BACKGROUND

In the arc volcanics, ¹⁰Be data, radiogenic isotopes, and trace element studies of Nicaragua lavas (Tera et al., 1986; Carr et al. 1990; Reagan et al., 1994; Patino et al., 2000) suggest that the entire sediment section is subducting to the depths of magma generation, producing in the lavas a strong signature from the hemipelagic sediments at the top of the incoming sediment section. In contrast, the Costa Rican lavas have a much weaker sediment signature, little contribution from the uppermost hemipelagic sediments of the incoming plate, and a proportionally larger contribution from the basal carbonate section. Such a difference between the two regions could be explained by sediment accretion or greatly enhanced subduction erosion off northwest Costa Rica.

Various workers have suggested that the changing nature of seismicity and arc volcanism may be due to variations in the incoming plate, the fate of incoming sediments as they traverse the forearc of the overriding plate, or a combination of the two. The bathymetry and thermal structure of the incoming plate and the active fluid flow both outboard and inboard of the trench may play a key role in deformation and sediment dynamics across the margin. In addition, differences in origin and chemistry of the subducting oceanic crust may also contribute to changing chemistry of the arc lavas. The nature of the incoming oceanic crust off Nicoya and the active hydrology of the margin are the primary foci of Leg 205.

The Incoming Plate
There are significant variations in the origin, morphology, and thermal structure of the incoming plate through the Nicaragua-Costa Rica segment of the Central American convergent margin (Fig. 1). Offshore Costa Rica, a tectonic boundary separating lithosphere formed at the East Pacific Rise (EPR) from that formed at the Cocos-Nazca spreading center (CNS) was identified using magnetic anomalies (Barckhausen et al., 2001) and confirmed by seismic reflection data. The boundary, called the fracture zone trace, enters the Middle America Trench (MAT) ~20 km south of the ODP Leg 170 drilling area. This means that the drill holes are underlain by crust that formed at the EPR. The crustal age in the Legs 170 and 205 area is ~24 Ma. The lithosphere to the southwest of the fracture zone trace formed at the CNS, and its crustal ages decrease to the southwest. The oldest crust directly at the fracture zone trace is 22.7 Ma, which corresponds to the break-up age of the Farallon plate.

The part of the Cocos plate that is presently being subducted offshore Costa Rica was overprinted by hotspot-related volcanism between 14 and 12 Ma. This is most evident in the area of the Cocos Ridge off southern Costa Rica, but rock samples dredged from seamounts have proven that the overprinting extended at least as far north as the southern tip of the Nicoya Peninsula (Fisher Seamount). It seems possible that the sills encountered at the base of some of the ODP Leg 170 holes, and also observed in nearby seismic reflection profiles, may be related to this volcanic event.

The topography of the incoming plate also changes along the length of the margin (Fig. 2) (Ranero et al, 2000b). The crust subducting beneath Nicaragua, formed at the East Pacific Rise, is pervasively faulted with offsets of up to 500 m (Kelly and Driscoll, 1998). Off central and southern Costa Rica the ocean plate formed at the Galapagos spreading center has thicker crust and is covered 40% with large seamounts (von Huene et al., 2000; Barckhausen et al., 2001). In the area of ODP Legs 170 and 205, the subducting plate is the smoother segment of ocean crust formed at the EPR, with shallower grabens than those off Nicaragua.

Heat flow studies and coring on the Cocos plate reveal that heat flow offshore the Nicoya Peninsula and to the north averages 30 mW/m², significantly lower than values of ~108 mW/m² expected for 20-25 Ma crust. The regionally depressed heat flow has been interpreted as evidence for vigorous fluid flow within the upper oceanic crust, which effectively refrigerates the incoming plate by advection (e.g., Langseth and Silver, 1996). Recent heat flow surveys (E. Silver and A. Fisher, pers. comm., 2001) reveal a more complex pattern of vigorous fluid flow in shallow portions of the incoming crust, with local heat flow highs as well as anomalously low values.

There are some parameters that vary little along the entire section shown in Figure 1. The convergence direction is constant with the subducting plate dipping to the northeast. The convergence rate changes only slightly along strike, with a rate of 82 mm/yr off southernmost Nicaragua and 88 mm/yr off southernmost Costa Rica (Kimura, Silver, Blum, et al., 1997). Figure 3 shows that the lithology and thickness of sediment entering the trench is similar at Deep Sea Drilling Project (DSDP) Site 495 (off Guatemala) and ODP Site 1039 (off Costa Rica) and that there is, thus, likely to be little variability in the sediment supply to the trench along the length of the margin.

Leg 170 originally planned basement penetration at Site 1039. This objective was not achieved because of the presence of sills above basement (Kimura, Silver, Blum, et. al., 1997) and the pressure of time needed to satisfy other high-priority Leg 170 objectives. The sills were drillable, but at a slow rate of penetration on a worn rotary core barrel (RCB) bit. For Leg 205, drilling and coring the sills plus a minimum of 100 m penetration into basement remains a highest-priority goal. The extent to which hotspot-related volcanism may have overprinted the EPR signature of the plate subducting beneath northern Costa Rica is an important issue. Some chemical changes in arc lavas between Nicaragua and Costa Rica have been attributed to the presence of more enriched mantle because of Galapagos hotspot volcanism beneath Costa Rica (Carr et al., 1990). If a hotspot chemical signature resides in the subducting oceanic crust, the role of a heterogeneous mantle wedge along the length of the Middle America arc (and geodynamic inferences that follow therefrom) may need closer scrutiny.

Moreover, little is known about the contributions from the altered oceanic crust to arc magmatism and long-term evolution of the mantle, although much work has focused on the role of sediment recycling (e.g., Kay, 1980; Tera et al., 1986; Morris et al., 1990; Plank and Langmuir, 1998). For many key tracer elements in volcanic arc lavas (e.g., K, Sr, U, Pb, B, and Li), alteration processes in the oceanic crust contribute a significant part of the budget subducted to the arc. A large fraction of the water and CO₂ that is deeply subducted will reside in veins and disseminated alteration minerals, much within the zone of oxidative alteration in the upper part of the oceanic basement. Constraining the igneous petrology and chemistry of the upper basement, as well as the style, magnitude, distribution, mineralogy, and chemistry of basement alteration is essential for constructing a complete mass balance for the subduction factory, and for the conclusions drawn therefrom.

The Overriding Plate
Extensive work has imaged the structure of the forearc. Limited Leg 170 coring, drilling, and seismic data show that the bulk of the Pacific margin is a wedge-shaped high-velocity body probably made of rocks similar to the igneous oceanic rocks cropping out along the coast (Shipley et al., 1992; Kimura, Silver, Blum, et al., 1997; Ranero and von Huene, 2000), which precludes the existence of any significantly large sediment mass being recently accreted. Only a small sediment prism (<10 km wide) is located at the front of the margin wedge. Initially, multichannel seismic (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 basically the entire sediment cover of the ocean plate is currently underthrust beneath the margin and that the frontal sediment prism can store 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., 2000a; von Huene et al., 2000).

Prior to Leg 170, the Nicaragua margin was believed to be a nonaccretionary margin while Costa Rica was believed to be a site of sediment accretion (von Huene and Scholl, 1991; Shipley and Moore, 1986; Shipley et al., 1992). Leg 170 drilling and subsequent research show no current or recent frontal sediment accretion off Costa Rica. The sediment section beneath the décollement at Site 1040 repeats the complete lithology and sequence of the incoming section cored at Site 1039 (Fig. 3), allowing little sediment accretion to the front of the prism at present (Kimura, Silver, Blum, et al., 1997). The thinning of the underthrust section seen between Sites 1039 and 1040 (Fig. 4) must be because of compaction and dewatering. Cosmogenic ¹⁰Be, which decays with a 1.5-Ma half life, also shows that there has been little, if any, frontal accretion at this site over the last several million years (Fig. 5) (Morris et al., in press). Surface sediments have very high ¹⁰Be concentrations, which decay exponentially with increasing depth and age in the sediment column. The very high ¹⁰Be concentrations in the incoming sediment section beneath the décollement are typical of young marine sediments. Were these incoming sediments to be frontally accreted, the prism sediments above the décollement would have measureable ¹⁰Be enrichments. The very low concentrations in the sediments above the décollement indicate that they are older than several million years and preclude construction of the prism from accretion of imbricate thrust packets over the last several million years. The prism is thus either a paleoaccretionary prism or is composed largely of slumped slope sediments rather than accreted marine sediments. In the absence of recent frontal accretion, the thinning of the underthrust hemipelagic sediments observed between Sites 1039 and 1040 must be because of dewatering and compaction.

The sediment signature in the arc lavas suggests complete sediment subduction to the depths of magma generation beneath Nicaragua, with only a limited sediment signature in the Costa Rica lavas. 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.

There is evidence for underplating to the base of the prism landward of the Legs 170 and 205 coring area. Christeson et al. (1999) use seismic reflection and refraction data to show stacked velocity duplicates, interpreted as repeated stratigraphic sections because of underplating. The low, but real, ¹⁰Be enrichments in the Costa Rican lavas could be explained if the upper 80-100 m of the incoming sediment section were underplated (Valentine et al., 1997).

In addition to evidence for sediment subduction and underplating beneath the Nicoya segment, the seismic stratigraphy of the slope off Nicaragua and the tectonic structure off Costa Rica indicate extension and subsidence of the margin during much of the Miocene (Ranero et al., 2000a; Ranero and von Huene, 2000; Walther et al., 2000). Multibeam bathymetry along the continental slope displays structures that indicate significant mass wasting off Nicaragua and a rugged morphology off Costa Rica (Ranero et al., 2000a, von Huene et al., 2000), which is consistent with tectonic erosion and thinning of the overriding plate. These results are further substantiated by Leg 170 coring and postcruse science. Coring at Site 1042, 7 km landward of the Middle America Trench, 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 nearshore setting (Vannucchi et al., 2001). Sr isotope ratios place the depositional age at 16-17 Ma—latest early Miocene—and establish that the breccia section is stratigraphically upright. 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, 1999; Vannucchi et al., 2001). Unfortunately, erosion rates over the last several million years are not well constrained. Speculation is that tectonic erosion has been controlled by the roughness of the subducting plate. The thinning of the overriding plate and the continental margin morphology suggests 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 off 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 past Site 1040 greatly simplifies estimating the mass and element fluxes. Specifically, changes in underthrust sediment thickness between Site 1039 and 1040 are due to compaction. To calculate the flux of elements out of the compacting sediment section and updip from the deeper décollement, steady-state conditions may be assumed.

Margin Hydrology
At the Costa Rican subduction zone, coring during ODP Leg 170 and subsequent postcruise studies have identified three fluid-flow systems: active flow of modified seawater within the upper oceanic crust, updip fluid flow within underthrust sediments, and deeply sourced fluid expulsion along the décollement and through the margin wedge (e.g., Silver et al., 2000; Kastner et al., 2000).

The existence of active fluid flow in the oceanic basement of the incoming plate is evidenced by both the heat flow anomalies discussed earlier and the chemistry of pore fluids sampled during Leg 170. Postcruise studies of pore fluid chemistry (Fig. 6A) show chemical characteristics (e.g., ⁸⁷Sr/⁸⁶Sr) typical of modern seawater in the uppermost sediments. Below this, values depart increasingly from seawater at increasing depth as a result of diagenesis. The deepest sediments, however, have pore fluid chemistry that trends back toward modern seawater values, particularly evident in the Sr isotope profile. The heat flow anomaly and pore fluid profiles have been modeled in terms of active fluid flow at rates of ~1-5 m/yr. While certainly model dependent, these results do indicate extensive contemporary flow of seawater to basement and require high permeability horizons, thought to be within the uppermost basement based on results from off-axis drilling along the Juan de Fuca and Mid Atlantic Ridges (Davis and Becker, 1994; Becker et al., 1997, 1998; Fisher, 1998; Davis et al., 2000; Kopf et al., 2000). In addition to cooling the uppermost part of the plate, the flow may further alter the basaltic crust. Subducted high-permeability horizons may provide conduits for fluids leaving the deeper subduction zone.

The décollement and faults cutting through the margin wedge of the upper plate serve as pathways for updip flow of deeply sourced fluids. Structural observations across the décollement indicate a zoned fault, with heavily fractured fault rocks above a ductile, plastic zone that may act as an aquitard to segregate the flow systems above and below the plate boundary (Vannucchi and Tobin, 2000; Tobin et al., 2001). Pore fluid chemistry studies since Leg 170 show that fluids sampled along the décollement zone at Sites 1040 (320-360 meters below seafloor [mbsf]) and 1043 (~130 150 mbsf) have distinct chemical signatures indicative of focused fluid advection. Similar chemical anomalies are also seen in an upper conduit at Site 1040 (~180-200 mbsf), which may occur along a thrust fault. Extremely high Li concentrations are observed along the thrust and décollement at Site 1040 (Fig. 6B). Enrichments in Ca and Sr, changing Sr and Li isotopic compositions, as well as higher concentrations of thermogenic heavy hydrocarbons C₃-C₆) are also observed. Collectively, these data indicate that some fraction of the fluids sampled along these localized horizons are derived from depths great enough that temperatures are ~150°C. This temperature corresponds to the updip limit of the seismogenic zone (e.g., Hyndman and Wang, 1993; Hyndman et al., 1997). The sharpness of the pore fluid anomalies at these horizons indicates updip advective flow and permits virtually no diffusion or vertical advection away from these intervals. The composition of deeply sourced fluids derived from the updip limit of the seismogenic zone may be useful in constraining the mineralogy of sediments at depth and the dehydration reactions that are thought to be important in governing the rheological properties of the subduction interface in the region of seismogenesis. Given that most seismogenic zones will be forever beyond the reach of even a riser drill ship, development of chemical proxies for "remote sensing" of processes occurring in the seismogenic zone is an important adjunct of Leg 205.

Structural and chemical studies indicate a third hydrologic system, which drains fluids from the underthrust sediment section. Results from Leg 170 documented the complete underthrusting of the incoming sedimentary section at the trench, with the important implication that observed changes in sediment thickness and porosity directly reflect the evolution of effective stress. Laboratory consolidation tests, combined with logging-while-drilling (LWD) data from Leg 170, show that the subducting sediments are effectively undrained at Site 1043 (Saffer et al., 2000; D. Saffer, pers. comm., 2001). At Site 1040, the lower carbonate section (Unit III) remains essentially undrained, whereas the upper hemipelagic units (Units I and II) are partially drained. Pore fluid profiles from sediments immediately below the décollement at Sites 1040 and 1043 show a distinct chemistry from that of the décollement, indicating little, if any, communication between the two hydrologic systems. A Ba spike immediately below the décollement (Fig. 6C) results from sulfate reduction in the uppermost underthusting section that mobilizes Ba out of barite. The very low Ba values in the décollement zone itself indicate that these fluids are not draining into the décollement. Other broad anomalies below the décollement also imply updip advective flow of locally derived fluids (Kastner et al., 2000).

The differences in pore pressure development downsection reflect nonuniform fluid escape. More rapid drainage of the uppermost ~100 m (Units I and II) than of Unit III 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. The nonuniform dewatering also has important mechanical implications. An inferred minimum in effective stress developed near the top of Unit III between Sites 1043 and 1040 results in a mechanically weak horizon and suggests that detachments may form below the décollement there. This is consistent with the down-stepping of the décollement at 2-3 km from the trench observed in this region (McIntosh and Sen, 2000) and illustrates the role of fluid pressure in mediating structural development. Based on observed changes in porosity, volumetric fluid sources (in [V_fluid/V_sediment]s^-1) range from ~10^-12 s^-1 at the top of the section to ~10^-13 s^-1 at the base. These values are one to two orders of magnitude larger than those calculated for underthrust sediments at the Nankai and Barbados subduction zones (e.g., Zhao et al., 1998; Screaton et al., in press). This dramatic difference likely reflects higher sediment permeabilities at Costa Rica, resulting in a more active fluid flow system.

Figure 7 shows schematically the three separate hydrologic systems, their locations in the Leg 205 coring area, and summarizes their physical and chemical characteristics. Hydrological and geological modeling (e.g., Saffer et al., 2000; Silver et al., 2000) suggests relatively high permeabilities in the oceanic basement, décollement, and underthusting section.

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