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doi:10.2204/iodp.sp.344.2012

Background

The Cocos Ridge and subducting plate

Offshore the western margin of Costa Rica, the oceanic Cocos plate subducts under the Caribbean plate, forming the southern end of the Middle America Trench (Fig. F1). A prominent feature of the Costa Rican segment of the Middle America Trench is its along-strike variability. Subduction parameters including the age, convergence rate, azimuth, obliquity, morphology, and slab dip all vary along strike. The age of the Cocos plate at the Middle America Trench decreases from 24 Ma offshore the Nicoya Peninsula to 15 Ma offshore the Osa Peninsula (Barckhausen et al., 2001). Subduction rates vary from 70 mm/y offshore Guatemala to 90 mm/y offshore southern Costa Rica (DeMets, 2001). Convergence obliquity across the trench varies from offshore Nicaragua, where it is as much as 25° oblique, to nearly orthogonal southeast of the Nicoya Peninsula (DeMets, 2001; Turner et al., 2007). This obliquity has implications for slip partitioning as indicated by focal mechanisms and GPS displacement data (Lundgren et al., 1999; McCaffrey, 2002; Norabuena et al., 2004; Turner et al., 2007; LaFemina et al., 2009).

The bathymetry and morphology of the incoming Cocos plate are largely a function of its origin and subsequent history. The Cocos plate was formed at two ridges, the fast-spreading East Pacific Rise (EPR) and the slow-spreading Cocos Nazca spreading center (CNS). The boundary separating EPR from CNS crust is a combination of a triple junction trace and a fracture zone, collectively comprising a “plate suture” (Fig. F1). EPR-generated crust has a generally smoother morphology than CNS-generated crust. Subsequent to the plate’s formation it was intruded by Galapagos hotspot volcanism. Passage of the Cocos plate over the Galapagos hotspot created the aseismic Cocos Ridge, an overthickened welt of oceanic crust. This ridge is ~25 km thick, greater than three times normal oceanic crustal thickness. The ridge stands 2.5 km high and is characterized by a distinctive Galapagos-type geochemistry. The area just northwest of the EPR/CNS plate suture (Barckhausen et al., 2001) was drilled during Deep Sea Drilling Project (DSDP) Leg 84 and Ocean Drilling Program (ODP) Legs 170 and 205 (Kimura, Silver, Blum, et al., 1997; Shipboard Scientific Party, 1985, 2003). Sills with a Galapagos-type geochemistry were cored at ODP Sites 1039 and 1253, indicating the great lateral extent of hotspot magma intrusion. Northwest of the Cocos Ridge, ~40% of CNS oceanic crust is covered by seamounts that also have a Galapagos-type geochemistry. These seamounts increase the roughness of the seafloor generated by slow spreading and have likely caused substantial subduction erosion of the outer forearc (Ranero and von Huene, 2000) and uplift of the Osa and Nicoya Peninsulas and the Quepos region (Gardner et al., 1992, 2001; Fisher et al., 1998; Sak et al., 2004).

Historical large magnitude plate interface earthquakes (Mw > 7) may correlate with the locations of subducted seamounts or bathymetric relief. Possible examples include the 1992 Nicaragua (McIntosh et al., 2007), 1950 and 1990 Nicoya (Husen et al., 2002), 1983 Osa (Adamek et al., 1987), and 1999 Quepos (Bilek et al., 2003) earthquakes.

The dip and depth of the Wadati-Benioff Zone decreases from Nicaragua to southern Costa Rica. At the Osa Peninsula, the overthickened Cocos Ridge is more buoyant than normal oceanic crust and causes a shallowing of the Wadati-Benioff Zone. The seismically active slab dips ~65° near the Nicaraguan border and shallows a few degrees inboard of the Cocos Ridge. At depths greater than 60 km there is no seismically defined slab landward of the Cocos Ridge (Vergara Muñoz, 1988; Protti et al., 1994).

An outstanding issue for the tectonics of the region is the timing of the Cocos Ridge impinging on the Middle America Trench. Estimates range from ~1 Ma (Hey, 1977; Lonsdale and Klitgord, 1978) to ~5 Ma (Sutter, 1985) to ~20–22 Ma (Lonsdale and Klitgord, 1978; van Andel et al., 1971). The 5 Ma age is based on the emplacement of adakitic arc rocks between 5.8 and 2.0 Ma (Abratis and Wörner, 2001) and thermochronological information on the uplift of the Talamanca Cordillera (Gräfe et al., 2002). However, marine deposition and volcanic flows in the Pliocene Terrabe forearc basin directly inboard of the Cocos Ridge (Kolarsky et al., 1995) raise concerns about this model.

The CRISP drilling area is located in a region where the incoming plate has relatively thin sediment cover, large variations in along-strike bathymetry, and a fast convergence rate. The plate interface is characterized by abundant seismicity.

Upper plate and onland geology

Seismic data along the margin and drilling offshore the Nicoya (Legs 170 and 205) and Osa (Expedition 334) Peninsulas show that the margin is composed of a thick slope sediment apron, a few hundred meters to ~2 km thick, unconformably overlying upper plate basement (von Huene et al., 2000). In the CRISP drilling area, the upper plate is underthrust by sediments and is buttressed by a small frontal prism (Fig. F2). Offshore the Osa Peninsula the frontal prism is 10–12 km thick but diminishes to 3–5 km offshore the Nicoya Peninsula. The forearc basement was not well sampled during Leg 170.

The basement comprising the margin along the CRISP transect is interpreted to be composed of a mélange of oceanic lithologies accreted to the overriding plate prior to the current phase of subduction erosion (Fig. F3). The basement is likely the offshore extension of igneous rocks cropping out onshore that consist of the Caribbean Large Igneous Complex (CLIP) and the Quepos and Osa terranes (Ye et al., 1996; Kimura, Silver, Blum, et al., 1997; Vannucchi et al., 2001). The CLIP is composed of accreted ocean islands and aseismic ridge terranes (Hauff et al., 1997, 2000; Sinton et al., 1997; Hoernle et al., 2002). The Quepos and Osa terranes are interpreted to represent rock accreted from subducted edifices generated by the Galapagos hotspot (Hauff et al., 1997; Vannucchi et al., 2006). On land and close to the CRISP transect, the seawardmost unit is the Osa Mélange, which is dominated by basalt, radiolarite, and limestone (Vannucchi et al., 2006). Short-wavelength magnetic anomalies observed on the Osa margin that are interpreted to be localized bodies of igneous rock mixed with sedimentary rocks lend additional support to this interpretation (U. Barckhausen, unpubl. data). The nature and significance of the Osa Mélange has been controversial. One interpretation is that it represents debris flows that were subsequently accreted to the margin (P.O. Baumgartner, pers. comm., 2002). Other interpretations are that the Osa terrane represents a tectonic mélange produced by subduction erosion (Meschede et al., 1999) or an old tectonic mélange developed within material that was accreted prior to the arrival of the Cocos Ridge (Vannucchi et al., 2006). There is no suggestion that the Osa Mélange reflects accretion from the currently subducting plate, and the evidence for active recent tectonic erosion of the forearc is compelling. The Osa Mélange is, to our best knowledge, the unit that forms the forearc basement and which we expect to drill as upper plate basement during CRISP.

A 45 m.y. gap exists in the rock record between the emplacement of the CLIP (74–94 Ma; Sinton et al., 1998), the Quepos and Osa terranes (60–65 Ma), and the dredged rock samples from the Cocos Ridge and related seamounts near the trench (13.0–14.5 Ma; Werner et al., 1999). Accretion during this period may be partially recorded beneath the Osa continental slope-forearc (Hoernle et al., 2002).

Within the margin, a major unknown is the nature of the high-amplitude landward-dipping reflectors cutting through the forearc basement (Fig. F2). They branch upward from the plate interface similarly to splay faults (Park et al., 2002). These surfaces may represent old faults, related to a middle Eocene–middle Miocene accretionary event, now sealed by the slope apron sediment. A few of these surfaces have offsets at the top of the forearc basement into the slope apron, indicating reactivation as normal faults. Similar reactivated normal faults are observed offshore the Nicoya Peninsula and Quepos terrane (McIntosh et al., 1993; Ranero and von Huene, 2000). The lack of a clear thrust sequence argues against the presence of out-of-sequence thrusts (OOSTs) cutting the forearc. The presence of these discontinuities across the forearc basement can offer preexisting planes of weakness, which can play a role in focusing fluid flow drained from the deeper part of the margin as suggested by the high reflectivity and heat flux. The nature and magnitude of permeability along these discontinuities is unknown. Identifying the nature and age of the landward-dipping reflectors is fundamental to understanding the tectonic history of the margin offshore Osa Peninsula and the process of subduction erosion.

The subduction of the Cocos Ridge is thought to have caused (1) the cessation of the arc volcanism and uplift of the Talamanca Cordillera; (2) the inversion of the middle Eocene–Pliocene forearc basin, now exposed along the Fila Costeña, a fold and thrust belt with peak elevations of 1000–1500 m; and (3) the exhumation of the Late Cretaceous–early Eocene ophiolitic rocks cropping out along the Osa Peninsula gulf and the middle Eocene–middle Miocene Osa Mélange. Directly inboard of the Cocos Ridge, geologic and GPS data reveal maximum uplift and shortening. Quaternary shortening exceeds 15 km (10–40 mm/y) across the Fila Costeña fold and thrust belt (Fisher et al., 2004; Sitchler et al., 2007). The Talamanca Cordillera located between the Fila Costeña fold and thrust belt and the northern Panama deformation belt exposes plutonic rocks as young as 6 Ma (MacMillan et al., 2004), implying rapid uplift.

Volcanic arc

In Costa Rica, ⁴⁰Ar/³⁹Ar dating indicates a maximum age of the volcanic arc of at least 24 Ma (Gans et al., 2002). Plutons intruded the Talamanca Cordillera until the late Miocene, ~7 Ma (Gans et al., 2002; Mora, 1979; Sutter, 1985), after which subduction-related calc-alkaline magmatism diminished. Backarc alkaline magmatism during the following ~6–3 m.y. produced lava flows, dikes, and sills (Abratis and Wörner, 2001). Just south of the central magmatic arc, lavas that erupted from 5.8 to 2.0 Ma have a trace element signature characterizing them as partial melting products of subducted oceanic crust with garnet residue, or adakites, and a plume-related isotope signature (Abratis and Wörner, 2001; Gans et al., 2002).

The Central America volcanic arc was a high-priority study area of the Subduction Factory initiative of the US MARGINS program. Here, variations in subduction dynamics result in sharp differences in the apparent sediment transport to depth, mirroring strong along-strike changes in trace element and isotopic chemistry, such as the ¹⁰Be deficit in Costa Rican volcanoes (Morris et al., 2002). The possibility of studying the tephra stratigraphy preserved in the slope apron sediments offshore Osa will help in the along-strike reconstruction of the margin and will open a window in the processes linked to the volcanic arc shutdown when compared to the ash stratigraphy already recovered offshore the Nicoya Peninsula.

Subduction erosion

Drilling and seismic data indicate active and long-lived subduction erosion from Guatemala to Costa Rica (Ranero and von Huene, 2000; Ranero et al., 2000; Vannucchi et al., 2001, 2003, 2004). This interpretation is based on the following:

• Long-term subsidence of the continental slope offshore Nicoya Peninsula. Leg 170 provided direct evidence of shallow-water sedimentary rocks, now located in 3900 m water depth in the forearc. These slope apron–forearc sediments lie unconformably on the basement, demonstrating that the margin offshore Nicoya Peninsula has experienced a net loss of crust since ~16 Ma (Vannucchi et al., 2001). Detailed analysis of the benthic fauna preserved in the slope apron sediment from Legs 84 and 170 indicates that a slow background subsidence of ~20 m/m.y. radically increased to ~600 m/m.y. starting at the Miocene/Pliocene boundary (Vannucchi et al., 2003). This acceleration in subsidence may be linked to the arrival of the Cocos Ridge at the subduction zone (Vannucchi et al., 2003). Faster subduction erosion may be expected to the south where ridge subduction caused severe damage to the margin, as suggested by the disrupted topography (von Huene et al., 2000). The slope offshore Osa has retreated as much as 20 km more than in the Nicoya area, where the subducting plate is smoother and the trench retreat has been estimated at ~50 km since 16 Ma (Vannucchi et al., 2001). Offshore Nicaragua, subsidence driven by tectonic erosion triggered the development of the Sandino forearc basin (Ranero and von Huene, 2000; Ranero et al., 2000).

• The regional extension of the slope apron–forearc unconformity across igneous basement in northern Costa Rica and the middle Eocene–middle Miocene mélange in southern Costa Rica is consistent with subduction erosion.

• Disrupted topography at the base of the slope and in the wake of seamounts. The trench inner slope of Costa Rica is punctuated by subducted seamount tracks reflecting a net loss of material, and at a larger scale, the whole margin has a broad concavity centered on the Cocos Ridge, testifying to the removal of material through ridge subduction.

Volatiles and fluids

Pathways of fluid flow through the Costa Rica margin include the margin, the décollement, and the underlying oceanic crust (e.g., Silver et al., 2000; Fisher et al., 2003; Hutnak et al., 2007; Sahling et al., 2008; Harris et al., 2010a, 2010b).

Along the entire Costa Rica margin, active fluid venting is indicated by elevated methane concentrations in the bottom water (Kahn et al., 1996; McAdoo et al., 1996; Bohrmann et al., 2002). Chemoautotrophic and methanotrophic communities mark cold vents at numerous localities, but higher concentrations have been found where subducted seamounts have triggered fractures, slides, and slumps that break a low-permeability, shallow sediment carapace, allowing ascending fluids to feed the communities that are particularly concentrated at the headwall scarps (Kahn et al., 1996; Bohrmann et al., 2002; Ranero et al., 2008). Mud volcanoes and mud diapirs have also been found, particularly across the middle slope, and are associated with a high density of chemosynthetic vents. The chemistry of the pore fluids sampled at these mid-slope features is indicative of dehydration reactions at depth, suggesting they are associated with structures that enable effective transport of fluids in the overpressured slope sediments (Shipley et al., 1992; Bohrmann et al., 2002; Grevemeyer et al., 2004; Hensen et al., 2004).

Along the décollement and the upper prism fault zone, Leg 170 coring and sampling revealed freshened pore waters containing elevated Ca, Li, and C3–C6 hydrocarbon concentrations and low K concentrations (Kimura, Silver, Blum, et al., 1997; Silver et al., 2000; Morris, Villinger, Klaus, et al., 2003). These fluids contrast with pore fluids from below the décollement and between the décollement and upper fault zone that have near-seawater chemistry (Kimura, Silver, Blum, et al., 1997; Morris, Villinger, Klaus, et al., 2003). Downhole temperatures measured during Legs 170 and 205 are insufficient to support in situ mineral dehydration and thermogenic methane. Collectively, geochemical data in the décollement offshore Nicoya Peninsula indicate that this flow system is active and a fraction of the fluid is derived from depths within the subduction zone where temperatures are ~80°–150°C (Chan and Kastner, 2000; Silver et al., 2000; Kastner et al., 2006; Solomon et al., 2009). The sharpness of the geochemical anomalies in the décollement and the estimated temperature of the fluid suggest updip flow from a source region ~38–55 km landward of the trench at ~9–14 km depth, near the updip limit of the seismogenic zone (Harris and Wang, 2002; Spinelli and Saffer, 2004; Kastner et al., 2006; Ranero et al., 2008).

The magnitude of the hydrological activity in the subducting oceanic plate is just beginning to be appreciated (Silver et al., 2000; Harris and Wang, 2002; Fisher et al., 2003; Hutnak et al., 2008; Solomon et al., 2009; Harris et al., 2010a, 2010b). Low heat flow values averaging ~30 mW/m² exist in the EPR-generated crust offshore the Nicoya Peninsula (Langseth and Silver, 1996; Fisher et al., 2003; Hutnak et al., 2007, 2008). These values reflect <30% of the expected value from conductive lithospheric cooling models for 24 Ma crust (Stein and Stein, 1994), indicating effective hydrothermal cooling of the upper oceanic crust in the study area with recharge and discharge occurring at distant igneous outcrops and seamounts (Fisher et al., 2003). This inference is corroborated by pore fluid chemical and isotopic profiles in basal sediments that show a return to approximate seawater values near the upper part of the igneous basement (Chan and Kastner, 2000; Silver et al., 2000; Morris, Villinger, Klaus, et al., 2003). In addition to the cooling effect, the vigorous lateral flow of seawater must also alter and hydrate the igneous crust, affecting chemical and isotopic mass balances as well as the transfer of volatiles through the subducting slab down to the depth of magma genesis.

Offshore the Nicoya Peninsula, two sealed borehole hydrologic observatories (CORKs) were installed to investigate the relationship between tectonics, fluid flow, and fluid composition (Jannasch et al., 2003; Morris, Villinger, Klaus, et al., 2003; Solomon et al., 2009). One of these CORKs was deployed at ODP Site 1255 with downhole instrumentation designed to monitor formation fluid flow rates, composition, pressure, and temperature in a screened interval in the décollement. The other CORK was deployed at ODP Site 1253 with downhole instrumentation to measure fluid pressure, temperature, and chemistry in the subducting igneous basement. The initial 2 y record was recovered in September 2004, and a second record was recovered in February 2009. The long-term pore fluid pressure record at Site 1255 showed a near-steady-state pressure that was only moderately superhydrostatic with a pore pressure ratio (λ*) of ~0.2 (Davis and Villinger, 2006), and flow rates averaged ~1.0 cm/y during the 2002–2004 deployment period (Solomon et al., 2009). Two positive transients in fluid pressure, flow rates, and composition were observed along the décollement between 2002 and 2004 (Davis and Villinger, 2006; Solomon et al., 2009). Both transients coincided with onshore deformational events recorded at continuously monitored GPS stations on the Nicoya Peninsula ~2 weeks prior to being recorded near the trench at the CORK (Protti et al., 2004). These two transients have been interpreted as the result of aseismic slip dislocations that propagated updip over the course of ~2 weeks, terminating before reaching Site 1255 and the trench (Solomon et al., 2009), and indicate that slow slip events propagate through the seismogenic zone to the trench at the Costa Rica subduction zone.

The continuous fluid pressure, temperature, and chemistry record obtained from the CORK at Site 1253 shows that the pressure in uppermost igneous basement is ~6 kPa subhydrostatic (Davis and Villinger, 2006), indicating the upper basement is highly permeable. The average fluid flow rate measured at the Site 1253 CORK is 0.3 m/y, and the fluid chemistry in the basement indicates that the basement fluid is actually a mixture between seawater (~50%) from the regional fluid flow system and a subduction zone fluid originating within the forearc (~50%) (Solomon et al., 2009). These results suggest that the uppermost basement offshore Nicoya Peninsula serves as an efficient pathway for fluid expelled from the forearc.

Offshore Osa Peninsula, heat flow values are much higher than at Nicoya Peninsula (averaging ~130 mW/m²) (von Herzen and Uyeda, 1963; Vacquier et al., 1967). The Cocos Ridge upper crust is well layered and probably very permeable (C.R. Ranero, pers. comm., 2003). The contribution from the lower plate to the fluid circulation could also be significant in the CRISP drilling area. Results from Expedition 334 and Expedition 344 drilling and sampling will help clarify fluid sources and pathways in this segment of the Costa Rica margin.

Seismogenic zone and earthquakes

CRISP Program A is preparatory to the seismogenic zone experiment and will define the tectonic reference for deeper drilling. The most recent seismic sequence along the Costa Rican seismogenic zone occurred offshore the Osa Peninsula in 2002 (Fig. F4). This sequence nucleated in the southeastern region of the forearc where a seamount has been thrust under the margin. A Mw = 6.9 earthquake sequence occurred in 1999 and collocated with a subducted ridge and associated seamounts. The 2002 Osa mainshock and first few hours of aftershocks began in the CRISP drilling area ~30 km of the 1999 sequence. In the 2 weeks following the mainshock, aftershocks migrated both into the 1999 aftershock area and updip of the mainshock (Arroyo et al., 2011).

GPS measurements on land indicate that over the subducted Cocos Ridge most of the plate interface in the seismogenic region is essentially fully locked (Dixon, 2003; LaFemina et al., 2009). In contrast, seismic profiles indicate fault geometries (i.e., angles between forethrusts, backthrusts, and the décollement) that suggest low values of plate boundary friction (von Huene et al., 2000, 2004; von Huene and Ranero, 2003). These values are comparable to the shear strength of marine sediment and may accommodate seafloor relief at the front of the margin without much deformation. One model has fluids draining from the subducting lower plate sufficient to hydrofracture and to mobilize about a 1–2 km thick and 20 km long section of the upper plate material every 1 m.y. in Central America (von Huene et al., 2004).

Site survey data

The supporting site survey data for Expedition 344 are archived at the IODP Site Survey Data Bank.

The regional framework of the Central America Trench off Costa Rica is well known from investigations since DSDP drilling in the early 1980s (Aubouin et al., 1982) and later, Legs 170 and 205 (Kimura, Silver, Blum, et al., 1997; Morris, Villinger, Klaus, et al., 2003). Recently, this margin has been the focus of two major scientific projects: the German Collaborative Research Center (SFB) 574 “Volatiles and fluids in subduction zones” (sfb574.ifm-geomar.de/​home/) and the US MARGINS National Science Foundation program (www.nsf-margins.org/​SEIZE/​CR-N/​CostaRica.html).

More than 10,000 km of bathymetric imaging has been acquired (swath bathymetry; Weinrebe and Ranero, in GeoMapApp and MARGINS Data Portal; www.geomapapp.org/) (Fig. F5). The extensive multibeam bathymetric mapping shows variable seafloor morphology between the Nicoya and Osa Peninsulas (von Huene et al., 1995). The multibeam bathymetry is complemented by several deep-towed instrument traverses. The towed ocean bottom instrument (TOBI) sidescan sonar system of the Southampton Oceanography Centre was used during SO-163 in the spring of 2002 to detect active fluid flow at seafloor mounds and mass wasting offshore Costa Rica (Weinrebe and Ranero, 2003). Together with the results of the TOBI survey during the SO-144 cruise in 1999, much of the continental margin from Costa Rica to southeast Nicaragua was imaged with a resolution on the order of 10 m. Parts of that surveyed area were imaged with greater resolution using the GEO-MAR DTS-1 deep-towed sidescan sonar system to map key areas with a resolution of better than 1 m (Klaucke et al., 2008; Petersen et al., 2009). Observations of the seafloor with a TV sled, gravity coring, and a TV-guided grab (Flüh et al., 2004) pinpointed areas of interest. Widespread mounds, some tens of meters high and a few hundred meters wide, have been monitored with current meters and hydrographic stations (Flüh et al., 2004). Outcropping carbonates on top and at the flanks indicate that these mounds are formed by chemoherm carbonates with abundant signs of fluid flow (Bohrmann et al., 2002; Hensen et al., 2004).

A local network of stations on land has recorded seismicity in the area for 2 decades (Fig. F4). Several marine seismological networks of ocean-bottom seismometers (OBS) and ocean-bottom hydrophones (OBH) have been deployed offshore Costa Rica. The Costa Rica Seismogenic Zone Experiment (CRSEIZE), run by University of California Santa Cruz, University of California San Diego, Observatorio Vulcanologico y Sismologico de Costa Rica, and University of Miami, established two seismic networks off the Osa and Nicoya Peninsulas. The first network was a 3 month (September–November 1999) onshore and offshore deployment between Quepos and the north shore of the Osa Peninsula, recording aftershocks from the 20 August 1999 Mw 6.9 underthrust earthquake. The second network operated onshore and offshore the Nicoya Peninsula from December 1999 to June 2000 (Newman et al., 2002; DeShon et al., 2006). CRSEIZE also included GPS campaigns across Costa Rica (Norabuena et al., 2004). German SFB 574 deployed OBS and land stations for >9 months (i.e., from the beginning of October 2002 [R/V Meteor Cruise M54-3B] to August 2003 [R/V Sonne Cruise SO173-1]) (Flüh et al., 2004). SO 173-1 also deployed another 2 months of OBS offshore in 2002.

Geophysical data acquisition in the proposed Osa drilling area is extensive. The proposed drilling sites are positioned on an OBS/OBH seismic refraction transect across the entire onshore/​offshore of Costa Rica (Ye et al., 1996; Stavenhagen et al., 1998) (Fig. F3). These data were acquired in 1995/1996 during the Trans Isthmus Costa Rica Scientific Exploration of a Crustal Transect (TICOSECT) project. The TICOSECT transect is coincident with three multichannel seismic reflection surveys. The first was shot in 1978 (IG2903 vessel Ida Green), later reshot by Shell Oil (Kolarsky et al., 1995), and shot again in 1999 (BGR99 vessel Prof. Polshkov) with a long streamer and an industry acquisition system (Fig. F2). Proposed sites have cross-lines of industry and academic heritage. Transducer and high-resolution sparker coverage are available. Conventional heat probe transects were acquired regionally and along the primary transect, which calibrate bottom-simulating reflector (BSR)-derived heat flow from the seismic records (Ranero et al., 2008; Harris et al., 2010a). Magnetic and gravity data cover the area (Barckhausen et al., 1998, 2001). GPS geodesy has been studied for more than a decade, and results show a locked Osa Peninsula area (LaFemina et al., 2009).

Seismic reflection data

Seismic reflection data across the margin are substantial (Fig. F5), mainly coming from Cruises Sonne SO-76, SO-81 (Hinz et al., 1996; von Huene et al., 2000; Ranero and von Huene, 2000; Ranero et al., 2007), and BGR99 (Ranero et al., 2008). Seismic Line P1600 is provided by Shell (von Huene et al., 2000). All seismic lines were collected with large tuned air gun arrays and multichannel streamers, as described in the original papers. Seismic data have been processed for signal enhancement, including deconvolution and multiple attenuation, and were poststack time migrated. Selected sections are prestack depth migrated. All lines provide good imaging of the structure of the overriding plate including the sediment cover strata, BSRs, and plate boundary reflections.

Seismic reflection data from SO-81 (Hinz et al., 1996) complement those acquired during Cruise BGR99. Two BGR99 records are processed in depth and the remainder in time domains. The principal site survey line (BGR99-7) is flanked on either side by two lines at 1 km spacing, then by lines at 2 km, 5 km, and 10 km spacing (Fig. F6). Although these are the most revealing seismic images, other industry and academic acquired records in the area are numerous. Unfortunately, the resources are not available to process them to their full potential. Seismic reflection images collected between Osa Peninsula and the Cocos Ridge (Fig. F6) show more stratified forearc basement and lower velocity material (~1 km/s less) than in equivalent areas along the Nicoya transect.

In April and May of 2011, a three-dimensional (3-D) seismic reflection data volume was acquired offshore Costa Rica, northwest of the Osa Peninsula and northwest of the Expedition 334 transect, together with high-resolution backscatter and multibeam data. The goal of the 3-D seismic survey is to illuminate the crustal structure and deformation history of this erosive margin and to image the plate-boundary fault from the trench into the seismogenic zone. The 3-D survey covered 55 km across the upper shelf and slope and into the trench. The survey extends 11 km along strike for a total survey area of 11 × 55 km². These data were acquired with the R/V Langseth using a 3300 in³ source shot every 50 m. Data were recorded on four 6 km long, 468-channel streamers with 150 m separation. At the time of this Scientific Prospectus, preliminary results from processing two-dimensional (2-D) seismic lines extracted from the 3-D volume and from initial 3-D volume processing were available (Bangs et al., 2011; Kluesner et al., 2011). Initial processing of the seismic data shows an upper plate structure with numerous faults, many extending down to the plate interface, and intense folding and faulting of the slope cover sequences (Bangs et al., 2011). Multibeam data across the shelf and slope correlate to faulting and folding sequences in the slope cover and deeper upper-plate faulting seen in preliminary 2-D and 3-D seismic reflection images. The arcuate structure of the shelf edge and structural bulge seen in seismic data just landward of the shelf edge are consistent with a site of uplift over a subducting ridge (Kluesner et al., 2011).

Heat flow data

Values of surface heat flow vary greatly offshore Costa Rica and have important implications for the thermal state of the shallow subduction thrust and thermally mediated processes along the subduction thrust (Fig. F7). Regional heat flow measurements on the incoming Cocos plate reveal large along-strike variations in heat flow (von Herzen and Uyeda, 1963; Vacquier et al., 1967). EPR-generated seafloor of the Cocos plate has low heat flow relative to the global mean for crust of the same age. The mean value and standard deviation of regional heat flow data on EPR seafloor are approximately 30 and 15 mW/m², respectively. For seafloor of this age, conductive predictions are ~100–130 mW/m² (Stein and Stein, 1992). In contrast, heat flow values on CNS-generated seafloor are close to the conductive prediction but above the global mean for crust of this age. These values exhibit large variability indicative of fluid flow with a mean and standard deviation of 110 and 60 mW/m² for EPR- and CNS-generated crust, respectively.

In 2000 and 2001, heat flow studies specifically designed to investigate the nature of the thermal transition between the cold EPR and warm CNS crust were undertaken (Fisher et al., 2003; Hutnak et al., 2007, 2008). Closely spaced heat flow values collocated with seismic reflection lines show a sharp transition (<5 km) between warm and cool seafloor seaward of the trench that grossly corresponds to the area of the plate suture (Fig. F7). The sharp transition indicates a shallow source consistent with fluid flow in the upper oceanic crust. The thermal boundary between warm and unusually cool values deviates from the plate suture and appears to be influenced by the proximity of basement highs that penetrate the otherwise ~300–400 m thick sediment cover. This exposed basement on EPR accreted crust focuses discharge and efficiently ventilates the oceanic crust.

Probe measurements and BSR estimates of heat flow on the margin (Fig. F7) are discussed by Harris et al. (2010a, 2010b). These profiles show strong variations along strike consistent with regional heat flow data on the incoming Cocos plate. Along the margin underthrust by EPR crust, heat flow is low (Langseth and Silver, 1996; Fisher et al., 2003; Hutnak et al., 2007), whereas along the margin underthrust by CNS crust, heat flow is generally high. The transition between low and high heat flow correlates with the extension of the plate suture and appears relatively abrupt. High wave number variations in the heat flow data are interpreted as focused fluid flow through the margin near the deformation front (Harris et al., 2010a, 2010b).

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