DIKING, DEFORMATION, AND DILEMMAS ALONG LYELL FORK IN THE HALF DOME GRANODIORITE, YOSEMITE NATIONAL PARK, CALIFORNIA

The Tuolumne Intrusive Suite (TIS) of Yosemite National Park comprises nested, inward-younging plutons that range from the tonalitic Kuna Crest series inward through the Half Dome Granodiorite to the central Cathedral Peak Granodiorite. Contact zones both between units and within the Half Dome (Coleman et al., 2012) locally display extreme petrologic and structural complexity. An example in the Half Dome on the east side of the TIS is exposed in slabs next to the Lyell Fork of the Tuolumne River. We mapped the slabs at 1:700 to investigate the magmatic and structural history responsible for the complexity.

Rock types include coarse-grained bio-hbl diorite, granodiorite, granite, aplite, pegmatite, and porphyritic andesite. Mapping defined a coherent sequence of events during assembly of this part of the Half Dome Granodiorite: 1) intrusion of equigranular Half Dome; 2) intrusion of commingled mafic-felsic dikes; 3) intrusion of felsic dikes; 4) folding of the dikes; 5) disruption of dikes and folds by sinistral shear zones; 5) a second generation of commingled mafic-felsic dikes; 6) a second generation of felsic dikes; 7) andesite dikes that form a stockwork; 8) more folding; and 9) pegmatite dikes. Two phases of post-magmatic deformation followed: 10) NW-striking reverse-sense brittle-ductile shear zones—lineations on shear surfaces indicate a dextral component, although offsets of variably oriented aplite dikes across one fault suggest net sinistral shear; and 11) conjugate NE-striking subvertical tension-gash arrays that trend ~060 (sinistral) and ~025 (dextral). The gash arrays together record NE-SW shortening, similar to but clockwise relative to N-S shortening seen in similarly complex Half Dome Granodiorite exposures on the west side of the TIS near Tenaya Lake. The synintrusive strain field in both areas consists of N-S or NE-SW shortening and subhorizontal extension perpendicular to this. Dikes that are at least 1 m thick were intruded parallel to the shortening direction and then gently folded by continued shortening.

The zones of structural complexity reflect incremental assembly of the TIS, but why incremental intrusion is so clearly expressed in some places and subtle elsewhere is not understood.