ELSEVIER Earth and Planetary Science Letters 168 (1999) 255±270 Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision M.-A. Gutscher a,Ł, J. Malavieille a, S. Lallemand a, J.-Y. Collot b a Laboratoire de GeÂophysique et Tectonique, UMR 5573, Universite Montpellier II, Place E. Bataillon, F-34095 Montpellier, Cedex 5, France b IRD, Geosciences Azur, Villefranche-sur-Mer, France Received 17 July 1998; accepted 2 March 1999 Abstract The North Andean convergent margin is a region of intense crustal deformation, with six great subduction earthquakes Mw ½ 7:8 this century. The regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the Andes. Segments of steep slab subduction alternate with aseismic regions and segments of ¯at slab subduction. This segmentation is related to heterogeneity on the subducting Nazca Plate. In particular, the in¯uence of the Carnegie Ridge collision is investigated. Four distinct seismotectonic regions can be distinguished: Region 1 ± from 6ëN to 2.5ëN with steep ESE-dipping subduction and a narrow volcanic arc; Region 2 ± from 2.5ëN to 1ëS showing an intermediate-depth seismic gap and a broad volcanic arc; Region 3 ± from 1ëS to 2ëS with steep NE-dipping subduction, and a narrow volcanic arc; Region 4 ± south of 2ëS with ¯at subduction and no modern volcanic arc. The Carnegie Ridge has been colliding with the margin since at least 2 Ma based on examination of the basement uplift signal along trench-parallel transects. The subducted prolongation of Carnegie Ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed, broad volcanic arc. These ®ndings con¯ict with previous tectonic models suggesting that the Carnegie Ridge entered the trench at 1 Ma. Furthermore, the anomalous geochemical (adakitic) signature of the volcanoes in the broad Ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant, shallowly subducting Carnegie Ridge from segments of steep subduction in Regions 1 and 3. It is further suggested that Carnegie Ridge supports a local `¯at slab' segment similar to that observed in Peru. The impact of the Carnegie Ridge collision on the upper plate causes transpressional deformation, extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the San Andreas fault system. The pattern of instrumental and historical seismicity indicates (1) great earthquakes on the northern and southern ¯anks of the colliding ridge, (2) a slight reduction in observed seismicity at the trench±ridge intersection, (3) increased stress far into the continent, and (4) a NNE displacement of the N. Andes block, to be further effects of the collision. 1999 Elsevier Science B.V. All rights reserved. Keywords: Northern Andes; plate collision; tectonics; seismicity; segmentation; earthquakes Ł Corresponding author. Fax: C33-467-523908; E-mail: [email protected] 0012-821X/99/$ ± see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0012-821X(99)00060-6 256 M.-A. Gutscher et al. / Earth and Planetary Science Letters 168 (1999) 255±270 1. Introduction and tectonic setting reactivated in three subsequent events from south to north, in 1942, 1958 and 1979 [1,2]. This region, The North Andean margin is a region of intense frequently cited when discussing models of segment crustal deformation, in particular where Carnegie rupture, earthquake propagation and recurrence in- Ridge is subducting beneath Ecuador (Fig. 1). This tervals [1±4], was also the site of the large Mw D 7:1 section of the subduction zone has produced six earthquake, August 4, 1998. The region of great- great (de®ned as Mw > 7:75) earthquakes this cen- est seismicity coincides with the subducted northern tury. The largest in 1906 (Mw D 8:8) had an esti- ¯ank of Carnegie Ridge. Unlike other subduction mated rupture zone of ca. 500 km length, partially zones, in the North Andes instrumental and histori- 10˚N Plate Motions Caribbean Plate Global Model NUVEL-1 (De Mets et al., 1990) 7.2 cm/a 8˚N GPS 9.1 cm/a Panama FZ (Kellogg and Vega, 1995) 5 6˚N Cocos Coiba R. North 4 7.0 cm/a Plate Cocos Ridge 5.0 cm/a Andes 6-7 3 Block cm/a 6-7 4˚N cm/a 2 Malpelo R. Panama 5B 2 4 Rift DGM 2 5A 3 Malp. Yaquina 3 Rift Graben 2˚N 2 2 5A 2500m 4 7.0 cm/a 5B 2 7.0 cm/a 5 0˚ Galapagos 3 Islands Coastal R. Explanation Carnegie Ridge Brazilian Shield 2˚S Subduction Zone Active Volcano Thrust Fault Mangrove Swamp Nazca GG Spreading Center Drainage Basin w. Plate (active) Outflow Direction 4˚S Spreading Center (inactive) Oceanic Plateau Grijalva FZ (2500 m contour) Figures 2,3 Alvarado R. Transform Sarmiento R.10 Coastal Relief 6˚S 200 m, 400 m Farallon Ocean Crust9 Strike Slip Fault 8 Inferred Fault or 3 Magnetic 7 Anomaly Subducted Structure 8˚S 95˚W 85˚W 80˚W 90˚W 75˚W Fig. 1. Tectonic setting of the study area showing major faults, relative plate motions according to GPS data [7] and the NUVEL-1 global kinematic model [8], magnetic anomalies [13] and active volcanoes [50]. Here and in Fig. 4, the locations of the 1906 (Mw D 8:8, very large open circle) and from south to north, the 1953, 1901, 1942, 1958 and 1979 (M ½ 7:8, large open circles) earthquakes are shown. GG D Gulf of Guayaquil; DGM D Dolores±Guayaquil Megashear. M.-A. Gutscher et al. / Earth and Planetary Science Letters 168 (1999) 255±270 257 cal seismicity M ½ 7 extend hundreds of kilometers place ca. 25 Ma, breaking the Farallon Plate into inland and beyond the volcanic arc. The current de- the Cocos and Nazca plates to the south and the bate on seismic coupling in subduction zones [5,6] Juan de Fuca Plate farther to the north [12]. Due and its relation to subducting bathymetric highs (as- to differential stresses on the northeastward-subduct- perities) bears strongly on the entire area and on the ing Cocos Plate and the eastward-subducting Nazca assessment of its seismic risk. Plate, spreading was initiated around 23 Ma be- Recent advances in instrumentation and computer tween the two plates in the vicinity of the Galapagos technology and availability of satellite-derived data hotspot and later evolved into the current-day Gala- present a unique opportunity to combine indepen- pagos Rift with N±S spreading [13,14]. The Grijalva dent, high resolution, digital data sets from a va- scarp, an old N60ëE fracture zone in the Farallon riety of ®elds (seismology, volcanology, morphol- Plate, is interpreted to represent the southern half of ogy=topography, satellite altimetry, GPS±geodetic the scar where the Nazca Plate tore off. Carnegie studies, structural geology) for application to geo- and Cocos ridges are mirror-image hotspot traces dynamic problems. The principal objective of this formed by the northeastward motion of the Cocos study is to re-examine the seismicity and tectonics Plate and the eastward motion of the Nazca Plate in this region of intense crustal deformation in view over the Galapagos hotspot [13±16]. According to of these newly available data. By studying the spa- this model both Cocos and Carnegie ridges arrived at tial distribution of hypocenters and examining focal their respective trenches 1 Ma [13]. mechanisms we aim to identify the principal fault Most models suggest that Malpelo Ridge is a planes, identify areas prone to large earthquakes and former continuation of Cocos Ridge separated by scrutinize apparent seismic gaps. Correlations be- dextral motion along the N±S-trending Panama FZ tween structural heterogeneity of the oceanic Nazca [13,14]. Malpelo and Carnegie ridges separated from Plate and uplift, deformation and volcanism in the one another during a period of rifting and sea¯oor overriding South American Plate should improve spreading from ca. 17 to 8 Ma [13,15,17]. The ex- our understanding of these processes and of the pressions of the extinct Malpelo Rift and adjacent overall lithospheric structure of an oceanic plateau± rift segments as well as the transform faults separat- subduction zone collision. ing them are clearly visible in the morphology of the Along the North Andean margin, the Nazca Plate sea¯oor (Fig. 2). is subducting eastwards beneath South America at arateof5±7cm=a [7,8] (Fig. 1). Simultane- ously, the North Andean block is being displaced 2. Seismicity database and observations to the northeast at a rate of ca. 1 cm=a along the Dolores±Guayaquil Megashear accounting for the Earthquake data in the study area were obtained discrepancy between the global model (Nazca mo- from Engdahl et al.'s recent global relocation effort tion relative to stable South America) [2] and the [18], improving hypocenter locations from the Inter- GPS measurements (Nazca motion relative to the national Seismological Centre (ISC) Catalog. Thus, North Andes block [7,9] (Fig. 1). Thus, the ENE-ori- for this study area 1230 events Mb > 4:0 from ented Carnegie Ridge is sweeping southwards along January 1964 to December 1995 were available. Cat- the Ecuador margin. Recent coastal uplift facing alogs of lesser quality but (1) covering a greater time Carnegie Ridge is indicated by Pliocene marine ter- span (the South American SISRA Catalog [19] ca. races (Tablazos) exposed at 200±300 m elevations 800 events Mb > 4:0 from 1900 to 1972), or (2) [10,11]. To the north, the morphology, widespread comprising more events (the USGS PDE ± Prelim- mangrove swamps and neotectonics of the South inary Determination of Earthquakes ± Catalog; ca.