Structures in the Jackass Lakes pluton–host-rock system, central Sierra Nevada, California, and inferred mid-Cretaceous Farallon–North America plate kinematics

Structures within the Jackass Lakes granodiorite pluton-host-rock system, central Sierra Nevada, California, record regional deformation that was contemporaneous with magma chamber construction and may have been related to mid-Cretaceous plate motions. Host rocks consist of deformed metavolcanic rocks of the mid-Cretaceous Minarets caldera sequence, older metavolcanic rocks, minor metasedimentary rocks, and slightly older granodioritic plutons. Structures in the large Sing Peak roof pendant, within the southwest portion of the Jackass Lakes pluton, include: (1) a penetrative, subvertical, north-northwest-striking foliation; (2) folded synmagmatic dikes; and (3) a dextral-oblique, reverse-sense ductile shear zone with a shallowly to moderately plunging (similar to 30 degrees-45 degrees), north-northwest-trending lineation. The Sing Peak shear zone may be traced into the Jackass Lakes pluton along the northern margins of the pendant and involves both subsolidus but predominantly hypersolidus fabric development. Structures within the Jackass Lakes pluton include a well-developed, north-northwest-striking, steeply west-dipping magmatic foliation and a moderately north-plunging (similar to 40 degrees) lineation. The magmatic structures are continuous across compositional zones within the Jackass Lakes pluton and are parallel to the elongated axis of mafic enclaves observed within the pluton. Abundant xenoliths of metavolcanic rocks occur throughout the pluton. Metamorphic foliations observed within the xenoliths are subparallel to those in the host rocks and the magmatic foliations in the pluton. Some xenoliths contain folded synmagmatic granodioritic dikes with axial planes that are subparallel to the metamorphic foliations in the xenoliths and host rocks and to the magmatic foliations in the pluton. We conclude that the formation of magmatic and metamorphic structures as well as synmagmatic folding of dikes occurred during pluton construction and was most likely due to regional deformation in a magmatic-to-crystal plastic shear zone with dextral-oblique shearing at ca. 98 Ma. At the latitude of the Minarets caldera, the Sing Peak shear zone represents a shallow-crustal manifestation of a discontinuous, but extensive network of braided intra-batholithic shear zones that are exposed along the spine of the Sierra Nevada batholith. Several published studies have demonstrated an episodic shift in the kinematics of these shear zones (oblique vs. dip slip) beginning around 90 Ma. We suggest that the Jackass Lakes pluton-host-rock system preserves the earliest evidence for Cretaceous regional dextral transpression and may be related to relative plate-motion coupling in the Sierran continental magmatic arc. Such an interpretation is consistent with recently published mid-to Late Cretaceous relative plate-motion vectors between the Farallon and North American plates, which support the initiation of dextral-oblique convergence around 100 Ma.