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Chapter 31: Crustal geologic processes of the United States

By
Warren B. Hamilton
Warren B. Hamilton
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Published:
January 01, 1989

The evolution of crystalline continental crust probably has been dominated by arc magmatism. Olivine, pyroxene, and garnet are largely crystallized from rising arc melts in the subcontinental mantle. Residual hot, dry gabbroic magmas cross the density filter of the Mohorovičić discontinuity and spread out in the basal crust, heating preexisting crustal rocks and producing widespread secondary melting. The basal crust is dominated by two-pyroxene gabbro, increasingly fractionated and contaminated upward, variably metamorphosed in granulite facies. Migmatites and restites, of granulite or uppermost amphibolite facies, dominate the higher part of the lower crust, and from these rise mixed and secondary melts, of intermediate to felsic compositions, that evolve with complex combinations of crystallization, fractionation, and assimilation. The rising melts heat the middle crust, causing hybridization, migmatization, and secondary melting. The melts absorb much water derived mostly by breakdown of wall-rock micas, in continental crust, and of hornblende in island-arc crust. A hydrated melt cannot rise past the level at which load pressure equals fluid pressure, so hydrous melts expel their volatiles and crystallize in the middle crust, often as two-mica granitic rocks in continental crust. Only melts not hydrated by middle-crust reactions remain hot and dry enough to rise to the upper crust to crytallize as shallow batholiths and to erupt as ash flows.

Rift-related continental magmatism has also been important, although criteria for identifying its products are much less definitive than generally assumed. Volcanic-rift assemblages are in many cases basaltic or bimodal basaltic and rhyolitic, but they can include voluminous rocks of intermediate compositions. Arc assemblages also can be strongly bimodal, on modest scales of space and time. Metaluminous rhyolites are abundant in both rifts and arcs, but peralkaline ones occur primarily in rifts. Ancient rift complexes include upper-crustal layered gabbro-granite-rhyolite complexes. Ancient and modern rift systems display distinctively layered basal crust on reflection profiles, likely recording widespread injection of gabbroic magmas from the mantle.

Foreland thrust belts, wherein preexisting stratal wedges are imbricated craton-ward, have formed in both collisional and noncollisional settings. Paleozoic imbrication in Appalachian and Ouachita-Marathon regions was a byproduct of arc-continent and continent-continent collisions; basement overthrusting also affected the southern Appalachians. Cretaceous thrust-belt imbrication in much of the Cordillera occurred in an Andean setting, and gravitational spreading due to magmatic thickening of the crust in the magmatic-arc belt may have been responsible.

The latest Cretaceous and early Paleogene Laramide shortening of the Rocky Mountain sector of the craton represents a clockwise rotation of the Colorado Plateau region of about 4° relative to the continental interior about a New Mexico Euler pole. This, and the synchronous tectonic erosion from beneath of continental crust farther southwest, may have been byproducts of drag on subducting Pacific lithosphere that did not sink out of the way of the advancing continent.

Early Basin and Range extension occurred in back-arc-spreading mode, behind a trench and subducting margin; late extension has been in oblique mode as the continent has adjusted its shape to that of the evolving San Andreas boundary. The dominant mode of extension has been normal faulting wherein footwalls rise and deform and hingelines migrate between inactivated, undulating sectors of faults and still-active dipping sectors. Basin and Range extension followed widespread magmatic heating of the crust; it has doubled the width of a broad region and has continued for 30 m.y.

Extension starting with cold crust may be geologically more common and may result in extension of much lesser extent and shorter duration before a continent is sundered and oceanic spreading begins. The Midcontinent Rift system is of this type, although extension there stopped before reaching the stage of normal oceanic spreading. The Atlantic continental shelf may be largely prograded over thick Mesozoic basaltic crust, rather than having been built atop severely thinned continental crust.

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Contents

GSA Memoirs

Geophysical Framework of the Continental United States

L. C. Pakiser
L. C. Pakiser
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Walter D. Mooney
Walter D. Mooney
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Geological Society of America
Volume
172
ISBN print:
9780813711720
Publication date:
January 01, 1989

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