Segmented Fore Arc Deformation Along the Nicoya Peninsula Seismic Gap, Costa Rica
Total Page：16
File Type：pdf, Size：1020Kb
Eos, Transactions, American Geophysical Union, v. 88, Fall Meeting Supplement, Abs T53A-1121, 2007. Segmented Fore Arc Deformation Along the Nicoya Peninsula Seismic Gap, Costa Rica Jeffrey S. Marshall1, Eli J. LaFromboise2, Thomas W. Gardner3, and Marino Protti4 1Geological Sciences Department, Cal Poly Pomona University, Pomona CA, 91768, USA 2Geological Sciences Department, California State University, Northridge CA, 91330, USA 3Geosciences Department, Trinity University, San Antonio TX, 78212, USA 4OVSICORI, Universidad Nacional, Heredia 3000, Costa Rica Abstract The Nicoya Peninsula, Costa Rica deforms in response to rapid NE subduction of the Cocos plate at the Middle America Trench (9-10 cm/yr). This emergent outer fore arc peninsula lies 60- 80 km inboard of the trench and coincides with a locked segment of the seismogenic zone. The Nicoya segment is a high-potential seismic gap, with a slip deficit of >5 m since the last major earthquake (M7.7, 1950). That event produced widespread damage and up to 1.0 m of coseismic coastal uplift. Net Quaternary deformation on the Nicoya Peninsula is recorded by emergent marine terraces at the coast, and by uplifted alluvial fill within interior valleys. Recent field mapping, surveying, and isotopic dating provide new constraints on deformation patterns and upper-plate faulting. Local uplift anomalies reveal upper plate faults that may accommodate a significant fraction of fore arc deformation (shortening and/or lateral sliver transport). At the peninsula’s southern tip (Cabo Blanco), a prominent uplifted marine erosion surface (Cobano surface) encompasses at least three distinct Pleistocene terraces at 30-220 m elevation. Optically Stimulated Luminescence (OSL) dating yields terrace ages consistent with OIS 3-5 sea level high stands (30-120 ka), indicating uplift at 1.0-2.0 m/k.y. A NW-striking thrust fault (Delicias fault) offsets the upper terrace by 40 m, thrusting Cretaceous basalt over late Pleistocene marine sediments. Radiocarbon ages for adjacent Holocene terraces (Cabuya surface) indicate recent uplift at 1.0-6.5 m/k.y. On the peninsula’s south-central coastline (Puerto Carrillo to Playa Camaronal) marine fluvial terraces (Carrillo-Camaronal surface) occur at 20- 40 m elevation. Correlations with dated Cobano terraces and Quaternary sea level curves suggest terrace formation during the OIS 5a-e high stands (80-120 ka) and uplift rates of 0.2-0.3 m/k.y. Along the northern Nicoya coast (Tamarindo to Nosara), a 3 km wide wave-cut surface (Iguanazul surface) includes two treads with paleo-shorelines at 10-45 m elevation. Age correlations (as above) suggest terrace formation during the OIS 5a-e high stands (80-120 ka) and uplift rates of 0.1-0.2 m/k.y. Radiocarbon ages for Holocene beachrock horizons are consistent with recent uplift at <0.5 m/k.y. Within the interior Morote Valley, geomorphic anomalies indicate deformation along the NW-striking Morote fault (e.g., uplifted and incised alluvial fill, irregular drainage networks, stream piracy, and abrupt mountain facets). Uplifted Pleistocene alluvium (La Mansion surface) is incised 5-40 m by stream networks. At one site, fluvial gravels overlie a paleosol on fine-grain wetland deposits, 10 m above local base level. In some areas, the paleo-valley gradient is opposite that of modern incised streams, indicating capture and drainage reversal. The Morote fault forms a prominent NW-trending lineament oriented sub-parallel to the plate margin. A composite focal mechanism for shallow seismicity along this trend (CRSEIZE) shows dextral slip for a NW-striking nodal plane, consistent with NW fore arc sliver escape. The observed differences in Quaternary deformation along the Nicoya Peninsula coincide with three contrasting domains of subducting seafloor offshore (EPR, CNS-1, CNS-2). Upper- plate segmentation may reflect along-strike variations in subducting plate roughness, coupling, and seismogenic zone geometry (e.g., dip angle, depth of up-dip and down-dip limits). This segmentation may have implications for understanding the rupture behavior and earthquake hazards along the Nicoya seismic gap. Introduction The Nicoya Peninsula, Costa Rica provides an optimal setting for the study of upper plate deformation along the Middle America convergent margin. Located only 60-80 km inboard of the Middle America Trench (Figs. 1 and 2), this outer fore arc peninsula sits directly above the seismogenic zone within a high potential seismic gap (Fig. 3). The last major earthquake centered beneath the Nicoya peninsula occurred on 5 October 1950 with a moment magnitude of M=7.7 (Protti et al., 2001). This event produced widespread damage and generated 1.0-1.5 m of coseismic uplift along the peninsula's central coast (Marshall and Anderson, 1995). The net pattern of upper plate deformation related to large subduction earthquakes is recorded by emergent Quaternary marine terraces along the peninsula’s coast and uplifted alluvial fill terraces within interior valleys (Hare and Gardner, 1985; Marshall and Anderson, 1995; Gardner et al., 2001; Marshall et al., 2003 and 2005). Here, we present new results from ongoing field studies of marine and fluvial terraces on the Nicoya Peninsula. Tectonic and Geologic Setting Costa Rica is part of the Central American volcanic arc (Figs.1 and 2), formed by NE subduction of the Cocos plate beneath the Caribbean plate and Panama block at the Middle America Trench (MAT). Plate convergence offshore of Costa Rica is nearly orthogonal and occurs at 9-10 cm/yr. The subducting Cocos plate consists of seafloor produced along both the East Pacific Rise and the Cocos-Nazca spreading center (Fig. 1; Hey, 1977). Hotspot volcanism at the Galapagos Islands generates a rough domain of thickened seafloor that includes the Cocos Ridge and adjacent seamounts. Two major segment boundaries on the subducting Cocos plate intersect the MAT offshore of the Nicoya Peninsula (Fig. 4; Barckhausen et al., 2001). These include an abrupt morphologic break between smooth and rough sea floor domains, and a fracture zone trace that divides crust derived at the East Pacific Rise (EPR) from that formed along the Cocos-Nazca spreading center (CNS-1 and 2). Contrasts in subducting plate thickness, roughness, and dip across these boundaries produce along-strike variations in seismicity, volcanism, and upper plate deformation (e.g., Wells et al., 1988; Gardner et al., 1992; Protti et al., 1995; Fisher et al., 1998; Marshall et al., 2000 and 2003; von Huene et al., 2000). The Nicoya Peninsula spans an emergent segment of the Costa Rican fore arc (Fig. 2), exposing Cretaceous seafloor basement (Nicoya Complex) overlain by an upward-shallowing sequence of late Cretaceous-Quaternary marine sediments (Fig. 3; Lundberg, 1982). Tectonic erosion at the trench has removed a substantial volume of the upper plate, inducing subsidence of the offshore margin wedge (Ranero and von Huene, 2000; Vannuchi et al., 2001). However, flights of Quaternary marine terraces along the Nicoya coast and uplifted alluvium within interior valleys attest to active uplift 60-80 km inboard of the trench (Hare and Gardner, 1985; Marshall, 1991; Marshall and Anderson, 1995; Gardner et al., 2001). This uplift suggests net crustal shortening beneath the Nicoya peninsula associated with repeated slip along the plate interface. The Nicoya segment of the MAT produces frequent large earthquakes interspersed by periods of low background seismicity (Protti et al., 2001). Historical large magnitude events (>M 7.0) include the earthquakes of 1826, 1853, 1900, and 1950. The 1950 earthquake (M 7.7) is thought to have ruptured the entire length of the Nicoya Peninsula (Fig. 3). In contrast, the M 6.9 event of 1978 ruptured only 20% of the Nicoya asperity, and is estimated to have released only 6% of the accumulated energy since the 1950 earthquake (Protti et al., 2001). Based on scarce background seismicity, a rapid convergence rate (9-10 cm/yr), and the relative frequency of large earthquakes (every 40-60 years), the Nicoya segment is considered a high potential seismic gap. The limits of the Nicoya seismic gap (Fig. 3) are defined by the aftershock areas of the 1990 and 1992 earthquakes centered in adjacent seismogenic segments SE and NW of the peninsula. Purpose and Methods The goal of this study is to constrain the lateral extent and elevations of uplifted Quaternary marine and fluvial terraces along the length of the Nicoya seismic gap. At present, uplifted marine terraces and paleo-shorelines have been mapped along three segments of the Nicoya Peninsula’s coastline (Fig. 5): 1) the northern coast (Iguanazul surface), 2) central coast (Carrillo-Camaronal Surface), and 3) the peninsula’s southern tip (Cobano surface). In addition, uplifted and incised alluvial fill terraces have been mapped in the peninsula’s interior (La Mansión surface) The results of this investigation allow us to assess the relationship between Quaternary deformation patterns, morphology and thermal state of the subducting seafloor, and the geometry of the seismogenic zone. The regional extent of the Iguanazul, Carrillo-Camaronal, and Cobano marine terraces (Fig. 5) was determined through field reconnaissance and examination of aerial photographs. We then selected three representative coastal segments for more detailed study (Figs. 6a-c). In the northern and central study areas (Fig 6a-b), the Iguanazul and Carrillo-Camaronal surfaces (10- 40 m elev.) consist of one to two discrete marine terrace treads located between the shoreline and the coastal mountains. In the southern study area (Fig. 6c), the highly dissected Cobano surface (30-220 m elev.) consists of three discrete terrace treads separated by well-defined risers. In all three areas, terraces were mapped and surveyed along a series of nine transects extending inland from the coast (Figs 6a-c). The location of terrace treads, risers, outer, and inner edges were recorded on base maps and the spatial coordinates for these features (map location and elevation) were determined using a Garmen 76S GPS unit and a Sokkia barometric altimeter.