are key locations for examining the crust that constitutes Tasman Sea. Within the WARS crust, there is also sparse the WARS and observing the structures responsible for the evidence of “Ross-aged” basement rocks with an affinity to Cretaceous extension. The oldest rocks exposed are Swan- the TAM (Fitzgerald and Baldwin, 1997; Pankhurst et al., son Formation metagreywacke and Ford Granodiorite of 1998; Bradshaw, 2007). Paleozoic age (Bradshaw et al., 1983; Weaver et al., 1991).

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94 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD -135 -140 -145 -150 -155 -75 Nickerson Demas Ice Shelf Range Southern Ocean . d Gl Amundsen Lan Province s Ba Ford R a n g e lch Phillips Mtns. Fosdick Mts Executive Comm. Range en -76 Gl. s s ce Marie Prestrud Rock, Sc Nunatak ott Cape o R vin Byrd Bo m Colbeck Ha yd ond Land -77 m o G Pr Allegheny l. Gl. Alexandra Mtns Mtns Edward VII Peninsula Rockefeller Mtns st ed cru Ice elevation -78 in meters Area of sub-glacial 2500 volcanoes, inferred Trans Ross t end Area of from aeromagnetics Sea 2000 figure anta f ex rc t 1500 ic -79? Mo it o nt u ain 1000 s l im 500 ed err 0 Inf -80 FIGURE 2 Eastern Ross Sea and western Marie Byrd Land location map. Inferred limits of extended crust and a subglacial volcanic field, determined from airborne geophysics (Luyendyk et al., 2003) are indicated. The Ross Province (Pankhurst et al., 1998) of the Ford Ranges comprises lower Paleozoic sedimentary rocks intruded by Devono-Carboniferous intermediate plutons. The rock exposures east of Land Glacier are dominated by intermediate to mafic arc-related plutonic rocks, with subsidiary, younger alkalic intrusions; an association termed the “Amundsen Province” by Pankhurst et al. (1998). Base map by D. Wilson. THE ACTIVE MARGIN OF EAST GONDWANA AND of intermediate plutonism spanning the interval 124 to 96 Ma FORMATION OF THE MESOZOIC WEST ANTARCTIC (Pankhurst et al., 1998; Mukasa and Dalziel, 2000). RIFT SYSTEM The timing of HT metamorphism in NZ is determined by U-Pb ages on metamorphic zircon or titanite sampled from gneisses at sites distributed along the convergent margin. Convergent Margin Plutonism These include the Paparoa range at 119-109 Ma (Kimbrough Mesozoic convergent tectonism with intermittent subduc- and Tulloch, 1989; Ireland and Gibson, 1998; Spell et al., tion-related plutonism and terrane accretion is recorded 2000); and Fiordland at 126-110 Ma (Ireland and Gibson, in West Antarctica (Vaughan and Livermore, 2005) and 1998; Hollis et al., 2004; Scott and Cooper, 2006). Granulite contiguous parts of NZ (e.g., Bradshaw et al., 1997). The metamorphism documented in Fiordland at 108 ± 3 Ma (Gib- calc-alkaline, I-type Median Batholith was emplaced in NZ son and Ireland, 1995) gives an indication that elevated and between 145 Ma and 120 Ma, with some ages older, to 170 compressed crustal isotherms developed during convergent Ma (Muir et al., 1998; Mortimer et al., 1999a,b; Tulloch tectonism (Figure 5). and Kimbrough, 2003); and tectonic reconstructions show In the Fosdick Mountains gneiss dome in MBL, new continuity of the magmatic arc, together with associated U-Pb SHRIMP ages of 115 ± 1 Ma have been acquired for tectonic terranes, into MBL-Thurston Island (Figure 3) (e.g., igneous zircon within K-feldspar leucogranite equated with Bradshaw et al., 1997). The arc province in MBL, termed the anatectic leucosome, that has been sampled at deepest struc- “Amundsen Province” (Pankhurst et al., 1998), was the site tural levels (Siddoway et al., 2006). Nd isotope data indicate

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95 SIDDOWAY M ed ia n Ba th ol 15 ith Lachlan 0E Belt c. 80 Ma opening Bo RBT S we 60 rs Ter ra Sw ne 180° 300° 120°E 30 E W 60 000° FIGURE 3 Tectonic correlation between terranes of north Victoria Land, West Antarctica, and New Zealand/Campbell Plateau, compiled from Bradshaw (1989) and Bradshaw et al. (1997). Reconstruction of the Cretaceous East Gondwana margin is based on Gaina et al. (1998) and Kula et al. (2007), with oceanic plates configuration based on Sutherland and Hollis (2001) and Larter et al. (2002). Representation of oceanic plateaus is based on Taylor (2006) and Hoernle et al. (2004). Lower Paleozoic orogenic sediments are shown in olive green and tan. Belt of Cretaceous magmatism is shown in violet. Paparoa metamorphic core complex in the Western Province (WP), Fosdick gneiss dome in Marie Byrd Land (MBL), and detachment systems are marked by ellipses. TAS = Tasmania; RBT = Robertson Bay terrane; EP = Eastern Province; Sw = Swanson Formation; LHR = Lord Howe Rise; CR = Chatham Rise; TI = Thurston Island terrane. a Ford Granodiorite source for the leucogranites (Saito et al., 1998), those of the Separation Point and Rahu suites in the 2007) at T, P conditions of 820-870°C and 6.5-7.5 kbar deter- Western Province and the deeper level Fiordland Orthog- mined from mineral equilibria modeling (Korhonen et al., neiss in Fiordland. The Rahu suite granites are interpreted to 2007a,b). There is evidence of metamorphic zircon growth as derive from crustal melting of preexisting rocks (Ireland and early as ca. 140 Ma. A summary of U-Pb SHRIMP analyses Gibson, 1998). Thus, the conditions for HT metamorphism of igneous and metamorphic zircon from Fosdick Mountains and granite genesis in the Fosdick Mountains were attained migmatites (Figure 6) reveals that there is a bimodal distribu- and overlapped in time with arc plutonism in the Median tion of ages. Whereas HT metamorphism and zircon growth Batholith and in the Amundsen Province. is recorded as early as 150 Ma, a majority of points analyzed By contrast, the alkaline plutonism attributed to back-arc thus far fall within the interval of 120-100 Ma. Anatectic extension occurred in eastern MBL (Figure 5) at 105-102 Ma leucosomes from sites in MBL’s Amundsen Province, the (Weaver et al., 1992, 1994; Mukasa and Dalziel, 2000), dis- Demas Range (Figure 2) yield ages of 128 Ma to 113 Ma tinctly later than onset of high temperature metamorphism. In for igneous zircon (Mukasa and Dalziel, 2000). The MBL western MBL the Ford Ranges experienced alkalic plutonism data fall within the 126-107 Ma age range of the youngest at 105-103 Ma and ca. 99 Ma (Richard et al., 1994) and in arc-related intrusions identifid in NZ by Muir et al. (1997, Edward VII Peninsula at 103-98 Ma (Mukasa and Dalziel,

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96 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD e 147 E 145 E 143 E ur ct ru st c oi oz PHILL IPS MOUNTAINS B en N AL C C H ed EN rr GL fe AC in IE ? BA R LCH FAULT GLACIER EN - Mt. Mt ? Lockhart Mitchell Pk Bitgood Mt. Bird 88 5 Ma ? Avers Bluff ? 13.7 1 m FOS DICK MOUNTAINS lt 76 30’S M u SF fa M DZ Mt. s Mt. in Richardson Getz rk e P t. M IER M AC Chester Mtns ? GL EY 95 6 Ma LL VA 13.5 1.6 m SE VAS SULZBERGER CRE A n = 33 MT. PASSEL M U D U D M Swanson Rg. 1000 m B IER AC n = 13 GL ARTHUR H ICE M A The M Billboard M Mt. Crow 91 4 Ma O 77 S SA N RN D B O U FF D O MT N S H E LF S.. Y G D LA ALLEGHENY G LA MTNS. C Mt. Darling CI IE ER CLARK C R Mt. Woodward. n = 42 MTNS. Mt. Douglass in 97 5 Ma fe 13.7 1 m rr ed fa ul t McKAY Contour interval, 200m MTNS. L= 5 D F E n = 21 n=6 n = 16 10 20 30 40 0 50 km Key to Units K Byrd Coast Granite K Q Fosdick migmatite D-C Ford Granodiorite ePz Swanson Formation Basalt, Pleistocene FIGURE 4 Structural-geological map of the Ford Ranges, western Marie Byrd Land. Inferred faults that are concealed by ice are mapped on the basis of contrasts in metamorphic grade between ranges, geophysical lineaments or boundaries, and zones of penetrative brittle deforma- tion in rock exposures. AFT cooling ages and track lengths (Richard et al., 1994; Lisker and Olesch, 1998) are indicated for selected sites. Brittle mesoscopic fault data are shown in stereographic plots (insets). Sites and kinematic sense are as follows: (A) southern Ford Ranges, normal oblique; (B) Mt. Darling, sinistral (Cenozoic); (C) southern Ford Ranges, sinistral oblique; (D) Sarnoff Range, normal oblique; (E) Mt. Crow, sinistral; (F) Mt. Woodward, sinistral oblique. The label “M” indicates sites of glacial deposits examined for clast provenance. Stereonet v. 6.3.3 and FaultKin 4.3.5, by R. Allmendinger 1989-2004, were used for plotting stereographic diagrams. Shaded relief ice topography and base map prepared from Antarctic Digital Database by G. Balco.

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97 SIDDOWAY FIGURE 5 Conceptualization for development of the West Antarctic rift system inboard of the Mesozoic convergent margin during oblique plate convergence and subduction of young oceanic lithosphere, including oceanic plateaus. Top: Crustal thickening and advective heating during development of Amundsen province magmatic arc; active margin undergoing transpression due to oblique convergence. Middle: Heat- ing of the lower crust causes partial melting and lateral flow in the middle and lower crust; thermal gradient is increased. Thickening of the crust continues but the lower lithosphere thins. Upper crust undergoes brittle faulting. Bottom: Change to transtension, with oblique opening across preexisting high-angle faults. Lateral flow of hot, weak, partially molten lower crust is accompanied by brittle deformation in shallow upper crust. Strain perturbation along faults allows localized gravity-driven vertical flow of lower-density migmatite-diatexite and formation of gneiss dome(s). Isotherms are elevated, tectonic exhumation and cooling are enhanced, next to transcurrent faults.

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98 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD 2000; Siddoway et al., 2004a). The dominant plutonic rock a view of crustal rheology, kinematics, and dynamics of is Byrd Coast Granite (Figure 4). Mafic alkalic dikes and Cretaceous tectonism that pertain to the West Antarctic rift syeno-granites were emplaced over a wide region. A dolerite system as a whole. dike swarm at 107 ± 5 Ma was followed closely by 102-95 Ma syenite and alkalic granite in the Amundsen province The Fosdick Mountains Gneiss Dome (Storey et al., 1999). A wider a range of dike ages, 142 to 96 Ma, comes from the Ross Province (Siddoway et al., 2005). The Fosdick Mountains form an elongate migmatite gneiss In the Ross province an early phase of Byrd Coast alkalic dome (Wilbanks, 1972; Siddoway et al., 2004b) delimited granite from the Allegheny Mountains is ca. 142 Ma, and by a S-dipping, dextral-oblique detachment zone on the another at Mt. Corey is 131 Ma (Figure 2) (Adams, 1987). south (McFadden et al., 2007) and by an inferred steep Subduction ceased in New Zealand at 105 ± 5 Ma (Muir et dextral strike-slip zone on the north, the Balchen Glacier al., 1994, 1995, 1997, 1998). fault (Siddoway et al., 2004b, 2005). From lower to higher Alkalic magmatism in NZ coincided with extension, structural levels, gneisses that exhibit features indicative of development of a regional unconformity, and dramatic melt-present ductile flow give way to mylonitic rocks exhib- sedimentation, including thick deposits of sedimentary iting mixed ductile-brittle deformation textures, indicative of breccia (Laird and Bradshaw, 2004). Metamorphic core solid-state deformation (McFadden et al., 2007). Kinematic complexes developed in South Island (Figure 3) (Tulloch and axes calculated from nappe-scale folds and subsidiary folds, Kimbrough, 1989; Forster and Lister, 2003), together with mineral lineation, and anisotropy of magnetic susceptibility deep level shear zones that were active in Fiordland (Gibson (AMS) fabrics within the 15 80 km dome are subhorizon- et al., 1988; Scott and Cooper, 2006). tal, 065 to 072. The orientation is oblique to the long axis of The Lachlan belt on continental Australia and Robertson the dome and to the Balchen Glacier fault. Bay terrane in NVL occupied an inboard position in middle The migmatite gneisses forming the core of the Fosdick Cretaceous time and did not experience tectonism related Mountains dome reached temperatures (T) and pressures (P) to the active margin (Figure 3), although Australia-EANT of the upper amphibolite to granulite facies (Siddoway et al., breakup was in its initial stages (Li and Powell, 2001, and 2004b; Korhonen et al., 2007a). Granite formed by biotite references cited). Remnants of the active margin are the breakdown (Saito et al., 2007) forms sheets, stocks, and submarine plateaus that border the Tasman Sea and provide extensive interconnected networks on a scale of hundreds a sparse geological record of the change in plate dynamics of meters. Leucogranite occupies structural sites—within (Tulloch et al., 1991; Mortimer et al., 1999a, 2006; Mortimer, foliation-parallel sheets, shear bands, and interboudin 2004). necks—suggestive of deformation-enhanced migration and coalescence of melt products (Sawyer, 2001). Concordant layers of leucogranite may exceed 10 m in thickness. New GEOLOGICAL STRUCTURE OF THE FORD RANGES, U-Pb SHRIMP zircon studies (Siddoway et al., 2004b, 2006; MARIE BYRD LAND: DATA BEARING ON TECTONISM McFadden et al., 2007) and isotope geochemistry (Saito et IN THE WEST ANTARCTIC RIFT SYSTEM al., 2007) aid in the task of determining the extent and distri- In MBL the general absence of dynamic fabrics in plutonic bution of anatectic granites formed in Late Cretaceous time, units and the elusive nature of crustal-scale faults in a region coincident with development of the WARS. with extensive ice cover have long hindered the understand- At intervals along the Fosdick Mountains dome, leuco- ing of the strain evolution. New progress has been made in granite sills form vertical sequences that reach 1000 m in the region through tectonic investigations focused on the thickness (“leucogranite sheeted complex” of McFadden et structure and metamorphic petrology of the Fosdick Moun- al., 2007). The thin layers (<1-3 m) of para- and orthogneiss tains gneiss dome (Siddoway et al., 2004b; Korhonen et al., that separate the sills contain microstructures indicative of 2007a; McFadden et al., 2007), the configuration of mafic the former presence of melt and of deformation mechanisms dikes representing a regional tensile array (Siddoway et al., dominated by melt-assisted grain boundary diffusion creep. 2005), and kinematic analysis of mesoscopic brittle faults Kinematic data obtained from the horizons of shallowly (Luyendyk et al., 2003). Airborne geophysical data over dipping paragneiss and orthogneiss include fold axes of the Ford Ranges (Luyendyk et al., 2003), and Edward VII symmetrical, tight to isoclinal recumbent folds that trend Peninsula (Ferraccioli et al., 2002) delineate regional-scale 062-242 (n = 118), and sparse mineral lineation aligned faults. Geochemical investigations reveal the granite petro- 072-252 (n = 38) (Siddoway et al., 2004b). The consistent genesis (Pankhurst et al., 1998; Mukasa and Dalziel, 2000; linear data suggest ENE-WSW stretching under suprasolidus Saito et al., 2007) and U-Pb geochronology provide critical conditions when the leucogranite sheets were emplaced. The age control (Siddoway et al., 2004a,b, 2006; McFadden et layers of paragneiss and orthogneiss in this setting are “dia- al., 2007). A review of the recent findings from MBL in the texitic” (Brown, 1973; Milord et al., 2001), in that the color next section will begin with the lower crustal exposure pro- distinction between leucosome and melanosome is subdued, vided by the Fosdick Mountains, where migmatites provide with melanosome a light grey color, and boundaries between

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99 SIDDOWAY the light- and darker-colored portions, indistinct. Diatexite dextral normal oblique shear sense, with top-to-the-SW textures indicate a high degree of chemical interaction of transport along azimuth 240. Foliation dips steepen from leucogranite melt with host gneisses under suprasolidus west toward east, and give way to strong subhorizontal L- conditions (Sawyer, 1998, 2004). Conventional thermo- tectonite fabrics trending 070-075 within Ford-phase grano- barometry carried out on the diatexitic paragneisses yielded diorite at Mt. Richardson (Figure 4). U-Pb SHRIMP zircon P = 4 kbar to 6 kbar and T = 680°C to 780°C (Smith, 1992, data bracket the time of deformation upon the South Fosdick 1997; Siddoway et al., 2004b). New results of comparative detachment zone between 107 Ma and 96 Ma (McFadden et thermobarometry using THERMOCALC indicate consider- al., 2007). ably higher conditions of 820-870°C and 6.5-7.5 kbar for the Cretaceous peak (Korhonen et al., 2007a,b). The leucogranites exhibit compositional layering and igneous microstructures, such as euhedral grains and til- 30 Bins, 2 Ma ing of large feldspars; together with evidence of magmatic solid-state deformation (Blumenfeld and Bouchez, 1988; 25 Fosdick Mountains, U-Pb zircon from leucogranite (igneous zircon Weinberg, 2006) such as mechanical kinking at grain-to- and metamorphic rims), n = 127 grain contacts in coarse-grained phases. The textures sug- 20 gest that the interstices between solid phases represented a permeable melt network through which melt flowed. The 15 migmatite structures of the Fosdick Mountains suggest that sills and leucosome networks are remnants of a melt transfer 10 system that allowed magma flux through a zone of anatexis (e.g., Olsen et al., 2005; Weinberg, 2006) within the middle 5 and lower crust (e.g., Brown and Pressley, 1999; Brown, 2007). Deformation aided melt-migration and melt enhanced 0 deformation in a mutually complementary process. 15 Buckland granite, Median Batholith, U-Pb zircon The extensive Cretaceous leucogranites within the (Tulloch and Kimbrough, 2003), U-Pb monazite (Ireland and n = 56 gneiss dome contain a dominant population of prismatic Gibson, 1998), 10 n = 20 igneous zircons. The bipyramidal, elongate zircon grains exhibit oscillatory zoning and lack inheritance, suggesting 5 that they crystallized from a melt. U-Pb SHRIMP ages deter- mined for the zircons are 115-101 Ma (Figure 6), suggesting 0 that elevated temperatures were attained and that melt trans- 30 fer and crustal flow initiated during middle Early Cretaceous Relative Probability (oblique?) plate convergence, then continued during the 25 Fiordland Orthogneiss and transition to extension/transtension in the WARS. paragneiss, U-Pb zircon (Hollis et al., 2004), n = 128 20 Number South Fosdick Detachment Zone 15 The leucogranitic sheeted complex passes upward into metatexite at highest structural levels on the southern flank 10 of the range. Metatexite (Brown, 1973; Milord et al., 2001; Sawyer, 2004) is a migmatite type that consists of meso- 5 scopic, cm- to dm-scale compositional layering with a sharp color distinction between light-colored quartzofeldspathic 0 leucosomes and dark, biotite-rich melanosomes. Leucosomes 90 100 110 120 130 140 150 Age, Ma that are volumetrically minor represent a mobile portion, or metatect, and melanosomes, a nonmobilized, depleted com- FIGURE 6 Comparison of U-Pb SHRIMP frequency distribu- ponent (Brown, 1973; Milord et al., 2001; Sawyer, 2004). tion for Fosdick Mountains leucogranites, with New Zealand data Sills and interconnected networks of leucosome that would including from the Fiordland Orthogneiss (Hollis et al., 2004), be indicative of melt transfer and coalescence are poorly Median Batholith (Tulloch and Kimbrough, 2003), and Buckland developed to absent in the melanosome. Granite (Ireland and Gibson, 1998). Fosdick Mountains data were Solid-state deformation is indicated by pervasive acquired on SHRIMP II at Australian National University under mylonitic microstructures indicative of plane strain-simple the direction of C. M. Fanning. The relative probability plots with shear, including C-S fabrics and asymmetric porphyroclasts stacked histograms of 206Pb/238U ages (207Pb corrected) were with tails (McFadden et al., 2007). Kinematic criteria show calculated using ISOPLOT/EX by K. Ludwig.

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100 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD Structural Analysis of Mesoscopic Brittle Fault Arrays planes (Figure 4, inset D). The array is expressed both as Throughout the Ford Ranges minor faults and shear fractures with kinematic criteria indicative of oblique slip, with top-to-the-ESE translation Geometrical and kinematic data have been gathered for sys- (Luyendyk et al., 2003). The dominant orientation in this tematic mesoscopic brittle structures, including dikes, faults, array is ESE-striking, with dextral normal oblique kine- shear fractures, and joints that cut the isotropic plutonic units, matic sense. A second generation of brittle structures in Ford Granodiorite, and Byrd Coast Granite. Structures that the Sarnoff and Denfield ranges consists of NNW-striking, cut Byrd Coast Granite or mafic dikes, the majority of which normal- to oblique-slip shear fractures (Figure 4, inset C). fall in the age range 104-96 Ma, are known to be middle The widespread NNW-oriented mesoscopic structures have Cretaceous or younger. Mesoscopic faults are striated planes strikes parallel to the regional mafic dike array, and to the accommodating >2 m offset or zones of cataclasis exceed- prevalent fault orientations offshore in the easternmost Ross ing 15 cm thickness. The term “shear fractures” refers to Sea (Luyendyk et al., 2001; Decesari et al., 2003). A late slickenside surfaces, sometimes mineralized but generally ENE-oriented array is strongly expressed in the Chester lacking gouge, and rarely associated with geological mark- Mountains (Figure 4), forming chloritic brittle shear zones ers that allow quantification of offset. In most instances, up to 15 cm in width, and chloritic and oxidized shear frac- therefore, brittle criteria are used for interpretation of shear tures. Brittle criteria on the planes oriented N75E (mean) sense (Marrett and Allmendinger, 1990). Few data come indicate normal dextral oblique slip upon SSW- and NNE- from Swanson Formation, because brittle shear planes typi- trending striae (Figure 4, inset A). cally reactivated bedding or preexisting cleavage, making the NE-oriented, sinistral strike-slip shears are spatially kinematic significance uncertain. associated with inferred Cenozoic faults that trend NE-SW Outside of the Fosdick Mountains migmatite dome and offset the Fosdick Mountains gneiss dome (Figure 4, most exposures of crystalline rocks in the Ford Ranges lack inset B). Pleistocene mafic lavas erupted from small volca- dynamic fabrics (Siddoway et al., 2005) and mylonitic zones nic centers along the trend (Gaffney and Siddoway, 2007) are found only rarely. Four sites hosting ductile shear zones (Figure 4). A brittle fault data set comes from the series are situated near locations for which thermochronology data of outcrops forming the easternmost exposures in the Ford are now available (Table 1). Narrow mylonitic shear zones Ranges, which is situated near a prominent NE-trending (1-5 m wide) cut Ford Granodiorite at Mt. Crow (Figure 4, escarpment imaged in the bedrock topography (Luyendyk inset E) and Mt. Cooper (Siddoway et al., 2005). High-tem- et al., 2003) that corresponds with a geophysical anomaly perature shear zones exist along the present-day Ross Sea arising from inferred sub-ice volcanic centers (Figure 7). coast, at Mt. Woodward (60 m exposed width) (Figure 4, These are NE-SW shear fractures with strike slip striae, inset F) and at Prestrud Rock (30 m minimum width). Each with consistent sinistral-sense offset from brittle kinematic of the sites is adjacent to an inferred crustal-scale strike-slip criteria (Figure 4, inset B). zone that is concealed by ice (Ferracioli et al., 2002; Luyen- With respect to timing, regional deformation caused by dyk et al., 2003; Siddoway et al., 2005). mid-crustal flow arising from melt accumulation in the lower Mafic dikes provide a very valuable kinematic dataset and middle crust (Figure 8) was under way as early as 115 due to their regional distribution and very consistent regional Ma, based on the ages determined for melt-present deforma- orientation of azimuth 344, subvertical, throughout the Ford tion in the lower crustal exposures in the Fosdick Mountains Ranges. Tensile opening perpendicular to the dike margins rocks. The older limit on the time of brittle deformation and is a reflection of ENE stretching at the time of emplace- formation of mylonitic zones in the upper crustal rocks of ment. The dike array cuts Byrd Coast granite of 102-98 the Ford Ranges is provided by Cretaceous plutonic rocks Ma age, and the range of 40Ar/39Ar ages for the majority of 104-96 Ma age (Byrd Coast granite and mafic alkalic of the mafic dikes is 104-96 Ma (groundmass concentrates dikes) that are cut by brittle faults. The cooling history of the on microcrystalline dikes; Siddoway et al., 2005). U-Pb block-faulted mountain ranges constrained by 40Ar/39Ar and titanite and 40Ar/39Ar hornblende ages for discordant mafic apatite fission track thermochronology (summarized below) dikes within the Fosdick range are 99-96 Ma (Richard et al., provides a younger age limit on regional tectonism. 1994; Siddoway et al., 2006). Mutually crosscutting relation- ships between mafic dikes and faults indicate that they are Crustal Structure from Airborne Geophysics contemporaneous. Brittle fault data offer the most tenuous data to inter- Airborne gravity and radar soundings over western MBL pret due to the paucity of offset markers and the need to indicate that the crustal thickness beneath the Ford Ranges use brittle criteria for kinematic shear sense. Nonetheless, is 22-25 km, increasing to the north and inland by 8-9 km consistent fault and shear fracture arrays are identified. for central MBL (Figure 2) (Luyendyk et al., 2003). The In the central and southern Ford Ranges a well-defined inferred steep gradient in crustal thickness coincides spa- ~NW-SE-oriented conjugate fault array hosts moderately tially with the linear northern front of the Fosdick Moun- oblique, SE-plunging striae on both SSW and NE dipping tains, where migmatites were exhumed from mid-crustal

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101 SIDDOWAY TABLE 1 Summary of 40Ar/39Ar and AFT Thermochronology Data for Sites in the Central and Eastern West Antarctic Rift System AFT Track Location Feature Age (Ma) Length (mm) Method Source of Data Field Association Kinematics 40 39 Ford Ranges Mafic dikes 104-96 n.a. Ar/ Ar Siddoway et Tabular, vertical to Tensile, 074-254 groundmass al., 2005 sub-vertical dikes throughout the Ford Ranges 40 Ar/39Ar biotite Mt. Cooper Mylonite zone 96.92 ± 0.34 n.a. Siddoway et 3- to 5-m-wide Normal sense, al., 2005 zone cutting Ford down to East Granodiorite Prestrud Rock Gneiss 91 ± 4 13.1 ± 0.2 AFT Lisker and Contrast in grade Strike oblique; Olesch, 1998; and fabrics; strong kinematic sense Smith, 1996 lineation suggest not determined shear zone The Billboard Unfoliated Ford 91 ± 4 n.a. AFT Lisker and Ford granodiorite Strike normal granodiorite Olesch, 1998 bounded by an oblique, inferred inferred east-west dextral fault; borders inferred east-west fault Mt. Douglass Unfoliated Byrd 97 ± 5 13.7 ± 0.2 AFT Lisker and Unfoliated Byrd Sinistral oblique Coast granite Olesch, 1998 Coast granite, sense; shear zone located 6 km >100 m wide from shear zone in calcsilicate gneisses at Mt. Woodward Chester Unfoliated Ford 95 ± 6 13.5 AFT Lisker and Ford granodiorite Hanging wall of Mountains granodiorite Olesch, 1998 South Fosdick detachment Mitchell Peak Migmatite gneiss 88 ± 5 14.2 ± 0.1 AFT Lisker and Migmatite gneiss Hanging wall of Olesch, 1998 South Fosdick detachment DSDP 270 Calc-silicate 90 ± 6 n.a. AFT Fitzgerald and Cataclasite/breccia Not possible to gneiss Baldwin, 1997 in detachment determine zone Colbeck Mylonite 86 ± 5 13.8 ± 1.4 AFT Siddoway et Dredged material Not possible to Trough al., 2004a from submarine determine Mylonite 71 ± 5 14.1 ± 1.3 AFT escarpment 40 Ar/39Ar Mylonite 98-95 n.a. K-feldspar, biotite NOTE: n.a. = not applicable; AFT = Apatite fission track; DSDP = Deep Sea Drilling Project. Kinematic determinations from Siddoway et al., 2005, or Siddoway, unpublished. depths. A steep gradient also is observed in the magnetics, with a postulated NE-SW strike slip fault of Cenozoic age and there is a well-defined lineament in the bedrock topog- (Siddoway et al., 2005) (Figure 4). raphy beneath Balchen Glacier (Figures 2 and 7). Present- Linear magnetic anomalies on Edward VII Peninsula day bed topography for much of the surveyed area defines have three dominant trends. They trend ~E-W, NNW, and distinct NNW-SSE and NE-SW-oriented lineaments that NE (Ferraccioli et al., 2002). Outlet glacier troughs in the are oblique to the density distributions. They are generally southern Ford Ranges and Edward VII Peninsula appear to be parallel to the NNW-oriented, normal-sense and NE-striking, controlled by the NNW trend (Luyendyk et al., 2001; Wilson sinistral-sense, second generation shear fractures measured and Luyendyk, 2006; Sorlien et al., 2007), and have a narrow, throughout the Ford Ranges and to the mafic dike array. deep linear morphology that suggests an association with the High-gradient magnetic anomalies inferred to be subglacial regional normal fault array. volcanoes of Cenozoic age are mapped to the east (Figures 2 Magnetic and gravity anomalies together with bed- and 4) (Luyendyk et al., 2003), and are spatially associated rock topography calculated from airborne geophysics data

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104 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD information was obtained for the structures since sample time of development of the eastern WARS. The younger retrieval was by drill core (Ford and Barrett, 1975; Hayes phase of rapid cooling at ~75 Ma reflects regional uplift and and Davey, 1975) and dredge (Luyendyk et al., 2001). DSDP cooling, coincident in time with and attributable to modest 270 yielded a multicomponent AFT sample from a few denudation in response to onset of seafloor spreading and grains of apatite extracted from calcsilicate gneiss (n = 16) separation between WANT and New Zealand, upon a new (Fitzgerald and Baldwin, 1997). The dominant component divergent plate boundary that continued in to the Tasman Sea is 90 ± 6 Ma in age. (Figure 3a) (Gaina et al., 1998; Sutherland, 1999; Kula et al., A recently discovered shear zone site that yields critical 2007). The observation that rapid cooling occurred first upon kinematic data is at Mt. Woodward, bordering a pronounced discrete WARS fault zones (101-92 Ma) suggests a localized lineament along the Haines Glacier (Figure 4, inset F). landscape response, reflected in the thermochronology cool- ing histories data. The affected area covers 250,000 km2 of The steep high-strain zone developed in high-temperature calcsilicate gneisses is oriented 160-340 and exceeds 100 western Marie Byrd Land and the neighboring Ross Sea. m in width. Asymmetrical folds indicate sinistral shear sense (Siddoway, unpublished). The thermochronology data DISCUSSION obtained from Mt. Douglass, 6 km away, yield the region’s oldest AFT cooling age of 97 ± 5 Ma on Byrd Coast granite The Role of a Hot Middle Crust in the Regional Structural (Lisker and Olesch, 1998). Northwest-southeast-oriented Evolution of the WARS bedrock faults are inferred to control the Sarnoff Range trend, where pronounced narrow troughs, oriented 150- The Fosdick Mountains gneiss dome is a structure of vast 330, are evident in the bedrock topography (Luyendyk et complexity pervaded by sills and discordant networks of al., 2003). A 3-m-wide mylonitic shear zone at Mt. Crow leucogranite. Crosscutting relationships and varying degrees (Figure 4, inset E) parallels this trend and offers kinematic of deformation suggest multiple cycles of melt migration information that possibly is representative of the concealed and emplacement within structurally controlled, dilatant fault. The Mt. Crow shear zone exhibits shallow-plunging sites. Thick sills of leucogranite containing microstructures sinistral-sense stretching lineation oriented 20, 138, on steep indicative of horizontal magmatic flow are interlayered with foliation. An AFT age of 91 ± 4 Ma (Lisker and Olesch, thin sheets of diatexitic gneisses that exhibit consistent ENE- 1998) came from Ford Granodiorite at The Billboard in the WSW kinematic sense, leading to the interpretation that the Sarnoff Range (Figure 4). Fosdick gneiss dome represents an exposure of deep middle The remaining sites with AFT data in the older age crust that underwent directional viscous, magma-like flow range (Table 1) are associated with the hanging wall of the (Figure 8). South Fosdick detachment zone. They are Mitchell Peak, Relationships in the Fosdick Mountains dome suggest the isolated nunatak forming the westernmost outcrop in that partial melting and rheological weakening of the crust the Fosdick range, and the Chester Mountains, south of the in MBL was a consequence of crustal heating during oro- Fosdick range (Figure 4). There is a pronounced contrast genesis, affecting “Lachlan”-type sedimentary rocks and in cooling age across the South Fosdick detachment zone middle Paleozoic intermediate plutons. Argillaceous rocks of (Figure 4), with 95 Ma to 88 Ma AFT ages obtained from Lachlan type are chemically fertile (e.g., Thompson, 1996) sites in the hanging wall, and 76 Ma to 67 Ma from sites in and may generate substantial quantities of melt. Subjected the gneiss dome core (Table 1). to a differential stress in a convergent orogen or to gravity Remaining AFT localities in western MBL record mod- forces in the region of thickened crust at the convergent erate to slow cooling between 83 Ma and 67 Ma. The ages margin, viscous flow commenced (Figures 5 and 8). The correspond to the time of initiation of seafloor spreading localization of strain at the interface between the region of between Campbell Plateau (NZ) and WANT (Figure 1) at hot versus cold crust caused detachment structures to initi- 83-79 Ma (chron 33r) (McAdoo and Laxon, 1997; Larter ate (Figure 8b) (e.g., Teyssier et al., 2005) and/or reactivated et al., 2002; Stock and Cande, 2002; Eagles et al., 2004), preexisting faults (Siddoway et al., 2004b, 2005), leading to suggesting that the second AFT cooling pulse was triggered gneiss dome emplacement (Figure 8c). by the lithospheric separation (Figure 9c) (Siddoway et al., The development of thermal perturbations of this type in 2004a; Kula et al., 2007). There is a good correspondence in a convergent margin setting has been noted as a characteristic timing and tectonic history of MBL localities with detach- of hot accretionary orogens (Collins, 2002a). The Lachlan ment structures in New Zealand (Kula et al., 2007). belt exemplifies this type of orogen as it has undergone In summary, examination of AFT data together with multiple cycles of contractional orogeny and extensional mapped structures shows that the early stage of rapid cool- collapse involving HT metamorphism (Foster et al., 1999; ing in western Marie Byrd Land at 95-85 Ma was localized Collins, 2002a,b; Gray and Foster, 2004; Fergusson et al., upon high-angle conjugate wrench zones. The timing of 2007). In MBL the elevated heat flow into the base of the fault activity determined from U-Pb zircon geochronology, continental crust may have arisen during subduction of hot 40 Ar/39Ar, and AFT thermochronology corresponds with the oceanic lithosphere newly formed at the Phoenix-Pacific

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105 SIDDOWAY FIGURE 8 Cross-sections illustrating the consequences of presence of a hot, weak, partial melt-rich horizon in the lower crust. Portrays the geometry and kinematics of brittle structures in the upper crust and propagation of viscous flow region within the middle to lower crust. Top: Crustal heating and crustal thickening within convergent margin setting, wrench faults active. Middle: Partially molten crust flows laterally, advecting heat into new regions and weakening the crust. Bottom: Transtension affects a large region of warm crust near the active margin. Sites of melt transfer and accumulation exhibit vertical translation if focused upon a fault. X, O symbols indicate motion out of and in to the plane of the profile. ridge (Figures 4 and 5b) (Bradshaw, 1989; Luyendyk, 1995), that had been thickened during Mesozoic convergence (e.g., or due to back arc extension and lithospheric thinning (Figure Collins and Hobbs, 2001; McKenzie and Priestly, 2007), 5b and 5c) (Weaver et al., 1991, 1994; Mukasa and Dalziel, augmented by heat advection by fluids. The rheological par- 2000); or from basal heating in the presence of a postulated titioning interpreted to exist in the Cenozoic WARS, with a mantle plume (Weaver et al., 1994). Singly or collectively brittle to ductile gradient across the Ross Sea (Salvini et al., these factors could promote magmatic underplating and 1997) may have been established at this time. advection of heat into the crust, with corresponding effects The question of whether the substantial volumes of leu- on crustal rheology (Regenauer-Lieb et al., 2006). Further cogranite derived from constituent gneisses of the Fosdick effects could arise from infiltration of fluids into the over- dome (phases of Ford Granodiorite, Swanson Formation) or riding plate due to dehydration of the newly subducted slab, were produced within other parts of a crustal zone of magma or to radiogenic heat production in the East Gondwana crust flux (e.g., Olsen et al., 2005) then migrated into the dome

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106 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD FIGURE 9 Development of the WARS in three hypothesized stages, based on the observations of strain accommodated upon wrench and extensional fault systems in Marie Byrd Land, a part of the WARS that resides above sea level. Basement graben configura- tion based on Cooper et al. (1991b) and Trey et al. (1997). (A) 130 Ma to 100 Ma, interpreted configuration of the East Gondwana margin. HT metamorphism and crustal melting (ellipse symbol) is contemporaneous with arc magmatism at inboard sites including Fiordland (F); Paparoa complex in Western Province, NZ (WP); and Fosdick Mountains gneiss dome in MBL. Diachroneity of calcalkaline plutonism in the Median Batholith and Amundsen province may be a reflection of subducted slab geometry or the configuration of the Phoenix-Pacific spreading ridge offshore (Fig- ures 3 and 5). Sites of alkalic magmatism may be an expression of dilational jogs in wrench zones or within detachment structures. TAS = Tasmania; EP = Eastern Province, NZ; CR = Chatham Rise; TI = Thurston Island. (B) 100-90 Ma, major phase of intracontinental deformation in the WARS. Tensile dike arrays and alkalic plutons were emplaced across the back-arc region. Blue shaded areas are gravity anomalies corresponding with high-density material along basin axes (Cooper et al., 1991b). Differential movement upon steep wrench zones is recorded by 40Ar/39Ar and AFT data that record rapid cooling between 97 Ma and 90 Ma for fault zone samples (Table 1). Based on the available data there appears to be an age progression from northeast toward southwest, with site DSDP 270 in the Ross Sea recording the youngest of the older subset of cooling ages, if the dominant population is accepted as the AFT cooling age (Fitzgerald and Baldwin, 1997). Dominant wrench deformation is documented in Marie Byrd Land, and prevalent normal faulting is inferred in the Ross Sea. The regional strain variation may be due to contrasts in competency of the pre-Mesozoic continental lithosphere, contrast- ing thermal conditions arising from lithospheric thinning or the dynamics of the convergent plate boundary (e.g., Figure 8), or the transition from a region undergoing oblique subduction of young continental lithosphere (eastern WARS) to one experiencing slow rifting between mature continental crust of Antarctica-Australia (western WARS). (C) Ca. 80 Ma, the time of breakup between WANT and NZ- Campbell Plateau. Continental extension across the WARS and Campbell Plateau exceeded 100 percent and was completed prior to onset of seafloor spreading. Blue shaded areas are gravity anomalies corresponding with high-density material along basin axes; red areas are bathymetric (and basement) highs (Cooper et al., 1991b). Extension direction for breakup was nearly orthogonal to that for WARS opening and the rifted margin cuts at a high angle across Ross Sea basins (Lawver and Gahagan, 1994). It is probable that a preexisting wrench fault structure was reactivated at the time of breakup. This would explain the exceptionally abrupt ocean-con- tinent boundary along the coast of Marie Byrd Land (Gohl, 2008, this volume).

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107 SIDDOWAY upon structurally controlled pathways, is being addressed by 2004b), suggests that a crustal to lithospheric-scale discon- isotopic and geochemical investigation. A petrogenetic link tinuity has a role in gneiss dome emplacement. between Ford Granodiorite (source) and Byrd Coast granite The predominance of subhorizontal fabrics (rather than (product) has been demonstrated in the region (Weaver et al., vertical geometry expected for strike slip faults in the brittle 1991; Pankhurst et al., 1998). New Nd isotope data from the upper crust) is considered to be either (1) an expression of Fosdick Mountains strengthen this interpretation (Saito et al., coupling between crustal layers of contrasting compentency 2007). Therefore it is plausible that regional melting of Ford (e.g., metatexite versus diatexite plus leucogranite) that Granodiorite contributed to anatectic granite magmas that accommodates strain by different mechanisms (e.g., Tikoff were capable of vertical or lateral migration during oblique et al., 2002), or (2) accentuation of vertical shortening at the convergence (Weaver et al., 1995) to transtension (Siddoway “melt propagation front” for melt-rich diatexite-leucogranite et al., 2005) along the Early Cretaceous plate margin. as melt-rich material migrated upward and was arrested at HT metamorphism and exhumation of deep crustal the thermal or permeability boundary (e.g., Sawyer, 2001) rocks on detachment structures are documented over a wide represented by metatexite (Figure 8) or (3) a change in ori- region proximal to the Gondwana margin arc in New Zealand entation of the shortening axis of strain due to unroofing, to and MBL (Kimbrough and Tulloch, 1989; Fitzgerald and coincide with direction of gravitational load. Baldwin, 1997; Forster and Lister, 2003; Siddoway et al., 2004b; Kula et al., 2007), suggesting pervasive middle to Overview of the West Antarctic Rift System lower crustal flow and advection of magmatic heat (Ehlers, 2005) over a large region (Figures 8 and 9). The existing Structural and geochronological data from sites throughout geochronological and thermochronological data reviewed the eastern WARS show a broad compatibility with respect above suggest a comparatively short and dynamic develop- to ENE coordinates for principal finite strain and timing of ment of the Fosdick gneiss dome and other strike slip shear crustal thinning deformation at ca. 105-95 Ma. The kine- zones active in the eastern WARS in Cretaceous time. U-Pb matic compatibility between structures of the brittle upper SHRIMP zircon data indicate that HT metamorphism was in and viscous lower crust is a great aid to interpretation of the effect by 140 Ma and growth of new igneous zircon within mechanisms of formation of the WARS. The best-exposed, the anatectic granites was under way by 115 Ma (Figure 6) crustal-scale structure with lateral extent in Marie Byrd (Siddoway et al., 2006); a possible indication that crustal Land is the South Fosdick detachment fault, which accom- melting and conditions favorable for viscous flow arose dur- modated dextral normal oblique translation of a pervasively ing convergent tectonism and crustal thickening along the brittlely deformed hanging wall block to the SW and WSW East Gondwana active margin. kinematic sense (dip dependent) along a mean direction of In the Fosdick Mountains gneiss dome syn- to post- 240° (McFadden et al., 2007). The transport direction agrees tectonic granite intrusions of 107 Ma to 96 Ma age delimit with stretching axes at deeper levels within the Fosdick the duration of detachment tectonics and exhumation, with dome, determined to be 060-240 to 070-250 (Siddoway et upward translation through ductile to brittle conditions and al., 2004b). development of a mylonitic shear zone at the transition Structural data that support an ENE direction for the (McFadden et al., 2007). Following the emplacement of the maximum principle finite strain axis for the Ford Ranges dome, mid-crustal magmatism and flow ceased and dome (Figure 4) include the regional mafic dike array (Siddoway rocks cooled rapidly from >700°C to <200°C at rates as high et al., 2005); mapped and inferred NW-SE dip-slip normal as 70°C/m.y. (Richard et al., 1994). Overprinting textures of faults in the southern Ford Ranges; and brittle kinematic cri- cordierite possibly record decompression of footwall rocks teria on ESE-striking dextral oblique minor faults and on NE- due to translation upon the detachment structure; and the striking sinistral shear fractures. Among mesoscopic brittle late-tectonic melt-filled, normal-sense shear bands that cut structures on land, strike slip faults are prevalent, forming all older structures may be an indication of a small volume of populations that accommodated both dextral and sinistral late leucogranite formed by decompression-induced melting motion (Figure 4). Mutually crosscutting relationships with (Siddoway et al., 2004b; Korhonen et al., 2007b). The very alkalic dikes indicate that the strike slip faults were active high rates of cooling recorded by 40Ar/39Ar mineral cooling during the 070-250-directed opening that is recorded by the data are comparable to rates of advective heat loss that arise mafic dike array. Regional mapping indicates that Byrd Coast from pluton emplacement in cold country rock (Fayon et plutons are spatially associated with inferred major faults and al., 2004). These observations are possible indications of a may occupy releasing bends (Figures 4 and 9). component of upward, gravity-driven flow (diapirism) during Consequently, the dextral and sinistral regional-scale emplacement of the dome (e.g., Teyssier and Whitney, 2002). faults in western Marie Byrd Land are viewed as contem- The association of the Fosdick Mountains gneiss dome with poraneous conjugate structures whose motion aided ENE- the Balchen Glacier fault, which is known to be an inherited WSW dextral transtension in the eastern WARS (Figure 4). Paleozoic structure (Richard et al., 1994; Siddoway et al., Kinematics of normal faults mapped offshore of Edward VII Peninsula (Luyendyk et al., 2001) are consistent, as is

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108 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD CONCLUSIONS the direction of margin-parallel divergence across a short- lived MBL-Bellingshausen plate boundary further east The determination of dextral transtensional strain in the (Heinemann et al., 1999). The stretching direction for the eastern WARS in Marie Byrd Land is consistent with the eastern WARS in MBL is generally parallel to that predicted current picture of tectonic plate interactions at the Phoenix- from the orientation and geometry of basement grabens in the East Gondwana (Pacific sector) boundary, with the final Ross Sea (Cooper et al., 1991a, 1997; Davey and Brancolini, stages of subduction marked by oblique convergence of 1995; Trey et al., 1997). The documentation of important young oceanic crust (Bradshaw, 1989; Luyendyk et al., 1995; wrench deformation in MBL supports past interpretations of Sutherland and Hollis, 2001; Wandres and Bradshaw, 2005), wrench and transfer faults within the WARS (e.g., Grindley as far east as Palmer Land (Figure 3) (Vaughan and Storey, and Davey, 1982) and for the first time determines their ori- 2000; Vaughan et al., 2002b). It is now clear that strike slip entation and kinematics. fault systems, thought to be in existence during oblique The apparent change from prevalent normal faults in convergence at the Early Cretaceous margin (Weaver et al., the Ross Sea portion of the rift (Cooper et al., 1991a,b; 1995), remained active and accommodated intracontinental Tessensohn and Wörner, 1991) to dextral strike slip in the örner, rner, extension in the WARS until ca. 90 Ma. The structural and eastern WARS implies a rotation of principal stress axes from thermochronology record from MBL and the eastern Ross vertical in the west to 2 vertical in the east. The spatial 1 Sea indicates that the intracontinental extension between variation in kinematics and dynamics across the WARS prob- EANT and WANT that brought about opening the WARS by ably is related to the geometry of subducted lithosphere at 90 Ma is distinct from the NZ-WANT breakup at 83 Ma and the active margin (e.g., Bradshaw, 1989; Luyendyk, 1995), later. There is compelling evidence that the sharp continent- or is an expression of a regional strain gradient between ocean boundary that distinguishes the MBL margin from the the East Gondwana convergent boundary and the Australia- other gradational continent boundaries of the Antarctic Plate Antarctica boundary, undergoing slow divergence since ca. (Gohl, 2008, this volume) is controlled by a wrench zone 125 Ma (Cande and Mutter, 1982; Tessensohn and Wörner, örner, rner, formed during opening of the WARS. 1991). It may be that relict subvertical transcurrent zones pen- The conjugate wrench zones active in MBL between etrating to the base of the crust provide a deep-seated conduit 107 Ma and 97 Ma indicate a vertical orientation for the for magmatism in the linear volcanic mountain ranges of intermediate axis of principle finite strain, with axis of the MBL volcanic province (LeMasurier and Rex, 1989), or minimum finite strain oriented NW-SE in the plane of the control the deep narrow lineaments in the subglacial topog- earth. Transcurrent strain in MBL is consistent with the raphy beneath the Pine Island and Thwaites ice streams, oblique convergence vector (Figures 9a and 9b) deter- 950 km to the east (Holt et al., 2006; Vaughan et al., 2006). mined for Late Cretaceous time (Vaughan and Storey, In this way the lithospheric-scale structures formed during 2000; Sutherland and Hollis, 2001; Vaughan et al., 2002a). development of the West Antarctic rift system continue to Therefore, the postulated tectonic boundary separating the exert fundamental influences on the long-term continental Ross continental province from the Amundsen arc province evolution of West Antarctica. (Pankhurst et al., 1998) probably corresponds to an intrac- ontinental dextral transform fault. The Amundsen province ACKNOWLEDGMENTS boundary is inferred on paleomagnetic grounds to have an approximately E-W trend in Marie Byrd Land (Figure Sincere thanks are extended to J. D. Bradshaw, F. J. Davey, 9b) (DiVenere et al., 1996). Restoration of dextral motion and F. Tessensohn who provided reviews; to A. K. Cooper for across an E-W transform fault potentially would place the his dedicated service as editor; and to the ISAES 2007 pro- Amundsen province magmatic arc outboard of the Ford gram committee for inviting this contribution. M. Brown and Ranges (Figure 9a). Such a reconstruction helps explain the N. Mortimer provided input on a prior manuscript. Interpreta- extent and degree of regional heating throughout the Ross tions represented here have arisen through collaborations with province that elevated crustal isotherms (Figure 5b and 5c), B. P. Luyendyk, C. M. Fanning, R. McFadden, C. Teyssier, induced extensive mid-crustal flow of the type documented D. L. Whitney, C. A. Ricci, D. Wilson, F. J. Korhonen, and in the Fosdick Mountains (Figure 8c), promoted rapid S. Saito. Other contributors include the Support Office for intracontinental extension across the WARS, and prevented Aerogeophysical Research (SOAR) (1998), A. Whitehead, development of orogenic topography. Dynamic subduction L. Sass III, S. Kruckenberg, J. Haywood, S. Fadrhonc, and (e.g., Giunchi et al., 1996) or a postulated mantle plume M. Siddoway. Mike Roberts (polar guide), Raytheon Polar (Weaver et al., 1994; Storey et al., 1999) may have been Services, the 109th Air National Guard, and Kenn Borek Air responsible for preventing dramatic subsidence and volumi- provided logistical support over several years. Warm thanks nous infilling of sedimentary basins of the Ross Sea (Wilson are also due to GANOVEX VII (1992-1993); Spedizione et al., 1998; Luyendyk et al., 2001; Karner et al., 2005). X, Italiantartide (1996-1997); J. Müller; and members of

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109 SIDDOWAY Bradshaw, J. D., B. Andrew, and B. D. Field. 1983. Swanson Formation and the South Pacific Rim Tectonics Expedition (SPRITE). S. related rocks of Marie Byrd Land and a comparison with the Robertson Borg and J. Palais are thanked for program leadership. The Bay Group of northern Victoria Land. In Antarctic Earth Science, eds. U.S. National Science Foundation provided support through R. L. Oliver, P. R. James, J. B. Jago, pp. 274-279. Canberra: Australian grants OPP-0338279, 0443543, 9702161, and 9615282. The Academy of Science. content of the article is the work of the author and does not Bradshaw, J. D., R. J. Pankhurst, S. D. Weaver, B. C. Storey, R. J. Muir, and T. R. Ireland. 1997. New Zealand superterranes recognized in Marie necessarily reflect the views of the National Science Foun- Byrd Land and Thurston Island. In The Antarctic Region, Geological dation. Colorado College faculty research awards provided Evolution and Processes, ed. C. A. Ricci, pp. 429-436. 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