Assessing vertical axis rotations in large-magnitude extensional settings: A transect across the Death Valley extended terrane, California

摘要

Models for Neogene crustal deformation in the central Death Valley extended terrane, southeastern California, differ markedly in their estimates of upper crustal extension versus shear translations. Documentation of vertical axis rotations of range-scale crustal blocks (or parts thereof) is critical when attempting to reconstruct this highly extended region. To better define the magnitude, aerial extent, and timing of vertical axis rotation that could mark shear translation of the crust in this area, paleomagnetic data were obtained from Tertiary igneous and remagnetized Paleozoic carbonate rocks along a roughly east-west traverse parallel to about 36°N latitude. Sites were established in ∼7 to 5 Ma volcanic sequences (Greenwater Canyon and Brown's Peak) and the ∼10 Ma Chocolate Sundae Mountain granite in the Greenwater Range, ∼8.5 to 7.5 Ma and 5 to 4 Ma basalts on the east flank of the Black Mountains, the 10.6 Ma Little Chief stock and upper Miocene(?) basalts in the eastern Panamint Mountains, and Paleozoic Pogonip Group carbonate strata in the north central Panamint Mountains. At the site level, most materials yield readily interpretable paleomagnetic data. Group mean directions, after appropriate structural corrections, suggest no major vertical axis rotation of the Greenwater Range (e.g., D = 359°, I = 46°, α_(95) = 8.0°, N = 12 (7 normal (N), 5 reversed (R) polarity sites)), little post-5 Ma rotation of the eastern Black Mountains (e.g., D = 006°, I = 61°, α_(95) = 4.0°, N = 9 N, 6 R sites), and no significant post-10 Ma rotation of the Panamint Range (e.g., D = 181°, I = −51°, α_(95) = 6.5°, N = 9 R sites). In situ data from the Greenwater Canyon volcanic rocks, Chocolate Sundae Mountain granite, Funeral Peak basalt rocks, the Little Chief stock, and Paleozoic carbonate rocks (remagnetized) are consistent with moderate south east-side-down tilting of the separate range blocks during northwest directed extension. The paleomagnetic data reported here suggest that the Panamints shared none of the 7 Ma to recent clockwise rotation of the Black Mountains crystalline core, as proposed in recent models for transtensional development of the central Death Valley extended terrane.