STRUCTURAL CONTROL AND TECTONIC SETTING OF MESOZOIC VOLCANISM IN HONG-KONG

Middle Jurassic to Lower Cretaceous silicic volcanic rocks and granito id plutons in Hong Kong form part of the broad, NE-trending Yanshanian tectonomagmatic belt, that extends through SE China. Pyroclastic depo sits in Hong Kong are typically proximal and infill linear flared vent s and ellipsoidal calderas. These pass down through elongate rift-like conduits into extensive multiple fault- and shear zone-hosted ('sheet ed') dyke complexes and granitoid plutons. Dykes also emphasize the li mits of calderas. The spatial distribution and geometry of the vents, calderas, dykes and plutons are structurally controlled during each of four temporally wen-constrained periods of petrogenetically related v olcanism and plutonism. E- and ENE-trending axes of extension, and tra nstension were dominant. NW-trending faults increased in importance wi th time, and their intersections with ENE-trending faults influenced t he loci of volcanic centres. Nesting of the calderas confirms persiste nt reactivation of some faults. The loci of magmatism migrated over 25 Ma from NW to SE. Zones of silicic crust created during these magmati c episodes have a broadly tripartite crustal 'stratigraphy' (pluton, d yke, volcanic) reminiscent in very general terms of ophiolites. The zo nes were created during periods of unusually rapid extension, as the c ontinental margin of SE China changed from an active margin, dominated by subduction, to a back-arc regime, and dextral strike-slip along EN E-trending faults gave way to sinistral strike-slip.