3: Regional gravity and tectonic patterns: Their relation to late Cenozoic epeirogeny and lateral spreading in the western Cordillera
Gordon P. Eaton, Ronald R. Wahl, Harold J. Prostka, Don R. Mabey, M. Dean Kleinkopf, 1978. "3: Regional gravity and tectonic patterns: Their relation to late Cenozoic epeirogeny and lateral spreading in the western Cordillera", Cenozoic Tectonics and Regional Geophysics of the Western Cordillera, Robert B. Smith, Gordon P. Eaton
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A new, simple Bouguer gravity map for that part of the United States west of long 109°W is examined in terms of its relationship to other geophysical and geological parameters. Unifying geophysical and tectonic characteristics define a large central province that has the Great Basin as its principal geographic element, but also includes parts of the Sierra Nevada, western Colorado Plateaus, Columbia Plateaus, and Middle and Northern Rocky Mountains. It is characterized by a high average elevation (>1.5 km), a low Bouguer gravity field (<-110 mgal) with a bilaterally symmetrical distribution of long-wavelength anomalies in its southern half (120°-opposed tracks of progressively outward-migrating silicic volcanism continue this symmetry in its northern half), high heat flow, the presence of two-thirds of all thermal springs in the conterminous United States, a concentration of Quaternary volcanic rocks at its east and west margins, and pervasive extensional faulting throughout. Much of the boundary of this geophysical province is moderately sharp, and parts of it cut across the interiors of classic physiographic provinces, as well as earlier geologic provinces. With the exception of the gravity and volcanic symmetry, none of these characteristics alone is unique to the province; uniqueness lies in their collective assemblage. The northern and southern ends of the province are not sharply defined; it narrows notably and some of its characteristics merge with those of adjoining regions both to the north and south.
Long-wavelength gravity-anomaly patterns within the province are interpreted as reflecting extensional, thermomagmatic episodes in the late Cenozoic history of the lithosphere. Kinematics and patterns of faults, dikes, and geophysical anomalies suggest that east-west spreading of late Cenozoic age was preceded by significant northeast-southwest spreading of greater latitudinal extent in Miocene time. The latter is interpreted as back-arc spreading. Northwest-southeast oblique spreading commonly attributed to the Great Basin as a whole appears to be restricted largely to its western one-third and to the northeast-trending Humboldt zone.
Most regional topographic features of the province are in approximate isostatic equilibrium. Compensating masses for some appear to be within the crust, which in the Great Basin is nearly coextensive with the lithosphere. Some are in the upper mantle. Load differences for individual basin and range structures are supported by the strength of the crust and lithosphere. Faults that block out basins and ranges do not penetrate deeply into the crust, but tend to dip less steeply with depth. This interpretation is supported both by maximum earthquake focal depths and by the observed local response to surface loading. Geothermal gradients and material properties suggest that lateral extension below depths of 15 to 20 km probably takes place by plastic flow aided perhaps by pervasive injection of basaltic magma.
Because the lithosphere of the Great Basin is thin, it probably does not greatly modify the temperature field of the upper asthenosphere. For this reason the origin of compensating masses and major regional gravity gradients is thought to be the complex sum of (1) lateral temperature distributions in the lithosphere and asthenosphere, (2) distribution of Cenozoic intrusive masses reflecting earlier thermal events, (3) high temperature metamorphism related to both injection and heating of the crust, and (4) variations in the degree of extension and resultant thickness of both lithosphere and crust.
Volcanic activity extended across the entire province in Cenozoic time, but is youngest at the east and west margins. In this aspect and in the details of heat flow, topography, broadly distributed extension, and the symmetry of its geophysical anomalies, the region contrasts sharply with both oceanic spreading ridges and major intracontinental graben systems. The difference may be attributed to active, as opposed to passive, spreading processes. Those features of the western Cordillera not related directly to subduction and regional dextral shear are interpreted as the products of lithospheric heating, injection, uplift, and basal traction resulting from the rise and divergent flow of hot, asthenospheric mantle. Associated phenomena include thermal tumescence of the lower lithosphere and brittle faulting of the upper lithosphere.
A “hot spot” of very large dimensions is thus identified near the western, transform boundary of the North American plate. Its proximity to this boundary has led to a complex, episodic and cyclic interaction of two profound stress fields, producing young structures on the west indicative of both thermal doming and dextral shear. To the east the principal deformation is simple east-west extension resulting from rapid spreading that accompanied major regional doming and collapse.