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Late Miocene transition between Basin and Range extension and Walker Lane tectonics, northern Pine Nut Mountains, Nevada: New insights from geologic mapping and 40Ar/39Ar geochronology
AuthorSay, Michael Christopher
AdvisorZuza, Andrew V
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The westward encroachment of Basin and Range extension on the relatively stable Sierra Nevada block occurred during the Miocene. To better bracket the timing, magnitude, and kinematics of this transition, I conducted new geologic mapping, 40Ar/39Ar geochronology, and geochemical analyses in the northern Pine Nut Mountains, NV, which are the westernmost structural and topographic expression of the Basin and Range extensional province. Structural mapping suggests that north-striking normal faults developed during the initiation of Basin and Range extension and were later reactivated as northeast-striking oblique-slip faults following the onset of Walker Lane transtensional deformation in the Carson Domain. Newly obtained 40Ar/39Ar radiometric dates collected from 30-36° NW-dipping intermediate to felsic (~55-65% SiO2) volcanic and sedimentary rocks in the northern Pine Nut Mountains show that deformation initiated after 6.83 0.01 Ma. Tilting of the range was accommodated by a major east-dipping normal fault that defines the eastern flank of the range. Extension magnitude recorded in the northern Pine Nut Mountains (14% extensional strain) and westward towards the rigid Sierra Nevada is significantly less than the highly extended Singatse and Wassuk Ranges (~150-180% extension) to the east. Subsequently, Walker Lane transtension initiated and dextral shear in the Carson Domain induced clockwise rotation of structural blocks bounded by northeast-striking left-slip faults orthogonal to the dextral shear zone. This resulted in a northeast-striking oblique-slip and transtensional structure in the northern Pine Nut Mountains called the Bull Canyon fault. Although this oblique left-slip normal fault system is covered by young alluvium and inactive today, it is likely this structure served as a major kinematic component of left-lateral shear similar to other left-slip faults identified in the Carson Domain to the north and south. It may have become inactive because it became mechanically unfavorable as the Carson Doman rotated and slip was accommodated on the other parallel left-slip faults. Recently active, north-striking east-dipping curvilinear faults on the western flank of the range show dip-slip to oblique right-slip normal kinematics that may have accommodated some dextral shear as conjugate Riedel shears of the Carson Domain.Results from the northern Pine Nut Mountains provide further insight into the timing and evolution of the Sierra Nevada-Basin and Range/Walker Lane transition zone. Extension east of the study area began ca. 15 Ma in the Singatse and Wassuk Range and post ca. 6.8 Ma to the west in the Carson Range. The new age of deformation initiation in the northern Pine Nut Mountains (ca. 6.8 Ma) agrees with prior studies in and around the Sierra Nevada-Basin and Range/Walker Lane transition zone that utilized apatite fission track thermochronology, timing of basin development, and geologic cross-cutting relationships.