Continental Tectonics (1980) / Chapter Skim
Currently Skimming:
IV. Characterization of Continental Crust
IV. Characterization of Continental Crust
Pages 115-158
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 117... ...
It is not that methods and tools for exploration of the continental basement are unavailable. We have them; many of them will be discussed in this paper.
|
From page 118... ...
By continental basement I mean the entire continental crust below the sediments and Me uppermost mantle. From the broadest perspective and for several reasons, it is clear that the potential of the seismic method for exploration of the earn, and particularly the continental basement, is by no means exhausted.
|
From page 119... ...
On the basis of the clear signs of life in current study of body waves, and the renewed emphasis that is likely to result from new efforts to predict earthquakes and reduce the earthquake hazard and to detect and identify nuclear explosions, one can foresee new results, particularly those emphasizing lateral variations, over the next decade at least. The keys for application ofthree-dimensional inversion of travel times are tightly spaced, probably movable, networks and developments in methods for using not only travel-time differences but also wave character resulting from attenuation, focusing, conversion, or other effects.
|
From page 120... ...
The results Tom seismic reflection profiling so far clearly indicate vast potential for this method in fixture studies of the continental basement. Figure 10.6 shows a seismic reflection profile across the Rio Grande nR.
|
From page 121... ...
continental crust.
|
From page 122... ...
(1977) on the FIGURE 105 Seismic reflection pro filing park in the field in Wyoming.
|
From page 123... ...
across westem part of Rio Grande no north of Socorro. Abscissa Is vibration point number or distance in decameters; ordinate is two-way travel time in seconds.
|
From page 124... ...
My purposes are (l ~ to draw attention to the large unrealized potential for understanding the continental basement through application of present-day seismic methods and ¢2J to point to means for improving that potential through continuing evolution and improvement of seismological methods. Suppose, for example, that a method could be found for artificially generating shear waves of sufficient amplitude so that the deep crust and upper mantle could be explored using such waves, or that movable, tightly spaced seismic arrays could be deployed so as eventually to cover the continent.
|
From page 125... ...
. to,; got Jon Toteraccle Butte \ f / Aikoli Otter i5 elf P~ p,450 l -~ ~ ~ =~ ~ ~ ::: :- : I; 'T' "I ~ Who r'~ ' "''2""."~ \~N \ ~~\~' :/~250 /~00 ~50 6' :~OC~,>~ Oregon Buttes OContlnenla Peal _ ,45' N 30' _ /5' 42-00' 0 10 FIGURE 10.9 Clap of southeastern Wind River Mountains in Wyoming showing location of COCORP seismic reflection line.
|
From page 126... ...
. A new model of the continental crust, in The Earth's Crust, J
|
From page 127... ...
In the continental crust, this may vary anywhere from 15 50 km, depending on heat flow and the thermal properties of the rocks.
|
From page 129... ...
deduces some important tectonic relationships from aeromagnetic data at the junction of the San Andreas Fault and Mendocino fracture zone. "Straight magnetic lineaments on the continental shelf west of the San Andreas fault between Point Arena and Cape Mendocino have northwest trends, and are interpreted to be caused by ophiolite belts.
|
From page 130... ...
, probably the most outstanding feature on the gravity map of the United States. A more detailed aeromagnetic map of this feature, FIGURE 11.2 Midcontinent gravity high (~m Zietz, 1969, copyrighted by American Geophysical Union)
|
From page 131... ...
Exploration of the Continental Crust Using Aeromagnetic Data lows FIGURE 11.3 Aeromagnetic map of midcontinent gravity high (from Zietz, 1969, copyrighted by the American Geophysical Union)
|
From page 133... ...
, belts of certain amphibolites and magnetite-beanng schists, or belts of mafic and ultrarnafic rocks that may represent rifts, sutures, or former subduction zones. Because of the linearity of the aeromagnetic trends, aeromagnetic maps have proven usefi~1 as an aid to field mapping over the Appalachian crystalline rocks.
|
From page 134... ...
crossing the lower Piedmont of South Carolina, passes beneath a segment of the Coastal Plains in the Carolinas, and then flanks the Raleigh belt in North Carolina and continues into Virginia. From east~entral Georgia to Virginia, cataclastic rocks along the faults of the system are bounded to the northwest and southeast by rocks of the Carolina slate belt, conning perhaps the most extensive fault system in eastern North America.
|
From page 136... ...
The magnetic data combined with gravity, heat flow, and drill-hole data show quite clearly that the Brunswick anomaly is axial to the Southeast Georgia Embayment, one of the largest Atlantic Coastal Plain sedimentary basins, with a history of subsidence that was active throughout the Cenozoic and is still taking place. Seismic profiles and computed profiles and models from the aeromagnetic and gravity maps indicate that the anomaly~ausing body is a mafic intrusion at or just below the top of the basement beneath the Coastal Plain.
|
From page 137... ...
Because crystalline rocks usually have a magnetic signature associated with them, the magnetic method can be used in geological mapping for both lithology and structure. In order to integrate on a regional scale detailed mapping of local areas, oRen done by different people, using vastly different approaches, aeromagnetic maps provide a common denominator by relating s~aat~graphic units defined using different criteria and tracing units through unmapped areas.
|
From page 138... ...
(1969~. Aeromagnetic investigations of the earths crust in the United States, in The Earth's Crust and Upper Mantle, P
|
From page 139... ...
A common later event may be intrusion and attendant heating of the crust by magmas in continental rift zones or convergent plate margins [e.g., Andean arc magmas intruding Precarnbrian cmst in Peru (Dalmayrac et al., 197711. In addition, we recognize that at present, new continental crust is being formed largely by magmatic activity along arcuate zones at convergent plate margins and within broad intraplate tensional zones, including rift zones.
|
From page 140... ...
However, evidence looks persuasive for interaction of magmas with deep continental crust in Italy and in the western United States, reinforcing the hypothesis that magmas can provide infonnation on the lower crust. CONSEQUENCES OF CRUSTAL MELTING: TWO TYPES OF LOWER CRUST RESIDUAL CRUST An important consequence of the contamination of magmas by a crusted low melting Faction is that a refractory residue complementary to the melt exists at depth This pleat Flow Contours @~2.5HFO 1.5-2.5 O 0.75-1 5 1.8-15 75 ~ 1.0 ; it, _,,'-t'~ ~ ,_.~.
|
From page 141... ...
Melts derived from continental crust are diverse, and different magmas leave different residual crusts. Thus, the residual crust concept is easily reconciled with lower crustal heterogeneity.
|
From page 142... ...
ICAY and SUZANNE MAHLBURG KAY ity that xenolith localities may represent unique or unusual conditions in the crust, a better approach might be to group the localities based on tectonic similarities: convergent plate margins, riPr valleys, areas of intraplate volcanism, and Precambrian cratonic regions. Xenoliths have not been reported from all tectonic regimes, e.g., continental collision zones and greenstone belts.
|
From page 143... ...
Both 143 metasedimentary and meta-igneous rocks of granulite metamorphic grade have been recognized among the abundant xenoliths at Kilbourne Hole (Padovani and Carter, 1977~. Metasedimentary rock types include sillimanite-bearing garnet granulites, which have been interpreted as high-temperature residues of pelitic sediments.
|
From page 144... ...
XENOLITHS FROM POLYMETAMORPHOSED PRECAMBRIAN LOWER CRUST Two of the most highly studied suites of probable lower crustal xenoliths come from kimberlites in regions where polymetarnorphism of Precambrian lower crust can be
|
From page 145... ...
These xenoliths may have resided in abnormally warm lower crust, although probably not so hot as the Rio Grande riPc. Colorado Plateau Tertiary kimberlite diatremes in the Four Corners region of the Colorado Plateau contain abundant xenoliths and xenocrysts from both the crust and upper mantle.
|
From page 146... ...
WHAT ARE THE AGE RELATIONSHIPS WITHIN VERTICAL AS WELL AS HORIZONTAL CRUSTAL SECTIONS? At present there is a lack of knowledge ofthe mechanisms of continental crustal thickening.
|
From page 147... ...
Since similar Nanette components are not found in surface granulites, they postulate that aluminum has re~quilibrated upon uplift if surface granulite outcrops were formerly in the deep crust. Determination of a lower crustal geothermal gradient has been a major concern of several xenolith studies.
|
From page 148... ...
I[AY and SUZANNE MAHLBURG KAY Me lower crust? If stress and temperature can be determined for a xenolith, Den strain rate can be calculated, and questions of lower crustal dynamics can be addressed.
|
From page 149... ...
. Preliminary transient heat flow model of the Rio Grande rift in southern New Mexico, Earth Planet.
|
From page 150... ...
. A model for lower continental crust, Earth Planet.
|
From page 151... ...
Both models acknowledge the more or less continuous addition of igneous rocks into or upon the upper continental crust. There are, however, two possible sources for this material, the mantle or the lower crust.
|
From page 152... ...
Thus, to understand crustal evolution it is essential also to understand the present convection regimes ofthe mantle, how these regimes may have evolved with time, and the possible interactions that various parts of the mantle may have had with the continental crust. As a first approximation, the upper mantle may be divided into two parts: the suboceanic mantle and the subcontinental mantle.
|
From page 153... ...
Likewise, further geochemical study of rocks derived from the mantle may allow a characterization of the scale of heterogeneities in the mantle, show how they are evolving through time, and help to distinguish parts ofthe mantle interacting with the continental crust at a given time. As convection models for the mantle improve, this inforTnation should allow a direct correlation between ancient tectonic regimes and convection in the mantle.
|
From page 154... ...
Based ore the trace-element abundances and the geological relations, the best model is that the gneisses and the later granites are derived from melting of similar sources, presumably the lower continental crust. This model is supported by Pb isotope data (Doe and Delevaux, in press)
|
From page 155... ...
Applying this model to a continental environment, there may be a continental mantle keel attached to the continental crust for hundreds to thousands of millions of years (Figure 13.41. In this model, starting from the leR side of Me figure and using the numbers in Figure 13.4: (1)
|
From page 156... ...
in the arc, volcanism and the intrusion of gabbroic through granitic plutons. Below the arc there may be melting of: the subducted plate to produce tonalities; the subcontinental mantle or the ocean-ridge-type mantle to produce basalts; magic rocks near the base of the crust to produce a`nor~ositic or gabbroic plutons and possibly andesites; and the intennediate-composit~on continental crust to produce granitic intrusions.
|
From page 157... ...
. A continental crustal model and its geothermal implications, Earth Planet.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.