Synvolcanic oligocene stress reorientation in Eastern Nevada: evidence from the Golden Gate range

Detailed mapping and kinematic analysis of faults in the northern Golden Gate Range provide new kinematic evidence regarding two temporally distinct episodes of extension within east-central Nevada. The Golden Gate Range sits astride the Blue Ribbon Lineament, an east-west trending belt of Tertiary rocks located in southwestern Utah and east-central Nevada. Previous work on the Blue Ribbon Lineament in east-central Nevada indicated that broadly W-striking faults that structurally define the lineament were active during volcanism along the lineament (31-22 Ma). However, the kinematics of these faults could not be reliably determined until this study. The earlier extensional event is characterized by W-striking faults that cut Mesozoic contractional structures. Contact relations within the northern Golden Gate Range and along-strike correlations to other ranges indicate that these faults were active during mid-Tertiary volcanism. In the Golden Gate Range, paleovalleys localized by movement a long these faults are filled with the 27.5 Ma Monotony Tuff. Postvolcanic reactivation of the W-striking faults is indicated by small displacements of 27.5-18 Ma tuffs and basalts. Offsets of the Paleozoic section across W-striking faults are between 100 and 2000 m and indicate normal displacement. Offsets of the Tertiary section across these same faults are less than 10 m. Alignment of piercing points defined by the hinges of Mesozoic folds allows at most 360 m of cumulative strike slip across three W-striking faults which have respective dip separations of 1500 m, 760 m, and 2100 m. These relations indicate that W-striking faults in the northern Golden Gate Range were active as normal faults prior to 27.5 Ma and were later reactivated after peak mid-Tertiary volcanism ceased. Fault kinematic data verify that the W-striking faults formed as conjugate normal faults during N-S extension and suggest that some were reactivated in strike slip during later extension in a NE-SW direction. Compaction foliations in the Tertiary volcanic section dip consistently 20° west, recording postvolcanic tilting of the range and of the W-striking faults. Removing this tilt also removes the westward plunge of M-poles of the W-striking faults, consistent with pure dip slip across those faults. N-striking highangle normal faults cut the Tertiary volcanic section and record postvolcanic E-W extension. Paleomagnetic data from Tertiary volcanic rock samples collected in the northern Golden Gate, northern Seaman, southern Schell Creek, and Timpahute ranges show a lack of significant vertical-axis rotations in structural blocks bounded by W-striking faults. Vertical-axis rotations are a common consequence of distributed strike-slip faulting, and the lack of rotations in these ranges is consistent with predominantly normal movement a long W-striking faults. Map relations and kinematic data from the northern Golden Gate Range combined with paleomagnetic data from east-central Nevada indicate that W-striking faults in the western half of the Blue Ribbon Lineament formed as normal faults during Oligocene synvolcanic N-S extension. In the northern Golden Gate Range, total N-S extension along a 14.36 km long balanced cross section was 1.9 km. These observations are consistent with a tectonic model linking N-S extension along the lineament to transient modification ofthe crustal stress field by passage of a southward-migrating magmatic belt.