Tom Schweich  

Bull Canyon fault and Cretaceous and Miocene unroofing of the Granite Mountains (Hess/Wells REF). Discuss offsets along the Bristol-Granite Mountains fault to the south (Buesch) and north (Brady).

Initial review suggests that coarse, east dipping (20°) reddish sandstone is the oldest unit. This is overlain by shallow-dipping (10°) coarse, sub-angular mixed lithology gravels in a deep red matrix. Deposition of relatively fresh granitic and gneissic breccias followed. The breccia is overlain by coarse conglomerate in a pale red matrix. In turn, this conglomerate is overlain by flat-lying tan arkosic sandstone with pedogenic carbonate. Return to vehicles, proceed east.

Cross the buried trace of the Bristol–Granite Mountains fault. It is marked here by linear truncation of pavement surfaces, but any scarps that may be present are very subtle (Figure) . This is the easternmost fault of the ECSZ, and has been subject to many interpretations in terms of its role in the shear zone. Langenheim and Miller (this volume) showed that it has about 12-17 km of right slip, with an apparent northward decrease in offset. The amount of right slip from correlating pairs of aeromagnetic anomalies in the Marble and Old Dad Mountains (15-17 km) is similar to the result, as modified below, from Lease et al. (2009). This suggests that all of the fault offset is Miocene and younger. Furthermore, gravity and aeromagnetic data indicate that the Bristol-Granite Mountains fault zone is connected to the Soda-Avawatz fault zone, with a curious pattern of alternating stretches of uplift and subsidence that may reflect coupling with faults farther west or east.

South Bristol Mountains fault at tower stub road. Mapping to the south (Harvey, this volume) has traced the South Bristol Fault northwest from the Bristol Lake area to near here. This fault appears to cut middle Pleistocene fan deposits but no younger deposits (Bedford et al, 201x), indicating that it has ceased activity for close to 100,000 yr. (Miller et al, 2007) or has had a very low slip rate in its recent history. A valley continues straight to the northwest, along which one or two parallel faults that place Miocene volcanic rocks against Cretaceous granitic rocks can be mapped. In a few places, these faults appear to cut old gravels of uncertain age, and one splay of the faults cuts and displaces the Broadwell Mesa basalt. We consider the South Bristol Fault to continue along this valley at least as far as the intersection with the Broadwell Mesa fault, but details of how the two faults interact are obscured by young stream and fan deposits, making its extrapolation farther north tentative at best.