Erosion rates and orogenic-wedge kinematics in Taiwan inferred fromEmail alerting services cite this article to receive free e-mail alerts when new articles www.gsapubs.org/cgi/alerts click Subscribeto subscribe to Geology www.gsapubs.org/subscriptions/ click Permission requestto contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click viewpoint. Opinions presented in this publication do not reflect official positions of the Society.positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political article's full citation. GSA provides this and other forums for the presentation of diverse opinions and articles on their own or their organization's Web site providing the posting includes a reference to the science. This file may not be posted to any Web site, but authors may post the abstracts only of their unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make GSA, employment. Individual scientists are hereby granted permission, without fees or further requests to

[1] Structural and geomorphic analyses of the Fila Costeña thrust belt in southwest Costa Rica indicate active thrusting within the inner forearc. The Fila Costeñ a exposes three major thrust faults that imbricate the late Tertiary forearc basin sequence of the Térraba basin. The frontal thrust of the Fila Costeña marks the boundary between an uplifting inner forearc and a subsiding outer forearc, with only local uplift astride the indenting Cocos Ridge. On the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low-angle décollement horizon near the contact between the basement and the cover sequence of the Térraba basin. This décollement lies at a depth of $4 km. The minimum shortening recorded by restoration of fault-related folds is 17 km, or 45%. Observations of late Tertiary marine sediments, tilted and faulted late Quaternary fluvial terraces, and raised Holocene marine terraces indicate that Fila Costeña uplift was likely initiated in the Quaternary and is ongoing. Given that the coastal mountains are separated from the Talamanca Range by a valley, the décollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal-scale ramp system, a structural geometry that has resulted in uplift of the Talamanca Range, the highest peaks in Central America. The dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough, subducting crust.

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Steady state models of overgrowth and vein formation are developed using kinetic data for quartz dissolution and precipitation and estimates of fluid advection, pore‐fluid and grain‐boundary diffusion. Application of these models to overgrowths and veins in the Kodiak accretionary complex suggests that the Kodiak Formation deformed continuously by a grain‐boundary diffusion‐limited mechanism, accompanied by episodic pore fluid diffusion of quartz from the matrix to vertical fluid‐filled fractures near the base of the accretionary wedge. These processes produced two types of syntectonic crystal textures within the Kodiak Formation: overgrowths containing displacement‐controlled fibers, and throughgoing veins composed of face‐controlled elongate blocky quartz crystals. Based on textural observations, displacement‐controlled quartz growth in overgrowths is rate‐limited by either diffusion along a cohesive interface or the rate of matrix strain. The magnitude of elongation recorded by displacement‐controlled crystal growth varies smoothly (elongation of 1 to 3) from the shallowest to the deepest structural levels of the Kodiak Formation, suggesting that the diffusional component of deformation in the accretionary wedge increases with depth. In contrast, face‐controlled quartz growth is largely restricted to Veins within the deepest level, where the cleavage is subhorizontal and deformation involves a component of simple shear, suggesting proximity to a decollement. The face‐controlled quartz veins represent mode I cracks which seal periodically and contain continuous planar solid inclusion bands, cracks which partially seal periodically and contain discontinuous solid inclusion bands, or cracks that remain open and contain euhedral quartz crystals with no solid inclusions. The initial crack aperture, inferred from spacing of inclusion bands, varies from 8 μm in crack seal features to much larger values in euhedral growth veins. Euhedral growth veins remain open throughout their development (105 to 106 years), and crack seal veins develop as a consequence of many crack‐seal events over a 103–106 year period. In both cases, textural evidence suggests that most transport of silica occurs by local pore‐fluid diffusion from matrix to vein. Wall rock inclusion bands suggest that “crack” events and “seal” events each occurred within periods of 102–104 years. A picture emerges of intermittent fluid flow upward from the decollement into a branching hierarchy of vertical fractures in the accretionary wedge during hydrofracturing events, followed by local transport and precipitation of silica causing sealing of the fractures at depth and propagation of pulses of fracture fluid upward.

The late Cretaceous to early Tertiary sediments of the Kodiak accretionary complex preserve the earliest deformation associated with underthrusting along a thickly sedimented convergent margin. Because of the thick trench fill, this early deformation resulted in the formation of two distinct structural terranes: (1) coherent terranes of layered turbidites and (2) melange terranes. The earliest deformation (D1) in the coherent terranes is marked by normal faults, mineralized layer‐perpendicular veins in sandstones, and microfaults in shales. These features suggest that σ1 was vertical and that σ3 was horizontal and NW trending during D1. We interpret D1 structures as hydrofractures that formed in flat‐lying sediments with high fluid pressures caused by rapid tectonic loading. In contrast, the melanges appear to record a more protracted and complex history that culminated in structures similar to the D1 structures above. We have recognized three successive, transitional stages in the melanges: (I) breakup of sand beds along cataclastic shear zones with dispersal of sand inclusions within a relatively ductile shale matrix, resulting in the blocks‐in‐matrix character of the melange, (II) development of ductile shear bands that crosscut the melange fabric at a low angle, and (III) hydrofracturing with the formation of calcite and quartz‐filled veins in sandstone inclusions and microfaults in the shales. The sequence of microstructures associated with stages I–III records increased cohesion of the sediments during deformation dominated by layer‐parallel shear. Intense mineralization and microfaulting during stage III suggest that the melanges are zones of concentrated fluid flow with a fracture dominated permeability. Stage III structures are texturally indistinguishable and contemporaneous with the less pervasive D1 structures in the coherent terranes. Based on these observations, we believe that the melanges are major shear zones that formed at the top of the subducting pile and immediately below a major decollement. The coherent terranes represent the structurally and stratigraphically lowest sediments, and they bypassed the stage I and II deformations of the melanges because they were the greatest distance from the decollement when they were deformed. Moreover, both the melange and coherent terranes appear to have bypassed the shallow zone of intense horizontal shortening that dominates the toe of the overriding accretionary prism.

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