2000 GSA Annual Meeting -- Reno, Nevada
2000 GSA Annual Meeting -- Reno, Nevada
Author(s): ANDERSON, R. Ernest; BERGER, Byron; and SNEE, Lawrence W., U. S. Geological Survey, MS964, Federal Center, Denver, CO 80225, anderson@usgs.gov
Keywords: structure, extension, late-Tertiary, Nevada, mineralization
Directly west of the Walker Lane in the Wassuk Range-Coal Valley area of Mineral and Lyon Counties, Nevada, late Tertiary tilting of Mesozoic granite and metasedimentary basement blocks is determined from the attitudes of sparse Mesozoic dikes within the blocks and late Tertiary strata deposited on them. Strain is heterogeneous, with some blocks tilted WNW and others NNW. Regardless of direction, diversely oriented tilting is associated with extensional faulting of basement blocks and a late Tertiary cover-rock sequence consisting of volcanic and sedimentary strata. Only the uppermost part of the cover-rock sequence is synkinematic, suggesting that prior to the extension the basement blocks were blanketed by a cover-rock sequence as much as 2 km thick. Some basement blocks are tilted and uplifted through the similarly tilted cover-rock sequences. Some blocks have contrasting amounts of internal tilting, with clearly defined homoclinal hinge folds located 2-3 km behind the trace of the main bounding normal faults. Although some of the main faults have the low to moderate dips of detachment faults; they have compound curvature (convex upward and convex toward hanging wall), imparting a lozenge shape to the footwall blocks. Displacement on faults with this shape produces nominal total extension as compared with displacements on true detachment faults. This style of basement-block uplift and tilting is not apparent in adjacent areas where tilting is less and was accompanied by magmatism. Also, none of the faults in the extending regime of the Wassuk Range/Coal Valley area controlled hydrothermal fluid flow and associated alteration and mineralization, as faults do in the adjacent magmatic terrain. Although the local forces responsible for heterogeneous uplift and tilting of basement blocks are unclear, an origin coupled to some form of thermal disturbance seems likely. If so, a single episode of regional extensional deformation and thermal disturbance is expressed in dramatically different ways in contiguous areas, and the differences have strong potential economic implications.
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