The detrital zircon geochronology and structural petrology of Pacheco Pass, Diablo Range, central California : implications for subduction zone timing and tectonics

The timing and tectonic history of blueschist facies rocks, though a widely studied problem, is still a topic of considerable debate because of poor age constraints in terranes with few fossils or marker beds. This study applies U-Pb detrital zircon geochronology by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), to a well-studied blueschist facies terrane, the area around Pacheco Pass, which is a window through the Great Valley forearc into the deepest coherent part of the Franciscan accretionary complex. 150 zircon grains were analyzed from each of sixteen samples, to determine their U-Pb ages. Additionally, a mineralogical and microstructural study of 103 thin sections was performed by traditional optical microscopy to supplement the geochronologic information. The maximum ages of deposition for coherent metagraywackes are as young as ca. 90 Ma, and ca. 70 Ma for blocks in mélange. Using the observed age distribution, the coherent rocks are divided into two units: Unit 2 (unimodal) and Unit 3 (bimodal). Unit 2 is structurally higher than Unit 3; thus, it was underplated first. Similar patterns are present in the Sacramento Valley of the Great Valley forearc basin (DeGraaff-Surpless et al., 2002), except that the base of the section is unimodal, and the upper parts are bimodal. This evolution is thought to reflect increased dissection of the arc over time due to the eastward stepping of the drainage divide. The source area for the coherent units is inferred to be similar to that of the Great Valley Group: the Sierran magmatic arc and associated terranes in northern California, western Nevada and southern Oregon. To explain the geochronology, as well as field observations, underplating must occur at the tens-of-meter scale, although larger or smaller packages of rocks are not precluded from being underplated. Minimum rates of underplating, based on the maximum ages of deposition for the two coherent units, are 10 to 30 m/m.y. The layer-parallel foliation is defined by sheet silicates and pressure solution selvages, with sodic amphibole, lawsonite and blocky jadeitic pyroxene parallel to it. The foliation formed immediately after underplating, and the rocks were affected by high shear strain in the subduction zone, evidence of which is seen in the phyllite (for example, cm-scale isoclinal folds). Near-static conditions were reached by the time continued underplating sequestered the rocks from the subduction channel. The crystallization of radiating sprays of jadeitic pyroxene and unoriented lawsonite tablets occurred at this time. These conditions are also recorded by the opening of anisotropy-controlled layer-parallel veins. Mélange genesis began after 90 Ma, and was active by at least 70 Ma, which corresponds to the onset of Laramide style orogenesis in western North America. By 60 Ma, however, the rocks were at 110°C based on apatite fission track data, which corresponds to a depth of ~15 km, with geothermal gradients of 10°C/km. This indicates rates of unroofing of 500 m/m.y. from depths of ~30 km. Localized folding and simple shear in an extensional regime is characteristic of this latter phase of deformation. Folds with axial planar pressure solution cleavages that are superposed upon one another indicate localized zones of sinistral shear in near-vertical shear zones. Ptygmatically folded veins have been shortened up to 70% in some cases. Pressure solution has removed up to 30% of the rock volume in some cases. When the rocks moved out of the regime of ductile deformation, layer-perpendicular veins and faults formed. Some of the unroofing, therefore, may have been accommodated by faulting in the latter part of the unroofing history. Based on the Quien Sabe volcanic rocks (11-9 Ma) that overlay Franciscan rocks of the Diablo Range, the rocks must have been at the surface by ~10 Ma. Therefore, the minimum rate of unroofing between 60-10 Ma is 300m/m.y. This indicates slow synsubduction unroofing.