Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 11
3
Needed Investigations
There are many gaps in our understanding of the structure and evolution of
North American continent-ocean transitions. The gaps and uncertainties have been
translated to problems whose solutions will cause major progress in the science of
the evolution of North America and of the growth and destruction of continents in
general. Focused investigations are necessary to achieve the solutions in a timely and
efficient manner. Moreover, they are required to bring together the broad spectrum
of scientific backgrounds and resources that must be pooled to address these complex
problems. Studies of continent-ocean transitions must bring together terrestrial and
marine scientists from varied disciplines: geologists, geophysicists, and geochemists for
observational, data processing, theoretical, and laboratory approaches.
Four principal avenues for focused investigations are outlined below: syntheses,
topical investigations, processes, and techniques.
SYNTHESES
Transect analyses show the value of regional synthesis and interpretation of existing
data by scientists of diverse background acting in concert. The gains in perspective,
problem definition, and in many cases, new ideas of continental margin evolution,
have been enormous. Equally important, each transect has conceived and displayed
graphically a mode] of the continent-ocean transition to Moho depths. Such models
should be tested by new data gathering, which in turn will lead to improved models
and further refinement in our understanding of continental evolution.
The USGC advocates continuing syntheses of North American continent-ocean
transitions in two modes: (1) with existing data as in the Transects Program, and (2)
with new data acquisition.
1. Syntheses with existing data: There are many regions of the North Ameri-
can continent-ocean transition that have not been sampled by the 23 transects of the
Transects Program. Studies of such regions comparable to those of the Transects Pro-
gram would produce a significantly more comprehensive understanding and problem
definition than exist now. Such regions contain features that may be unique and vital
11
OCR for page 12
12
to continental science and that are fundamental to analysm of lateral gradients and
discontinuities in orogenic belts.
Candidate regions for a new phase of transect studies are: Aleutian Islands, north-
western British Columbia, Santa Barbara-southern Sierra Nevada, northern Labrador
to Greenland, northern Mexico across Gulf of California, west Texas to the Gulf of
Mexico, Florida across southern Blake Plateau, Yucatan to Cayman Trench, Bahamas
to Puerto Rico, Mackenzie Delta to Canada Basin, and the eastern Baffin Islands.
2. Syntheses with data acquisition: Some or Al of the models of the 23 tran-
sects should be tested and refined by acquisition and synthesis of new, comprehensive
datasets that are profiled along the transects. The new investigations should include
seismic profiling with source-receiver parameters designed for penetration and resolu-
tion that can maximize analysis of model features: potential field data with uniform
high- and low-frequency contents; isotopic dating that will resolve protolith ages and
tectonic events; detailed analyses of kinematics and conditions of deformation in dim
placement zones and strained rocks; and determinations of latitude anomalies and
rotations of well dated magnetizations. Data from scientific drilling on land and at sea
should be incorporated.
TOPICAL INV1:STIGATIONS
The USGC recommends new investigations that focus on problems in three
broad tectonic divisions of North American continent-ocean transitions: (1) forearcs,
(2) lower crust and Muffs in t.r?~.n~it.inn~1 r-~ir~n~ ~nr1 {.~1 t.-rr~n~c~ and f~r~lan~o rep the
orogenic belts.
~ ^ ~ __ ace_ ~~~ _ ~ ~~_~ ~~ ^~^ _~— ~~ user
1. Forearcs: There should be concentrated data acquisition that will lead to
greatly enhanced understanding of the deep structure and processes of forearcs of
North America, both those active today and those in the Cenozoic. The objective is to
evaluate what may be the prime sites of tectonic progradation and/or attrition of the
continental edge and at which the continental edge appears to thicken. A prime tech-
nique is seismic profiling, which may reveal the characteristics of underplated packets,
underthrustin~ sediments. basement reliefs ramrod anr1 It. of rh~na" of ~l~t.z~rkrn,~nt.
level, out of sequence faults, diapirs, duplexes, lateral velocity changes indicating rhe-
ologic gradients, and the downdip length of sediment cover on the downgoing plate
below the forearc and into the crystalline subduction zone. Other important studies of
forearcs should include seismicity, fluid and heat fluxes, progressive deformation, and
deflection of the downgoing plate.
2. Lower crust and Moho: The scientific objectives to be addressed include
the origin of reflector sets in the lower crust of the transitional region, nature and
thickness of the Moho, existence of motions along and mass transfer across the Moho,
protoliths on both sides of the Moho, and age of the Moho. Also included here are the
structures of the deeply buried, greatly extended North American crust of the Mesozoic
and Cenozoic passive margins and the nature of specific features such as the outer
ridge, possible mantle protrusions at the continent-ocean join, continental microplate
structure (Orphan Knoll, Flemish Cap, Blake Plateau), rift basin configurations, and
estimation of amount of magma added to the crust.
Investigations of this problem may follow several lines, which should be coordi-
nated: (a) geophysical studies, chiefly seismic, that are tuned to desired depths, are set
out in three-dimensional arrays so as to determine true dips and velocity anisotropies
and are flexible enough to track deep structures to shallow depths; (b) studies of
OCR for page 13
13
magmas and xenoliths derived from
to the surface of plunging lower crustat structures, ana fat
lower crustal or upper mantle levels; (c) tracking
~ 'I' study of greatly exhumed
rocks that were once at depths equivalent to today's Moho in transitional regions.
3. Terranes and forelands of orogenic belts: There should be continuing study of
the erogenic belts of North America with traditional goals of defining basic tectonic
units and dating their times of movement and attachment. Added emphasis should
be placed on analysis of kinematics of erogenic belts, on the relationships between
motions in the zone of terranes and those of the continental foreland, and on subsurface
structure in the zone of terranes. Further goals of this topical study are a comprehensive
picture of the closure history of Laurasia and Gondwana to form Pangea and the
Tosses of Precambrian North America in tectonic interactions at active and passive
margins since the Precambrian. The reconstruction of terrane migration in Mesozoic
Mexico and western North America is also a goal. Also included here is the objective
of understanding the widths of modern and ancient boundary zones of the North
American plate over which boundary-related motions occur.
The methods to be employed in this investigation include all those for kinematic
analysis: brittle and ductile fault slips, strain to displacement calculation, satellite
geodesy, paleomagnetics, seismic parameters, and barometry of metamorphism. They
include extensive dating by multiple techniques, and identification of protolith kindred
by isotopic signatures (Nb, Nd, Sm. Sr, Pb, etc.~. They also include geophysical
studies, mainly seismic, and drilling to identify and track subsurface horizons that may
be terrane boundaries, especially those horizons that lead from the zone of terranes
onto the edge of sialic North America.
PROCESSES OF CONTINENT-OCEAN TRANSITIONS
Aside from the areal investigations of the synthesis and topical types discussed
above, studies of certain basic processes have been recognized to play major roles In
tectonic zones. Examples of such processes are detachment, unroofing, reactivation
in fault zones, underthrusting and wedging, accretion, attrition, diapirism, intrusion,
foliation generation, defluidization, extensional failure, anatexis, initiation of subduc-
tion, onset of drifting, predrift elevation, and transform faulting. Another process that
requires extensive research Is continental separation and the definition of the rift-drift
transition from sedimentary successions and tectonic structures.
The objectives are improved comprehension of the conditions under which each
process occurs and the mechanics and kinematics. The study of processes should
include observational, theoretical, and experimental means.
TECHNIQUES
Major advances in understanding come not only from the recognition of new
problems or new insights, but also from the development or ~rnprovement of techniques
that yield new data. The study of the continent- ocean transitions has benefited greatly
in the twentieth century from new techniques, such as paleomagnetem and isotopic
dating. Therefore, research on improvement of techniques is an important component
of the study of continental evolution.
Specifically, advances in techniques in the following areas would be important:
dating of protolith ages; dating of thermal ~d straining events and of unroofing times;
resolution of displacement components, especially of rotations and margin-paraBel an
OCR for page 14
14
latitude-parallel components; measures of bulk finite strain and small-domain strains;
barometry and thermometry; detections of major rise of fluids and fluid sources;
methods of delineating structures in and tracking displacement zones to Arid- and
lower crustal depths; improved analysis of the structure of the Moho; and more precise
geodetic analyses. Advances should be made in techniques of digitization of data
and in software that are compatible with commonly used computers and that allow
combination and manipulation in three dimensions.
Representative terms from entire chapter:
data acquisition
