Unmetamorphosed sedimentary mélange with high-pressure metamorphic blocks in a nascent forearc basin setting

Abstract

Mélanges crop out within unmetamorphosed basal Great Valley Group (GVG) forearc basin strata and between GVG and the underlying Coast Range Ophiolite (CRO) in the San Francisco Bay region of coastal California. These mélanges include high-pressure (HP) metamorphic blocks of the Franciscan subduction complex that structurally underlies the unmetamorphosed CRO as well as blocks of GVG and CRO. The matrix consists of foliated shale and serpentinite, locally interleaved at centimeter scale. The mélanges strike and dip parallel to bounding GVG sandstones and conglomerates. The matrix locally consists of sedimentary breccia and conglomerate made up of clasts of serpentinite and shale. GVG sandstones within and bounding the mélanges have detrital serpentinite clasts. The field relationships indicate a sedimentary origin of the mélanges as olistostromal deposits within latest Jurassic to earliest Cretaceous basal GVG. The mélanges correlate to units along the eastern margin of the northern Coast Ranges, about 250km to the north with restoration of post-subduction dextral faulting, and differ from the latter in the higher proportion of shale, stronger matrix foliation, and common occurrence of HP blocks. Similar units may have mistakenly been assigned to the Franciscan owing to the foliated nature of the matrix and occurrence of HP blocks. This and the broad distribution of localities indicate that these deposits are more widespread than previously believed. Exhumation rates of coarse-grained HP mélange blocks may have been 2 to 10mm/yr or higher based on the ages of similar blocks in the Coast Ranges, burial depth of the blocks, and depositional age of the enclosing strata. Exhumation and deposition of Franciscan blocks in these GVG mélanges predates preserved accretion of similar materials in the adjacent Franciscan by at least 30m.y., suggesting subduction erosion of previously accreted material, or exhumation of the blocks in forearc serpentinite–shale diapirs.