Geologic signature of early Tertiary ridge subduction in Alaska

摘要: A mid-Paleocene to early Eocene encounter between an oceanic spreading center and a subduction zone produced wide range of geologic features in Alaska. The most striking effects are seen the accretionary prism (Chugach-Prince William terrane), where 61 50 Ma near-trench granitic gabbroic plutons were intruded into accreted trench sediments that had been deposited only few million years earlier. This short time interval also saw genesis ophiolites, some which contain syngenetic massive sulfide deposits; rapid burial these ophiolites beneath turbidites, followed immediately by obduction; anomalous high-T, low-P, metamorphism; intense ductile deformation; motion on transverse strike-slip normal faults; gold mineralization; uplift above sea level. magmatic arc experienced brief flare-up quiescence. In Alaskan interior, 100 600 km landward paleotrench, several Paleocene sedimentary basins underwent episodes extensional subsidence, accompanied bimodal volcanism. Even as far 1000 inboard ancestral Brooks Range its foreland basin pulse about 40 All events-but especially those prism-can be attributed with varying degrees confidence center. this model, allied ore deposits at soon-to-be subducted ridge. Near-trench magmatism, metamorphism, deformation, mineralization took place slab window, hot asthenosphere welled up gap two subducted, but still diverging, plates. Deformation critically tapered adjusted shape changes bathymetry incoming plate, convergence direction before after ridge subduction, strength it was heated then cooled. events interior would have taken more distal, deeper parts window. Extensional (or transtensional) subsidence driven subducting plates each exerted different tractions upper plate. lull along presumably marks when hydrated lithosphere not being axis. absence may explain North Slope: Geodynamic models predict long-wavelength magnitude will take from Andean-type margins is absent. Precise correlations hampered, however, palinspastic problems. During since Tertiary, margin-parallel faulting has offset plutonic belt-i.e., very basis for locating triple junction window-from backstop, amount remains controversial. magmatism began Sanak Island west until 51 Baranof Island, 2200 east. west-to-east age progression suggests migration trench-ridge-trench junction, we term Sanak-Baranof junction. Most workers held separated Kula Farallon As possible alternative, suggest plate another east, termed Resurrection