The general tectonic setting for Legs 170 and 205 is shown in Figure F1, and the bathymetry and topography of the margin are shown in Figure F2. Sites for the two legs were drilled across the deformation front outboard of the Nicoya Peninsula. Sites 1039 (Leg 170) and 1253 (Leg 205) penetrate the sedimentary and upper igneous sections of the incoming plate. They are located ~1 and 0.2 km, respectively, outboard of the deformation front. Holes 1040C and 1254A are located within 50 m of each other, ~1.5 km inboard of the deformation front. Both penetrate the forearc sediment prism and the décollement zone; coring in Hole 1040C recovered the entire underthrust sediment section and the top of a gabbro sill on the subducting plate. Sites 1043 and 1255 are again within 50 m of each other, ~0.4 km inboard of the deformation front. Both penetrated the sediment prism and the décollement zone. Leg 205 CORKs were installed at Sites 1253 and 1255. The lithology and structure of incoming and underthrust sediments are shown in Figure F3.
The convergence rate of the Cocos plate relative to the Caribbean plate increases only slightly from 83 mm/yr offshore Guatemala to 85 mm/yr offshore Nicaragua, reaching 88 mm/yr offshore southernmost Costa Rica (DeMets et al., 1990). The convergence direction offshore the Nicoya Peninsula is almost perpendicular to the trench with the subducting plate dipping to the northeast (25°N–30°E). The maximum depth of seismicity gradually becomes shallower from Nicaragua (~200 km) to southern Costa Rica (~45 km). The dip angle of the slab in the upper 100 km is similar from Nicaragua to Central Costa Rica (~30°), becoming steeper beneath Nicaragua at ~100 km depth, with a dip of ~80° (Protti et al., 1994).
There are significant variations in the origin and morphology of the incoming plate through the Nicaragua–Costa Rica segment of the Central American convergent margin (Figs. F1, F2). Large-scale tectonic features, such as the Carnegie and Cocos Ridges and the subduction trench, show up clearly in the regional bathymetry (Ranero et al., 2000a). Crust subducting beneath Nicaragua, formed at the East Pacific Rise (EPR), is pervasively faulted with offsets of up to 700 m on back-tilted normal faults (Kelly and Driscoll, 1998; Kelly, 2003), possibly associated with extensional tectonics caused by the flexure of the crust as it is subducted. The offsets become smaller moving south toward the vicinity of the Nicoya Peninsula, where they are <200 m. Farther southeast, seafloor relief in general is more pronounced (being southeast of the so-called "rough/smooth boundary" after Hey, 1977) and the seafloor is covered by numerous seamounts (Figs. F1, F2). Crustal thickness increases slightly from ~5 km offshore Nicaragua to ~6 km offshore the Nicoya Peninsula (Ranero and von Huene, 2000). The thickness of the incoming sediments is generally in the range of 400–500 m along the entire length of the margin (Fig. F3).
Analysis of marine magnetic measurements in Figure F4 (Hey, 1977; Lonsdale and Klitgord, 1978; Barckhausen et al., 1998, 2001) shows that ~20 km southeast of the Leg 205 transect, a fracture zone trace (FZT) separates lithosphere formed at the EPR from that formed at the Cocos-Nazca spreading (CNS) center. This means that the drill holes of Leg 205 are underlain by crust that formed at the EPR at ~24 Ma. Wilson (1996) indicates that the crust at this location was formed at a full spreading rate of ~130 mm/yr. Seismic images of the FZT (Barckhausen et al., 2001) confirm the location of the boundary and reveal that the top of basement is ~100–200 m shallower on CNS-generated seafloor than on EPR-generated seafloor. The lithosphere southeast of the FZT formed at the CNS center, with decreasing age to the southeast. The oldest CNS crust abutting the FZT is 22.7 Ma, which corresponds to the breakup age of the Farallon plate. ODP Leg 206 Site 1256 (6°44.19´N, 91°56.06´W) is located on crust generated at the EPR at a full spreading rate of ~200 mm/yr in the immediate vicinity of the rough/smooth boundary (Fig. F1).
Seismic Profile BGR-99-44 (Fig. F5) reveals the general structure of the seaward side of the trench, the trench itself, and the toe of the forearc prism. All drill sites of Leg 205 are located on this multichannel seismic (MCS) line (C. Reichert and C. Ranero, pers. comm., 2001), shot in 1999. The data were acquired with a 6-km, 1024-channel digital streamer using a 3-m3 tuned air gun array and differential Global Positioning System (GPS) navigation. Data shown in Figure F5 are a near-trace (171-m offset) time-migrated section of the complete profile. The incoming pelagic sediments with a thickness of ~400 m show faults with offsets of 50–100 m seaward of the trench. A prominent reflector at 0.25 s two-way traveltime (TWT) below the seafloor marks the base of a late Miocene sedimentary unit. Beneath the sedimentary sequence, the strong reflection at 0.5 s (TWT below seafloor) images the top of a gabbro sill as revealed by drilling results from Leg 170 at Site 1039. The top of oceanic basement below the sill is very difficult if not impossible to identify based on the seismic records.
Heat flow data from the Global Heat Flow Database and from more detailed studies in the area (Langseth and Silver, 1996; Ruppel and Kinoshita, 2000; Fisher et al., 2003b; Hutnak et al., in press) reveal a clear overall picture. North of the FZT, heat flow values average ~30 mW/m2, about one-third of the conductive lithospheric value (Stein and Stein, 1992); values jump to an average of ~110 mW/m2 south of the FZT, consistent with lithospheric cooling models. Given the similar plate age north and south of the FZT, these data imply that a substantial amount of heat is being removed by hydrothermal circulation within the EPR crust to the north. A detailed heat flow study prior to Leg 170, focusing on the trench and the prism offshore the Nicoya Peninsula (Langseth and Silver, 1996), confirmed the observation of a cool plate subducting under Costa Rica. Two more recent heat flow surveys (TicoFlux I and II) investigated in detail the thermal structure of the incoming plate seaward of the Leg 205 area by mapping heat flow along seismic lines (Fisher et al., 2003a, 2003b; Hutnak et al., 2006). Three major conclusions can be drawn:
New heat flow values at the prism from northern Nicaragua to southern Costa Rica (Meteor Cruise 54-2) clearly support the idea that there is a major thermal boundary in the vicinity of the FZT, not only seaward of the trench but also underneath the prism.
Seismic data from the Costa Rica forearc and coring during Leg 170 (within 7 km of the trench) show that the bulk of the Pacific margin is a wedge-shaped high-velocity body probably made of rocks similar to the Nicoya ophiolite complex cropping out along the coast (Shipley et al., 1992; Kimura, Silver, Blum, et al., 1997; Ranero and von Huene, 2000). The proximity of the presumed ophiolitic basement to the trench precludes the existence of any significant sediment mass derived from recent accretion; only a small sediment prism (<10 km wide) is located at the front of the margin wedge. Initially, MCS images were interpreted in terms of sediment accretion to the Costa Rica margin (Shipley et al., 1992). More recently, however, and in the wake of Leg 170 drilling, seismic images have been interpreted to show that essentially the entire sediment cover of the ocean plate is currently underthrust beneath the margin and that the frontal sediment prism is storing very little, if any, of the incoming material (Kimura, Silver, Blum, et al., 1997; Christeson et al., 1999, 2000; McIntosh and Sen, 2000; Moritz et al., 2000; Silver et al., 2000; Ranero et al., 2000b; von Huene et al., 2000).
Part of the prism relevant to Leg 205 is imaged in the seismic line shown in Figure F5. Northeast of the deformation front (shotpoint 3210) the décollement is clearly visible as a boundary separating the underthrust sediment sequence from the overlying poorly structured prism sediments. Detailed analysis of a three-dimensional seismic data set (Shipley et al., 1992) shows that the décollement structure is quite diverse across the lowermost part of the prism (within an 8.5 km transect). Shipley et al. (1992) were able to identify numerous thrust faults, mostly in the deeper part of the prism, possibly acting as fluid conduits, but clear tectonic structures are less evident approaching the deformation front. Only one of these faults is imaged offsetting the underthrust sequence (Fig. F5) (common midpoint 3155) and appears to continue up into the prism sediments.
As shown in Figure F3, the sediment section beneath the décollement at Site 1040 repeats the complete lithology and sequence of the incoming section cored at Site 1039, allowing little sediment accretion to the front of the prism at present (Kimura, Silver, Blum, et al., 1997). Cosmogenic 10Be, which decays with a 1.5-m.y. half-life, also shows that little, if any, frontal accretion has taken place at this site over the last several million years (Morris et al., 2002). The sediment wedge is thus either a paleoaccretionary prism or is composed largely of slumped slope sediments rather than accreted marine sediments. Sedimentological and chemical data (Kimura, Silver, Blum, et al., 1997; Morris, Villinger, Klaus, et al., 2003) are more consistent with the latter interpretation.
The composition of arc lavas often records a sediment contribution from the downgoing plate, constraining sediment dynamics at depths greater than those that can be reached by drilling or seismic imaging. Chemical differences between the arc lavas from Nicaragua and Costa Rica suggest that the entire sediment section is subducting to the depths of magma generation beneath Nicaragua, with the carbonate section dominantly subducting beneath Costa Rica (Morris et al., 1990; Carr et al., 1990, 2003; Patino et al., 2000; Reagan et al., 1994). The seismic and lithologic observations indicate complete sediment subduction past the prism front in both regions. The arc and prism observations can be reconciled if sediments are underplated to the base of the prism beneath Costa Rica or if greatly enhanced subduction erosion occurs beneath the Nicoya segment. Christeson et al. (1999) used seismic reflection and refraction data to show stacked velocity duplicates, interpreted as repeated stratigraphic sections due to underplating beneath the deeper part of the Costa Rica prism.
In addition to evidence for sediment subduction and underplating beneath the Nicoya segment, the seismic stratigraphy and multibeam bathymetry of the slope offshore Nicaragua and the tectonic structure offshore Costa Rica indicate significant mass wasting and extension and subsidence of the margin during much of the Miocene (Ranero et al., 2000b; Ranero and von Huene, 2000; Walther et al., 2000; Clift and Vannucchi, 2004; Meschede et al., 1999), which is consistent with tectonic erosion and thinning of the overriding plate. These results are further substantiated by Leg 170 coring at Site 1042, located 7 km landward of the Middle America Trench, which encountered a ~30-m-thick sequence of fossiliferous well-lithified calcarenite breccia at a depth of ~4000 meters below sea level (mbsl) (Kimura, Silver, Blum, et al., 1997). Fossil, textural, cement paragenesis, and sedimentological observations document that the calcarenite was formed, brecciated, and cemented in a shallow nearshore setting (Vannucchi et al., 2001). Sr isotope ratios place the depositional age at ~16–17 Ma (latest early Miocene). It is overlain by ~320 m of unconsolidated slope mud showing the complete Pleistocene to Miocene sequence, where benthic foraminifers indicate the subsidence of the margin from the upper bathyal to abyssal depths (Meschede et al., 1999; Vannucchi et al., 2001). Unfortunately, erosion rates over the last several million years are less well constrained. Speculation is that tectonic erosion has been controlled by the roughness of the subducting plate. Thinning of the overriding plate and the continental margin morphology suggest that subduction erosion increases in intensity from Nicaragua to southern Costa Rica (Ranero and von Huene, 2000).
For the purposes of Leg 205, an important result is that the margin offshore Nicoya is not currently accreting sediments and has not done so over the last several million years. The conclusion that all sediments fed to the trench over this time frame have been subducted below Site 1254/1040 greatly simplifies estimating the mass and element fluxes in this shallow part of the system. Changes in underthrust sediment thickness between Sites 1039/1253 and 1040/1254 are due to compaction and dewatering in the earliest stages of subducting, with underplating playing a role farther downdip. Subduction erosion is likely to be an episodic rather than steady-state feature of the margin, as discussed below.
Coring during Leg 170 and subsequent postcruise studies identified three distinct fluid flow systems. These included flow of modified seawater within the upper oceanic crust, lateral updip fluid flow within underthrust sediments, and fluid expulsion along the décollement and through faults in the margin wedge (e.g., Silver et al., 2000; Kastner et al., 2000; Saffer et al., 2000; Chan and Kastner, 2000).
The existence of fluid flow in the oceanic basement of the incoming plate (Site 1039/1253) is evidenced by both the heat flow anomalies discussed earlier and the chemistry of pore fluids sampled during Legs 170 and 205. Postcruise studies of pore fluid chemistry (Fig. F6) indicate that chemical characteristics typical of modern seawater are sampled in pore waters from the uppermost sediments. Deeper in the sediment section, values depart increasingly from seawater values as a result of diagenesis, largely ash alteration. The consequence is that measured pore fluid isotope ratios at a particular depth are lower than those of seawater at the comparable age, as determined from the paleoseawater Sr isotope curve. Pore fluid chemistry of the deepest sediments, however, trends back toward modern seawater values. Similar reversals in pore fluid chemical profiles at the base of the sedimentary section are seen for a wide variety of tracers (see fig. F20 in Shipboard Scientific Party, 2003). The heat flow anomaly and pore fluid profiles have been modeled in terms of lateral fluid flow in basement at rates of ~1–5 m/yr (Silver et al., 2000), with fluids having a residence time of at least ~15,000 yr. Thermal modeling from Fisher et al. (2003a) of the EPR segment is consistent with advective heat extraction in the uppermost 100–600 m of basement, with flow rates modeled in the range of 3–30 m/yr, suggestive of high-permeability horizons in the shallow part of the incoming plate, similar to that inferred for off-axis ridge flanks (Becker et al., 1997, 1998; Fisher, 1998; Davis et al., 2000; Kopf et al., 2000).
The décollement and faults cutting through the forearc sediment wedge of the upper plate serve as pathways for updip flow of fluids whose chemistry point to a source region with temperatures of 80°–150°C. Structural observations across the décollement (Fig. F7) indicate a zoned fault with heavily fractured fault rocks above a ductile plastic zone that may act as an aquitard, segregating the flow systems above and below the plate boundary (Vannucchi and Tobin, 2000; Tobin et al., 2001). At Sites 1040 and 1254, deformation increases gradually downward. Shipboard structural geologists placed the top of the décollement zone at 333 meters below seafloor (mbsf) and 338 mbsf at Sites 1040 and 1254, respectively. The base of the décollement is a sharp boundary at 371 and 368 mbsf at the two sites, respectively (see fig. F24 in Shipboard Scientific Party, 2003).
Pore fluid chemical profiles measured during and since Legs 170 and 205 at Site 1040 show three distinct intervals. Above ~190 mbsf, Li, propane, and Ca concentrations are relatively uniform, as they are again in the underthrust sediments below the décollement. Generally high Li, propane, and Ca concentrations are observed between ~200 mbsf and the décollement, with peaks typically coincident with fault zones. Similar chemical anomalies, albeit of smaller magnitude, are also seen along the décollement zone at Sites 1043 and 1255 (~130–150 mbsf). Enrichments in Ca and Sr, depletions in K, and changing Sr and Li isotopic compositions, as well as higher concentrations of thermogenic heavy hydrocarbons (propane to hexane), are also observed in these faulted horizons. Collectively, these data indicate that some fraction of the fluids sampled along these localized horizons is derived from depths great enough that temperatures are 80°–150°C. The chemical variations shown in Figure F7 suggest that the zone between 200 and 370 mbsf is heavily infiltrated by fluids originating from a depth where temperatures are between 80° and 150°C; the sharp peaks at ~200 and 350–360 mbsf indicate that the anomalies are supported by relatively recent advective flow along the upper fault and the décollement.
Chemical and structural studies indicate a third hydrologic system, which drains fluids from the underthrust sediment section (Saffer et al., 2000, Saffer, 2003; Silver et al., 2000; Morris, Villinger, Klaus, et al., 2003). Abrupt changes in pore fluid chemistry (Fig. F7) across the décollement indicate that this drainage is not primarily into the décollement zone but rather is likely accommodated by lateral flow. More rapid drainage and greater compaction of the uppermost ~100 m (Units 1 and 2) than Unit 3 may result from (1) more abundant coarse-grained high-permeability ash layers that focus flow, (2) higher permeability within the hemipelagic sediments, or (3) significant permeability anisotropy within the hemipelagic sediments (Saffer et al., 2000; Saffer, 2003). Overall, hydrological and geological modeling (e.g., Saffer et al., 2000; Silver et al., 2000; Fisher et al., 2003a, 2003b) suggests relatively high permeabilities in the oceanic basement, décollement, and underthrusting section, with the décollement being locally more permeable than the underthrusting sediments. Leg 205 CORKs are monitoring fluid flow within the igneous section and the décollement zone.