FABRIC AND PALEOMAGNETIC ANALYSES OF THE SW LOBE OF THE TUOLUMNE INTRUSIVE SUITE, SIERRA NEVADA BATHOLITH, CALIFORNIA, USA

The Sierra Nevada batholith is an upper crustal exposure of a Mesozoic magmatic arc. A series of en échelon north-northwest trending Late Cretaceous intrusive suites along the Sierra Crest are among its youngest intrusive suites: from north to south, these are the Tuolumne, Mono Pass, and Mount Whitney intrusive suites. Each of these intrusive suites display lobes that deviate from the NNW trend of the arc; the largest of these bodies is the SW lobe of the Tuolumne intrusive suite, which extends 25 km out from the suite’s main body and includes exposures in Yosemite Valley. We present field-based, anisotropy of magnetic susceptibility (AMS), and paleomagnetic data collected to constrain the emplacement mechanism of the SW lobe. Samples were collected in a series of transects across the boundary of the Tuolumne intrusive suite, from the internal unit of the lobe (Half Dome granodiorite) to the surrounding units (Kuna Crest, El Capitan, Sentinel, and Illilouette Creek granodiorites). Field and AMS foliations both strike sub-parallel to the local boundary of the SW lobe and dip sub-vertically. AMS lineations are moderate to steeply plunging, with their maximum density showing a vertical orientation. AMS ellipsoids vary in shape around the boundary of the lobe. Plane strain ellipsoids are located on the NW and SE margins, oblate ellipsoids are found on the SW margin, and prolate ellipsoids are scattered around the margin of the bulge. Microstructural analysis of the Half Dome unit shows magmatic fabrics, with equant quartz and euhedral biotite books, feldspars, and amphiboles. All other units exhibit solid-state deformation, with elongate quartz and deformed biotite. Paleomagnetic analysis was conducted along the same transects. Preliminary analysis indicates some variation in paleonorth vector orientations which seems focused on the locations where the lobe boundary abruptly changes orientation (i.e., corners). Moreover, some of the paleonorth orientations are discordant with the Cretaceous paleopole of Hillhouse and Grommé (2011), based on the cooling histories of the intrusive bodies. We interpret this data in the context of forceful emplacement of the Half Dome in the SW lobe of the Tuolumne intrusive suite into the surrounding granodiorites.