Basal Subduction Erosion and the Formation of the Aleutian Terrace and Underlying Forearc Basin
Proceeding of the 3rd International Biennial Workshop on Subduction Processes, University of Alaska, Fairbanks, June 10-14, 2002
BASAL SUBDUCTION EROSION AND THE FORMATION OF THE ALEUTIAN TERRACE AND UNDERLYING FOREARC BASIN.
David W. Scholl; Department of Geophysics, Stanford University, Stanford, CA, 94305, and U.S. Geological Survey, Menlo Park, CA, 94025 (firstname.lastname@example.org or email@example.com)
Roland von Huene; Department of Geology, University of California, Davis, CA, USA 95616, and GEOMAR, Kiel, Germany (firstname.lastname@example.org)
Holly F. Ryan; U.S. Geological Survey, Menlo Park, CA 94025 (email@example.com)
INTRODUCTION: It is generally supposed that the structural depression forming large forearc basins, like that underlying the Aleutian Terrace, is caused by the upward and outward growth of an accretionary prism. However, an improved base of offshore data reveals that many forearc basins are underlain by continental or arc basement rock that has been thinned in place. Thinning is best explained by basal subduction erosion. We explore here the notion that the Aleutian forearc basin (AFB) records a late Cenozoic episode of enhanced crustal thinning caused by the underthrusting Pacific plate.
BACKGROUND INFORMATION: The Aleutian Terrace is a broad, 40-50-km wide mid-slope bench that runs along virtually the length (~2000 km) of the Aleutian forearc. This prominent geomorphic feature extends seaward from a depth of 3000-4000 m at the based of the upper landward trench slope to a depth of about 4500 m at the southern edge of the terrace. Bathymetric highs are common along the southern side of the terrace, beyond and below which the lower landward trench slope descends to the trench floor (~7000 m). In cross-slope profile, the surface of the terrace is either gently basinal in contour or, more typically, sloping irregularly downward toward the top of the lower slope.
Seismic reflection and gravity data document that the terrace is the surface expression of a thickly (2-3 km) sedimented forearc basin (Grow, 1973; Harbert et al., 1986; Scholl et al., 1987; Ryan and Scholl, 1989; Ryan and Scholl, 1993). Structurally, the AFB is a broad depression or swale in the surface of the ridge's basement of igneous rock. The axis of the basin strikes parallel to the ridge. Major faults do not border the ridge side of the AFB, but faulting and folding are typical of its outer, forearc-high margin. Drilling and dredging establish that the basin began to form rapidly about 5-6 Myr ago in the latest Miocene (Scholl et al., 1987). The basin's fill of mostly Pliocene and Quaternary beds is a richly diatomaceous sequence of turbidite, hemipelagic, and ash debris shed from the Aleutian Ridge. The basinal sequence overlies older coarse-grained volcaniclastic beds (sandstone and siltstone) of Miocene and Oligocene age. This section, ~1 km thick, drapes the upper trench slope but does not thicken where it passes beneath the AFB. The clastic sequence is thus a pre-basinal forearc accumulation. Beneath the crust of the AFB, at a sub-sea level depth near 15 km, the underunning slab of Pacific lithosphere is virtually horizontal.
BASAL SUBDUCTION EROSION: At erosional convergent margins, lower plate underthrusting thins forearc crust by detaching rock from the upper plate and transporting this material to the mantle. Evidence for basal subduction erosion of a forearc is (1) rapid (0.3-0.5 km/Myr) and substantial (3-5 km) subsidence, (2) offshore truncation of cratonic rock, (3) retrograde (landward) migration of the arc-magmatic front, and (4) the coastal and offshore occurrence of arc magmatic rocks. Globally, at convergent margins bordered by no observable or small- and medium-width accretionary prism (~5-40 km), the long-term (~10's of Myr), the average rate of subduction erosion is at least ~40 km3/Myr/km of trench.
EVIDENCE FOR SUBDUCTION EROSION ALONG THE ALEUTIAN RIDGE: Presently, only circumstantial evidence exists that subduction erosion has thinned the forearc crust of the Aleutian Ridge. This docket of observations includes: (1) the landward (northward) migration of the arc magmatic front by ~30 km since the early Oligocene and 20 km since the middle Miocene; (2) the regional occurrence of a deeply (~1 km) subside shelf edge bordering the southern side of the ridge's wave-planed summit platform, and (3) the recovery along the inner side of the terrace of coarse clastic sediment from the pre-basinal deposits of Oligocene and Miocene age (Scholl et al., 1987). It is inferred that the older of these units has subsided 3-4 km similar to the shallow-water deposits recovered by drilling or dredging the deeply submerged (2-5 km) outer forearcs of northern Japan, Tonga, northern Chile?, Peru, Costa Rica, Guatemala, and Mexico.
THE HYPOTHESIS: It is recognized that immediately seaward of the AFB a sizable accretionary prism forms the lower landward trench slope. But we propose that the tectonic mechanism that formed the AFB basin is not only the addition of an accretionary prism but more dominantly the consequence of subcrustal erosion (Ryan and Scholl, 1995). If we are correct in supposing that the older pre-basin volcaniclastic deposits are shallow water deposits, then it seems likely their subsidence to a depth of at least 6 km began well before the late Cenozoic formation of the AFB. Pre-basin subsidence can be explained by a background or long-term rate of subduction erosion of 40 km3 / Myr / km of ridge. However, construction of the AFB calls for an enhanced rate of basal erosion during the past 5-6 Myr that most prominently thinned the mid-slope area.
We speculate that the underthrusting of a nearly horizontal slab covered by a thick sequence of subducted sediment (McCarthy and Scholl, 1985) caused the enhancement that built the structural depression of the AFB. The heightened transport of fluids beneath the base of the forearc is presumed to be involved. Increased sediment subduction can be linked to rapid trench-axis sedimentation initiated by uplift and glaciation of eastern Gulf of Alaska drainages. Slab flattening suggest the Aleutian Ridge has been driven seaward over a mantle-anchored Pacific slab. We note that the axis of the AFB is situated above the inner edge of the flat slab, where it bends sharply downward to plunge below the crestal mass of the ridge. Great subduction zone earthquakes occur just beneath the inner edge of the AFB.
REFERCENCES: Grow, J., 1973, Crustal and upper mantle structure of the central Aleutian arc: Geol Soc. Amer. Bull. v.84, p. 2169-2192; Harbert, W. P., D.W. Scholl, T.L. Vallier, A.J. Stevenson, and D.M. Mann, 1986, Major evolutionary phases of a forearc basin of the Aleutian terrace--relation to north Pacific tectonic events and the formation of the Aleutian subduction complex: Geology, v. 14, p. 757-761; McCarthy, J., and D.W., Scholl 1985, Mechanisms of subuction accretion along the central Aleutian Trench: Geol. Soc. Amer. Bull. v., 96, p. 691-701; Ryan, H.F., and D.W Scholl,. 1989, The evolution of forearc structures along an oblique convergent margin, central Aleutian Arc: Tectonics, v. 8, p. 497-516; Ryan, H.F., and D.W. Scholl, 1993, Geologic implications of great interplate earthquakes along the Aleutian Arc: Jour. Geophys. Res., v. 98, p. 22,135-22,146; Ryan, H. F., and D.W Scholl,1995, Deep reflectors beneath the central Aleutian forearc: Implications for the geometry of the subducting slab [abs]: Eos, AGU, v. 76; Scholl, D. W., T.L.Vallier, and A.J.Stevenson,, 1987, Geologic evolution and petroleum geology of the Aleutian Ridge, in Scholl, D. W., Grantz, A., and Vedder, J. G., (eds), Geology and resource potential of the continental margin of western North America and adjacent ocean basins--Beaufort Sea to Baja California: Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series, v. 6, p. 124-155, Houston, Texas.