Detrital zircon geochronology and petrography of Franciscan graywacke blocks at San Simeon, California : implications for mélange genesis

Morisani, Anna Marie
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The Central Melange Belt of the Franciscan comprises a tectonically produced mixture of graywacke blocks with lesser amounts of greenstone, chert, and graphite schist blocks, in a shale matrix. Rare exotic blocks of blueschist and eclogite are also found dispersed within the Central Belt exposures. South of San Francisco, it is exposures at San Simeon that give the best sense of the chaotic nature of melange. The nature of melange formation has been debated since Hsu (1968) applied the term to describe the chaotic rocks in the central California Coast Ranges. The age of the most common type of block in the Franciscan melange, graywacke, has never been directly determined. Knowing the depositional age of the ubiquitous graywacke blocks at San Simeon will provide new constraints on the origin of melange. The age distribution obtained in the process will reveal a new understanding of the character of the source terranes of the sediment being fed to the trench. Petrographic information obtained from melange graywacke blocks in this study includes the occurrence of pumpellyite in many samples, indicating temperature and pressure conditions consistent with depths of 10-15 km in an active subduction zone with subnormal geothermal gradients. Thin sections also revealed the pinch-and-swell structure of melange graywacke blocks was attained by particulate flow. The style of deformation in the melange graywacke blocks as well as the generation of pumpellyite is consistent with the process described in the flow melange model. On the basis of age distributions obtained for the 16 melange graywacke block samples analyzed in this study, three groups have been identified: Graywacke Group One (12 samples), Graywacke Group Two (two samples), and Graywacke Group Three (two samples). The timing of deposition for Graywacke Group One has been constrained to a window between 87 and 92 Ma. The timing of deposition for Graywacke Group Two has been constrained to between 84 and 87 Ma. The timing of deposition for Graywacke Group Three has been constrained to between 119 and 127 Ma. New age data for two samples of the Bedded Slope Cover (135-140 Ma), one sample of the Green Sand Unit (123-126 Ma), and two samples of Graphite Schist (175-183 Ma) has also been obtained. Additional constraint on the maximum age of the Bedded Slope Cover comes from two interbedded ash layers, with dates of 148 Ma and 150 Ma. The age data has shed new light on the model of melange emplacement at San Simeon. Two scenarios are proposed to account for the new petrographic and age information obtained in this study. Sediment was transported to the trench axis, subducted to depths of 10 to 15 km, and then returned towards the surface where it was accreted. The three Graywacke Groups are interpreted to represent three distinct phases of melange generation and or diapirism at San Simeon. The second scenario postulates that all three Graywacke Groups were deposited at the trench axis and subsequently subducted and deformed together. The variable age data is the result of the complexities of sedimentation at the trench axis.