Implications of paleomagnetic data on Miocene extension near a major accommodation zone in the Basin and Range Province, northwestern Arizona and southern Nevada

摘要

Paleomagnetic data from volcanic and crystalline rocks elucidate the evolution of a major Miocene accommodation zone in the northern Colorado River extensional corridor. The accommodation zone is a 10‐km‐wide belt of intermeshing conjugate normal faults that facilitates reversals in the dominant tilt direction of fault blocks and dip direction of major normal fault systems. Tilt‐corrected means (e.g., N = 28 sites, D = 353.4°, I = 61.3°, α95= 6.7°, k = 17.4) from Miocene volcanic strata overlap expected Miocene directions at the 95 confidence level. These data and geologic relations suggest that at exposed structural levels the accommodation zone did not facilitate distributed strike‐slip displacement between opposing tilt block domains. Vertical axis rotations are probably negligible in most of the corridor, as the Miocene structural grain generally mimics that in the zone. Discrepancies between characteristic remanent magnetizations (ChRM) in crystalline rocks and expected directions are therefore attributed to rotations about horizontal (i.e., tilting or flexing) axes. ChRMs from Cretaceous and Miocene intrusions suggest 50°–90° of tilting of large crystalline terranes on either side of the accommodation zone. The magnitude of tilting inferred from the paleomagnetic data is similar to that of Tertiary strata in nearby fault blocks, implying that these crystalline terranes are parts of highly tilted fault blocks rather than flexed lower plate rocks. Major low‐angle normal faults that bound highly tilted parts of these crystalline terranes probably nucleated at steep dips and were rotated to gentle dips by block tilting. Paleomagnetic data indicative of negligible tilting (e.g., Miocene intrusions, northern Black Mountains crystalline terrane, N = 13 sites, D = 359°, I = 55°, α95= 9°, k = 24) and geologic relations imply that lower plate rocks may surface in both the east and west tilted domains 35–50 km away from the zone. The trend toward shallower structural levels, with respect to Miocene extension, and tapering of highly extended terrane toward the accommodation zone imply that the magnitude of upper crustal extension decreases toward the zone. Temporal patterns of major extension, especially an apparently continuous northward younging across both the east and west tilted domains, further suggest that the accommodation zone served as a long‐lived rupture barrier between conjugate normal fault systems rather than as a short‐term boundary between opposing systems that propagated towar