An Exhumed Strike-Slip Duplex Formed at the Seismogenic Base of the Crust

Abstract:

In strike-slip systems, the orientation of bedding influences local fault zone architecture. New 1:5,000 and smaller scale mapping of a paleoseismic shear zone in the central eastern Sierra Nevada, California, provides insight into the behavior of strike-slip faults deforming like-oriented bedding at a dextral shearing, pseudotachylyte-bearing brittle-ductile transition (BDT) in the Sawmill Canyon/Saddlebag Lake segment of the Sierra Crest shear zone. In and near this map area, a strike-slip duplex system, a Riedel shear system, and shear zone-parallel major fault strands all comprise a heavily quartz and silicate vein-sealed, brittle, and partially ductilely overprinted fault system. The strike-slip duplex system is defined by spectacular restraining and releasing bend duplex geometries. New and published stable isotopic compositions of these veins (𝛿¹⁸O = +14.5 to –3.2‰) and their fluid inclusions (𝛿D = –79 to –137‰) monitor mixing between three fluid sources. The main mixing trend includes an earlier local (likely hot magmatic) to a later distal (likely cool meteoric) fluid source over time.

We interpret the following. (1) Strike-slip faulting subparallel to stratigraphy at the BDT can result in strike-slip duplexes and semi-ductile Riedel shears. As planar mechanical anisotropies are folded, continued offset of the fold requires either rotation of the material about an axis, or extension of the inverted limb. In homogeneous units, rotation becomes exceedingly difficult with thickness. Thus the folded material may be subjected to enhanced tension relative to outside limbs, which in turn nucleates shear zone-subparallel lateral ramps along which folds are transported. (2) This mechanism may promote the fault-assisted descent of fluids from the surface to the seismogenic base of contractional arc crust, where earlier deformation has transposed stratigraphy toward the shear plane in a simple or transpressive wrench shear zone. (3) Duplex fractures in both restraining and releasing bends can be sinks for descending hydrothermal fluids.