Abstract

Structural and paleomagnetic data from the 66 to 70 Ma Hall stock and its environs suggest that extension-related tilting has continued in the northern San Antonio Mountains of Nevada since at least 17–18 Ma. Evidence includes both surface and subsurface mapping and paleomagnetic data. The stock and its related molybdenum deposit, most likely magnetized during a reversed-polarity chron, have been significantly tilted to the east, as have nearby 16 to 29 Ma volcanic rocks. The data do not preclude the possibility of counterclockwise crustal rotations related to dextral motion of the San Andreas and related faults. Similar horizontal tilt axes for the stock (N30°W) and the volcanic rocks (N-N20°W) and similar tilt magnitudes among all units, however, suggest that much or all of the tilting in the district occurred between late Oligocene and latest Miocene time during west-southwest–east-northeast pre–Basin-Range extension. Maximum pre-Oligocene, post-Cretaceous tilt is clearly <50° and probably on the order of 30°. The shallowly westward-dipping (35°–40°) range-front Liberty fault has probably also been tilted eastward 25°–50°. As much as 15° of tilt in the district occurred in the past 7–10 m.y., during “true” Basin-Range extension, accounting for 5°–15° of southeastward dip in mid-Pleistocene alluvium. The youngest structural features include (1) north-south– to northeast-southwest-trending antithetic fault sets having minor displacements and (2) Holocene alluvial beds having normal drag along the Liberty fault. These are of post-tilting age but imply that “true” Basin-Range extension in the northern San Antonio Mountains has continued into the Holocene.

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