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doi:10.2204/iodp.proc.344.101.2013

Background

Seismogenesis at convergent margins

Convergent margins produce the world’s largest earthquakes and devastating tsunamis, as most recently and tragically demonstrated by the 2011 Tohoku earthquake. Studies of interplate seismicity have shown that large earthquakes generally initiate within specific depth limits, generally between ~10 and 40 km (Byrne et al., 1988; Pacheco et al., 1993; Tichelaar and Ruff, 1993; Scholz, 2002), but the precise limits vary with temperature (Hyndman et al., 1997). This observation led to a conceptual model in which the subduction thrust is divided into three zones (e.g., Scholz, 1988). In the shallowest zone, plate convergence is accommodated by stable (aseismic) slip. Here, fault material is fresh, unconsolidated, and characterized by velocity strengthening. The middle section, where large earthquakes initiate, is characterized by unstable (stick-slip) behavior and is termed the seismogenic zone (Scholz, 2002). Fault zone rocks in this region have transitioned from velocity-strengthening to velocity-weakening friction, and shear becomes localized. The onset of seismogenic behavior is correlated with the intersection of the 100°–150°C isotherm and the subduction thrust (Hyndman et al., 1997; Oleskevich et al., 1999). With increasing depth down the subduction thrust, the frictional characteristics undergo a second transition either due to the juxtaposition with the forearc mantle or because the rocks are heated to 350°–450°C and can no longer store elastic stresses needed for rupture. Transitional regions between the three zones have conditional stability and can host rupture but are generally not thought to be regions where large earthquakes initiate.

Although this three-zone two-dimensional view of the subduction thrust provides a reasonable framework, it is simplistic. Rupture models for large subduction earthquakes suggest significant fault plane heterogeneity in slip and moment release that in three dimensions is characterized as patchiness (Bilek and Lay, 2002). Additionally, we now know the transition zone from stable to unstable sliding is not simple but hosts a range of fault behaviors that includes creep events, strain transients, slow and silent earthquakes, and low-frequency earthquakes (Peng and Gomberg, 2010; Beroza and Ide, 2011; Ide, 2012).

Fundamentally unknown are the processes that change fault behavior from stable sliding to stick-slip behavior. Understanding these processes is important for understanding earthquakes, the mechanics of slip, and rupture dynamics. For a fault to undergo unstable slip, fault rocks must have the ability to store elastic strain, be velocity weakening, and have sufficient stiffness. Hypotheses for mechanisms leading to the transition between stable and unstable slip invoke temperature, pressure, and strain-activated processes that lead to downdip changes in the mechanical properties of rocks. These transitions are also sensitive to fault zone composition, lithology, fabric, and fluid pressures.

The composition of the material in the fault zone and its contrast with the surrounding wall rock play a key role in rock frictional behavior. The frictional state of the incoming sediment changes progressively with increasing temperature and pressure as it travels downdip. Important lithologic factors influencing friction are composition, fabric, texture, and cementation of rocks, as well as fluid pore pressure (Bernabé et al., 1992; Moore and Saffer, 2001; Beeler, 2007; Marone and Saffer, 2007; Collettini et al., 2009). For example, fault rocks with high phyllosilicate content are generally weaker than rocks with low phyllosilicate content (Ikari et al., 2011). Sediment properties including porosity, permeability, consolidation state, and alteration history also exert a strong influence on fault zone behavior. At erosive margins, where the plate boundary cuts into the overriding plate, the composition and strength of the upper plate is also important (McCaffrey, 1993).

Field observations and laboratory experiments suggest that a prerequisite for unstable sliding is that the incoming sediment is consolidated and lithified (Davis et al., 1983; Byrne et al., 1988; Marone and Scholz, 1988; Marone and Saffer, 2007; Fagereng and Sibson, 2010). This hypothesis posits that the updip limit of the seismogenic zone occurs near a threshold of consolidation and/or lithification in which fault zone behavior changes from distributed shear (stable sliding), where shear is accommodated by granular processes, to localized shear (unstable sliding) along discrete surfaces. Among other factors, the processes of lithification and consolidation are influenced by fault slip rate, mineralogy, and heat flow.

Pore pressure within the fault zone is a principal factor controlling effective stress and strength along the plate interface (Sibson, 1981; Scholz, 1988). Pore pressures in excess of hydrostatic pressures decrease the effective normal stress, thereby limiting the shear stress, and, coupled with the low consolidation and lithification state of sediment in the updip aseismic portion of the fault, may explain the lack of seismicity. A downdip decrease in pore fluid pressure increases the normal stress, leading to greater instability in plate sliding (Scholz, 1988).

Diagenetic and low-grade metamorphic reactions likely contribute to both lithification and consolidation processes, as well as changes in pore pressure (Moore and Saffer, 2001). These reactions lead to clay transformation, carbonate and zeolite cementation, the initiation of pressure solution, and quartz cementation. These processes lead to an increase in lithification, a decrease in fluid production that increases the effective stress, and strengthening of the overriding plate, which increases its ability to store elastic strain.

Variation in subducting plate relief has also been invoked as a cause for the change in the effective stress, the mechanics of the plate boundary, and the frictional stability of the fault zone (e.g., Scholz and Small, 1997). Detailed bathymetric and seismic data collection in several subduction zones has led to correlations between large earthquake rupture patterns and subducting features such as seamounts and ridges (Bilek et al., 2003; Bilek, 2007). Subducting oceanic ridges and seamounts have been suggested as asperities that can be the loci of earthquake nucleation (e.g., Lay and Kanamori, 1981; Lay et al., 1982; Ruff, 1992) or as barriers to along-strike earthquake rupture (Kodaira et al., 2000; Hirata et al., 2003; Wang and Bilek, 2011). In fact, all of these processes may act in concert to change the frictional properties of the fault zone. Pore fluid pressure, fault rock mineralogy, diagenesis, consolidation, lithification, and subducting plate relief may all play an important role.

Chief among the many reasons that the active seismogenic zone is poorly understood is that it has been inaccessible to geologic sampling. Testing these hypotheses requires obtaining samples of fault zone material in the updip aseismic zone, in the transition zone, and within the seismogenic zone. Advances in drilling technology and the capabilities of the Japanese drilling vessel Chikyu make drilling into the transition and seismogenic zones possible.

Erosive and accretionary convergent margins

Fault behavior needs to be understood in the context of regional tectonics. Convergent margins may be divided into two end-member types that are termed erosive and accretionary plate boundaries (e.g., von Huene and Scholl, 1991; Clift and Vannucchi, 2004). The fundamental difference between these styles of subduction is the transfer of material between the overriding and downgoing plates. At accretionary margins, material above the décollement is transferred from the downgoing plate to the upper plate through either frontal accretion or underplating at the base of the forearc accretionary prism, leading to a net growth of the margin. Accretion rates vary between ~7 and 29 km³/m.y. (Clift and Vannucchi, 2004). At erosive margins, all incoming sediment is subducted and the upper plate is eroded at its front and base such that the upper surface of the margin subsides and retreats (Lallemand et al., 1994). Basal erosion rates of 25–50 km³/m.y. per kilometer of trench length are estimated for northeast Japan, northern Chile and Peru, Ecuador, Costa Rica, and Alaska (Scholl et al., 1980; von Huene and Lallemand, 1990; von Huene and Scholl, 1991; Ranero and von Huene, 2000; Collot et al., 2004; Clift and Vannucchi, 2004). The process by which subduction erosion occurs is unclear.

Catalogs of convergent margins (Clift and Vannucchi, 2004; Heuret et al., 2011) indicate subduction erosion is favored in regions where the incoming sediment thickness is <1 km and convergence rates exceed 60 mm/y. Conversely, subduction accretion is favored in areas where the incoming sediment thickness is >1 km and convergence rates are <60 mm/y. It is important to note that these are end-member models and in reality margins likely transition back and forth along strike and in time with phases of accretion, nonaccretion, and erosion.

One reason it is important to understand the contrasting behavior of erosive and accretionary margins is that the style of seismicity associated with each of these margins may differ. Notwithstanding the short instrumental record, Bilek (2010) suggests tsunami earthquakes are more likely to be generated at erosive margins. These events arise from slip in the shallowest portion of the subduction zone that produce large tsunamis relative to their seismic moment (Kanamori, 1972; Kanamori and Kikuchi, 1993; Satake and Tanioka, 1999; Polet and Kanamori, 2000; Abercrombie et al., 2001; Bilek and Lay, 2002; Ammon et al., 2006). These earthquakes generate large shallow slip on the subduction thrust that uplifts the seafloor, displacing the overlying water. In contrast, the largest events (M > 9) are more likely to be generated at accretionary margins (Bilek, 2010). This observation suggests the difference in seismic style between subduction accretion and erosion may be systematically different.

The Cocos Ridge and subducting Cocos 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). Here, the Middle America Trench exhibits prominent variability that includes changes in the incoming plate age, convergence rate and obliquity, morphology, and slab dip. The age of the Cocos plate at the Middle America Trench decreases from 24 Ma offshore the Nicoya Peninsula to 14 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°, to nearly orthogonal convergence southeast of the Nicoya Peninsula (DeMets, 2001; Turner et al., 2007). The variations in plate age, convergence rate, and slab dip influence the thermal structure of the margin (Harris et al., 2010b), and the variation in the convergent 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 varies substantially between the Nicoya and Osa Peninsulas (Fig. F1). This variation is a function of the plate’s 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” (Barckhausen et al., 2001) (Fig. F1). EPR-generated crust is north of the plate suture and has a generally smoother morphology. CNS-generated crust is south of the plate suture and has a generally rougher morphology.

After the Cocos plate formed, it was intruded by Galapagos magmatism that generated the aseismic Cocos Ridge and many seamounts that together contribute to large variations in bathymetric relief. The Cocos Ridge is an overthickened welt of oceanic crust that stands ~2.5 km higher than the surrounding seafloor and has a maximum thickness of ~25 km, which is greater than three times normal ocean crust thickness (Walther, 2003). Northwest of the Cocos Ridge, seamounts 1–2 km high and 10–20 km in diameter cover ~40% of the seafloor (von Huene et al., 1995, 2000). The subduction of the Cocos Ridge and seamounts has caused substantial subduction erosion of the outer forearc (Ranero and von Huene, 2000). Additionally, the overthickened Cocos Ridge is more buoyant than normal oceanic crust and has uplifted the Osa Peninsula and the Quepos region (Gardner et al., 1992, 2001; Fisher et al., 1998; Sak et al., 2004). Cocos Ridge subduction is also manifested in changes to the dip and depth of the Wadati-Benioff Zone. The seismically active slab dips ~65° near the Nicaraguan border and shallows by a few degrees inboard of the Cocos Ridge. At depths below 60 km there is no seismically defined slab landward of the Cocos Ridge (Vergara Muñoz, 1988; Protti et al., 1994).

Upper plate and subaerial geology

Active subduction erosion from Guatemala to Costa Rica is indicated by drilling, seismic, and bathymetric data (Ranero and von Huene, 2000; Ranero et al., 2000; Vannucchi et al., 2001, 2003, 2004). Disrupted bathymetry at the base of the margin slope indicates frontal erosion (Fig. F1). These embayments are thought to mark the location where seamounts or other elevated bathymetric features have tunneled into the margin and indicate a net loss of material. Offshore the Osa Peninsula, the margin has a broad concavity centered on the Cocos Ridge, testifying to the removal of material through ridge subduction.

Basal erosion is suggested by shallow-water sedimentary rocks recovered at deep-water depths that lie unconformably on upper plate framework rock (Vannucchi et al., 2001). Drilling data offshore the Nicoya Peninsula suggest the margin has undergone basal erosion and subsidence since ~16 Ma (Vannucchi et al., 2001). Benthic fauna preserved in the slope apron sediment coupled with changes in facies are interpreted to suggest that a slow background subsidence of ~20 m/m.y. radically increased to ~600 m/m.y. initiating at the Miocene/Pliocene boundary at 2.4 Ma (Vannucchi et al., 2003). The short-term rate of removal of rock from the forearc is estimated to be 107–123 km³/m.y. per kilometer of trench. This acceleration in subsidence may be linked to the arrival of the Cocos Ridge at the subduction zone (Vannucchi et al., 2003).

Seismic data along the margin show that it is composed of a thick slope sediment apron, a few hundred meters to ~2 km thick, unconformably overlying upper plate framework material (von Huene et al., 2000). In the Costa Rica Seismogenesis Project (CRISP) drilling area, the upper plate is underthrust by sediment and buttressed by a small frontal prism (Fig. F2). The frontal prism offshore the Osa Peninsula is ~6 km wide, a similar width to the frontal prism offshore the Nicoya Peninsula.

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. Geologic and GPS data reveal uplift and shortening is at a maximum directly inboard of the Cocos Ridge. 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.

Because of its influence on the upper plate, an outstanding issue for the tectonics of the region is the timing of the Cocos Ridge impinging on the Middle America Trench. Estimates range between ~1 Ma (Hey, 1977; Lonsdale and Klitgord, 1978), ~5 Ma (Sutter, 1985), and ~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 Terraba forearc basin directly inboard of the Cocos Ridge (Kolarsky et al., 1995) raise concerns about this model.

The lithology of the upper plate framework rock is not known. One hypothesis is that the upper plate basement is composed of a mélange of oceanic lithologies accreted to the overriding plate prior to the current phase of subduction erosion (Fig. F2). This hypothesis posits that the basement is the offshore extension of the Caribbean Large Igneous Province (CLIP) and the Quepos and Osa terranes that crop out onshore (Ye et al., 1996; Kimura, Silver, Blum, et al., 1997; Vannucchi et al., 2001). The CLIP is composed of accreted oceanic 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 as accreted material derived 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). Alternatively, upper plate basement may be composed of paleoaccretionary prism material. This hypothesis is consistent with the relatively low seismic velocities (3.5 km/s) observed below the erosional unconformity (Fig. F2B).

Within the upper plate basement, a major unknown is the nature of the high-amplitude landward-dipping reflectors cutting through the forearc (Fig. F2). These reflectors branch upward from the plate interface similar to “splay faults” in Nankai Trough (Park et al., 2002). These surfaces may represent old faults, related to a middle Eocene–middle Miocene accretionary event, now covered by the slope apron sediment. A few of them 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). These discontinuities across the forearc basement can offer preexisting planes of weakness and may play a role in focusing fluid flow drained from the deeper part of the margin. 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.

Seismogenic zone and earthquakes

Costa Rica is a seismically active area with a history of Mw > 7 earthquakes and along-strike changes in the seismogenic zone (Protti et al., 1995; Newman et al., 2002). Offshore of the Osa Peninsula, the 20 August 1999 Mw 6.9 Quepos interplate thrust earthquake (Fig. F3) generated aftershocks recorded by a network of ocean-bottom seismometers (OBSs) and land seismometers deployed during the Costa Rica Seismogenic Zone Experiment (CRSEIZE) (DeShon et al., 2003). The 1999 Quepos earthquake initiated at the downdip extension of the incoming Quepos Plateau at a depth of 21 ± 4 km. This area coincides with the southern edge of the Costa Rica margin seamount province and bathymetrically elevated crust. These observations led Bilek et al. (2003) to suggest that this event represented rupture of topographic highs that act as asperities (Lay and Kanamori, 1980; Lay et al., 1982). Offshore the Osa Peninsula, earthquake hypocenters and aftershock clustering are located in regions of seismically imaged lower plate relief (Bilek et al., 2003; Husen et al., 2002; Protti et al., 1995).

Aftershocks to the 1999 Quepos earthquake delineate a plate dipping at 19° and are interpreted to mark the interface between the Cocos and Caribbean plates (DeShon et al., 2003). Most of the aftershocks occurred between 10 and 30 km depth and at a distance 30 to 95 km landward of the trench. DeShon et al. (2003) found that the sequence of aftershocks appeared to be influenced by the morphology of the downgoing plate, consistent with the suggestion of Bilek et al. (2003). The depth distribution of aftershocks yielded updip and downdip seismicity limits at depths of ~10 and 30 km below sea level (DeShon et al., 2003).

More recently, a 16 June 2002 Mw 6.4 event and its aftershocks were recorded by a temporary network of ocean-bottom hydrophones (OBHs) (Arroyo et al., 2011). This event nucleated 40 km west of the Osa Peninsula. Preliminary analysis shows this event to be a shallow underthrusting event that likely occurred on the plate boundary.

GPS measurements on land indicate interseismic strain accumulation on the plate interface and highlight the important influence that the subducting Cocos Ridge has on the tectonics of the area (Norabuena et al., 2004; LaFemina et al., 2009). Regional surface velocity data are consistent with the Cocos Ridge acting as a rigid indenter that leads to arc-parallel forearc motion (LaFemina et al., 2009).

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. F4). Regional heat flow measurements on the incoming Cocos plate reveal a large along-strike variation in heat flow (Von Herzen and Uyeda, 1963; Vacquier et al., 1967). EPR-generated crust of the Cocos plate has low heat flow values 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 ~30 and 15 mW/m², respectively. Conductive predictions for seafloor of ages between 24 and 14 Ma 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, respectively.

In 2000 and 2001, heat flow studies specifically designed to investigate the nature of the thermal transition between 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 generally corresponds to the area of the plate suture (Fig. F4). 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 bottom-simulating reflector (BSR) estimates of heat flow on the margin were analyzed 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 the low and high heat flow generally correlates with the extension of the plate suture but is also influenced by the proximity of seamounts (Fisher et al., 2003) and appears relatively abrupt. High wavenumber variations in the heat flow data are interpreted as focused fluid flow through the margin near the deformation front (Harris et al., 2010a, 2010b).

Volatiles and fluids

Fluid flow pathways through the Costa Rica margin include the upper plate basement and overlying sediment, 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., 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; Sahling et al., 2008). Chemoautotrophic and methanotrophic communities mark cold vents at numerous localities, but higher community abundances 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. These communities are particularly concentrated along headwall scarps (Kahn et al., 1996; Bohrmann et al., 2002; Sahling 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 midslope features is indicative of clay dehydration reactions at depth (Shipley et al., 1992; Bohrmann et al., 2002; Grevemeyer et al., 2004; Hensen et al., 2004; Sahling et al., 2008).

Along the décollement and the upper prism fault zone, coring and sampling during Ocean Drilling Program (ODP) Legs 170 and 205 offshore the Nicoya Peninsula 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; Chan and Kastner, 2000; Morris, Villinger, Klaus, et al., 2003; Kastner et al., 2006). 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 too low to support in situ mineral dehydration and thermogenic hydrocarbon generation. Collectively, the geochemical data in the décollement offshore Nicoya Peninsula indicate an active fluid flow system with a fraction of the flow 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 et al., 2006; Kastner et al., 2006; Harris et al., 2010b).

The magnitude of the hydrological activity in a subducting oceanic plate setting is just beginning to be appreciated (Silver et al., 2000; Fisher et al., 2003; Hutnak et al., 2008; Spinelli and Wang, 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) and indicate effective hydrothermal cooling of the upper oceanic crust. Seaward of the trench, recharge and discharge occur through exposed basement and seamounts (Fisher et al., 2003). This inference is corroborated by pore fluid chemical and isotopic profiles in basal sediment fluids 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.

Site survey data

The Central American margin offshore Costa Rica is one of the best studied subduction zones, with a wide spectrum of data from seismicity, land geology, volcanic petrology, geodesy, seismic imaging, submersible dives, and five deep-sea drilling and long-term monitoring cruises (Deep Sea Drilling Project Leg 84, ODP Legs 170 and 205, and Integrated Ocean Drilling Program (IODP) Expeditions 301T and 334). 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 U.S. MARGINS National Science Foundation program (www.nsf-margins.org/​SEIZE/​CR-N/​CostaRica.html). The supporting site survey data for IODP Expedition 344 are archived at the IODP Site Survey Data Bank.

More than 10,000 km of bathymetric imaging has been acquired (GeoMapApp and MARGINS Data Portal) (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 Helmholtz-Center for Ocean Research Kiel (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 authigenic precipitation at sites 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 over the last two decades (Figs. F3, F5). Several marine seismological networks of OBSs and OBHs have been deployed offshore Costa Rica. CRSEIZE, run by University of California Santa Cruz (USA), University of California San Diego (USA), Observatorio Vulcanologico y Sismologico de Costa Rica, and University of Miami (USA), 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., 2003). CRSEIZE also included GPS campaigns across Costa Rica (Norabuena et al., 2004; LaFemina et al., 2009). German SFB 574 provided infrastructure for deploying OBS and land stations for >9 months (October 2002–August 2003) (Fig. F3) (Flüh et al., 2004).

Geophysical data acquisition in the Osa drilling area is extensive. The CRISP drilling sites are positioned on an OBS/OBH seismic refraction transect that crossed the onshore and offshore region of Costa Rica (Ye et al., 1996; Stavenhagen et al., 1998) (Fig. F5). 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. The drilling sites have crosslines of industry and academic heritage. Transducer and high-resolution sparker coverage is available. Conventional heat probe transects that calibrate BSR-derived heat flow from the seismic records were acquired regionally and along the primary transect (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 offshore the Osa Peninsula are substantial (Fig. F6), mainly coming from R/V Sonne Cruises 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). This combination of data greatly expanded swath mapping, seismic reflection, and refraction coverage from the area off the Nicoya Peninsula for ~250 km southeast where the crest of the Cocos Ridge is subducted. 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.

The seismic reflection data from Cruise SO-81 (Hinz et al., 1996) complement those acquired during Cruise BGR99. Cruise BGR99 records are processed in depth and the remainder in the time domain. The principal site survey line is flanked on either side by two lines at 1 km spacing, 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. Seismic reflection images collected between the Osa Peninsula and the Cocos Ridge (Fig. F6) show more stratified forearc basement and lower velocity material (by ~1 km/s) than in equivalent areas along the Nicoya transect.

In April and May of 2011, a 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 (Fig. F6). The goal of the 3-D seismic survey was to image the crustal structure and the plate-boundary fault from the trench into the seismogenic zone. The 3-D survey covers a distance of 55 km across the upper shelf, slope, and trench and extends 11 km along strike for a total survey area of 605 km². These data were acquired with the R/V Langseth using a 3300 inch³ source shot every 50 m. Data were recorded by four 6 km long, 468-channel streamers with 150 m separation. When the Expedition 344 Scientific Prospectus was written (Harris et al., 2012), only preliminary results from processing 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

Heat flow data along seismic Line BGR99-7 (Fig. F5) are measured with marine probes and estimated from the depth of the BSR (Harris et al., 2010a). In general, probe and BSR values of heat flow agree (Fig. F7). Near the deformation front, high wavenumber variability indicative of fluid flow is observed. A simple analytical calculation suggests fluid flow rates of 7 to 15 mm/y. The high wavenumber variability is superimposed on a trend of decreasing landward heat flow consistent with the downward advection of heat with the subducting plate. Two-dimensional finite element thermal models of subduction were used to understand the heat flow data (Harris et al., 2010b). This analysis indicates that models using a conductive geotherm based on a plate model of lithospheric cooling produce values systematically lower than the heat flow data. Instead, the data are most consistent with a thermal model incorporating fluid flow within the upper oceanic crust of the subducting plate. These models predict much lower temperatures along the subduction thrust than the depth-extrapolated surface values used by Ranero et al. (2008).

Preliminary results from Expedition 334 (CRISP-A1)

The primary goal of Expedition 334 was to characterize the shallow upper plate at two sites along the BGR99-7 seismic reflection line (Fig. F2). Sites U1378 on the midslope and U1379 on the upper slope were characterized with logging-while-drilling (LWD) data and later cored, allowing samples of fluids and sediment to be collected. A third midslope location (Site U1380) was attempted but abandoned because of poor drilling conditions. In addition, the sediment and basement were cored on the incoming plate (Site U1381). These sites are briefly reviewed starting at the input site and continuing landward. Additional details are given in the Expedition 334 Proceedings volume (Vannucchi, Ujiie, Stroncik, Malinverno, and the Expedition 334 Scientists, 2012).

Site U1381 reveals the characteristics of the incoming sedimentary section and upper basement and serves as a reference site. This site is on the incoming Cocos plate, located ~6 km seaward of the trench on a local basement high. The primary objective at the site was to core basement material, and because of time constraints the entire sediment section was drilled using the rotary core barrel (RCB). Expedition 344 recored the sediment using the advanced piston corer (APC).

Midslope Site U1380 is 14.5 km landward of the trench (Fig. F2). Earthquake relocations (DeShon et al., 2003) and geodetic data (LaFemina et al., 2009) suggest this site is seaward of the locked portion of the plate interface. The site was cored between ~395 and 480 meters below seafloor (mbsf) during Expedition 334 as a contingency to Site U1378, which was drilled to ~524 mbsf (Expedition 334 Scientists, 2012a). During Expedition 334, both Sites U1380 and U1378 (1 km seaward) were abandoned before science goals were achieved, and a fundamental reason for returning to Site U1380 was to attain the science goals.

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