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1. Introduction1 Kimura, G., Silver, E.A., Blum, P., et al., 1997 Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 170 1. INTRODUCTION1 Shipboard Scientific Party2 The planet is profoundly affected by the distributions and rates of al., 1990), which extends from the Caribbean coast of Colombia to materials that enter subduction zones. The material that is accreted to Limon, Costa Rica. In Costa Rica, the boundary consists of a diffuse the upper plate results in growth of the continental mass, and fluids left lateral shear zone from Limon to the Middle America Trench squeezed out of this accreted mass have significance for biologic and (Ponce and Case, 1987; Jacob and Pacheco, 1991; Guendel and geochemical processes occurring on the margins. Material that is not Pacheco, 1992; Goes et al., 1993; Fan et al., 1993; Marshall et al., accreted or underplated to the margin bypasses surface residency and 1993; Fisher et al., 1994; Protti and Schwartz, 1994). The north- descends into the mantle, chemically affecting both the mantle and trending boundary between the Cocos and Nazca Plates is the right- magmas generated therefrom. Subduction of the igneous ocean crust lateral Panama fracture zone. West of this fracture zone is the Cocos returns rocks to the mantle that earlier had been fractionated from it Ridge, a trace of the Galapagos Hotspot, which subducts beneath the in spreading centers, along with products of chemical alteration that Costa Rican segment of the Panama Block. occur on the seafloor. Sediments that are subducted include biogenic, The Nicoya Peninsula is composed of Late Jurassic to Late Creta- volcanogenic, authigenic, and terrigenous debris. These sources pro- ceous ophiolitic rocks, and Late Cretaceous and younger sedimentary vide characteristic geochemical signatures that may be discerned in rocks (Fig. 2). The older magmatic sequence has a mid-ocean-ridge the erupting arc magmas and that may alter the composition and pos- basalt (MORB) chemistry (Meschede and Frisch, 1994), suggesting sibly the behavior of the eruptions. The partitioning between accreted an origin as oceanic crust. The younger magmatic sequence has a and subducted sedimentary materials, along with the roughness of the mixture of arc and ocean-island chemistry, which Meschede and subducting plate, may fundamentally affect the earthquake rupture Frisch (1994) interpret as part of a regional Cretaceous sill event that process and tsunami generation. It is thus of primary importance to affected the whole Caribbean region. Sedimentary rocks overlying understand the nature of the partitioning between accreted and sub- the Nicoya Complex are thin and include radiolarian cherts, black ducted sediments. shales, pelagic limestones, sedimentary breccias, and minor sand- The Costa Rica Margin presents an excellent opportunity to study stones. Abundant ash beds occur in the early part of the section. Sev- this partitioning process. Many subduction environments suffer from eral kilometers of age-equivalent strata on the northeast side of the rapid fluctuations in the delivery of terrigenous debris to the trenches peninsula include volcaniclastic turbidites that probably accumulated because of Pleistocene sea-level changes. Such fluctuations make it in a forearc basin (Lundberg, 1983). Overall, the strata overlying the difficult to extrapolate the volume of incoming material back in time. Nicoya Complex indicates shallowing through time at irregular rates, Costa Rica and wide regions of the Middle America Trench have lit- and uplift continuing into the Holocene (Gardner et al., 1992). tle or no turbidites, and the incoming sedimentary sections on the Co- cos Plate are relatively constant. In addition, the Costa Rica Margin Seismicity has been exceptionally well imaged with two-dimensional (2-D) and three-dimensional (3-D) seismic reflection data and recently with Costa Rica is seismically very active. Shallow seismic events (z < wide-angle seismic refraction data. Intensive studies of the arc volca- 40 km) occur (1) associated with the subduction of the Cocos Plate noes have been carried out and are continuing. Detailed submersible, under the Caribbean Plate and Panama Block; (2) along the Panama heat-flow, and bathymetric observations have recently added to our fracture zone; (3) as intraplate activity within the Cocos and Caribbe- store of knowledge of this margin. an Plates and the Panama Block; (4) as interplate activity between the Caribbean Plate and Panama Block, both along the North Panama Deformed Belt as well as along the central Costa Rica shear zone; and REGIONAL SETTING (5) associated with the volcanic arc (Protti et al., 1995). Intermediate- depth earthquakes (40−220 km) occur as internal deformation of the Costa Rica lies at the southern end of the Central American island subducted portion of the Cocos Plate (Protti et al., 1995). Since April arc system, formed by subduction of the Cocos Plate on the Pacific 1984, the Costa Rica Volcanological and Seismological Observatory Margin (Fig. lA). The surface manifestation of this subduction—the at the National University (OVSICORI-UNA), has been recording Middle America Trench—has its southern end off southernmost Cos- activity from all these seismic sources and has located, in just a de- ta Rica, where it intersects the Panama fracture zone. The regional cade, over 20,000 earthquakes. tectonic setting of southeastern Central America is controlled by the The intermediate-depth seismic activity under Costa Rica reveals convergence of the Cocos, Caribbean, and South American Plates. a tear on the subducted Cocos Plate under central Costa Rica (the The oceanic Cocos Plate subducts beneath the Caribbean Plate along Quesada Sharp Contortion [Protti et al., l995]), which is recognizable the Middle American Trench at rates ranging from 70 mm/yr off below 70 km (Fig. lB). To the northwest the Cocos Plate dips 80°, and Guatemala to nearly 90 mm/yr off southern Costa Rica (Fig. lB). The seismicity attains depths of 220 km beneath the Nicaragua border to southeastern part of Central America is called the Panama Block, 135 km at the tear. To the southeast the Cocos Plate dips 60°, and which includes all of Panama and southeast Costa Rica. Its northern seismicity does not exceed 125 km (Protti et al., 1995). The projec- boundary is the convergent North Panama Deformed Belt (Silver et tion of the tear to the trench coincides with an abrupt change in the bathymetry of the Cocos Plate along a northeast alignment (the rough/smooth boundary of Hey [1977]). This boundary projects to 1Kimura, G., Silver, E., Blum, P., et al., 1997. Proc. ODP, Init. Repts., 170: College the entrance of the Nicoya Gulf just southeast from Ocean Drilling Station, TX (Ocean Drilling Program). Program (ODP) Site 1039. This bathymetric change marks the 2 Shipboard Scientific Party is given in the list preceding the Table of Contents. boundary between lithosphere of the Cocos Plate created along the 7 Acknowledgments Table of Contents Next Chapter SHIPBOARD SCIENTIFIC PARTY A 16 No 5 Middle America CARIBBEAN PLATE Tehuantepec Ridge Clipperton Fracture Zone Trench 25 Hess Scarp 5 QSC 20 10 o Fracture Zone N Siqueiros 15 COCOS PLATE 20 10 15 5 RSB Cocos Is. 15 10 5 Cocos Ridge 5 EAST PACIFIC RIFT PACIFIC EAST 10 EAST PACIFIC RIFT PACIFIC EAST Malpelo 5 CORCCR 5 Ridge GALAPAGOS RIFT Panama Fracture Zone ECR Galapagos Islands Carnegie o Ridge 0 RSB NAZCA PLATE Trench SOUTH AMERICAN Chile-Peru PLATE 100ooW 90 W Figure 1. A. Regional tectonic map of southern Central America (modified from Protti et al., 1995), showing major tectonic boundaries and geographic features discussed in the text. Also shown is the distribution of lithospheric age (in Ma) of the Cocos Plate, after Hey (1977). RSB = rough/smooth boundary; QSC = Quesada Sharp Contortion of the Wadati-Benioff Zone; CRR and ECR = Costa Rica and Ecuador rifts, respectively. Darker shades of gray show greater age. B. Tectonic setting of southern Central America and the geometry of the top of the Wadati-Benioff Zone (from Protti et al., 1995). Isodepth contours are at 20-km intervals, from 40 km. Filled triangles = active volcanoes; and filled circles = location of large (magnitude >7.0) earthquakes that occurred in this century. Darker shades of gray show greater depth. Each shade increment is 1000 m. East Pacific Rise and that created along the Galapagos spreading cen- 1992. A spatial and temporal seismic gap is defined by the nonoccur- ter. The northeast extension of the rough/smooth boundary, which rence of an important earthquake on the Nicoya segment since 1950, may become the Quesada Sharp Contortion after subduction, was in- coupled with the occurrences of the 1990 earthquake at the entrance terpreted as a relict of the original transform fault along which the of the Nicoya Gulf and the 1992 earthquake off Nicaragua. A lack of Farallon Plate broke into the Cocos and Nazca Plates either 25 Ma seismicity in this Nicoya gap is evident at magnitudes of 2.5 or above. (Hey, 1977) or 27 Ma (Lonsdale and Klitgord, 1978). On the other Based on the experience of the 1990 and 1992 earthquakes and the hand, recent studies of the magnetic pattern in the Cocos Plate have aftershock relocations of the 1950 earthquake (Guendel, 1986), elas- inferred faster spreading rates than were previously recognized, so tic coupling begins about 15-km arcward from the trench. The poten- the age of the Cocos Plate subducting beneath the Nicoya Peninsula tial rupture area of a future Nicoya earthquake is estimated between may be younger. 3900 and 9600 km2 (J.M. Protti, pers. comm, 1996). Earthquakes of a magnitude >7 occurred beneath the Nicoya Pen- Accumulated strain has also been recorded by repeated global po- insula in 1827(?), 1853, 1900, and 1950. The 1978 Samara earth- sitioning system (GPS) campaigns in the region. The Nicoya Penin- quake of magnitude 6.8 ruptured only a small portion of the Nicoya sula moves to the northeast with respect to the rest of the Caribbean segment (Guendel, 1986).
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