LATE-CRETACEOUS SHEAR ZONES FORMED BY EMPLACEMENT, REGIONAL DEXTRAL SLIP, AND STRAIN PARTITIONING AROUND THE TUOLUMNE BATHOLITH: IMPLICATIONS FOR SHEAR ZONE DEVELOPMENT IN ARCS AND CRETACEOUS TECTONICS

Our new 1:10,000 mapping combined with published studies, indicate that three types of Late-Cretaceous shear zones formed in the central Sierra Nevada, particularly around the Tuolumne Batholith (TB).

The first are small displacement, submagmatic to moderate T subsolidus shear zones developed in and subparallel to the margins of plutons and are common in SE and SW lobes and older host rocks of the ~ 94.5 to 93 Ma Kuna Crest TB. These shear zones are steeply dipping, have lineations pitching steeply on shear planes , and have pluton-side-up kinematics.

The second are large displacement shear zones recording transpressional motion. They are subparallel to the arc and despite variations in lineation plunges, have kinematic indicators in subhorizontal planes indicating largely dextral oblique-slip movement. Examples of these shear zones, include the NNW-striking, ~98 Ma syn-magmatic Sing Peak shear zone with moderately N-plunging lineations (Yoshinobu, pers comm), and newly mapped NW-trending high to low T Virginia Canyon and Steelhead Lake shear zones with steep SE plunging lineations along the eastern margin of the TB. The latter are likely connected to the Gem Lake shear zone (91 Ma? - 80 Ma) which also displays dextral strike-slip kinematics. These dextral transpressive shear zones remained active during cooling and evolved into brittle, dextral, strike-slip faults now found in the center of the ductile shears in the Steelhead Lake and Virginia Canyon areas.

The third, probably related to strain partitioning between group two shears, have NE-SW pure-shear kinematics. The 95-90 Ma syn-magmatic phase of NW-trending Bench Canyon shear zone and 99 Ma NW-trending moderate-T Quartz Mountain ductile shear zone may belong to this type.

Our studies indicate that the large shears have complicated spatial and temporal histories forming secondary shears with various kinematics. Difficulties in fully understanding the regional kinematics of these shear zones are the loss or modification of old kinematics indicators and poor constraints on the initiation/duration of shearing. Our present results support a complex pattern of dextral transpression from at least 110 Ma to 75 Ma, locally altered by magma emplacement: proposed temporal changes in shearing kinematics based on lineations and shear zone ages are worth reconsidering.