STRUCTURE OF THE YOSEMITE VALLEY INTRUSIVE SUITE: REGIONAL STRAIN FIELDS AND INFLUENCE ON EMPLACEMENT OF YOUNGER INTRUSIONS IN THE CENTRAL SIERRA NEVADA BATHOLITH

The ~103-99 Ma Yosemite Valley Intrusive Suite (YVIS) formed during voluminous silicic (leucogranite to granodiorite) magmatism that predated the large zoned granodioritic suites of the Sierra Nevada batholith. The structure of the YVIS, although generally not considered in regional syntheses, provides insights into the regional strain field, and its rheology influenced the geometry and emplacement style of younger intrusions. The suite consists of older, generally coarser-grained, and higher-color-index rocks of the El Capitan Granite and similar units, the younger Taft Granite, and comagmatic diorites that are more abundant in the south. Thin (<10 m wide) metasedimentary screens to km-wide pendants separate the YVIS from younger plutons for ~35 km in the north, and form the eastern extent of the suite. The YVIS apparently acted as a rheological barrier to younger magmas of the Tuolumne Intrusive Suite, which only penetrate the YVIS for short distances. The YVIS is also extensively fractured and surrounded by younger intrusive rocks along parts of its eastern margin (cf. Calkins, 1985), a relationship not shown by other units in the region.

Steep magmatic foliation in the suite is commonly margin-parallel, but in some >10 km² domains, N- and NW-striking foliations are at high angles in map view to contacts with host rocks and are interpreted to record regional NE-SW shortening. This shortening direction is compatible with the overall Sierran strain field, but contrasts with that of the E-W-striking regional foliation in the Tuolumne suite and Cretaceous plutons in the Tahoe area. The major solid-state structures are ductile shear zones (~5 cm to >50 m thick) that formed at >450⁰C and include NW-striking and larger NE-striking zones. They are much better developed in the El Capitan than the Taft Granite and indicate deformation shortly after emplacement and before ~98 Ma. The shear zones have moderate to steep dips, roughly down-dip lineations, and record reverse slip. Slightly younger (~98-80 Ma) map-scale reverse and dextral strike-slip zones in the region strike N to NNW in contrast to the large NE-striking zones in the YVIS. The anomalous NE strikes speculatively record strain refraction near NE-trending margins of the suite, but their origin remains a significant unanswered question.