Long-term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench. Since 47 Ma (Eocene) up to 148 km of the plate margin ...
TECTONICS, VOL. 22, NO. 3, 1023, doi:10.1029/2002TC001386, 2003
Tectonic erosion of the Peruvian forearc, Lima Basin, by subduction and Nazca Ridge collision Peter D. Clift Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA
Ingo Pecher Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand
Nina Kukowski and Andrea Hampel GeoForschungsZentrum Potsdam, Potsdam, Germany
Received 18 March 2002; revised 11 December 2002; accepted 27 February 2003; published 4 June 2003.
[1] Subsidence of Lima Basin, part of the Peruvian forearc, is controlled by tectonic erosion by the subducting Nazca plate. Multichannel seismic reflection data coupled with age and paleowater depth constraints derived from Ocean Drilling Program (ODP) coring now allow the rates of erosion to be reconstructed through time. In trenchward locations the forearc has experienced limited recent relative uplift (700–850 m) likely due to preferential basal erosion under the center of Lima Basin. Long-term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench. Since 47 Ma (Eocene) up to 148 km of the plate margin have been lost at an average rate of up to 3.1 km myr1. Appoximately 110 km of that total appears to be lost since 11 Ma, implying much faster average rates of trench retreat (10 km myr1) since collision of the Nazca Ridge with the Lima Basin at 11 Ma. Although there is no clear subsidence event at ODP Site 679 during the time at which Nazca Ridge was subducting beneath this part of the forearc (4–11 Ma), the more trenchward ODP Sites 682 and 688 show significant deepening after 11 Ma indicating that subduction of the ridge accelerates tectonic erosion. Long-term rates of crustal erosion in the region of Lima Basin are greater than estimates of regional arc magmatic productivity, implying that such margins INDEX TERMS: 3025 are net sinks of continental crust. Marine Geology and Geophysics: Marine seismics (0935); 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 8015 Structural Geology: Local crustal structure; 0935 Exploration Geophysics: Seismic methods (3025); KEYWORDS: Peru, subduction, tectonics, subsidence. Citation: Clift, P. D., I. Pecher, N. Kukowski, and A. Hampel, Tectonic erosion of the Peruvian forearc, Lima Basin, by subduction and Nazca Ridge collision, Tectonics, 22(3), 1023, doi:10.1029/2002TC001386, 2003.
Copyright 2003 by the American Geophysical Union. 0278-7407/03/2002TC001386
1. Introduction [2] The tectonic erosion of crust in the forearc of convergent plate margins represents an important part of the mass budget within subduction environments. Understanding the fate of the sedimentary cover and oceanic crust of a subducting plate is important if global geochemical cycles are to be understood. Does material extracted from the upper mantle get recycled back into this reservoir through deep subduction, or is this material merely reworked along convergent margins, either being off-scraped within accretionary complexes, or re-melted and incorporated into the arc magmatism itself? Although large accretionary complexes formed along the frontal edges of continental lithospheric plates are known from many margins (e.g., Barbados) [Nankai, Makran and Cascadia [Moore and Biju-Duval, 1984; Davis and Hyndman, 1989; Moore et al., 1990; Minshull and White, 1989], there are more significant lengths of modern convergent margin, mostly located around the periphery of the Pacific, including large parts of the Peruvian margin, where minor or no accretion is observed [Rutland, 1971; Hilde, 1983; von Huene and Scholl, 1991]. In these areas it is often unclear as to whether the sediment on the oceanic plate is being subducted deep into the mantle, or if accretion is occurring by basal underplating under the forearc, but at a depth that is not readily imaged by seismic reflection surveys. Study of the vertical tectonics in forearc basins in such areas can help to estimate the rates of subduction erosion or accretion by underplating because the character of the sedimentary record can constrain the bathymetry of the forearc over long periods of subduction. Although forearc basins do not cover the entire forearc, they do provide information from areas lying tens of kilometers landward of the trench region. [3] In this study we quantify the rates of tectonic erosion of the Peruvian forearc in the area of Lima Basin (Figure 1) in order to understand the mass budget of this convergent margin over significant lengths of geologic time. To do this we use seismic and drilling data to reconstruct the subsidence and uplift history of the forearc (Figure 2). In particular, we examine the rates of vertical motion during normal subduction of oceanic crust and compare this with the dynamics related to subduction of the Nazca Ridge
7-1
7-2
CLIFT ET AL.: TECTONIC EROSION OF THE PERUVIAN FOREARC
Figure 1. Bathymetric map of South American forearc and Pacific Ocean showing location of Lima Basin and major bathymetric features discussed in the text. Water depth is in meters. [Pilger, 1981], an aseismic volcanic edifice that is in oblique collision with the South American margin and the crest of which is considered to have been subducted below the Lima Basin after 11 Ma [Hampel, 2002].
2. Controls on Subduction Erosion [4] The factors governing accretion versus tectonic erosion along a given margin are understood in outline, but not in detail. Rates of accretion and brief phases of tectonic erosion have been estimated in margins where an accretionary complex juxtaposed against much older rock is preserved as a record of the margin’s development [e.g., von Huene et al., 1994, 1996]. In non-accretionary margins the removal of the forearc sediment record has resulted in estimates of tectonic erosion rates that vary widely. In the central Andes at 21°S, average rates of trench retreat have been estimated at 1.5– 2.0 km Myr1 [Scheuber and Reutter, 1992]. Ballance et al. [1989] suggested that tectonic erosion in Tonga has been generally minor, except during the subduction of major seamount features, most notably the Louisville Ridge. Conversely, Lallemand [1998] has argued that strong coupling between the down-going and overriding plates around the Pacific results in rapid trench retreat of 4– 10 km Myr1 in such settings. As a result, several hundred kilometers of forearc material would have been lost
since the initiation of subduction in the western Pacific at 45 Ma. Clift and MacLeod [1999] reconstructed the subsidence history of Tonga forearc from sedimentary and structural data, concluding that long-term tectonic erosion of the forearc was mostly by slow removal of material from the base of the forearc crust, causing the trench to retreat at
