VARIETY OF OROGENIC ULTRAMAFIC BODY INTERNAL CHARACTER AND CONTACT RELATIONSHIPS REFLECTS DIVERSE EMPLACEMENT MECHANISMS

Most serpentinitized peridotite found in orogenic belts is derived from oceanic lithosphere, but the emplacement mechanisms of these bodies vary greatly, as illustrated by the nature of their contacts. The diverse apparent emplacement mechanisms have important implications for connecting ophiolitic rock occurrences to large-scale orogenic processes. In the California Cordillera, the largest sheets of ultramafic rocks comprise intact sheets associated with ophiolites, such as the Coast Range ophiolite, that structurally overlie subduction complexes, such as the Franciscan Complex. The basal contact of such sheets is a subduction initiation horizon, commonly reactivated by later faulting that exhumed the subduction complex relative to the serpentinite. The upper contact of such occurrences may be intrusive (gabbro/diabase), tectonic (detachment fault), or sedimentary (volcanic or pelagic sedimentary rocks). Intra-subduction complex ultramafic rocks occur as nearly block-free sheets (in the Franciscan Complex), and as a part of mafic-ultramafic duplexes or broken formations (in the Shoo Fly Complex Sierra City mélange). For such occurrences, their basal contact is an intra-subduction complex megathrust horizon and the upper contact may be tectonic or depositional (pelagic or clastic sedimentary rocks). Intra-subduction complex serpentinite also occurs as sedimentary serpentinite mélange (in the Franciscan Complex) that was partly subducted after deposition in the trench via submarine sliding and includes blocks that record older and higher-grade metamorphism than the matrix. For such units, their original basal contact is depositional and their upper contact may be, too (in contact with siliciclastic rocks). Sedimentary serpentinite mélange that includes HP metamorphic blocks is also found in the basal Great Valley Group forearc basin deposits. Such exposures may represent forearc serpentinite mud volcano deposits. Their basal contacts and upper contacts are sedimentary, but they may also be associated with diapiric structures that fed them. Diapiric structures have proved difficult to document. Although many of the ultramafic bodies have a regionally low-angle geometry, they have been folded so that exposed limbs commonly have steep dips.