Tectonic evolution of the northern Sierra Nevada-Basin and Range transition zone: A study of crustal evolution in extensional provinces

Structural and thermochronologic data from the northern Sierra Nevada—Basin and Range transition zone at 39°N constrain the Tertiary thermal and structural evolution of the western margin of the Basin and Range province. A geologic and geophysical transect has been constructed and encompasses a cross-section from relatively unextended crust in the Sierra Nevada on the west to highly-extended crust in the Singatse and Wassuk Ranges on the east. Multiple generations of east-dipping normal faults in the central Wassuk Range, western Nevada, have accommodated major ENE-WSW oriented extension. Extensional deformation has resulted in exhumation of large fault blocks which display westward tilts of ∼60% Westward fault block tilting initiated following extrusion of 15 Ma. andesites and was accommodated by two major generations of normal faults with minor post-6.9 Ma. high-angle normal faults accommodating less than 12° westward tilting. Extension has exposed approximately 8.5 km of the pre-extensional upper crust in the Wassuk Range fault block. Apatite fission track data from the Wassuk Range indicate a pre-extensional geothermal gradient on the order of 26 ± 5°C/km, a value corroborated by apatite (U-Th)/He data. Both fission track and (U-Th)/He data suggest reduced rates of cooling following major extension, as well as renewed high-angle extensional faulting starting at approximately 4 Ma and continuing to the present. The modern geothermal gradient beneath the central Wassuk Range is on the order of 35°C/km, while the Tertiary geothermal gradient in the Sierran arc was on the order of 10 ± 5°C/km. This indicates heating of the upper crust through time, with the 26 ± 5°C/km value at 15 Ma. a probable transient value which has increased since the onset of large-scale extension due to compression of isotherms during crustal thinning. Present-day heat flow values and the depths of seismic events across the Sierra Nevada—Basin and Range transition zone indicate a probable thermal encroachment of Basin and Range-type values into the Sierra Nevada. Apatite fission track and (U-Th)/He data help define a westward thermal migration which occurred in concert with the structural encroachment of Basin and Range-type deformation into the Sierra Nevada over the last 15 Ma.