MÉLANGE TYPES AND FORMATION IN CONTRASTING SETTINGS ASSOCIATED WITH CONVERGENT MARGIN TECTONICS

Researchers commonly classify mélanges based on structural, lithologic or sedimentary features, but the regional tectonic setting is useful in linking mélange development to convergent margin processes. Coastal California & the Tethyan region offer good examples of various tectonic settings of mélanges. The Franciscan subduction complex has several distinct settings & types of mélanges: shale matrix mélanges occur both within & bounding coherent nappes, whereas serpentinite matrix mélanges occur at the structurally highest levels & commonly cut blueschist & higher-grade coherent nappes. These mélanges are rare/absent bounding lower grade nappes or cutting them. Mixed serpentinite & shale matrix mélanges occupy ambiguous tectonic settings & contain blocks of both Franciscan material & the overlying Coast Range ophiolite & Great Valley Group. Sedimentary serpentinites exist in the Great Valley Group but not in the Franciscan. In the Tethyan system mélanges occur as sub- & supra-ophiolitic units & syn-collisional mélanges straddling the suture zones. Sub-ophiolitic mélanges (SBOM) are wedged between ophiolites above & passive margins below and include clasts & blocks or thrust sheets of volcanic & hemipelagic sedimentary rocks & serpentinite breccia in a clastic matrix; lavas with MORB-WPB affinities are derived from rifted margins and/or seamounts. SBOM form during SSZ ophiolite emplacement via trench-continent collisions. Supra-ophiolitic mélanges (SPOM) rest unconformably on ophiolitic units & include rocks derived from the ophiolites, metamorphic soles & continental margins in a locally stratified shaly matrix. Matrix-supported SPOM occur as debris-flow deposits developed during advanced stages of basin collapse. Syn-collisional mélanges (SCM) along suture zones comprise olistostromal blocks and/or thrust wedges of ophiolitic & sole rocks, blueschist assemblages (exhumed, partially subducted passive margin rocks) & continental margin units in a reworked serpentinite or shaly matrix. SCM form during the terminal closure of basins & their structural fabric is characterized by thrusts-backthrusts in hard collision zones & by transpressional faults in oblique collision zones. The collision geometry & kinematics & the relative motions of bounding continents are critical in SCM evolution.