DOES HOTSPOT MELTING PACE GREAT BASIN EXTENSION?

Crustal extension in the northern Great Basin (GB) occurs between Newberry Caldera to the west and Yellowstone Caldera to the east; those calderas mark active melting fronts of two opposing, coeval hotspot tracks. The track along the eastern Snake River Plain, punctuated by active volcanism at Yellowstone, is mirrored by a second, trending northwest across the Oregon Plateau that culminates at the historically active Newberry craters (Christiansen and McKee, 1977). Because these magmatic centers coincide with the margins of the GB, a link between extension and volcanism seems tenable and raises the question of whether hotspot-related melting and GB extension have kept pace throughout the 17 Ma history of northern GB development. Tertiary E-W extension has produced the regional N-S elongate basin and range fabric that characterizes GB morphology. Overlooked, is the fact that the trends of ranges fan out, spanning a >50 degree arc. Greater extension in the southern GB could account for the pattern of range trends, but it would result in clockwise crustal rotations that are inconsistent with paleomagnetic data that show widespread counter-clockwise (CCW) rotations in northern Nevada. Similarly, a mechanism, in which extension controls subduction and follows the migrating remnant of the Farallon plate (Stewart, 1998), could account for the range-trend fanning, but fails to produce CCW rotations. Alternatively, collision between Pacific Northwest crustal blocks, which drives westward extension there (Wells et al., 1998), can produce the CCW rotations, but cannot account for the fanning of range trends. This problem is reconciled if the pattern of range trends was established before extension. Structures that presently form many of the range-front bounding faults may have been inherited and re-activated during Tertiary extension. The range-trend pattern resembles a basin-wide bilateral symmetry that is evident in topography, gravity, and crustal thickness (Eaton et al., 1978), the axis of which lies close to where the two hotspot tracks converge, suggesting that origin of GB symmetry may be related to nascent hotspot magmatism. If subsequent extension has kept pace with melting then it follows that extension would have initiated in the central GB, near the axis of GB symmetry ~12-17 Ma and migrated outwards with time.