Vargas and Mann 2013 Caldas Tear Bullseismsocamer
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Bulletin of the Seismological Society of America, Vol. 103, No. 3, pp. 2025–2046, June 2013, doi: 10.1785/0120120328 Tearing and Breaking Off of Subducted Slabs as the Result of Collision of the Panama Arc-Indenter with Northwestern South America by Carlos A. Vargas and Paul Mann Abstract We present two regional, lithospheric cross sections that illustrate east- ward- and southeastward-dipping, subducted slabs to depths of 315 km beneath the surface of Colombia in northwestern South America. These cross-sectional interpre- tations are based on relocated earthquake hypocentral solutions, models supported on gravity and magnetic regional data, and coda-Q (Qc) tomography. The method of tomographic imaging based on spatial inversion of the coda wave has advantages of providing information on the lateral variations of the anelastic properties and ther- mal structure of the lithospheric system. Mapping of earthquake-defined Benioff zones combined with tomographic imaging reveals the presence of an ∼240 km long east–west-striking slab tear, named here the Caldas tear. The proposed Caldas tear separates a zone of shallow, 20°–30°-dipping, southeastward subduction in the area of Colombia adjacent to Panama and the Caribbean Sea, which is not associated with subduction-related volcanism, from an area of steeper, 30°–40°-dipping, slab adjacent to the eastern Pacific Ocean that is associated with an active north–south chain of active arc volcanoes. We propose that the Caldas slab tear separating these two distinct subducted slabs originally formed as the southern boundary of the Panama indenter, an extinct island arc that began subducting beneath northwestern South America about 12 Ma. The area south of the Panama indenter is Miocene oceanic crust of the Nazca plate, which subducts eastward beneath northwestern South America at normal angles and melts to form a north–south-trending active volcanic arc. In addition to the for- mation of the Caldas tear, we propose that impedance of the thicker crustal area of the Panama arc-indenter over the past 12 Ma may have led to down-dip break-off of previously subducted oceanic crust that is marked by an extremely concentrated and active earthquake swarm of intermediate-depth earthquakes beneath east-central Colombia. Introduction and Tectonic Setting Hypocentral solutions recorded by the Colombian Na- Regional compilations of Global Positioning Systems tional Seismological Network (CNSN) show an ∼240 km (GPS) data provide a quantitative tectonic framework for long, right-lateral offset of intermediate to deep events with understanding the widespread crustal effects of the Panama azimuth of 102° (Fig. 1a,b). We infer this discontinuity in arc collision on large areas of northwestern South America earthquakes to be a major slab tear which we have named (Calais and Mann, 2009; Fig. 1a). GPS vectors in western the Caldas tear based on the location in the Caldas department Colombia show a marked decrease in velocities consistent of Colombia and the alignment of fault-related surface features with the ongoing collision of the Panama arc with north- (e.g., volcanism, faulting, mineral deposits, geothermal anoma- western South America along a north–south-trending suture lies, etc.). Using the distribution of earthquakes >80 km, zone roughly parallel to the international boundary between Ojeda and Havskov (2001) proposed that the discontinuity Panama and Colombia (Adamek et al., 1988; Trenkamp along the Caldas tear represented a boundary between two sub- et al., 2002; Corredor, 2003; Fig. 1a). The east–west direc- ducted slabs with differing dips and strikes: the northern sub- tion of GPS vectors shows that the effects of east–west duction zone, called the Bucaramanga subduction zone, has shortening and indentation related to the collision of the Pan- a shallower dip (27°) and more northeasterly strike, and the ama arc remains relatively constant over a large, V-shaped, southern, called the Cauca subduction zone, has a steeper fault-bounded area of Colombia due east of the Panama arc- dip (35°–40°) and a more northerly strike (Fig. 1a). indenter (Fig. 1b). GPS vectors on the Maracaibo block of 2025 2026 C. A. Vargas and P. Mann Figure 1. (a) Tectonic map of northwestern South America and Panama showing plate boundaries, neotectonic fault systems, and se- lective distribution of hypocentral solutions of ∼30;000 earthquakes extracted from the entire catalog of the CNSN (∼102;000 events) during – 1993 2012 with these criteria: mL ≥ 0:5; GAP ≤ 200; rms ≤ 0:5; error in latitude ≤10:0 km; error in longitude ≤10:0 km; and error in depth ≤10:0 km. Color scale indicates depth of earthquakes. The north and south profiles symbolize the tomographic sections presented in this study. SMM, Santa Marta massif; CB, Choco block; WC, Western Cordillera; CC, Central Cordillera; EC, Eastern Cordillera; PR, Perija Range; GB, Guajira basin; LB, Llanos foreland basin; MMVB, Middle Magdalena Valley basin; RFZ, Romeral fault zone; SMBF, Santa Marta– Bucaramanga fault; PF, Palestina fault; CF, Cimitarra fault; MGF, Mulato–Getudo fault; HF, Honda fault; SFS, Salinas fault system; GF, Garrapatas fault; LFS, Llanos fault system; IF, Ibague fault; SR, Sandra ridge; BN, Bucaramanga nest; CN, Cauca nest; MN, Murindo nest; PIVC, Paipa–Iza volcanic complex; RSDV, Romeral and San Diego volcanoes. Yellow stars correspond to (1) the Tauramena earthquake (19 January 1995, Mw 6.5); (2) the Armenia earthquake (25 January 1999, Mw 6.2); and (3) the Quetame earthquake (24 May 2008, mL 5:7). Sections AA0 and BB0 correspond to tomographic profiles presented in Figures 5 and 6. (b) Crustal isochron pattern of the Sandra ridge; pink- colored line, Caldas tear zone; arrows, station velocity GPS vectors relative to stable South America (after Calais and Mann, 2009). CHEP and BOGO are GPS stations used as reference to estimate the onset of the Panama-arc and South American plate collision. Other GPS stations in the Panama-arc collision area are MANZ, RION, BUCM, MONT, and CART. Faded blue arrow enclosing 102° azimuth of the approximately 240 km long, right-lateral offset of intermediate to deep events associated with the Caldas tear. Colombia and Venezuela show a more northerly direction of 20°–30° in the area north of ∼5:6°N (Ojeda and Havskov, plate motions related to northward tectonic escape of the 2001; Vargas et al., 2007). Maracaibo block into the southern Caribbean (Trenkamp Two nests of concentrated intermediate-depth earth- et al., 2002). In contrast to this fairly uniform GPS velocity quakes are present beneath Colombia (Fig. 1a). The Bucara- field of deformed crustal rocks produced by the Panama col- manga earthquake nest (BN) is found at a depth of ∼160 km lision, underlying, eastward-dipping slabs change abruptly on the down-dip extension of the southern (Bucaramanga) across the Caldas tear from dip angles of 30°–40° between subduction zone and has an estimated volume dimension latitudes ∼3:0°–5:6° N in southern Colombia, to dip angles of of ∼13 × 18 × 12 km (Schneider et al., 1987; Frohlich et al., Tearing and Breaking Off of Subducted Slabs as the Result of Collision of the Panama Arc-Indenter 2027 1995). Previous tectonic interpretations of the origin of the and a high-resolution seismic profile, seeking to define the Bucaramanga nest vary from a zone of two slabs in contact geometry of the Caldas tear and its geotectonic implications (van der Hilst and Mann, 1994), two slabs overlapping in the northwestern corner of South America. (Taboada et al., 2000), or a single slab undergoing extreme bending (Cortés and Angelier, 2005) all occurring in the boundary area of the subducted northern (Bucaramanga) Hypocentral Solutions and Estimation and southern (Cauca) subduction zones (Fig. 1a). The Cauca of the Coda-Wave Attenuation ∼400 intermediate-depth earthquake nest (CN) is located km A catalog has been compiled of ∼102;000 earthquake southwest of the Bucaramanga nest on the trend of our pro- locations calculated by the CNSN during the period 1993– posed Caldas tear and has been previously interpreted 2012 (mL ≤ 6:8). Hypocentral solutions were estimated by by Cortés and Angelier (2005) as a bend in the slab in this using a seismological array of 17 short-period instruments area (Fig. 1a). There is no clear consensus among seismol- (T 1 s) of the CNSN and complemented by 13 stations ogists for the tectonic interpretation of the two concentrated associated with local volcanic monitoring systems and also Colombian intermediate earthquake nests (Frohlich, 2006; foreign networks (Panama, Ecuador, and Venezuela). Final Zarifi, 2006). solutions were calculated with the HYPOCENTER program The Caldas tear defines the northern limit of the active and the velocity model proposed by Ojeda and Havskov volcanic front of the northern Andes that has formed as a (2001). Then, 9338 waveforms associated with 7645 regional consequence of the steeper subduction of oceanic slab of – earthquakes (3:0 ≤ mL ≤ 6:5; 1993 2012) were selected for normal thickness of the Nazca plate (Fig. 1a). Moreover, estimating the decay rate of the coda amplitudes (Q−1,coda – c associated with active and inactive volcanoes, the east west attenuation). The selected events, on basis of a significant – projected surface trace of the Caldas tear localizes an east number of stations that recorded them (Table 1), have epicen- west alignment of some unusual volcanic rocks including tral distances to stations ranging between 22.6 and 690.0 km adakites (Borrero et al., 2009; Fig. 1a). Other volcanic rocks and depths varying between 0 and 222.0 km. Figure 2 shows in the vicinity of the east–west-trending Caldas tear include −1 all events used for the Qc plotted on a map of northwestern the Plio-Pleistocene Paipa-Iza volcanic complex in the South America along with tectonically significant earth- Eastern Cordillera of Colombia and the Romeral and San quake focal mechanisms including the Quindio, Quetama, Diego volcanoes (Pardo et al., 2005).