Extension of the Basin and Range Province: Late orogenic collapse or something else?

Phillip B. Gans and Elizabeth L. Miller

Dept. of Geology, UC Santa Barbara, Santa Barbara, Calif. USA
and Dept. of Geology, Stanford University, Stanford, Calif. USA

The Basin and Range province of the western U. S. is widely recognized as a classic example of extended continental crust, yet the ultimate cause(s) of this extension remain controversial. Three principal “driving forces” for extension have been proposed: (1) Gravitational collapse following thermal re-equilibration of crust that was over thickened during Cretaceous to early Tertiary crustal shortening (Sevier and Laramide Orogenies), (2) Relaxation of confining stresses at the North American/Pacific plate margin due to changes in plate motions and/or plate margin configuration during the mid to late Tertiary, and (3) “Active rifting” in response to asthenospheric upwelling, either by lithospheric delamination or a mantle plume. These mechanisms are not mutually exclusive but they bring into focus an important question: Was extension of the Basin and Range province the inevitable consequence of previous shortening and thickening - i.e. late orogenic collapse, or did the earlier thickening play a subsidiary role to changes in plate boundary conditions? The answer to this question must come from the geologic record and requires a detailed understanding of the timing and magnitude of extension, how extension was distributed with respect to earlier thickening, the relative timing of extension and magmatism (and the character of that magmatism), and the thermal evolution of the crust prior to extension. There remain serious gaps in our knowledge, but a number of observations can be made which bear on any dynamic model for Basin and Range extension.

The history of extension in the Basin and Range province is much more complex and episodic than originally thought; Evidence exists locally for extension in the Jurassic, late Cretaceous, Eocene-Oligocene, Miocene, and continuing today. Many of these “episodes” do not appear to be regionally extensive and may not have involved significant strains. A growing body of data suggests that a significant proportion of the extension in the Basin and Range province, including the extensional unroofing of many of the metamorphic core complexes and inception of the present basin-range physiography took place during a relatively short time interval in the mid-Miocene (~20 to 10 Ma).

Throughout all of the shortening history and much of the extensional history, oceanic crust was being subducted beneath western North America. However, the rate of convergence and age of subducted lithosphere varied significantly with time and along the strike of the orogen. One of the most profound changes to affect the region was the change from rapid subduction of relatively old oceanic lithosphere during the late Mesozoic and early Tertiary to much slower subduction of young warm lithosphere. This led to progressive enlargement of a slab free area beneath much of what is now the Basin and Range province. The subsequent evolution of the Pacific/North America plate margin from subduction to the San Andreas transform system post dates most of the extensional tectonism (at least in the northern half of the province) and thus was not likely a controlling factor.

The western U.S. has been the locus of voluminous magmatic activity throughout much of the Mesozoic and Cenozoic. Distinctions between “subduction related”, “extension or rift related”, and "plume related" magmatism are not clear. Virtually all of the magmatic activity is fundamentally basaltic in that it documents the influx of mantle derived (both lithospheric and asthenospheric sourced) melts into the crust. Relative timing of extension and magmatism generally does not support “passive” decompression melting during extension, but rather indicates melting in the mantle prior to and during extension.

Peak metamorphism and crustal melting over much of the region were achieved during late Cretaceous shortening and thickening in the back arc region of the Mesozoic magmatic arc. Inferred very low-angle subduction during the latest Mesozoic and early Tertiary (Laramide) shut off this magmatic arc and moved the region that is now the Basin and Range province into a forearc position. This was likely accompanied by “refrigeration” of the overlying lithosphere during the latest Mesozoic and early Tertiary - i.e., the time span preceding most of the extension.

These observations make it difficult to view Basin and Range extension as the inevitable late to post orogenic collapse of previously thickened crust. There is indeed evidence for locally significant extension synchronous with and/or closely following the latter stages of crustal shortening, but this extension should be distinguished from the main phase of Basin and Range extension, as there is commonly a prolonged hiatus in deformation during which the Basin and Range lithosphere was cooled and strengthened. Crustal thickening almost certainly played a role in localizing large-magnitude extension but was not a sufficient driving force. Widespread extension during the Miocene may be attributed instead to the combined effects of (1) renewed magmatic activity in the upper mantle and associated thermal weakening of the lithosphere, (2) relaxation of confining stresses due to changes in plate motions, and (3) increased buoyancy forces associated with the presence of a slab-free window and shallow asthenosphere beneath the region. Thus, extension of the Basin and Range province is probably a poor example of “late orogenic extension”.

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