CONTROLS ON STRUCTURAL PATTERNS IN THE YOSEMITE VALLEY INTRUSIVE SUITE, NORTHWESTERN YOSEMITE NATIONAL PARK

The voluminous Cretaceous Yosemite Valley Intrusive Suite (YVIS) of the central Sierra Nevada batholith displays a complex magmatic foliation pattern in Yosemite National Park. Research conducted in a 25 km² area centered along the Tuolumne River focused on the structure of the YVIS, and structural relationships with the younger Tuolumne Intrusive Suite (TIS). The study area is dominated by two units of the YVIS, the ~98-102 Ma Taft Granite, a homogenous, medium-grained biotite granite, and the 102-103 Ma Granodiorite of Mt. Hoffman (correlated with El Capitan Granite), a coarse-grained, locally porphyritic granodiorite to granite and associated mafic rocks. The YVIS regionally shares a variably oriented contact with the 93-85 Ma TIS, which is exposed in the eastern most part of the study area. Narrow (<50 m) metasedimentary bodies separate the YVIS and TIS in places and are interpreted to mark the original extent of the YVIS. Magmatic foliations and solid state ductile shear zones in the YVIS directly south of the study area strike NE and are parallel to the TIS contact, compatible with rotation of foliation by emplacement of the younger rocks. A critical area to evaluate this hypothesis is in a large salient of YVIS into the TIS in the study area, which is associated with a series of 90° swings in the trend of the contact. Steeply dipping magmatic foliation in the study area generally strikes NW and swings abruptly to N-S next to the northern contact of the salient. All foliations are discordant to the margin of the YVIS and to contacts between the Taft Granite and Granodiorite of Mt. Hoffman. We infer that this discordance in foliation orientation reflects regional strain and argues against reorientation during emplacement of the TIS. Further, the pronounced bends in the salient likely represent the original shape of the YVIS and we speculate that the geometry controlled the western extent of the TIS. Structural patterns in the study area demonstrate the potential importance of weak regional strain on the orientation of foliation in the central Sierra Nevada batholith.