Donald M. Fisher

Models of quartz overgrowth and vein formation: Deformation and episodic fluid flow in an ancient subduction zone

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 magfiitude 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 facecontrolled 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 gm in

Erosion rates and orogenic-wedge kinematics in Taiwan inferred from fission-track thermochronometry

New apatite and zircon fission-track ages and previously published thermochronometric data are used to evaluate erosion rates and particle paths within the active Taiwan arc-continent collision. We present 20 new apatite fission-track ages and 6 new zircon fission-track ages. Apatite and zircon ages are all reset in the northern and eastern parts of Taiwan, although the region of reset apatite ages is larger. We interpret this pattern as resulting from crustal accretion at the western margin of the orogenic wedge combined with southward propagation of the collision zone. A onedimensional thermal model including erosion provides prediction of the fission-track ages. The distribution of reset ages is best explained with an erosion rate of 4‐6 mm/yr. Given a propagation velocity of 60 mm/yr, this erosion rate implies that nearly 25 km of material has been eroded from northern Taiwan. The lack of reset 40 Ar/ 39 Ar ages from muscovite and biotite suggests that rockparticle paths have a large horizontal component, a result consistent with an eroding orogenic-wedge model.

Effect of subducting sea-floor roughness on fore-arc kinematics, Pacific coast, Costa Rica

Fault kinematics and uplift in the Costa Rican fore arc of the Middle America convergent margin are controlled to a large extent by roughness on the subducting Cocos plate. Along the northwest flank of the incoming Cocos Ridge, seafloor is characterized by short wavelength roughness related to northeast-trending seamount chains. Onland projection of the rough subducting crust coincides with a system of active faults oriented at high angles to the margin that segment the fore-arc thrust belt and separate blocks with contrasting uplift rates. Trunk segments of Pacific slope fluvial systems typically follow these margin-perpendicular faults. Regionally developed marine and fluvial terraces are correlated between drainages and across faults along the Costa Rican Pacific coast. Terrace separations across block-bounding faults reveal a pattern of fore-arc uplift that coincides roughly with the distribution of incoming seamounts. Magnitude and distribution of Quaternary uplift along the Costa Rican Pacific coast suggests that, despite a thin incoming sediment pile, the inner fore arc shows an accumulation of mass—a characteristic that may be due to underplating of seamounts beneath the fore-arc high.

Duplex accretion and underplating in an ancient accretionary complex, Kodiak Islands, Alaska

The landward belt of the Kodiak Formation represents a sequence of lower Maastrichtian turbidites that was underplated to the Kodiak accretionary complex by duplex accretion. Evidence for duplexes exists on the mesoscopic and map scale. At the microscopic scale, incremental strain histories are consistent with early shear during underthrusting abrupt landward rotation of individual thrust slices over fault ramps, and locking of thrust packets in steeper orientations. Duplex geometries at the mesoscopic scale are represented by high-angle reverse faults bounded either above or below by low-angle, southeast-verging detachments. At the map scale, different structural belts can be related to a low-angle detachment zone (central belt) flooring a more landward duplex (landward belt). Duplex underplating can result in the uplift and thickening of accretionary prisms without necessarily deforming the overlying, previously accreted material and slope sediments.

Central Costa Rica deformed belt: Kinematics of diffuse faulting across the western Panama block

Fault kinematics, seismicity, and geodetic data across central Costa Rica reveal a diffuse fault zone, here named the Central Costa Rica Deformed Belt (CCRDB). The CCRDB defines the western margin of the Panama block and links the North Panama Deformed Belt (NPDB) along the Caribbean coast with the Middle America Trench (MAT) along the Pacific coast. The junction of the CCRDB and the MAT coincides with an abrupt transition from smooth to rough crust on the subducting Cocos plate (rough-smooth boundary). Shallow subduction of rough, thickened oceanic crust associated with the Cocos Ridge shifts active shortening into the volcanic arc along faults of the CCRDB. Variable fault kinematics along this zone may reflect three combined deformation mechanisms: horizontal shortening and shear from oceanic ridge indentation, basal traction from shallow subduction, and localized block uplift from subducting seamount roughness. Within the forearc (domain 1), mesoscale faults express transtension where steep NE striking regional-scale faults intersect the Pacific coast. Across the volcanic arc (domain 2), mesoscale faults exhibit mostly sinistral and dextral slip on NE and NW striking conjugate faults, respectively. Approaching the NPDB in the back arc (domain 3), transcurrent faulting is modified by transpression and crustal thickening. Fault kinematics are consistent with earthquake focal mechanisms and Global Positioning System (GPS) measurements. Radiometric age constraints confirm that faulting postdates the late Neogene onset of shallow subduction. The ensuing deformation front has propagated northward into the volcanic arc to its present position along the seismically active CCRDB. Within the forearc, the effect of shallow subduction is overprinted by local uplift related to underthrusting seamounts.

Cyclic fluid flow through a regionally extensive fracture network within the Kodiak accretionary prism

Two types of periodic textures observed in veins from the Kodiak accretionary prism attest to cyclic fluid flow through a regionally extensive fracture network buried at 8-12 km depth : (1) crack-seal microstructures with bands of mica inclusions and (2) collapse microstructures with jagged bands of residue embedded within euhedral crystals of quartz. The difference in texture reflects the closure of cracks : crack-seal microstructures record the complete chemical sealing of the crack after each fracture event, whereas the collapse features record longer fluid-filled periods followed by more rapid draining of fractures. Collapse features consist of pressure solution selvages trapped within veins and in the wall rock adjacent to euhedral growth terminations ; the high concentrations of immobile elements in these selvages indicate that these fractures closed by collapse and penetration of quartz crystals into wall rock. Analysis of chemical composition on either side of four large euhedral growth veins and whole rock analysis of slates across the Kodiak Formation reveal local depletion of silica adjacent to veins but no evidence for long-distance silica transport within the system. Both crack-seal and collapse textures are observed in a regionally extensive vein system that displays a regular geometry, with thin, closely spaced (0.5-3 cm), near-vertical crack-seal veins that connect vertically and laterally with thicker euhedral growth veins arranged in widely spaced (-500 mm) southeast dipping en echelon sets. The mesoscopic distribution and textural variability of the vein network suggests that the development of the vein system involved early nucleation and growth of vertical hydrofractures. As the fracture density increased, arrays of fractures locally provided zones of weakness and southeast dipping brittle-ductile shear zones nucleated. These en echelon cracks remained open and provided small reservoirs of fluid. Textures show that en echelon fractures remain open but periodically grow by upward and downward propagation. Crack tips are then sealed with locally derived silica, and fluid drains back into en echelon fracture arrays. This local fluid movement is punctuated by less frequent events where the system links up over a greater distance, fractured reservoirs become interconnected, and the fluid within reservoirs is drained upward or laterally. Periodic inflation and deflation of en echelon arrays may reflect periodic slip on crosscutting faults and rupture of the seals that separate reservoirs.

Active thrusting in the inner forearc of an erosive convergent margin, Pacific coast, Costa Rica

(1) Structural and geomorphic analyses of the Fila Costena thrust belt in southwest Costa Rica indicate active thrusting within the inner forearc. The Fila Costena exposes three major thrust faults that imbricate the late Tertiary forearc basin sequence of the Terraba basin. The frontal thrust of the Fila Costena 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 decollement horizon near the contact between the basement and the cover sequence of the Terraba basin. This decollement 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 Costena 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 decollement 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. INDEX TERMS: 8150 Tectonophysics: Plate boundary—general (3040); 8123 Tectonophysics: Dynamics, seismotectonics; 8010 Structural Geology: Fractures and faults; 8015 Structural Geology: Local crustal structure; 8005 Structural Geology: Folds and folding;

Rough crust subduction, forearc kinematics, and Quaternary uplift rates, Costa Rican segment of the Middle American Trench

Orthogonal subduction of bathymetrically rough oceanic lithosphere along the northwestern flank of the Cocos Ridge imprints a distinctive style of deformation on the overriding Costa Rican forearc. We divide the Costa Rican forearc into three 100–160-km-long deformational domains based on the bathymetric roughness and thickness of the Cocos plate entering the Middle American Trench, the dip of the subducting plate, the variation in surface uplift rates of late Quaternary coastal deposits, and the orientations and types of faults deforming Paleogene and Neogene sedimentary rocks. In the ~100-km-long Nicoya domain, coastal deposits show localized surface uplift and arcward tilting above the downdip projections of the fossil trace of the Cocos-Nazca-Panama (CO-NZ-PA) triple junction and the Fisher seamount and ridge. In the ~120-km-long central Pacific forearc domain between the Nicoya Peninsula and Quepos, shallower (~60°) subduction of seamounts and plateaus is accompanied by trench-perpendicular late Quaternary normal faults. Steeply dipping, northeast-striking, margin-perpendicular faults accommodate differential uplift associated with seamount subduction. Uplift and faulting differ between the segments of the forearc facing subducting seamounts and ridges. Inner forearc uplift along the seamount-dominated segment is greatest inboard of the largest furrows across the lower slope. Localized uplift and arcward tilting of coastal deposits is present adjacent to subducting seamounts. In contrast, inboard of the underthrusting aseismic Cocos Ridge, along the ~160-km-long Fila Costena domain between Quepos and the Burica Peninsula, mesoscale fault populations record active shortening related to the ~100-km-long Fila Costena fold-and-thrust belt. The observed patterns of faulting and permanent uplift are best explained by crustal thickening. The uplifted terraces provide a first-order estimate of permanent strain along the forearc in Costa Rica. The permanent strain recorded by uplift of these Quaternary surfaces exceeds the predicted rebound of stored elastic strain released during subduction-zone earthquakes.

Kinematic analyses of the Hsüehshan Range, Taiwan: A large-scale pop-up structure

The Hsuehshan Range, exposed in the northern and central Taiwan slate belt, is a fault-bounded structural high cored by biotite grade slates and metasandstones. Syntectonic overgrowths in pyrite pressure shadows indicate that much of the eastern Hsuehshan Range experienced coaxial strain histories and that finite strain magnitudes generally increase toward the hinterland. Near the eastern boundary of the Hsuehshan Range, however, pressure shadows record noncoaxial strain histories consistent with a top-to-the-east sense of shear along a steep NW dipping shear zone. This noncoaxiality is attributed to SE directed backthrusting on the Lishan fault, which separates the higher-grade, Eo-Oligocene rocks of the Hsuehshan Range from the lower-grade Miocene rocks of the Backbone Range. Because it is bounded to the east by the SE-vergent Lishan fault and to the west by a series of NW-vergent thrusts (e.g., the Chuchih fault), the Hsuehshan Range is envisaged as a pop-up structure. Strain magnitudes measured from pressure shadows in the coaxial part of the range are consistently lower than those predicted by steady state wedge models that assume all deformation is accommodated by penetrative strain. Departure from the model predictions is attributed primarily to strain localization along discrete fault surfaces (e.g., the Lishan fault). The Hsuehshan Range tapers in width to the south; thus the pop-up may be buried or die out to the south where the collision is younger.

Holocene forearc block rotation in response to seamount subduction, southeastern Península de Nicoya, Costa Rica

The southeastern tip of the Peninsula de Nicoya, Costa Rica, on the Caribbean plate margin lies inboard of the rough bathy- metric terrain on the subducting Cocos plate and along the land- ward projection of the convergence vector for the Fisher seamount group. The southern tip of the peninsula has nearly orthogonal coastlines and extensive, well-preserved, Holocene marine terraces,