T13 Integrated Sedimentary Systems: Applying Source-to-Sink Concepts across Geologic Time

Sunday, 9 October 2011
8:00 AM - 12:00 PM
Room 211CD

TECTONIC SINK OF THE TURBIDITE SYSTEMS OF TRENCHES AND DEEP SEA FANS--BUILDING THE COASTAL ROCK RECORD AND THE RUPTURING OF GREAT AND GIANT SUBDUCTION ZONE EARTHQUAKES

Paper number: T13-9
Program: Technical Sessions
Time: 10:05 AM – 10:25 AM

Authors

Abstract

OFFSHORE OBSERVATIONS: Seaward of modern subduction zones turbidite systems thickly fill (1 to >5 km) long sectors of some trenches (e.g., Nankai, Aleutian, Cascadia, and S. Chile) and also form large fan complexes adjacent to them (e.g., Surveyor Fan, Gulf of Alaska). Because trench axes slope away from major sediment input areas, sediment-filled trenches can be 1000->2000 km in length and equivalent in volume to large abyssal fans. At convergent margins, ocean-basin turbidite systems are transported tectonically into subduction zones (SZs).

PATH OF THE TECTONIC SINK: Geophysical observations at Pacific-rim SZs document that incoming trench and fan sections thicker than 1-2 km form a 20-40 km wide frontal prism of accreted sediment but that the bulk (~80 %) continues landward to enter the subduction channel separating the upper and lower plates. The fate of most oceanic turbidite systems is thus to travel tectonically beneath the submerged forearc to either underplate the inner forearc or continue on into mantle. Plate-boundary megathrust earthquakes (Eqs) rupture within or along the top of the subduction channel.

COASTAL OBSERVATIONS: Exhumed accretionary complexes of deeply (15-30 km) underplated turbidite systems of Cretaceous and E. Tertiary age are voluminously exposed around the periphery of the north Pacific, e.g., the Catalina-Pelona-Orocopia-Rand schist of California, the Chugach-Kodiak complex of Alaska, and Shimanto complex of Japan. The Kodiak complex exhibits pseudotachylytes, generally recognized as evidence of past great (Mw8.0 and higher) and giant (Mw8.5 and higher) megathrust Eqs.

SEISMIC CONSEQUENCE OF THE TECTONIC SINK: Most great and giant megathrust Eqs occur in SZs tectonically connected to thickly (>1-2 km) filled trenches. For example, of all well-recorded Mw8.0 and higher megathrust Eqs (N=23), ~70% of Mw8.5 and higher events (n=12), and 100% of those greater than Mw9.0 (N=3), nucleated at sedimented trenches. Subduction of a thick sedimentary section is inferred to smooth the roughness of subducting sea-floor relief and rupture-arresting asperities. This circumstance, first explored by Ruff (1989, Pure and Applied Geophysics, v. 129, p. 263-282), favors lengthy (300 to >1000 km) interplate rupturing that is characteristic of great and giant megathrusts Eqs.