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SF State GEOL 426 - Fort Point

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Franciscan rocks of Fort Point and the Hunter’s Point shear zone May 2, 2006 Field trip guide compiled by Mary Leech Name: Submit this field guide with the “Basalt lab”Driving directions Time Mileage Directions 13:00 0.0 Meet in the COSE van parking lot behind Thornton Hall, SFSU 0.5 R on Lake Merced Drive 1.3 Veer Right onto Sunset 3.8 R on Martin Luther King, Jr. Drive in Golden Gate Park 4.9 L on Park Presidio Blvd. toward Hwy 1 north Note: 19th Ave. becomes Park Presidio at GG Park Stay in the right lanes for Hwy 1 north 8.4 Exit at the last SF exit before the Golden Gate Bridge (signage is poor) to Lincoln Blvd. 8.5 R on Lincoln Blvd. 8.8 Hard R onto Merchant Road 9.0 Park in the large dirt lot with concrete bunkers visible in front — we will take the trail that leads northwest between two bunkers, then turns right along the edge of the cliffs. After descending some wooden stairs, branch left and take the steep path that heads straight down to the beach. This path exploits the only easy break in the cliffs in this area. Description of stop From J. Wakabayashi, Geologic Field Trips in Northern California, 1999, California Department of Conservation, Division of Mines and Geology, Special Publication 119, p. 10-13. This stop examines shoreline exposures of the Hunters Point shear zone, a serpentinite and shale matrix mélange zone that is one of several discrete mélange horizons that separates coherent Franciscan nappes in the Bay Area. Serpentinite matrix mélange, including gabbro pods that reach tens of meters in size, is generally restricted to the structurally higher part of the shear zone. The serpentinite appears to be intact, rather than disaggregated as in the sedimentary serpentinites of the basal Great Valley Group (Phipps, 1984), and contains massive as well as sheared rock. Locally, relatively unserpentinized harzburgite and clinopyroxenite can be found. Gabbro appears to have been metamorphosed at amphibolite-facies conditions, probably in a sea-floor setting, with replacement of most pyroxene by olive green to brownish calcic amphibole. Prehnite-pumpellyite-facies (probable burial) metamorphism overprints the higher-temperature sea-floor metamorphism. The structurally lower part of the mélange zone has a shale matrix. Sandstone is the most common block type in the shale matrix mélange, with fairly common basalt and chert blocks and rare high-grade metamorphic rocks. With the exception of the high-grade blocks, the blocks were metamorphosed at prehnite-pumpellyite conditions. Based on inferred incorporation ages of units structurally above and below the mélange zone, the Hunters Point shear zone probably formed about 100 million years ago. This zone is interesting because it is an example of an ultramafic-bearing structural horizon within the Franciscan in contrast to the Coast Range Ophiolite that structurally overlies the Franciscan. This serpentinite may have been scraped off the subducting plate (Coleman, 1996). At the upper rim of the beach cliff we pass through intact serpentinite before starting down a steep trail to the beach over colluvial and landslide debris. Many of the exposures at the base of the cliffare not in place, having slid down the cliff face to some extent, but a few exposures appear to be in place. Classic block-in-matrix mélange structures can be seen in the intact shale matrix mélange exposures. Although there is ample evidence of shearing contributing to mélange development, we will also see evidence for soft-sediment deformation, particularly in the fluid-like appearance of metatuff layers in shale. Thus, this mélange, similar to many others in the Franciscan, resulted from olistostromal deformation and mixing, followed by tectonic deformation and further mixing (Aalto, 1976; Page, 1978; Cowan, 1985; Macpherson and others, 1990). We will walk south along the beach to a rocky point that is composed of several blocks of unusually high-grade amphibolite. These blocks in the mélange zone are clues to the early history of the Franciscan, a history we will explore further at the Tiburon peninsula. The amphibolites are locally garnet-bearing, yield garnet-hornblende temperatures of about 700°C, and overprinted with blueschist assemblages. Textures suggestive of partial melting are preserved in some samples. Quartz and garnet-rich interlayers are apparently metachert beds. The occurrence of metachert is critical because it ties the surrounding metabasaltic rocks to a setting in the top layer of the ocean crust. High-temperature metamorphism in an ocean-floor setting has been recognized in fracture zones and deep hydrothermal settings associated with lower levels of the oceanic crust such as gabbros, but not the upper levels of the ocean crust, such as basalts or overlying cherts. The high-temperature metamorphism of the metachert and metabasalt is probably a product of the early stages of subduction (explained more fully in the stop description for the Tiburon peninsula). The high-grade blocks of the Franciscan Complex have a uniformly old age (about 160 Ma). The presence of such a block in a mélange that probably accreted at 100 Ma, suggests the block may have been originally introduced as an olistostrome block, consistent with the evidence for soft sediment deformation noted above. Alternatively, the introduction of the high-grade block into this mélange zone may be a result of mélange return flow that plucked blocks from a structurally higher horizon in the accretionary complex (Cloos, 1984). List and describe the various rock types we find at this stop:How does this example of Franciscan mélange compare to the exceptional Bodega Bay example we saw on our earlier field trip? Describe the Hunters Point shear/mélange zone:Below figures are from “Geology of the Golden Gate Headlands” by William P. Elder, National Park Service, Golden Gate National Recreation Area,


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