IMPLICATIONS OF PALEOMAGNETIC DATA ON DISPLACEMENT GRADIENT ACCOMMODATION ALONG A MAJOR DETACHMENT FAULT, WHITE HILLS, NORTHWEST ARIZONA

The South Virgin-White Hills detachment fault (SVWHD) is a major gently W-dipping normal fault that bisects the White Hills and flanks the west side of the South Virgin Mts in the northern Colorado River extensional corridor (NCREC) of northwest Arizona and southern Nevada. Displacement on this fault increases northward from ~5-6 km in the southern White Hills to as much as 17 km in the South Virgin Mts. As brittle deformation gives way northward to mylonitization, the fault acquires many characteristics of a classic detachment fault. Based on tilt-fanning in half grabens, extension is bracketed between ~16 and 9 Ma. Capping, little faulted basalt at Senator Mt (9.9 Ma) and Table Mt Plateau (8.7 Ma) effectively place a younger age constraint on extension. To assess strain accommodation in hanging-wall blocks, 86 paleomagnetic sites (~710 samples) and 8 new ⁴⁰Ar/³⁹Ar dates were obtained from mid Miocene volcanic rocks, including the 15.2 Ma tuff of Bridge Spring (TBS), 15.0 Ma tuff of Mt Davis (TMT), and 13-15 Ma mafic lavas. Paleomagnetic reference directions have been established for both TBS (D=199, I=-47, a₉₅=5.7) and TMT (D=170, I=-41, a₉₅=3.8) in non-rotated parts of the NCREC.

Compared to the expected mid Miocene direction (D=359, I=56, a₉₅=6.3) and reference directions for the tuffs, remanent magnetizations averaged from mafic lavas (D=10, I=-47, a₉₅=5.1, 58 sites), TBS (D=194, I=-49, a₉₅=7.4, 9 sites) and TMT (D=173, I=-48, a₉₅=5.6, 12 sites) throughout the White Hills indicate negligible vertical-axis rotation in the upper plate of the SVWHD. However, when viewed at the scale of small sets of fault blocks, data suggest as much as 15^o of local anti-clockwise (e.g., TBS, 5 sites, D=183, I=-46, a₉₅=6.3) and 10^o of clockwise rotation. Despite these minor, barely discernible, local rotations, it would appear that systematic anti-clockwise rotation did not accommodate the bulk of northward increasing displacement on the SVWHD. A series of E-striking sinistral faults, with ~5 km of cumulative slip, and possibly obscure footwall adjustments may account for much of the displacement gradient. Such relations suggest that the upper plates of major gently dipping normal faults cannot be viewed as relatively coherent, regionally extensive allochthons but are instead kinematically complex, segmented 3D entities.