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Lafemina, P. C., T. H. Dixon, W. Strauch, Bookshelf Faulting In Downloaded from geology.gsapubs.org on June 3, 2010 Geology Bookshelf faulting in Nicaragua Peter C. La Femina, T.H. Dixon and W. Strauch Geology 2002;30;751-754 doi: 10.1130/0091-7613(2002)030<0751:BFIN>2.0.CO;2 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geology Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes Geological Society of America Downloaded from geology.gsapubs.org on June 3, 2010 Bookshelf faulting in Nicaragua Peter C. La Femina T.H. Dixon Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, USA W. Strauch Instituto NicaraguÈense de Estudios Territoriales, Managua, Nicaragua ABSTRACT Oblique subduction at a high rate of convergence along much of the Middle America Trench results in northwest-directed trench-parallel block motion. Accommodation of this motion along northwest-striking dextral strike-slip faults has been postulated; however, in Nicaragua such faults are not well developed. We suggest instead that this motion is accommodated by bookshelf faulting that includes northeast-striking left-lateral faults. We present earthquake epicenter and focal mechanism data and mapped fracture and fault data consistent with this model. Trenchward migration of the volcanic arc since the Mio- cene and reactivation of northeast-striking Miocene structures may have led to the de- velopment of this arc- and trench-normal fault system. Keywords: bookshelf faulting, block rotation, Nicaragua, neotectonics, seismic hazards. INTRODUCTION at ®rst glance appear to be mechanically less favorable, may have de- Trench-parallel motion of crustal blocks or slivers in response to veloped, and discuss geological and seismic hazard implications. oblique subduction has long been recognized as an important aspect of crustal deformation (Fitch, 1972; Jarrard, 1986). In general, motion NEOTECTONICS OF CENTRAL AMERICA AND of these blocks, located between the subduction zone and volcanic arc, NICARAGUA is likely to be signi®cant in areas of rapid subduction and high obliq- Pleistocene-Holocene deformation in Central America includes uity, especially where coupling between subducting and overriding three active fault trends: N458±658W right-lateral strike-slip faults, plates is high (Beck, 1991; McCaffrey, 1992). In Central America, N308±458E left-lateral strike-slip faults, and N158W±N108E normal where present-day subduction of young (younger than 25 Ma) litho- faults (Carr, 1976; Weyl, 1980; Manton, 1987; Weinberg, 1992). In a sphere of the Cocos plate beneath the Caribbean plate occurs along the dextral shear zone these fault trends are consistent with Reidel and Middle America Trench at a rate of ;8 cm/yr (Dixon, 1993; DeMets anti-Reidel shear, and east-west extension, respectively. Northwest- et al., 1994; DeMets, 2001), obliquity varies due to changes in strike striking right-lateral faults are parallel to the arc, and have been of the trench, from essentially zero in southern Costa Rica to more mapped in El Salvador, Guatemala, and Costa Rica (Carr, 1976; Man- than 158 in Nicaragua (Barckhausen et al., 2001). DeMets (2001) used ton, 1987; Marshall et al., 2000). However, in Nicaragua this fault trend slip vectors of subduction-zone earthquakes and a new model for Cocos- is not well developed and northeast-striking faults dominate in the fore- Caribbean motion to estimate trench-parallel motion of the forearc at arc and arc (Carr, 1976; Weyl, 1980), offset northwest-striking right- a rate of 14 6 5 mm/yr. lateral faults in the arcs of El Salvador, Guatemala, and Costa Rica Despite this relatively high rate of motion, the crustal structures (Carr, 1976; Marshall et al., 2000), and are associated with segment that accommodate forearc motion in Nicaragua remain unclear. In Su- breaks along the Central American volcanic arc (Stoiber and Carr, matra, probably the best described example of forearc sliver transport, 1973; Carr, 1976). The approximately north-striking faults show nor- a major strike-slip fault parallel to the trench and located within the mal to oblique offset, bound north-striking grabens, such as the Ma- thermally weak volcanic arc accommodates the majority of trench-parallel nagua graben (Fig. 2), and are associated with the alignment of vol- motion (Fitch, 1972; McCaffrey, 1992; Sieh and Natawidjaja, 2000). canic vents (McBirney and Williams, 1965; Weinberg, 1992). These However, such a mechanism does not appear to be appropriate for faults accommodate ;8 mm/yr of east-west extension in northern Cen- Nicaragua. While focal mechanisms of forearc crustal earthquakes are tral America (Guzman-Speziale, 2001). The spatial and temporal dis- consistent with right-lateral motion on northwest-striking (i.e., trench tribution of earthquake focal mechanisms and epicenters suggests that parallel) faults (Molnar and Sykes, 1969; White, 1991; Fig. 1), such active faults are con®ned to within 25 km of the volcanic arc and that faults are not well expressed; northeast-striking faults are more com- seismic events are characterized by slip on multiple, parallel fault mon (Carr, 1976; Carr and Stoiber, 1977). Because of the well-known planes (Brown et al., 1973; Molnar and Sykes, 1969; White, 1991; this ambiguity in focal mechanism interpretation, the latter faults are also study, Figs. 1±3). Elongate damage zones from historical earthquakes consistent with the focal mechanism data, provided that they are left indicate that all three fault trends are active along the arc (Carr and lateral rather than right lateral. In this paper we present new seismic Stoiber, 1977; White and Harlow, 1993). data collected by the Instituto NicaraguÈense de Estudios Territoriales (INETER) local seismic network, and summarize focal mechanism and EVIDENCE FOR NORTHEAST-STRIKING LEFT-LATERAL mapped fracture and fault data from the literature. These data suggest FAULTS IN NICARAGUA that trench-normal faults play an important role in accommodating In Nicaragua, northeast-striking faults have been mapped within trench-parallel motion of the forearc in Nicaragua via vertical axis the forearc, volcanic arc, and highland regions (Carr and Stoiber, 1977; block rotation. We describe a model to explain why such faults, which Weyl, 1980; Weinberg, 1992; van Wyk de Vries, 1993; Cowan et al., q 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; August 2002; v. 30; no. 8; p. 751±754; 4 ®gures. 751 Downloaded from geology.gsapubs.org on June 3, 2010 Figure 1. A: Location map of Nicaragua and study area. B: Digital elevation model of western Nicaragua with focal mechanisms of his- torical strike-slip earthquakes from Molnar and Sykes (1969), White Figure 2. Map of Quaternary faults within Managua graben and Nejapa- (1991), and Harvard centroid moment tensor catalog. Stars mark lo- Mira¯ores volcanic alignment. Faults are from Brown et al. (1973), cations of destructive earthquakes (White and Harlow, 1993). Num- Weyl (1980), and Cowan et al. (2000). Focal mechanisms are from bers located next to focal mechanisms and stars are dates of events White (1991) and Harvard centroid moment tensor catalog. Black given in month-day-year notation. ESÐEl Salvador; CRÐCosta Rica. star is epicenter of March 31, 1931, earthquake. 2000). Stoiber and Carr (1973) and Carr and Stoiber (1977) discussed 1999, eruption of Cerro Negro by several hours (Fig. 3A). Aftershock the segmentation of the Central American volcanic arc along northeast- sequences of these earthquakes correlate well with mapped faults of trending transverse structures located at right steps in the volcanic arc the La Paz Centro fault zone (van Wyk de Vries, 1993; Cowan et al., and between volcanic complexes. In Nicaragua, right steps in the arc 2000; Fig. 3A). A similar pattern of earthquake epicenters located in are located at the Gulf of Fonseca, Managua, and southeastern Lago the La Paz Centro fault zone was presented by McNutt and Harlow de Nicaragua (Carr and Stoiber, 1977; Fig. 4). These right steps may (1983). Mw 5.4 and Mw 6.0 earthquakes with left-lateral focal mech- be caused by lateral variations in magma production, resulting from anisms preceded the Cerro Negro eruption and earthquake swarm by bending of the subducting slab (Carr et al., 1982). Weinberg (1992) four months and were located in the Gulf of Fonseca (Figs. 1 and 4). suggested that northeast-trending structures formed during an earlier On July 6, 2000, an earthquake swarm caused surface ruptures along Miocene phase of deformation have been reactivated. several northeast-trending faults, northwest of Laguna de Apoyo and Focal mechanisms and epicenter distributions of earthquakes dur- east-southeast of Masaya (INETER, 2000). Fracture data demonstrate ing the past century correlate well with these transverse structures and that the trends of surface ruptures are consistent with slip along northeast- segment breaks (Molnar and Sykes, 1969; White, 1991; White and trending faults (INETER, 2000; Fig.
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