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ARTICLE IN PRESS TECTO-124901; No of Pages 10 Tectonophysics xxx (2010) xxx–xxx

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Tectonophysics

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Pleistocene to Present North Andean “escape”

Obi Egbue ⁎, James Kellogg

Department of Earth and Ocean Sciences, University of South Carolina, USA article info abstract

Article history: This study compiles 20 published field geologic estimates of displacement rates for the northern Andes, such Received 2 June 2009 as displaced glacial moraines and offset pyroclastic flow, and compares them to published Global Positioning Received in revised form 25 March 2010 System (GPS) measurements. Dated displacements compiled in this study were obtained from the Gulf of Accepted 20 April 2010 Guayaquil, Pallatanga, Chingual-la Sofia, and Cayambe-Afiladores-Sibundoy fault systems in Ecuador and Available online xxxx southern Colombia and the Boconó fault system in Venezuela. Right-lateral slip estimates on the individual fault segments range from 2 mm/a to 10 mm/a. The mean estimated geologic slip rate for the last Keywords: Tectonic escape 86,000 years is 7.6 mm/a. This estimate is very similar to the GPS measurements of Present day motion at the North Andes 2 sigma level. Published GPS results suggest that a large part of the northern Andes is “escaping” to the Carnegie ridge northeast relative to stable South America at a rate of 6±2 mm/a. The GPS displacement rates of seven sites in Venezuela, Colombia, and Ecuador are statistically identical at the 95% confidence level. Four geologic estimates indicate that slip rates of 4 to 10 mm/a continued back to 1.8 Ma. No geologic slip estimates have been reported for Ecuador prior to that time period. The “escape” of the North Andes is believed to be a result of increased coupling between the obliquely subducting Nazca plate and the overriding South American plate due to the subduction of the Carnegie Ridge in the Ecuador–Colombia trench. If this is correct, the slip estimates for the North Andes suggest that the Carnegie Ridge arrived at the trench prior to 1.8 Ma. In the Eastern Cordillera of Colombia, strike-slip crustal earthquakes reflect slip partitioning on high angle faults located above crustal detachment ramps across a 200 km wide zone. Intermediate depth mantle earthquakes indicate that brittle shearing extends to the base of the lithosphere. © 2010 Elsevier B.V. All rights reserved.

1. Introduction America at a rate of 6±2 mm/a. The relative northeastward motion of the northern Andes is also demonstrated by earthquake focal Northwestern South America is a broad convergent plate boundary mechanism solutions (Fig. 2) as well as numerous geologic field zone characterized by active seismicity, a volcanic arc, subduction, measurements of dated offset glacial features and pyroclastic flow. and an on-going arc–continent collision. The North Andes is bounded Tapponnier et al. (1982), using plane indentation experiments on by the Colombian–Ecuador trench and the Panama block to the west, unilaterally confined blocks of plasticine were able to model the South Caribbean deformed belt to the north, and the Boconó fault intracontinental deformation and the evolution of strike-slip faults and East Andean fault zones to the east (Pennington, 1981; Kellogg due to the collision of India and Asia. The faults that develop, allow the and Vega, 1995). Cenozoic deformation in this broad zone has been “escape” of the detached block in the direction of the free boundary. produced by the converging Nazca, South American, and Caribbean Proposed driving mechanisms for the “escape” of the North Andes plates, and the Panama microplate (Kellogg and Bonini, 1982). include subduction of the Caribbean plate, collision with the Panama Geodetic measurements using GPS from the Central and South arc, rapid oblique subduction of the Nazca plate, and the subduction of American (CASA) GPS project show that the Nazca oceanic plate is the aseismic Carnegie Ridge at the Ecuador–Colombia trench. The rapidly converging with stable South America (Freymueller et al., geologic history of the “escape” might therefore shed light on the 1993; Trenkamp et al., 2002). The convergence direction is slightly driving mechanisms. If subduction of the Carnegie Ridge is an oblique to the Colombia–Ecuador trench (Fig. 1). The aseismic important factor as has been proposed (Lonsdale, 1978; Pennington, Carnegie Ridge, produced by the passage of the Nazca plate over 1981; Gutscher et al., 1999; Witt et al., 2006), then the initiation of the Galapagos hotspot, is being subducted in the Ecuador–Colombia northeastward displacement could indicate when the Ridge arrived at trench. CASA measurements also suggest that a large part of the the trench. northern Andes is “escaping” to the northeast relative to stable South This study provides the first compilation of published geologic estimates of displacements with reliable ages from the entire East Andean frontal fault zone. The results include 20 geologic estimates ⁎ Corresponding author. obtained from transverse faults underlying the pull-apart basin in the E-mail address: [email protected] (O. Egbue). Lechuza depression, on Puna Island, and from the Zambapala Fault

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Fig. 1. Structural map of the North Andes. Numbers are the locations of geological displacement estimates (Table 2). CASA station GPS velocity vectors relative to stable South America are shown with 95% confidence error ellipses (Trenkamp et al., 2002). The Galapagos vector (GALA) has been relocated so that it is visible on this map. AUFZ: Atrato-Uraba fault zone; CASF: Cayambe-Afiladores-Sibundoy fault; CSF: Chingual-la Sofia fault; DGM: Dolores Guayaquil megashear; EAFZ: East Andean Frontal fault zone; ESF: Espiritu Santo fault; SAFFZ: Sub-Andean Frontal fault zone; SMBF: Santa Marta-Bucaramanga fault.

Zone, both in the Gulf of Guayaquil, as well as from the Pallantanga include coseismic, postseismic, and interseismic deformations associ- and Chingual-la Sofia faults in Ecuador, the Cayambe-Afiladores- ated with plate motion and crustal deformation at the plate boundaries. Sibundoy fault in southern Colombia, and the Boconó fault system in The demonstrated repeatability of horizontal position estimates for northwestern Venezuela. These estimates were compared with the regional GPS networks is in the order of 1–5mm(Jaldeha et al., 1996; Present day displacement rate for the North Andes determined from Segall and Davis, 1997). The CASA GPS project measured plate motions CASA GPS measurements. The results were then used to make and crustal deformation of the Nazca, Cocos, Caribbean, and South inferences regarding the driving forces for the northeastward “escape” American plates. Fig. 1 includes velocity vectors from geodetic of the North Andes as well as to attempt to explain the lack of geologic measurements conducted in the field from 1991 to 1998 in Venezuela, measurements of northeastward strike-slip motion in the Eastern Colombia, and Ecuador (Trenkamp et al., 2002). The data was analyzed Cordillera of Colombia. using GIPSY OASIS and GIPSY OASIS II software. Deformation vectors in the northern Andes reflect strain associated with convergence at the 1.1. GPS results for the North Andes broad Nazca–South American plate boundary, Caribbean–South American plate boundary, and Panama arc–South America boundary. GPS measure- Space geodetic studies are now the primary method of studying ments are consistent with rapid active subduction (58±2 mm/a) of the the kinematics of plate boundary zones on land, because, since space oceanic Nazca plate and the Carnegie aseismic ridge at the Ecuador– geodetic measurements are in a global reference frame, they can be used Colombia trench beneath stable South America (Trenkamp et al., 2002). to measure motions within the boundary zones as well as relative global Approximately 50% of the Nazca–South America convergence is locked motions of plate interiors (Stein and Sella, 2002). These measurements at the subduction interface, causing elastic strain in the overriding plate

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Table 1 “Escape” velocities. Component of GPS velocity vectors (Trenkamp et al., 2002) parallel to the Boconó/EAFZ (043°) at one standard error, showing latitude of nearest point on Boconó/EAFZ and distance from Boconó/EAFZ. For station locations see Fig. 1 and Trenkamp et al. (2002).

GPS stations Escape velocity parallel Latitude of nearest Distance from to EAFZ (mm/a) point on EAFZ (°) EAFZ (km)

VDUP 9±3 8.5 320 CART 8±2 6.4 610 ELBA 5±6 8.8 50 MONT 9±2 5.6 530 MERI 4±3 8.8 0 BARI 1±6 8.8 −40 URIB 2±5 7.8 10 BUCM 7±2 6 190 RION 10±5 4.4 300 BHSL 10±3 3.2 450 BOGO 5±2 4.4 90 VILL −2±6 4.2 −20 BUEN 6±2 2.3 240 CALI 4±2 2.5 160 PPYN 4±4 1.9 100 CONE −1±4 0.5 −50 LAGO −1±5 0.4 −60 JERS 7±2 −0.3 70 LATA 4±4 −1.1 55 AMBT 4±4 −1.4 20 MULM 3±5 −1.5 40 RIOP 2±5 −1.6 0

viscoelastic relaxation associated with the rupture zone of the 1979

(Mw 8.2) trench earthquake (White et al., 2003), and permanent deformation associated with the Panama collision (Trenkamp et al., 2002). For a complete list of the GPS vectors and their velocities, please refer to Table 2 in Trenkamp et al. (2002).

1.2. Seismicity

Fig. 3 shows focal mechanism solutions for earthquakes with “ ” Fig. 2. Escape velocities. Component of GPS velocity vectors (Trenkamp et al., 2002) M N4 from 1979 to 2008 (Ekström et al., 2005). Focal mechanisms in fi w parallel to the Boconó/EAFZ (043°) at one standard error (Table 1). (a) Pro le perpendicular – – to Boconó/EAFZ. (b) Profile parallel to the Boconó/EAFZ. The lines are best-fitting third the northern Andes are generally bimodal, showing NW SE and ENE order polynomials. WSW maximum compressive stress directions (Ego, et al., 1996; Corredor, 2003; Cortés and Angelier, 2005). The NW–SE compression has been associated with Caribbean–South American convergence that will be released in large interplate thrust earthquakes (Trenkamp and Panama–South America collision (Kellogg and Vega, 1995; et al., 2002). Some of the locked strain may be released in aseismic slip Taboada et al., 2000; Trenkamp et al., 2002; Colmenares and Zoback, after large interplate earthquakes, and some are masked by postseismic 2003; Corredor, 2003), and the ENE–WSW compression has been viscous relaxation in the upper mantle (White et al., 2003). Another part ascribed to Nazca–South America convergence at the Colombia– of the plate motion occurs by stable interseismic slip at the trench. Ecuador trench (Ego et al., 1996; Trenkamp et al., 2002; Colmenares The remainder causes permanent shortening and mountain building and and Zoback, 2003; Corredor, 2003). Many of the earthquakes with rigid margin-parallel “escape” of the North Andean margin. strike-slip solutions (black and white in Fig. 3) are compatible with Seven GPS sites in the North Andes: LATA, JRS, PAST, EBUEN, BUCM, northeast-trending right-lateral displacement along the East Andean URIB, and MERI (Fig. 1) have statistically zero motion at two sigma relative Frontal fault system including the Boconó fault zone in Venezuela, to station BOGO located in Bogota, Colombia. The velocity of BOGO relative the Eastern Cordillera in Colombia, and the Gulf of Guayaquil, toSouthAmericais6±2mm/ainaneast-northeastdirectionandthisis Pallatanga, Chingual-la Sofia, and Cayambe-Afiladores-Sibundoy interpreted as the Present “escape” velocity of the North Andean block fault systems in Ecuador and southern Colombia. Only in the Eastern relative to stable South America. Fig. 2 and Table 1 show the “escape” Cordillera of Colombia are these earthquakes not associated with any velocity vectors for 22 GPS stations in the north Andes, i.e., the component major mapped fault trace. As in other strike-slip fault zones, most of of velocity parallel to the Boconó/East Andean Fault Zone (EAFZ). A these earthquakes occurred in the crust, from 15 to 40 km depth. The northwest–southeast profile (Fig. 2a) shows that the escape velocities two largest earthquakes occurred at a depth of 15 km in the brittle vary from 2 mm/a along the fault zone to 10 mm/a 300 km northwest of upper crust, June 6, 1994 in the Central Cordillera of Colombia and the fault zone. Shear strain, therefore, appears to be distributed over a May 24, 2008 east of Bogota. A cluster of mantle earthquakes known broad zone. A northeast–southwest profile parallel to the Boconó/EAFZ as the Bucaramanga nest (Pennington, et al., 1979) occurs at a depth (Fig. 2b) exhibits greater scatter, reflecting varying distances from the of 163±10 km. Although these earthquakes are not primarily caused fault zone. However, velocities vary from 4 to 10 mm/a along the length by North Andean escape, they may represent a maximum thickness of the fault zone, with no clear fault-parallel changes in the “escape” rate. for the overriding North Andean block. Three strike-slip earthquakes In addition to the northeastward “escape”, the observed GPS also occurred in the mantle at depths of 57 km, 67 km, and 96 km. velocities reflect elastic locking and permanent deformation associ- These three intermediate depth mantle earthquakes are not associated with the earthquake cycle at the Colombia–Ecuador trench, ated with subducting slabs, and they suggest that the East Andean

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Fig. 3. Map of the North Andes showing earthquake focal mechanism solutions (Ekström et al., 2005) in the northern Andes with Mw N4 from 1979 to 2008. Strike-slip focal mechanism solutions are shown in black and white. All other solutions are gray. Earthquake solutions compatible with right-lateral northeast-trending shear occur along a broad zone from Ecuador north to Venezuela. frontal fault zone extends to the base of the lithosphere. Brittle failure The estimates include offset drainage features like post-glacial strike- may be induced by rapid vertical movements associated with on- slip movement, offset glacial moraines, offset pyroclastic flow and lahar, going compressional orogenesis. The crustal earthquakes reflect slip and estimates of slip necessary to produce some pull-apart basins in partitioning on high angle faults located above crustal detachment the northern Andes. These measurements have been conducted in the ramps. The dextral northeast-trending shear follows a broad 200 km field and in some instances from detailed aerial photographs and radar wide zone of weakness associated with Mesozoic rift basins. Surface imagery. Other displacement estimates without well constrained age displacement in the Eastern Cordillera of Colombia is obscured by estimates have been excluded from this study. rapid NW–SE permanent shortening and uplift associated with the on-going Panama Arc–South America collision. Stevens et al. (2002) 2.1. Boconó fault show that continental strike-slip faults are often broad zones up to 350 km wide. Five reliable slip rate measurements for the Boconó fault have been used and they range in values from 2 mm/a to 10 mm/a. The 2. Geologic slip estimates Boconó fault (Fig 1) is the best developed strike-slip fault in northern Venezuela. It was first described by Rod (1956), as forming part of a The geologic estimates of right-lateral displacement for the North major system of faults in northern Venezuela, the Oca, El-Pilar, and Andes used in this paper are from published work and have been Boconó faults. It extends along the axis of the Venezuelan Andes estimated from a broad spectrum of geologic evidence all across the from the Tachira depression near San Cristobal to a point near the North Andes (Table 2). To be considered reliable, a displacement Caribbean Sea west of Puerto Cabello in the northeast (Rod, 1956, estimate must have a well constrained age estimate associated with it. Giegengack et al., 1976). Since the pioneering work of Rod, several

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Table 2 Geologic displacement estimates, ages, and calculated displacement rates with references used in this study. For locations see Fig. 1.

# Displacement (m) Age Displacement rate (mm/a) Fault/region Source

160–100 13 ka BP ∼4–8 Boconó fault Soulas et al. (1986) 2 100 10,575 ±165 BP ∼9–10 Boconó fault Giegengack et al. (1976) 3 40 1.5 ±2 ka BP 2.0–3.0 Boconó fault Audemard et al. (1999) 485–100 1.5 ±2 ka BP 5.0–7.7 5 69 10.7 ka BP ∼6 Boconó fault Schubert and Sifontes (1970) 6 7000–9000 1.8 Ma (?) ∼3–5 Boconó fault Schubert (1980) 7 132±27 14–16 ka BP 8.9±3 8 233±24 27–32 ka BP 8.0±2 Cayambe-Afiladores-Sibundoy Fault Tibaldi et al. (2007) 9 331±29 40–46 ka BP 7.4±2 (Colombian segment) 10 630±70 75–86 ka BP 8.0±2 Cayambe-Afiladores-Sibundoy fault Tibaldi et al. (2007) (Ecuadorian segment) 11 36–64 8600 ±60 BP 6±2 Chingual-la Sofia fault Ego et al. (1996) 12 270–344 11–12 ka BP 8±2 13 30–60 11–12 ka BP 4.0±1.0 Pallatanga fault Winter and Lavenu (1989) 14 960±70 240–250 ka BP 3.6±4 Pallatanga fault Winter et al. (1993) 15 590±65 120–135 ka BP 4.4–6mm 16 27±11 4–7 ka BP 2.6–6 17 41.5±4 10–13 ka BP 2.9–5 18 35–40 5–6 ka BP 5.8–8 Gulf of Guayaquil Dumont et al. (2005a) 19 2900 ±200 440 or 550 ka BP 5.3–6.6 Gulf of Guayaquil Dumont et al. (2005b) 20 13,500–20,000 1.6–1.8 Ma 9.9±3 Gulf of Guayaquil Witt et al. (2006); Witt and Bourgois (2009) Present day 6±2 North Andes Trenkamp et al. (2002)

authors have made estimates of slip rates along the Boconó fault The Ecuadorian segment has been described as the Chingual-la Sofia (Schubert and Sifontes, 1970; Schubert and Henneberg, 1975; Giegengack fault (Ego et al., 1996). The CASF extends about 270 km from northern et al., 1976; Schubert, 1980; Soulas et al., 1986; Audemard et al., 1999). Ecuador to southern Colombia along the Sub-Andean zone, and its They have described displacement of lateral moraines, offset drainage and surface expression is represented by one or more parallel fault traces stratigraphic features, and seismic slip evidence. The most recent age for striking NNE to NE (Tibaldi et al., 2007). Local names such as Algeciras, displacement along the fault is about 10,000 BP, based on radiocarbon Suaza, Garzon, Pitalito, and Altamira have been given to several related dating of displaced moraines. Schubert and Sifontes (1970) estimated a faults in southwest Colombia with similar strikes as the CASF. The name displacement rate of about 6 mm/a for the Boconó fault based on Algeciras Fault System has been used by Velandia et al. (2005),to measurements of post-glacial right-lateral strike-slip displacement on minimize confusion and preserve the genetic relationship between detailed topographic maps. Giegengack et al. (1976) reported offsets on these faults. Using stereoscopic aerial photos and geological-structural the order of 100 m. They conclude that since the event that displaced field data on offset geomorphic features, Tibaldi et al. (2007) estimated these moraines occurred after the formation of those features and the slip rates for both the Colombian and Ecuadorian segments of the CASF fact that no stratigraphically younger moraines have been identified, from measurements of dated offset geologic deposits and landforms the moraines must have been deposited during Mucubaji time, about for two different time spans: Late Pleistocene–Holocene and Holocene. 10,575±165 BP. The estimated rate of right-lateral displacement along Slip rates were also computed for the same time span along different the Boconó fault is 9–10 mm/a. Schubert (1980) estimated 7 to 9 km of segments of the fault system with multiple measurements along each slip necessary to produce the La Gonzalez pull-apart basin and two other segment. Offset land forms were also measured with calibration points pull-apart basins between Mérida and Mucuchies and between based on dated displaced deposits. These methods are useful in areas Mucubaji and Las Mesitas. Assuming that the displacement is Pleistocene where collecting samples for absolute dating such as radiocarbon is in age (b1.8 Ma) gives a slip rate of about 3–5 mm/a. However, because difficult or impossible: the time span involved in the CASF displace- of the rather large uncertainties in the age of this estimate it was not ment is larger than the 40 ka range for 14C and outcrops showing faulted plotted in Fig. 4. Soulas (1986) reported dextral offsets ranging from 60 buried paleosoils are rare in this area. In the northern segment to 100 m for glacial moraines dated at 13,000 BP northeast of Mérida (Colombia), the right-lateral components of slip cluster into three Andes along the Boconó fault, thus a displacement rate of 4 to 8 mm/a groups, 9±3 mm/a, 8±2 mm/a, and 7±2 mm/a, and the Ecuadorian for this segment of the fault. A portion of the Boconó fault that extends segment has a mean slip rate of 8±2 mm/a. From an offset pyroclastic for about 15 km in the area of Laguna Mucubaji examined during the flow (37.22±0.63 ka BP) and lahar (8.6±0.6 ka BP) of the Soche lava South America Workshop on Paleoseismology (SAWOP) 1997, showed flow, Ego et al. (1996) determined a long term slip rate for the Chingual-la right-lateral offset of lateral and terminal moraines of Mérida glaciation Sofia fault of 8±2 mm/a for the offset pyroclastic flow and a short term (Audemard et al., 1999). The displacement rates along the segment slip rate of 6±2 mm/a for the offset of the younger lahar. varied from 40 m in the northern strand to 85–100 m in the southern strand. Assuming the timing of the last glacial maximum advance of about 15±2 ka (Salgado-Laboriau et al., 1977), the Late Pleistocene– 2.3. Pallatanga fault Holocene slip rates for the northern and southern strands are 2–3 mm/a and 5–8 mm/a respectively. The Pallatanga fault (Fig. 1) is part of the Dolores Guayaquil megashear. It marks the southern end of the East Andean Front Fault Zone east of the Gulf of Guayaquil in Ecuador (Winter and Lavenu, 2.2. Cayambe-Afiladores-Sibundoy fault 1989; Winter et al., 1993) and extends about 200 km across the Western Cordillera of central Ecuador up to the foot of Chimborazo Dextral slip rate measurements have also been obtained for the volcano, following the entrenched valley of the Rio Pangor. In northern Cayambe-Afiladores-Sibundoy fault (CASF, Fig. 1). The CASF is a major Ecuador, the Pallantanga fault crosses to the eastern side of the Andean active structure in northern Ecuador and southern Colombia and Cordillera and is called the Chingual-la Sofia fault (Tibaldi and Ferrari, represents part of the eastern tectonic boundary of the North Andes. 1992; Ego et al., 1996). Morphological evidence for right-lateral slip is

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Fig. 4. Geological displacement estimates (Table 2) and Present day GPS slip rate for the North Andes (a) 0–20,000 years BP, (b) 0–90,000 years BP, (c) 0–600,000 years BP and (d) 0–2Ma.

well documented. Winter et al. (1993), reported short term offsets of this, calculated a displacement rate of 6–8 mm/a for the Zambapala 27±11 m and 41±4 m, and long term offsets of 590±65 m and 960± Fault Zone. Also radar imagery analysis of two transverse faults un- 70 m based on topographic measurements of offset morphological derlying the Lechuza depression, a pull-apart basin, shows evidence features such as tributaries of Rio Pangor and intervening ridges. As- of 2900±200 m of displacement since Marine Isotope Stage (M.I.S.) suming that slip on the fault has been continuous over several thousands 11 or 13 (440 ka or 550 ka), resulting in a mean displacement rate of years and punctuated by major glacial events which formed coeval of 5–7 mm/a (Dumont et al., 2005b). topographic features, Winter et al. (1993) estimated that these displacements have corresponding ages of 4–7kaBPand10–13 ka BP, and 120– 3. Results and discussion 135 ka BP and 240–250 ka BP respectively. Glacial structures of the last glacial maximum estimated to be about 10,000 BP to 12,000 BP have also Fig. 4 shows the displacement rates for the major North Andean been offset by about 30–60 m (Winter and Lavenu, 1989) giving a slip rate strike-slip faults discussed in the previous section. The four graphs show of 4±1 mm/a. the Present day slip rate from GPS results and the geological estimates of displacement for the last 20,000 years (Fig. 4a), 90,000 years 2.4. Dolores Guayaquil megashear (Gulf of Guayaquil-Tumbes Basin) (Fig. 4b), 600,000 years (Fig. 4c), and 2 Ma (Fig. 4d).Fromthese,mean displacement rates of 6 mm/a, 8 mm/a, 6 mm/a, and 10 mm/a were Witt et al. (2006) propose that the northward drift of the North obtained for the last 20,000, 90,000, 600,000, and 2 million years Andean block created space for the tectonic evolution of the Gulf of respectively. These estimated mean slip rates for the middle to late Guayaquil area as a trailing edge “pull-apart” basin. Using industry Pleistocene are all consistent with Present day GPS measurements (6± seismic data and well logs, Witt et al. (2006) and Witt and Bourgois 2 mm/a) at the 95% conﬁdence level, suggesting that the rate of “escape” (2009) estimate 13.5 to 20 km of extensional lengthening from for the North Andes has remained fairly constant for the last 1.8 Ma. beginning Pleistocene (1.8 Ma) to Present time. The resulting displace- Since most of the displacements referred in this paper were measured ment rate is 10±3 mm/a. The Zambapala Fault Zone is located in Puna, a on individual faults, which may represent a fraction of the overall dis- 50 km long, 25 km wide island off the Guayas estuary and the Gulf of placement in a fault zone, it is possible that our slip rate is an under- Guayaquil. The Zambapala Fault Zone has a similar strike, and connects estimation, and the true slip rates are probably closer to the higher with the Pallatanga fault in the northeast (Deniaud et al., 1999). It runs estimated values. As discussed earlier, GPS results (Fig. 2a) and along the Zambapala Cordillera which is characterized by a morphology seismicity (Fig. 3) also indicate that shear strain is distributed across a of pull-apart basins and Quaternary uplift (Dumont et al., 2005b). Based broad zone. The measurements shown in Fig. 4 suggest that maximum on offset drainage in the vicinity of the fault, Dumont et al. (2005a) slip rates of about 8 mm/a may have been reached between 20,000 reported 35–40 m of displacement dated at 5–6 ka BP and based on and 90,000 years ago and 10 mm/a between 0.6 and 2 Ma.

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There also appears to be a correlation between the rate of nearly complete absence of intermediate depth seismicity between displacement on the faults and the obliquity of the fault and the trench 2.5° N and 1° S. Gutscher et al. (1999) and Bourdon et al. (2003) to the Nazca–South America convergence direction. The highest strain hypothesized that this shallow subduction zone was the subducted rates are measured on the Cayambe-Aﬁladores-Sibundoy fault system extension of the Carnegie Ridge. Pennington (1981), Gutscher et al. (7–8 mm/a) and the Dolores Guayaquil megashear (10 mm/a), the fault (1999), Kellogg and Mohriak (2001), and Trenkamp et al. (2002) systems oriented closest to the convergence direction. The lowest proposed that the subduction of the thick buoyant crust of the displacement rates (3–5 mm/a) are measured on the Pallatanga fault Carnegie Ridge resulted in increased coupling with the overriding system in Ecuador, the fault system oriented most orthogonal to the South American plate. Scholz and Small (1997) proposed that even convergence direction. the subduction of a large seamount would increase the normal stress As discussed previously in this paper, all the dated slip estimates for across the subduction interface, thereby increasing seismic coupling. the Boconó fault zone in the Mérida Andes of Venezuela are for only The result is a large increase in the recurrence intervals of earth- the last 15,000 years, except for one less reliable estimate based on the quakes, up to twice as long as normal. The inferred continuation of the age of formation of three pull-apart basins (Schubert, 1980). Based Paleo-Carnegie Ridge beyond the trench (Gutscher et al., 1999) has a primarily on the analysis of radar satellite and Digital Elevation Model northeast orientation and is compatible with the displacement imagery, complemented by structural ﬁeldwork, Backé et al. (2006) direction of the North Andes. GPS measurements of east-northeast described two stages of regional deformation during the Neogene– rapid subduction (58±2 mm/a) of the Nazca plate and Carnegie Quaternary: 1) Mio-Pliocene NW–SE compression followed by 2) strike- Ridge under South America at the Ecuador trench are also consistent slip shearing along the Boconó, Burbusay and Valera faults. They with the northeastward displacement of the North Andes (Trenkamp propose that the wrenching started at some point between the Pliocene et al., 2002). and the Quaternary.

3.1. Panama arc collision 3.3. Mechanical model

Several mechanisms have been proposed for the northeastward Mechanically we propose that stable South America (Guyana “escape” of the North Andes. Pindell and Dewey (1982) proposed that shield) is acting as a rigid buttress for the margin-normal component the collision of Panama with northwestern Colombia is driving the of Nazca–South America convergence, while the margin-parallel escape of the North Andes. The Panama arc arrived at the North component of the Nazca–South America convergence is driving the Andean margin about 12 Ma, and the GPS results show a Present day North Andes northeastward toward the relatively free Caribbean– collision rate of approximately 30 mm/a (Trenkamp et al., 2002). The North Andes boundary (Fig. 5). For extrusion tectonics to occur, the collision has been linked by geometry and timing to basin inversion, block being impinged on cannot be bilaterally confined (Tapponnier crustal shortening and thickening, and uplift of the Eastern Cordillera et al., 1982). The escape direction is always toward the free boundary, of Colombia (Taboada, et al., 2000). Total shortening in the Eastern which in the northern Andes is towards the Caribbean. In fact, at the Cordillera has been estimated from volume-balanced geologic profiles Caribbean–North Andes boundary in Venezuela, the Caribbean is as 68 to 180 km (Colletta et al., 1990; Dengo and Covey, 1993; Kellogg moving eastward at 20 mm/a relative to stable South America, even and Duque, 1994; Cooper et al., 1995). The timing of uplift of the faster than the North Andes, and hence assisting with the “escape”. Eastern Cordillera determined by climatic and apatite fission track Also, due to the east–west convergence of the rigid South American studies suggests that most of the shortening occurred in the last 6 Ma backstop and the rigid Panama arc indenter, the relatively ductile (Van der Hammen, 1957; Wijninga, 1996; Gregory-Wodzicki, 2000; sediments of the inverted basin of the Colombian Eastern Cordillera Mora, 2007). At the Present rate of collision, this would require are compressed as in a vice. that most of the Panama–South America convergence has gone into permanent deformation and mountain building in the last 6 Ma. The fact that none of the reliable dated slip estimates for the Boconó 3.4. Timing of Carnegie Ridge collision and North Andean “escape” fault zone in the Venezuelan Andes are older than 15,000 BP suggests that the primary driving mechanism for the displacement on the Gutscher et al. (1999), based on examination of the basement Boconó fault is younger than the Panama–North Andes collision. uplift signal along a trench-parallel transect, and adakite volcanism Furthermore, because the Panama–North Andes suture zone is located along the Ecuador arc, proposed that the ridge reached the trench and in northwestern Colombia, although the collision may partially account has been colliding with South America for at least 2 Ma and most for the “escape” in northern Colombia and Venezuela, the collision could likely for the last 8 Ma. Subsequently, Garrison and Davidson (2003) not explain northeastward “escape” in southern Colombia and Ecuador, among others, documented that adakites in the Andes can be and so cannot be the sole driving force behind the northeastward explained by the state of equilibrium that exist between the mantle displacement of the North Andes. wedge derived arc magma and the thickened garnet-bearing continental crust and therefore do not require melting of subducted 3.2. Carnegie ridge collision oceanic slabs. Pedoja (2003) and Cantalamessa and Di Celma (2004), based on analyses of marine terrace uplift independently postulated Pennington (1981) and Gutscher et al. (1999) proposed that the that ridge subduction began at the Pliocene–Pleistocene boundary arrival of the aseismic Carnegie Ridge at the Ecuador–Colombian (1.8 Ma). Lonsdale and Klitgord (1978) proposed that the Carnegie trench initiated the “escape”, while Kellogg and Mohriak (2001) Ridge arrived at the trench about 1 Ma, based on interpretation of proposed that the rapid oblique subduction of the Nazca plate and the magnetic anomalies and bathymetry of the Cocos and Nazca plates. Carnegie Ridge may together be driving the northeastward “escape”. They concluded that the Malpelo, Cocos, and Carnegie Ridges were Elastic modeling of observed horizontal displacements in the Ecuador formed by a hot spot that originated about 20 to 22 Ma. Isotopic and forearc is consistent with partial locking in the subduction zone and geochemical similarities and paleomagnetic latitudes suggest that the partial transfer of motion to the overriding South American plate Caribbean Large Igneous Province (139–69 Ma) may have been (Trenkamp et al., 2002; White et al., 2003). Gutscher et al. (1999) and formed by melting in the Galapagos mantle plume (Duncan and Bourdon et al. (2003) noted an apparent shallowing in the subduction Margraves, 1984; Hoernle, et al., 2004). If this is true, the Galapagos of the Nazca plate in northwestern Ecuador based on the distribution hot spot may have undergone a quiescent phase prior to reactivation of hypocenters obtained from a local seismic network as well as the about 20 to 22 Ma.

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Fig. 5. Schematic 3-D tectonic model for the northern Andes showing principal plate boundaries. Cross-section through 2° S latitude shows the subducting Nazca plate and the detached North Andean block. Arrows show Present day GPS-determined relative velocities of major plates (white) and the North Andes (black) relative to stable South America (Trenkamp et al., 2002).

3.5. Eastern Cordillera overriding plate (McCaffrey, 2002). Sumatra is a classic example of the slip partitioning process, where the Australian plate is subducting The Eastern Cordillera of Colombia is a NE-trending fold belt obliquely 35°–50° relative to the normal to the Java trench beneath bounded by the Llanos foreland basin to the east and the Magdalena the Eurasian margin of southwest Sumatra. Using rigid-body rotations Valley basins to the west (Julivert, 1970; Colletta et al., 1990; Dengo to describe the motions of forearc blocks and calculating surface and Covey, 1993; Cooper et al., 1995). It has been interpreted as a deformation with elastic halfspace dislocations, McCaffrey (2002), Cretaceous extensional basin that was inverted during the Cenozoic estimated a velocity of 13.7±6.9 mm/a WNW close to the Sumatra (Colletta et al., 1990; Dengo and Covey, 1993; Cooper et al., 1995; fault. The oblique subduction of the aseismic Cocos Ridge at the Mid- Mora et al., 2006. Most of the structures in the Eastern Cordillera are America Trench may also be driving the detachment of the Panama related to NE–SW-trending thrust faults. The lack of a clearly deﬁned arc from the Caribbean plate as well as driving Panama's eastward northeast-trending strike-slip fault zone in the Eastern Cordillera of collision with South America. Colombia may be because the rapid range-normal permanent shortening and uplift obscures the strike-slip signal. We estimate 4. Conclusions the average range-normal shortening rate for the Eastern Cordillera over the last 6 Ma as approximately 20 to 30 mm/a. This is 3 to 5 If subduction of the Carnegie Ridge is the driving mechanism for times greater than the Present range-parallel “escape”,andthetotal the northeastward displacement of the North Andes, then the shortening of 120 to 180 km is 15 to 20 times greater than the largest initiation of the “escape” could indicate when the ridge arrived at dated measurements of northeastward strike-slip displacement. the trench. The compilation of geologic estimates presented in this Despite the lack of evidence of clear surface displacement in the paper suggests that the North Andean “escape” has been occurring for Eastern Cordillera, both earthquake focal mechanisms discussed earlier the last 1.8 Ma. If increased coupling, resulting from the subduction of and the GPS measurements indicate northeastward displacement the Carnegie Ridge beneath the South American plate, is driving the consistent with “escape.” northeastward “escape” of the North Andes, the Carnegie Ridge must have reached the trench at least 1.8 Ma. Well dated evidence for 3.6. Other examples of slip partitioning and oblique oceanic plate and earlier northeastward displacement of the northern Andes has not yet ridge subduction been reported. Since the East Andean fault zone passes near the cities of Quito, Bogota, and Merida, the on-going northeastward slip of the The margin-parallel displacement of a block or sliver of the northern Andes poses a continued seismic risk for the populations of overriding plate above a subduction zone is not unique to the North these cities. Andes. McCaffrey (1996) has shown that about half of all modern subduction zones have mobile forearc blocks. Slip partitioning into Acknowledgements margin-parallel and margin-normal components within the overriding plate at oblique subduction zones frequently results in litho- This study was supported by the University of South Carolina and a spheric blocks being detached from the overriding plate. The forearc grant from BP Colombia. The authors are grateful to Robert Trenkamp blocks are strained by plate coupling and are displaced relative to the for his input and suggestions, as well as Editor Mian Liu, and the

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