Decrition pétrographique et pétrochimique des roches cristallines et ophiolites . Description minéralogiques des inclusions dans les serpentinites.

The aim of the San Andreas Fault Observatory at Depth (SAFOD) project, a component of the NSF Earthscope Initiative, is to directly observe active fault processes at seismogenic depths through the drilling of a 3 km deep (true vertical depth) inclined borehole across San Andreas fault. Preliminary subsurface models based on surface mapping and geophysical data predicted different lithologies than were actually encountered. At 1920 meters measured depth (mmd), a sequence of well-indurated, interbedded arkosic conglomerates, sandstones, and siltstones was encountered. We present a detailed lithologic and structural characterization as a step toward understanding the complex geologic history of this fault-bounded block of arkosic sedimentary rocks. We divide the arkosic section into three lithologic units with different compositional, structural, and sedimentary features: the upper arkose, 1920-2530 mmd, the clay-rich zone, 2530-2680 Illtlld, and the lower arkose, 2680-3150 mmd. We interpret the section to have been deposited in a Salinian transtensional basin, in either a subaqueous or subaerial fan setting. We suggest four different possibly equivalent sedimentary units to the SAFOD arkoses, the locations of which are dependent on how the San Andreas fault system has evolved over time in the vicinity of the SAFOD site. Detailed analysis of three subsidiary faults encountered in the arkosic section at 1920 mmd, 2530 mmd, and 3060 mmd, shows that subsurface faults have similar microstructures and composition as exhumed faults at the surface, with less evidence of alteration from extensive fluid flow.

The ~2 km-thick Late Triassic Sugarloaf Arkose is the basal unit of the half-graben Deerfield basin, Massachusetts. Valley-river, piedmont-river, and alluvial-fan depositional facies within the arkose are defined by paleocurrent data and style of sedimentation. The valley rivers flowed from northeast to southwest, and the facies is present from the bottom to the top of the formation. Piedmont rivers built a megafan eastward into the basin, beginning about in the middle of the arkose. The local alluvial fan built from east to west in the upper-third of the formation. The petrology of the medium sand and conglomerate was used to delineate the source areas for each facies. The medium sand in the valley rivers is mostly granite and granite gneiss fragments, coarsely-polycrystalline quartz grains, and twinned plagioclase. This assemblage is a mixture of granite from continental basement uplift, granite gneiss from a dissected magmatic arc, and phyllites and schist from a recycled collision orogen. The medium sand in the piedmont-river facies lacks granite fragments, and untwinned plagioclase is more abundant than twinned: the provenance is continental basement uplift and recycled collision orogen. The alluvial-fan provenance is similar to the valley rivers, combining recycled collision orogen and dissected magmatic arc. Unlike the valley rivers, granite gneiss and untwinned plagioclase in the alluvial fan are dominant over granite and twinned plagioclase. Quartz provenance in the three facies was granite, trending to granite gneiss in the piedmont-river and alluvial-fan facies. In all facies, plagioclase feldspar is more common than K-feldspar in the medium sand. The conglomerate pebbles, however, are dominated by K-feldspar, most likely due to erosion of pegmatites in the source terrane. Gray quartzite, white and translucent varieties of quartz, and pink granitoid pebbles are also common. The post-depositional diagenesis of the Sugarloaf Arkose affects provenance determination. Diagenetic events include: hematite grain coats, mechanical compaction, albitization of feldspars, albite and quartz overgrowths, authigenic hematite cement, carbonate cement, and illite replacement of feldspars. Within the dry-dominated monsoonal paleoclimate, each facies formed in response to tectonism. The initial appearance of each facies is used to determine the timing of tectonic events. The valley rivers flowed from the northeast in an early NNE-SSW-trending ‘sag’ basin, associated with minor normal faulting. The initial appearance of the east-flowing piedmont rivers about half way up the section implies an early, down to the west, basin-bounding normal fault, which formed perpendicular to N70E-S70E extension. This fault propagated, and, on reaching the northeast corner of the basin, the alluvial fan built to the west off the fault scarp. The Amherst block is a relay ramp between basin-bounding faults in the Deerfield and Hartford basins. Linkage of the two basin-bounding faults through the Amherst block created an integrated basin linking the Triassic strata in the early Hartford and Deerfield basins, and may have caused the unconformity present at the top of the arkose.