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doi:10.2204/iodp.sp.334.2010

Background

Subducting plate and the Cocos Ridge

The oceanic Cocos plate subducting beneath Costa Rica has been formed at two ridges: the East Pacific Rise (EPR) and the Cocos-Nazca Spreading Center (CNS), and it has been intruded by Galapagos hotspot volcanism. The Cocos Ridge (Fig. F1) was formed by the passage of the Cocos plate over the Galapagos hotspot. The ridge stands 2.5 km high and has crust of Galapagos-type geochemistry three times the normal oceanic thickness (i.e., 25 km) (Stavenhagen et al., 1998). Bordering the ridge to the northwest is regular CNS oceanic crust, 40% of which is covered by younger seamounts (Fig. F1) having Galapagos geochemistry. In this area, the seamounts impart a rough morphology to the Cocos plate. Further north, the EPR-generated crust has a smoother morphology. The area drilled during Deep Sea Drilling Project Leg 84 and Ocean Drilling Program (ODP) Legs 170 and 205 (Kimura, Silver, Blum, et al., 1997; Shipboard Scientific Party, 1985, 2003) lies just northwest of the EPR/CNS crustal boundary (Barckhausen et al., 2001). Sills with a Galapagos-type geochemistry drilled at ODP Sites 1039 and 1253 show the great lateral extent of hotspot magma intrusion.

The influence of Cocos Ridge subduction increases from the Nicoya Peninsula in the northwest to the Burica Peninsula to the southeast (~400 km; Fig. F1) and corresponds to the onset of 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 Miocene time (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 information 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 first formed. Several investigators have proposed a date of ~20–22 Ma, synchronous with the formation of the CNS (Lonsdale and Klitgord, 1978; van Andel et al., 1971).

Upper plate and onland geology

On the landward side of the Middle America Trench offshore the Osa Peninsula, the lower slope consists of a 10–12 km wide frontal prism (Fig. F3A). The same feature, 3–5 km wide, is also present offshore the Nicoya Peninsula, where it is composed of slope sediment redeposited into the trench and buttressed against a forearc basement that, although poorly sampled during Leg 170, is generally accepted to be composed of the same igneous rock exposed onshore (Kimura, Silver, Blum, et al., 1997; Ye et al., 1996). 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), as well as accreted ocean islands and aseismic ridge terranes (Hauff et al., 1997, 2000; Sinton et al., 1997). Crucially, there is no evidence that the forearc comprises a complex of tectonized sediments 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). Moving south from the area of operations for Legs 84, 170, and 205 and focusing on the CRISP transect, the forearc basement is interpreted to be composed of a middle Eocene–middle Miocene mélange of oceanic lithologies accreted to the overriding plate. The Osa Mélange, dominated by basalt, radiolarite, and limestone, is the most seaward unit found on land close to the CRISP transect. The nature and significance of the Osa Mélange is still under discussion, being variously interpreted as debris flows that were subsequently accreted to the margin (P.O. Baumgartner, pers. comm., 2002), 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 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 can 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. F3A). They 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, and with only a few of them reactivated as normal faults. Only a few of them have offsets at the top of the forearc basement into the slope apron, and these indicate normal faulting, as commonly occur offshore the Nicoya Peninsula and Quepos (McIntosh et al., 1993; Ranero and von Huene, 2000). The lack of a clear thrust sequence, instead, argues against the presence of out-of-sequence thrusts (OOSTs) cutting the forearc. The presence of such discontinuities into the forearc basement can offer preexisting planes of weakness and play a role in focusing fluid flow drained from the deeper part of the margin as suggested by the high reflectivity and the 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 of the margin offshore Osa Peninsula and its modern functioning.

Finally, the CRISP drilling area has experienced the subduction of the Cocos Ridge, which has caused (1) the extinction 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.

Volcanic arc

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 (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 is a high-priority study area of the Subduction Factory initiative of the U.S. 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 shut down 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). The interpretation is based on

• 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 on the forearc and marking the slope apron–forearc basement unconformity, and proved that the margin offshore Nicoya Peninsula has experienced a net loss of crust since ~16 Ma (Vannucchi et al., 2001). Detailed analysis on 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 subsidence acceleration, probably linked to the arrival of the Cocos Ridge at the subduction zone (Vannucchi et al., 2003), is our best proxy for faster subduction erosion 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 up to 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.

• 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

Active fluid venting indicated by elevated methane concentrations in the bottom water have been observed along the entire Costa Rican margin (Bohrmann et al., 2002; Kahn et al., 1996; McAdoo et al., 1996). 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 (Bohrmann et al., 2002; Kahn et al., 1996; Ranero et al., 2008). Mud volcanoes and mud diapirs have also been found, particularly across the middle slope. They are associated with a high density of chemosynthetic vents, indicating that they may be effective in transporting fluids in the overpressured slope sediments (Bohrmann et al., 2002; Shipley et al., 1992; Weinrebe and Flüh, 2002). Drilling will help clarify fluid sources and pathways. Sampling during Leg 170 revealed freshened pore waters containing thermogenic methane, propane, and heavier hydrocarbons along the décollement, which, along with freshwater from dissociated gas hydrate, was dispersed into the upper plate sediment (Kastner et al., 1997; Silver et al., 2000). These fluids contrast with water from below the décollement that has near-seawater salinity (Kimura, Silver, Blum, et al., 1997). Because downhole temperatures measured during Leg 170 are insufficient to support mineral dehydration and thermogenic methane, a lateral flow from depths of 15–20 km within the subduction system is implied (Kimura, Silver, Blum, et al., 1997). Offshore Nicoya Peninsula, measurements of diffuse fluid flow near the seafloor indicate complex circulation patterns attributed to the underlying structure of this margin (Tryon and Brown, 2000), although we recognize that much of the diffuse flow is likely from local, rather than deep, sources.

The importance of the hydrological activity in the subducting oceanic plate is just beginning to be appreciated (Silver et al., 2000), and because the Cocos Ridge upper crust is well layered and probably very porous (C.R. Ranero, pers. comm., 2003), the contribution from the lower plate to the fluid circulation could be significant in the drilling area.

Seismic reflection data

Seismic reflection images collected between Osa and the Cocos Ridge (Fig. F3A) show more stratified forearc basement and lower velocity material (~1 km/s less) than in equivalent areas along the Nicoya transect. Contact between the Osa Mélange and a separate forearc igneous basement is indicated in wide-angle seismic data, reflection data (Fig. F3B), 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 gap may be partially recorded in rock accreted beneath the Osa continental slope-forearc (Hoernle et al., 2002).

Heat flow

Two recent heat flow surveys (Ticoflux I and II) offshore Nicoya investigated in detail the thermal structure of the incoming plate seaward of the trench, whereas the R/V METEOR Cruise 54-2 concentrated on the upper plate from Nicaragua to southern Costa Rica. The heat flow values measured support the idea that the boundary between EPR- and CNS-generated crust is a major thermal boundary, not only seaward of the trench, but also below the upper plate (Kimura, Silver, Blum, et al., 1997; Langseth and Silver, 1996).

In the proposed drilling area offshore the Osa Peninsula, considerable thermal data can be inferred from 10 closely spaced seismic lines that show bottom-simulating reflectors (BSRs) and which are arranged along the drilling transect (Ranero et al., 2008). These BSR depths may be converted to temperature and compiled for analysis within a three-dimensional (3-D) model of the plate boundary using conductivity data from Leg 170 (Fig. F4). Uncertainties are introduced by the major thermal boundary between the Leg 170 area and the CRISP area, but nonetheless, estimated temperatures at the plate boundary beneath the proposed midslope site fall within the range 141°–200°C. Conductivity measurements to be made during Expedition 334 will allow improved temperature estimates at the plate boundary.

Seismogenic zone and earthquakes

CRISP Program A is preparatory to 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 document. Here we want to stress that from teleseismic waveform modeling the main shock of the June 2002 Mw 6.4 underthrusting earthquake and its aftershock sequence (Fig. F5), which occurred just to the south of the August 1999 event, appears to occur at ~9 km depth (S.L. Bilek, pers. comm., 2003), shallower than the 1999 earthquake sequence. This may reflect along-strike variations in the updip extent of the seismogenic zone or its transitional nature.

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

Site survey data

The supporting site survey data for Expedition 334 are archived at the IODP Site Survey Data Bank.

The regional framework of Central America Trench off Costa Rica is well known from investigations since Deep Sea Drilling Project 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, it 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 U.S. MARGINS National Science Foundation program (www.nsf-margins.org/SEIZE/CR-N/CostaRica.html). The results are more than 10,000 km of seismic data acquisition and extensive bathymetric imaging (swath bathymetry: Weinrebe and Ranero, in GeoMapApp and MARGINS Data Portal) (Fig. F6). The extensive multibeam bathymetric mapping started after the results from SO-76 of the German R/V Sonne, which showed a 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 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 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).

A local network of stations on land has recorded seismicity in the area for 2 decades. Several marine seismological network 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 more than 9 months (i.e., from the beginning of October 2002 [METEOR cruise M54-3B] to August 2003 [Sonne cruise SO173-1]) (Flüh et al., 2004). SO 173-1 also deployed another 2 months of OBS offshore in 2002. They recorded the Mw 6.4 main shock and aftershock sequence northwest of the Osa Peninsula (I. Arroyo et al., unpubl. data). The latter sequence surrounds the drilling scheduled in 2011.

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. F3B) 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). During 1991 and 1992, the German research vessel Sonne made two cruises (SO-76 and SO-81) that greatly expanded swath mapping, seismic reflection, and refraction coverage from the area off the Nicoya Peninsula for ~250 km to the southeast where the crest of Cocos Ridge is subducted (Fig. F1). The interpretation of the seismic reflection data from SO-81 (Hinz et al., 1996) complement those acquired in 1999. Two of BGR99 records are processed in the depth (Fig. F3A) and the remainder in time domains. The principal site survey line is flanked on either side by 2 lines at 1 km spacing, then by lines at 2 km, 5 km, and 10 km spacing (Fig. F2). 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 is available. Conventional heat probe transects were acquired regionally and along the primary transect, which calibrate BSR-derived heat flow from the seismic records (Ranero et al., 2008; R. Harris et al., unpubl. data). 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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