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doi:10.2204/iodp.proc.334.101.2012

Background

Subducting plate and the Cocos Ridge

The oceanic Cocos plate subducting beneath Costa Rica was formed at two different spreading centers, the East Pacific Rise (EPR) and the Cocos-Nazca spreading center (CNS), and has been largely influenced by Galapagos hotspot volcanism. The largest feature formed by the passage of the Cocos plate over the Galapagos hotspot is the 2.5 km high Cocos Ridge (Fig. F1). The oceanic crust beneath the ridge is three times thicker than normal oceanic crust (25 km; Stavenhagen et al., 1998) and has a Galapagos-type geochemical composition. Bordering the ridge to the northwest is regular CNS oceanic crust. Younger seamounts also being formed by the Galapagos hotspot cover 40% of this area of the Cocos plate (Fig. F1), resulting in a rather rough plate morphology. Further north, EPR-generated crust has a smoother morphology. The area drilled during Deep Sea Drilling Project (DSDP) Leg 84 and Ocean Drilling Program (ODP) Legs 170 and 205 (von Huene, Aubouin, et al., 1985; Kimura, Silver, Blum, et al., 1997; Morris, Villinger, Klaus, et al., 2003) lies just northwest of the EPR/CNS crustal boundary (Barckhausen et al., 2001). Sills with Galapagos-type geochemistry drilled at ODP Sites 1039 and 1253 show the great lateral extent of hotspot volcanism.

The influence of Cocos Ridge subduction increases from the Nicoya Peninsula in the northwest to the Burica Peninsula in the southeast (~400 km; Fig. F1) and is accompanied by morphologic changes along the margin in response to shallowing of the Wadati-Benioff Zone. The seismically active slab dips at ~65° near the Nicaraguan border and shallows a few degrees inboard of the Cocos Ridge. The timing of the Cocos Ridge impinging on the Middle America Trench is an unresolved issue, with estimates ranging from ~1 Ma (Hey, 1977; Lonsdale and Klitgord, 1978) to ~5 Ma (Sutter, 1985). 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 constraints on the uplift of the Talamanca Cordillera (Gräfe et al., 2002). However, marine deposition and volcanic flows in the Pliocene Terraba forearc basin directly inboard of the Cocos Ridge (Kolarsky et al., 1995) raise serious concerns about this model. A second question is when the Cocos Ridge started to form. Several investigators have proposed a date of ~20–22 Ma, synchronous with the formation of the CNS (van Andel et al., 1971; Lonsdale and Klitgord, 1978); however, this remains an unresolved problem.

Upper plate and onland geology

Arcward of the Middle America Trench offshore the Osa Peninsula, the lower slope consists of a 10–12 km wide frontal prism (Fig. F2). A similar 3–5 km wide frontal prism is also present offshore the Nicoya Peninsula, where it is composed of slope sediment redeposited into the trench and buttressed against forearc basement. The forearc basement, although poorly sampled during Leg 170, is generally accepted to be composed of the same igneous rock exposed onshore (Ye et al., 1996; Kimura, Silver, Blum, et al., 1997; Vannucchi et al., 2001). The igneous complexes exposed in Costa Rica represent parts of the Caribbean Large Igneous Province (CLIP) (emplaced between 74 and 94 Ma; Sinton et al., 1998) and accreted ocean islands and aseismic ridge terranes (Hauff et al., 1997, 2000; Sinton et al., 1997; Hoernle et al., 2002). Crucially, there is no evidence that the forearc is composed of a complex of tectonized sediment offscraped from the currently subducting plate, although the 60–65 Ma Quepos and Osa terranes are interpreted to reflect rocks accreted from subducted edifices generated by the Galapagos hotspot (Hauff et al., 1997; Vannucchi et al., 2006).

The forearc basement (Costa Rica Seismogenesis Project [CRISP] transect) southeast of the operation area of Legs 84, 170, and 205 is interpreted to be composed of a middle Eocene–middle Miocene mélange of oceanic lithologies accreted to the overriding plate (Vannucchi et al., 2006). The Osa Mélange, dominated by basalt, radiolarite, and limestone, is the most seaward unit exposed on land close to the CRISP transect. The nature and significance of the Osa Mélange remains a subject of debate. It has alternatively been interpreted as debris flows that were subsequently accreted to the margin (Buchs and Baumgartner, 2003), as a tectonic mélange produced by subduction erosion (Meschede et al., 1999), or as 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 evidence to suggest that the Osa Mélange reflects accretion from the currently subducting plate, and the evidence for ongoing tectonic erosion of the forearc is compelling. The Osa Mélange is, to our best knowledge, the unit that forms the forearc basement, which we could expect to drill as upper plate basement during CRISP. A major unknown is the nature of the high-amplitude landward-dipping reflectors cutting through the forearc basement (Fig. F2). The reflectors branch upward from the plate interface similarly to “splay faults” (Park et al., 2002). Our interpretation, though, suggests that these surfaces represent old faults related to a middle Eocene–middle Miocene accretionary event, now sealed by the slope apron sediment. Only a few of these faults have been reactivated as normal faults, as indicated by offsets at the top of the forearc basement, similar to observations offshore the Nicoya Peninsula and Quepos (McIntosh et al., 1993; Ranero and von Huene, 2000). Thus, the lack of a clear thrust sequence argues against the presence of out-of-sequence thrusts cutting the submarine portions of the forearc.

Seismic reflectors extending into the forearc basement have been interpreted as faults that are potential planes of weakness, which could play a role in focusing the flow of fluids drained from the deeper part of the margin, as suggested by the high reflectivity and high heat flux. However, the nature of permeability along these discontinuities is unknown. Identifying the nature and age of the landward-dipping reflectors is fundamental to understanding the tectonic history and the modern functioning of the margin offshore of the Osa Peninsula. The near-orthogonal subduction of bathymetrically rough oceanic lithosphere along the northern flank of the Cocos Ridge imprints a distinctive style of deformation on the overriding Costa Rica forearc. The CRISP drilling area has experienced the subduction of the Cocos Ridge, which has caused

• The extinction of the arc volcanism and uplift of the Talamanca Cordillera;

• 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

• The exhumation of the Late Cretaceous–early Eocene ophiolitic rocks cropping out along the Gulfo Dulce and the middle Eocene–middle Miocene Osa Mélange.

In correspondence to the onland projection of the Cocos Ridge axis, mesoscale fault populations and field mapping record active shortening related to the Fila Costeña fold and thrust belt (Fisher et al., 2004). Magnitudes of shortening decrease northwest and southeast of the onland projection of the Cocos Ridge axis. Locally, the fold and thrust belt accommodates at least 36 km of post–middle Pliocene shortening, which translates to a shortening rate of ~40 mm/y, or nearly 50% of the total plate convergence rate (Sitchler et al., 2007).

Across the northwest coast of the Osa Peninsula, uplifted late Quaternary marine deposits have been dated (Sak et al., 2004). These Quaternary deposits disconformably overlie exposures of semilithified late Tertiary and Quaternary sediments of the Charco Azul and Armuelles Formations and the Paleogene Osa Mélange (Sprechmann, 1984; Corrigan et al., 1990; Di Marco et al., 1995; Vannucchi et al., 2006). Exposures of late Paleocene deposits inboard of the axis of the subducting Cocos Ridge provide a detailed record of a complex history of vertical tectonism. Uplifted accumulations of fining-upward marine sands indicative of increasing water depth were deposited during an interval of eustatic sea level fall. This complex, yo-yo-like history of rapid syndepositional subsidence followed by rapid uplift observed across the northwestern Osa Peninsula may be related to the morphology of the underthrusting Cocos plate (Sak et al., 2004). The permanent strain recorded by uplift of these Quaternary surfaces exceeds the predicted rebound of stored elastic strain released during subduction-zone earthquakes.

Volcanic arc

Throughout the Tertiary, and especially during the Miocene, frequent volcanism in Central America produced very large eruptions of highly siliceous magma (Sigurdsson et al., 2000; Jordan et al., 2006). The majority of this magma formed the ignimbrites that extend from southern Mexico to southern Nicaragua, forming the Central American highlands (Sigurdsson et al., 2000). Some of the Miocene ignimbrites are present as far south as Costa Rica (Vogel et al., 2004, 2006). At ~8 Ma, the volcanic front in Nicaragua shifted, probably in response to a change of the subduction direction (Ehrenborg, 1996; Barckhausen et al., 2001; DeMets, 2001), to its present position. In contrast, the volcanic arc in Costa Rica and Guatemala has maintained a more or less stable position.

In Costa Rica, new 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) (Mora, 1979; Sutter, 1985; Gans et al., 2002), after which subduction-related calc-alkaline magmatism diminished. Although normal arc magmatism ceased in southern Costa Rica and western Panama from the late Miocene to Pliocene (i.e., ~6–3 Ma), backarc alkaline magmatism produced lava flows, dikes, and sills (Abratis and Wörner, 2001). Volumetrically insignificant Pliocene to Quaternary (5.8–2.0 Ma) volcanic rocks erupted just south of the central magmatic arc. These lavas 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; MacMillan et al., 2004).

One of the few younger large-magnitude eruptions of central Costa Rica is represented by the 322 ka Tiribí Tuff (Pérez et al., 2006). However, there are additional widespread Plinian fall deposits from Costa Rican volcanoes that can be identified in the marine sediment, although no onshore correlatives have been identified (Kutterolf et al., 2008).

Variations in the nature of the incoming plate, in crustal thickness and composition (von Huene et al., 1995; Barckhausen et al., 1998), and in the tectonic setting are accompanied by arc-parallel variations in the composition of the volcanic rocks (Carr, 1984; Feigenson and Carr, 1986; Carr et al., 1990, 2003, 2007; Patino et al., 1997, 2000; Hoernle et al., 2002; Feigenson et al., 2004) and the magnitudes of eruptions (Rose et al., 1999). Such compositional variations are very helpful when correlating volcanic depositions on land with marine ash beds (Kutterolf et al., 2008). Compositional differences like these probably also exist for the Neogene volcanism of Central America.

The arc volcanic rocks from Costa Rica have a composition that is similar to ocean-island basalt (OIB), resembling volcanic rocks found along the Galapagos hotspot (Reagan and Gill, 1989). The origin of this geochemical signature is discussed. Different models exist that favor the origin of this signature either by residual Galapagos-type mantle after formation of the large igneous province or flow of OIB-type astenospheric mantle: (1) through a slab window or (2) from the northwest margin of South America (Herrstrom et al., 1995; Abratis and Wörner, 2001; Feigenson et al., 2004). Others explain the OIB signature by subduction erosion of older Galapagos and CLIP terranes in the Costa Rican forearc (Goss and Kay, 2006) or by it being primarily derived from the subducting Galapagos hotspot track (Hoernle et al., 2008). Because the Galapagos hotspot tracks (and islands) are chemically zoned (Hoernle et al., 2000; Werner et al., 2003), radiogenic isotope ratios that are not modified by melting processes can be used to distinguish between magmas influenced by the seamount province and the Cocos and Coiba Ridges. These characteristics allow estimates of arc-parallel mantle flow rate in the wedge of 63–190 mm/y from Costa Rica to Nicaragua (Hoernle et al., 2008).

The Central American volcanic arc was a high-priority study area of the Subduction Factory initiative of the US MARGINS program. Along this arc, variations in subduction dynamics result in sharp differences in the apparent depth of sediment transport, 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 tephra stratigraphy preserved in the slope apron sediment offshore Osa, when compared to that already recovered offshore of the Nicoya Peninsula, will help in the along-strike reconstruction of the margin and will open a window into the processes linked to the volcanic arc shutdown.

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 long-term subsidence of the continental slope offshore of Nicoya Peninsula. Leg 170 provided direct evidence of shallow-water sedimentary rocks, which are now located in 3900 m water depth on the forearc and mark the slope apron–forearc basement unconformity, proving 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 sediments from Legs 84 and 170 indicates that the slow background subsidence of ~20 m/m.y. dramatically increased to ~600 m/m.y. starting at the Miocene/Pliocene boundary (Vannucchi et al., 2003). This acceleration in subsidence, linked to the arrival of the Cocos Ridge at the Middle America Trench (Vannucchi et al., 2003), is our best proxy for faster subduction erosion offshore the Osa Peninsula. Here, ridge subduction caused severe damage to the margin, as suggested by the disrupted topography (von Huene et al., 2000). The whole margin, in fact, has a broad concavity centered on the Cocos Ridge, reflecting the removal of material through ridge subduction. 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). The inner slope trench of Costa Rica is punctuated by subducted seamount tracks that reflect a net loss of material. There, particularly disrupted topography is present at the base of the slope and in the wake of seamounts.

The slope apron–forearc unconformity extends regionally across the igneous basement in northern Costa Rica and the middle Eocene–middle Miocene mélange in southern Costa Rica.

Offshore Nicaragua, subsidence driven by tectonic erosion triggered the development of the Sandino forearc basin (Ranero and von Huene, 2000; Ranero et al., 2000).

Fluids and volatiles in the forearc

Elevated methane concentrations in the bottom water have been observed along the entire Costa Rica margin, indicative of active fluid venting at the seafloor (Kahn et al., 1996; McAdoo et al., 1996; Bohrmann et al., 2002). Chemoautotrophic and methanotrophic communities mark cold seeps at numerous localities. These communities are particularly concentrated at the headwall scarps, where subducted seamounts have triggered fractures, slides, and slumps that break a low-permeability, shallow sediment carapace and allow ascending fluids to feed the cold-seep biota (Kahn et al., 1996; Bohrmann et al., 2002; Ranero et al., 2008). Mud volcanoes and mud diapirs have also been found offshore Costa Rica and Nicaragua, particularly across the middle slope, and are associated with a high density of chemosynthetic organisms. The chemistry of the pore fluids sampled at these midslope features is indicative of dehydration reactions at depth, suggesting they are associated with structures that enable effective transport of deep-sourced fluids (Shipley et al., 1992; Bohrmann et al., 2002; Grevemeyer et al., 2004; Hensen et al., 2004).

Coring and sampling during Leg 170 revealed freshened pore waters along the décollement and the upper fault zone in the prism, which also show 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, which have near-seawater chemistry (Kimura, Silver, Blum, et al., 1997; Morris, Villinger, Klaus, et al., 2003). The downhole temperatures measured during Legs 170 and 205 are insufficient to support in situ mineral dehydration and generation of thermogenic methane. Collectively, the geochemical data in the décollement offshore Nicoya Peninsula indicate that this flow system is active and that 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).

During Leg 205, two sealed borehole hydrologic observatories (CORKs) were installed offshore Nicoya Peninsula 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 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 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). The 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 were 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, at the Costa Rica subduction zone, slow slip events propagate through the seismogenic zone to the trench.

The importance of the hydrological activity in the subducting oceanic plate is just beginning to be appreciated (Silver et al., 2000; Fisher et al., 2003; Hutnak et al., 2008; Solomon et al., 2009; Harris et al., 2010a). Low heat flow values averaging ~30 mW/m² exist in the EPR-generated crust offshore the Nicoya Peninsula (see “Heat flow”) (Langseth and Silver, 1996; Fisher et al., 2003; Heesemann et al., 2006). 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 with recharge and discharge zones 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 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.

The continuous fluid pressure, temperature, and chemistry records obtained from the CORK at Site 1253 show that the pressure in uppermost igneous basement is ~6 kPa subhydrostatic (Davis and Villinger, 2006), indicating that it 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 of seawater (~50%) 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 offshore 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 will help clarify fluid sources and pathways in this segment of the Costa Rica margin.

Seismic reflection data

Seismic reflection images collected between Osa and the Cocos Ridge (Fig. F2) indicate a more stratified forearc basement and lower velocity material (~1 km/s) than equivalent areas along the Nicoya transect. The contact between the Osa Mélange and a separate forearc igneous basement is indicated in wide-angle seismic data, reflection data (Fig. F3), and magnetic modeling. Short-wavelength magnetic anomalies beneath the Osa continental shelf are interpreted as localized bodies of igneous rock mixed with sedimentary rocks (U. Barckhausen, unpubl. data). Dredged rock samples from the Cocos Ridge and related seamounts give ages of 13.0–14.5 Ma near the trench (Werner et al., 1999). This leaves a 45 m.y. gap in the geologic record between the Galapagos hotspot activity preserved in the Cocos Ridge and the CLIP (74–94 Ma). Rocks emplaced during this interval may be partially recorded in rock accreted beneath the Osa continental slope-forearc (Hoernle et al., 2002).

Heat flow

Offshore Costa Rica regional values of heat flow show marked changes along strike. Offshore the northern Nicoya Peninsula, heat flow values are anomalously low compared to global averages of similarly aged crust and predictions based on conductive cooling models. In contrast, regional heat flow values offshore the southern Nicoya Peninsula and to the south are scattered, but the mean is consistent with conductive cooling models. Two heat flow surveys (Ticoflux I and II) mapped the thermal transition between these crustal regions seaward of the trench and found that the thermal transition was quite sharp, indicative of a shallow source and consistent with more vigorous circulation. Locally, the location of the thermal transition zone is influenced by the presence of seamounts that act as sites of recharge and discharge (Fisher et al., 2003; Hutnak et al., 2006, 2007). Detailed profiles along the margin were made by the R/V Meteor Cruise 54-2 from Nicaragua south to southern Costa Rica (Harris et al., 2010a). These values, coupled with an earlier survey (Langseth and Silver, 1996) and with values from Leg 170 (Ruppel and Kinoshita, 2000), document the thermal structure of the incoming plate and margin. Additionally, bottom-simulating reflector (BSR) depths used as a proxy for heat flow extend the spatial coverage of seafloor measurements. Comparisons of collocated seafloor heat flow measurements and BSR-derived heat flow are in excellent agreement, adding confidence to the use of BSRs as a heat flow proxy in this area (Harris et al., 2010a). The combination of these data shows the profound effect of fluid flow through faults that cut the margin and within the upper oceanic basement as the plate subducts (Harris et al., 2010a, 2010b). Temperatures beneath the midslope drilling sites are estimated to range between 60° and 90°C.

Seismogenic zone and earthquakes

CRISP Program A is preparatory for the seismogenic zone experiment and will define the tectonic reference for deeper drilling. A full overview of the seismogenesis studies offshore the Osa Peninsula is provided in the CRISP Complex Drilling Project documents (iodp.tamu.edu/scienceops/precruise/costarica/537A-Full5_Vannucchi.pdf and iodp.tamu.edu/scienceops/precruise/costarica/537A-Add.pdf). Here we want to emphasize, using teleseismic waveform modeling, that the Mw 6.4 June 2002 underthrusting earthquake (including its aftershocks) (Fig. F4) occurred at a shallower depth (~9 km) (S.L. Bilek, pers. comm., 2003; I. Arroyo, pers. comm., 2009) than the 1999 earthquake event to the north. This may reflect along-strike variations in the updip extent of the seismogenic zone or its transitional nature.

GPS measurements on land indicate high stress over the subducted Cocos Ridge with most of the plate interface in the seismogenic region, which is essentially fully locked (Dixon, 2003). In contrast, seismic profiles show 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 sediments and are able to accommodate seafloor relief at the front of the margin without much deformation. Fluids draining from the subducting lower plate in Central America are sufficient to hydrofracture and mobilize about a 1–2 km thick and 20 km long section of the upper plate material every million years.

Site survey data

The regional framework of the Middle America Trench off Costa Rica is well known from investigations based on DSDP expeditions in the early 1980s (Aubouin, von Huene, et al., 1982; von Huene, Aubouin, et al., 1985) and subsequent drilling during ODP Legs 170 and 205 (Kimura, Silver, Blum, et al., 1997; Morris, Villinger, Klaus, et al., 2003). Recently, this region has been the focus area 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). The results are >10,000 km of seismic data acquisition, detailed seismological studies, and extensive bathymetric imaging (swath bathymetry; Weinrebe and Ranero, in GeoMapApp (geomapapp.org/) and MARGINS Data Portal (www.marine-geo.org/portals/margins/) (Fig. F5). The extensive multibeam bathymetric mapping started after the results from Cruise SO-76 of the German R/V Sonne, which showed varying seafloor morphology from offshore the Nicoya Peninsula to offshore the Osa Peninsula (von Huene et al., 1995). The multibeam bathymetry is complemented by several traverses with a deep-towed instrument. The towed ocean bottom instrument (TOBI) sidescan sonar system of the Southampton Oceanography Centre was used during Cruise 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 Cruise SO-144 in 1999, much of the continental margin from Costa Rica to southeast Nicaragua was imaged at a resolution of 10 m. Parts of that surveyed area were imaged at greater resolution using the GEOMAR 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).

Two permanent seismological networks have recorded seismicity in the area for the last three decades. Because offshore coverage is necessary to obtain high-quality locations for earthquakes originating at the seismogenic zone, 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 the 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, which recorded 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 and the Red Sismológica Nacional used Cruises SO-163, SO-173, and M-54 (Meteor) to deploy and recover two networks of OBS and land stations between the Nicoya and Osa Peninsulas, each running during a period of 6 months from April 2002 to March 2003 (Arroyo et al., 2009; Dinc et al., 2010). The first of these networks (April–October 2002) happened to record the Mw 6.4 main shock and ~400 aftershocks to the west of Osa Peninsula (Aden-Arroyo, 2008). The latter sequence surrounds the drilling area.

Geophysical data acquisition in the proposed Osa drilling area is extensive. Besides the already mentioned CRSEIZE transect (Newman et al., 2002; Norabuena et al., 2004), the proposed 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), which was acquired in 1995/1996 during the Trans Isthmus Costa Rica Scientific Exploration of a Crustal Transect (TICOSECT) project. TICOSECT 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. F6). More recently, from 2004 to 2006, the German project SFB 574 operated a transect of broadband seismological stations following the same orientation as TICOSECT (Dzierma et al., 2010).

During 1991 and 1992, the Sonne made two cruises (SO-76 and SO-81) that greatly expanded swath mapping, seismic reflection, and refraction coverage to ~250 km, extending from the Nicoya Peninsula to the southeast, where the crest of Cocos Ridge is subducting (Fig. F1). The interpretation of the seismic reflection data from Cruise SO-81 (Hinz et al., 1996) is complemented with data acquired in 1999. Two of the BGR99 records are processed in depth (Fig. F2) and the remainder in time domains. The principal site survey line is flanked on either side by two lines at 1 km spacing and then by lines at 2, 5, 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. Proposed sites have cross-lines of industry and academic heritage. Transducer and high-resolution sparker coverage are available. 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).

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