P. Vannucchi | Geology Reports

USGS2003

Fast rates of subduction erosion along the Costa Rica Pacific margin: Implications for nonsteady rates of crustal recycling at subduction zones

At least since the middle Miocene (???16 Ma), subduction erosion has been the dominant process controlling the tectonic evolution of the Pacific margin of Costa Rica. Ocean Drilling Program Site 1042 recovered 16.5 Ma nearshore sediment at ???3.9 km depth, ???7 km landward of the trench axis. The overlying Miocene to Quaternary sediment contains benthic foraminifera documenting margin subsidence from upper bathyal (???200 m) to abyssal (???2000 m) depth. The rate of subsidence was low during the early to middle Miocene but increased sharply in the late Miocene-early Pliocene (5-6.5 Ma) and at the Pliocene-Pleistocene boundary (2.4 Ma). Foraminifera data, bedding dip, and the geometry of slope sediment indicate that tilting of the forearc occurred coincident with the onset of rapid late Miocene subsidence. Seismic images show that normal faulting is widespread across the continental slope; however, extension by faulting only accounts for a minor amount of the post-6.5 Ma subsidence. Basal tectonic erosion is invoked to explain the subsidence. The short-term rate of removal of rock from the forearc is about 107-123 km3 Myr-1 km-1. Mass removal is a nonsteady state process affecting the chemical balance of the arc: the ocean sediment input, with the short-term erosion rate, is a factor of 10 smaller than the eroded mass input. The low 10Be concentration in the volcanic arc of Costa Rica could be explained by dilution with eroded material. The late Miocene onset of rapid subsidence is coeval with the arrival of the Cocos Ridge at the subduction zone. The underthrusting of thick and thermally younger ocean crust decreased the subduction angle of the slab along a large segment of the margin and changed the dynamic equilibrium of the margin taper. This process may have induced the increase in the rate of subduction erosion and thus the recycling of crustal material to the mantle. Copyright 2003 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗

USGS2001

Tectonic erosion and consequent collapse of the Pacific margin of Costa Rica: Combined implications from ODP leg 170, seismic offshore data, and regional geology of the Nicoya Peninsula

The convergent margin off the Pacific coast of the Nicoya Peninsula of Costa Rica exhibits evidence for subduction erosion caused by the underthrusting Cocos plate. Critical evidence for efficacy of this process was recovered at the Ocean Drilling Program (ODP) drilling Site 1042 (Leg 170), positioned ???7 km landward of the Middle America trench axis off the Nicoya Peninsula. The primary drilling objective at this site was to identify the age and origin of a regionally extensive and prominent seismic discontinuity, the so-called base-of-slope sediment (BOSS) horizon or surface. The BOSS horizon, which can be traced landward from near the trench to the Nicoya coastal area and parallel to it for hundreds of kilometers, separates a low-velocity (??? 2.0-2.5 km s-1) sequence of slope sediment, from an underlying sequence of much higher-velocity (> 4-4.5 km s-1) rock. Site 1042 reached the acoustically defined BOSS horizon at a below sea level depth of ??? 3900 m and yielded a carbonate-cemented calcarenitic breccia of early-middle Miocene age. Sedimentological, geochemical, paleontological, and cement paragenesis data document that the breccia accumulated in a shallow water depositional environment. On the basis of coastal exposures, the BOSS horizon, as a margin-wide geologic interface, can be temporally and lithostratigraphically correlated to a regional angular unconformity. This unconformity, known as the Mal Pais unconformity, separates Neogene and younger shelf-to-littoral beds from the underlying mafic units of the Mesozoic Nicoya Complex and Cretaceous and early Tertiary sedimentary sequences. At Site 1042 it is inferred that tectonism caused the vertical subsidence of the early Neogene breccia from a shallow to a deep water setting. The Mal Pais unconformity of the BOSS horizon thus connects the rock fabric of the outermost part of margin to that of coastal Nicoya and implies that in the early Neogene the Nicoya shelf extended seaward to near the present trench axis. This circumstance requires that the early Neogene trench axis was at least 50 km seaward of where it is now located. The long-term effects of subduction erosion, similar to those described for the scientifically drilled Japan, Tonga, and Peru margins, best account for offshore and onshore evidence for a post-Paleogene history of crustal thinning and landward trench migration of Costa Rica's Pacific margin. During the past 16-17 Myr the calculated mass removal and landward migration rates are 34-36 km3 Myr-1 km-1 of margin, and 3 km Myr-1, respectively. These values are similar to those found for other Pacific margins dominated by nonaccretionary subduction zone processes.

Tectonics↗