Impact of the Carnegie Ridge Collision

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ELSEVIER Earth und Plnnciary Scicncc Letters 16K II9YY) 255-270

I Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision -

I? J. M.-A. Gutscher a**, Malavieille "'S. Lallemand a, J.-Y1 Collot ': Luhoruroirt de Ginphpique er %CJOIli~lIC,UMR 5573, liniversiJi Monrpellier II. Pluce E. Buiuillon. F-34UY5 Montpellier; Cedex 5. France IRD, Gcwciences AXE Villtfranchr-sur-Me%France

Received i 7 July 1998: accepted 2 March 1999

Abstract

The North Andean converfent margin is a region of intense crustal deformation, with six great subduction earthquakes M, 2 7.8 this centun.. The regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the .4ndes. Serments of steep slab subduction alternate with aseismic regions and secgents of flat slab subduction. This segmentation is related to heterogeneity on the subducting Nazca Plate. In particular, the influence of the Carnegie Ridge collision is investigated. Four distinct seismotectonic regions can be distinguished Region 1 - from 6"N to 5"N with steep ESE-dipping subduction and a narrow volcanic arc: Region 3 - from 3.5"N to 1"s showing an intennediate-depth . seismic gap and ;I broad volcanic arc: Region 3 - from 1"s to 3"S.wit.h steep NE-dipping subduction. and a narrow volcanic arc: Region 3 - south of 2"s with flat subduction and no modem volcanic arc. The Carnegie Ridge has been colliding with the margin since at least 3 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 pzturbed. broad volcanic arc. These findings conflict with previous tectonic models suggesting that the Carnegie Ridge entered the trench at I 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. shallo\vly subducting Carnegie Ridge from segments of steep subduction in Regions 1 and 3. It is further suggested that Cmegie Ridge supports a local 'flat 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 uith a modem level of seismicity comparable to the San Andreas fault system. The pattern of instrumental and historical seismicit! indicates (11 great earth-quakes on the northern and southern flanks of the colliding ridge. (3) a slight reduction in obsend seismicity at the trench-ridge intersection. (3)increased stress far into the continent. and (4)a %E displacement of the S. Andes block. to be further effects of the collision. C' 1999 Elsevier Science B.V. .411 rights resened. 5' '* k~*wrds:Nonhrm Andes: plate collision: tectonics: seismiciry: segnientation: earthquakes

@I 1. Introduction and tectonic setting reactivated in three subsequent events from south to north, in 1942, 1958 and 1979 11.2''. This region, The horth 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 bencath Ecuador (Fig. 1 ). This tervals [ 1-41, was also the site of the large M,. = 7.1 section of the subduction zone has produced six earthquake, August 4, 1998. The region of great- great (defined as M, > 7.75)earthquakes this cen- est seismicity coincides with the subducted northern tury. The largest in 1906 (M, = 8.8) had an esti- flank of Carnegie Ridge. Unlike other subduction mated rupture zone of ca. 500 km length, partially zones, in the North Andes instrumental and histori- M cal seismicity 2 7 extend hundreds of kilomcters 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 heismic coupling in subduction zones 15.61 Juan de Fuca Plate farther to the north 1131. Due and its relation to subducting bathymetric highs (as- to differential stresses on the northeastward-subduct- perities) brdrs strongly on the entire area and on the ing Cocos Plate and the eastward-subducting Nazca assessmefk6f '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 opponunity 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 N60"E fracture zone in the Farallon riety of fields (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 [ 13-1 61. 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 MaJ 131. 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 [ 13.141. Malpelo and Carnegie ridges separated from Plate and uplift. deformation and volcanism in the one another durin2 a period of rifting and seafloor 8 overriding South American Plate should improve spreading from ca. 17 to 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 seafloor (Fig. 2). is subducting easnvards beneath South America at a rate of 5-7 cmja [7.S] (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 Mepashear accounting for the Earthquake data in the study area were obtained discrepancy between the global model (Nazca mo- from Engdahl er al.'s recent global relocation effort tion relative to stable South America) [?] and the [IS]. 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 1. Thus. the ENE-ori- for this study area 13-30 events M,, > 4.0 from ented Camepie Ridge is sweeping southwards along Januay 1964 to December 1995 were available. Cat- the Ecuador margin. Recent coastal uplift facing alogs of lesser qualit!. bur ( 1) covering a greater time Carnegie RidFe is indiaxed tr! Pliocene marine ter- span (the South American SISRA Catalog [19] ca. (11 races (Tablazos) exposed at 200-300 m elevations SOO events Alb > 4.0 from 1900 to 1971). or 110.1 13. To the north. the morpholosy. ividespread comprising more events (the USGS PDE - Prelim- nlangrove swanips and neotectonics of the South inary Determination of Earthquakes - Catalo,:lT ca. Cohnbian coastal blo~SU,, "wst active subsidence 1800 events Alb > 4.0 from 197.3 to 19971. were also 1113. examined and revealed the sanie overall distribution The most \videl! accepted tecronic model for the patterns. though with inferior spatial resolution and fomlation of the oceanic crust of' the E. Panama \vel1 kno\vn 'artifacts' such as large concentrations Basin sugyests that :i nujor plate reorganizarion took of events ;ir depths of' -3? kni or IS km. 2%

BO sleep [kml ESE dipping sla 120 160 200

.- 40 e.,-.. .-*... . +;; a------.. BO Ikml ---:--!@!----’.? 120 Carnegie %at dab’ ? 160 168 (1999) M.-A. Gutscher et nl. /Euah und Plunetun Scicncc, Letren 255-270 259 .. The EHB database [IS] is shown sorted by mag- age a steepdipping slab segment with its narrow nitude and hypocenter depth and was used to cal- overlying arc. culate depth contours to the Wadati-Benioff zone Region 4, Section D-D'. South of 2"s the Peru (Fig. 2). Four different regions, with distinct pat- 'flat slab' segment begins, with a volcanic gap all the terns of seismicity and volcanic activity [15,20] are way to 15"s in southern Peru [23-261. Nearly hor- observed from north to south. Representative litho- izontal subduction is observed, despite the greater spheric cross-sections based on the seismicity, vol- age and presumably greater density and thickness canism and topography databases are presented to of Farallon crust. Recent work suggests that this illustrate these main differences Fig. 2). 1500 km long 'flat slab' is supported by two buoy- Region I, Secrion A-A'. The northemmost region ant oceanic plateaus, the completely subducted Inca (in west-central Colombia) shows a steep ESE-dip- Plateau in N. Peru and the currently subducting c(r- *pingWadatj-Benioff zone down to a depth of about Nazca Ridge in S. Peru [27]. Since no intervening 300 km in good agreement with earlier seismolog- wedge of asthenospheric material is present between ical [I51 and recent tomographic resdts [21]. The the downgoing Nazca Plate and the overriding South volcanic arc is narrow and located above a slab at American Plate, partial melting is inhibited and no ca. 160 km depth. Malpelo Ridge and the active arc develops. Shallow events (<40 km) in the E. Panama FZ lie west. Lithospheric ages (4Ma) and .4ndes (between km 700 and 780) appear to repre- thicknesses (420 km) are least to the west since sent inuacontinental 'subducdon' where the Brazil- nearby at the Panama Rift. new oceanic lithosphere ian shield underthrusts the Andes at a rate of ca. 1.5 is currently forming (Fig. 1). "/a. producing crustal-scale thrust slices [28]. Region 3. Secrion B-B'. Between 2.5"N and 1"s While the northern two sections are oriented there is a nearly complete absence of intermediate nearly parallel to the relative convergence direction depth seismicity fonly one event >90 km depth). between the North Andes block and the Nazca Plate The seismic gap is confirmed by the SISR4 and [7,9], the southern sections are oriented ENE-WSW PDE catalogs as well as by a local seismological in order to better sample the particular region. Note network of 54 stations operated during the Litho- that the 'seismological thickness' of the subducting scope experiment [I!].Carnegie Ridge enters the oceanic lithosphere in all profiles is ca. 30 km, as trench across from the broad ( 160 km wide). adakitic pointed out by earlier investigators [23). This cor- volcanic arc. The inferred subducted continuation of responds roughly to the effective elastic thickness Carnegie Ridge is interpreted to be a 'flat slab' con- (Te)of the lithosphere as related to its thermal age fi-pration. but is constrained by only a single major through well known cooling relationships. For a 40 event Mh = 5.8 at 130 km depth. Ma oceanic lithosphere an effective elastic thickness Region 3, Secrion C-C'. Between 1"s and 2"s of ca. 25 km is predicted for a wet olivine mantle there is cluster of intermediate-depth seismicity rheology. uhile a 100 Ma lithosphere is predicted to defining a steep hT-dippin: slab. South of Carnegie have T, = 35 km [29]. Ridge the transition from young to older oceanic Finally. the Andes parallel section F-F' (Fig. 3) lithosphere (Grijalva FZ) is crossed. The narrower also clearly illustrates the principal differences be- 1.2" sampling width and EhZ alisnment clearly im- tween the four regions. Furthermore, it implies that 260 M.-A. Gutschrr et ul. /Eunh und Plunetan Science Laiers 168 (1999) 255-270

Table I Listing of events and fault plane solutions obtained from the Hantard CMT catalog 1301

- ~~ ~~ ~~~ ~ Event No. Date Time Lat. Long. Depth Mh M, Fault plane 1. Fault plane 2 NOES (d/m/y) GMT (") (4 (km) strike/dip/slip suike/dip/slip mcch./loc.

(7 (O) 1 30/01/80 06:56 5.18 -82.10 18.0 5.8 6.3 41121- 111 212/81/- 19 ss PanFz 2 26/11/83 09:22 3.96 -82.52 32.1 5.4 5.6 114/83/ 173 26518318 ss PanFz 3 12/15/88 13:32 5.31 -81.49 6.2 5.3 5.6 181/90/180 271/90/0 ss PanFz 4 04/02/89 19:24 5.81 -82.63 10.9 5.8 6.3 93/16/10 0/80/166 ss PanFz 5 20/07/90 04:05 4.70 -81.63 1.o 5.0 5.1 116/90/ 180 266/90/0 ss PanFz 6 15/08/11 2023 2.66 -84.51 17.1 5.2 5.0 251 /45/-90 I1 1451-90 N PanRift I 18/09/92 1050 3.39 -81.96 22.2 5.1 5.4 241/45/-90 74/45/40 N PanRift .O8 8 08/05/71 16:45 -1.25 , -81 29.6 5.1 5.5 336116163 184/16/91 T subd 9 01/03/79 14:33 0.61 -80.06 9.6 5.6 5.7 23/24/116 115/69/19 T subd 08:OO 7.9 10 12/12/79 1.57 -79.36 26.6 6.4 3011 611 18 181/16/83 T subd 11 26/01/80 15:17 2.21 -79.57 26.1 5.0 5.6 12/22/16 207/69/95 T subd 13 06/05/81 21:36 -1.92 -80.94 15.1 6.0 6.4 339/17/81 168/73/93 T subd 13 10/06/85 0323 3.00 -18.51 29.2 5.6 5.5 3211 9/13 116/74/19 T subd 25/08/90 14 11:43 5.75 -77.53 28.4 5.1 5.3 350/36/19 c 183154198 T subd 15 02/09,'90 04:21 -0.16 -80.25 14.6 5.8 6.2 22/27/115 114/65/78 T subd 77.79 16 19/11/91 -e.- 4.53 -77.38 15.0 6.5 7.2 13113/95 188/11/89 T subd 11 01/01/81 1025 2.11 -79.82 11.8 5.1 5.3 161431-I8 180/48/-101 N Yaq. B 18 12/02/89 20:03 2.77 -79.16 4.9 5.0 5.1 3401461-1 16 195/50/-66 N Yaq. B 19 09/09/89 01:40 2.46 -79.73 14.8 6.0 5.1 1/19/-123 21 81661-73 N Yaq. B 10 26/11/94 0448 2.81 -79.43 15.9 5.1 5.3 3571311-114 206/51/-73 N Yaq. B 21 1311 1/95 12:36 2.92 -79.39 11.6 5.3 5.3 2081451-90 281451-90 N Yaq. B 71 I 02/01/81 07:31 2.12 -79.16 14.2 5.7 5.9 76/32/-128 25511581-61 N tear? 23 07/01/81 0201 1.98 -79.26 42.5 5.7 5.9 32/33/- 114 240/60/-75 N tear? 24 18/08/80 15:08 -2.02 -80.04 55.5 5.5 5.9 19/17/36 156/15/-1 12 tear? 1 s '5 11/06/81 1:54 -3.06 -80.28 53.1 5.1 5.3 198/30/-20 45/80/-119 s tear? 26 10/02/90 17:12 -3.20 -80.73 54.7 5.6 5.5 138/58/161 235/79/32 s tear? 21 19/05/83 1901 0.14 -77.10 24.1 5.6 5.2 60/49/129 189/54/54 ST EAnd 28 06/03/SI 01:.s4 -0.01 -77.66 12.2 6.1 6.4 198/20/118 348/73/81 T EAnd 29 06,!03:s7 01:lO 0.04 -77.78 19.5 6.5 7.1 i gspips 7/64/86 T EAnd 30 06/03/81 08:13 0.07 -71.85 8.6 5.4 6.0 176/40/-166 125/81/-51 SS EAnd 31 ll!08!90 02:59 -0.18 -78.53 20.2 5.0 5.1 323!45/53 190/55/ 112 ST Quit 32 21 109 '87 1621 - 1.O6 -78.01 19 5.8 6.0 197!41/129 330/59/61 ST IAD 33 26/12/92 1157 -1.01 -78.01 6.8 5.8 5.4 200/16/166 300/80/45 ST IAD :4 17!01/&4 16:19 -3.93 -81.40 35.4 5.9 5.5 303/45/78 139/41/101 T hot 35 19!07,'90 1529 -4.94 -81.00 41.5 5.4 5.1 10/17/58 23/16/99 T Amo1 36 '9!03,r9 1 20: -3.9s -80.91 38.2 5.1 5.6 55/17/ 171177166 T hot 13 -'.U 15 1 31 11 'o1'82 1221 -78.63 15 5.1 5.1 39/16/56 156/68/106 TASA j 8 o3 ,Io '9.5 0151 -2.83 -77.85 43.5 6.5 7.0 2.;3/39/110 18/51/68 T Sbhd 39 0-3 '10'95 1245 -2.81 -77.S2 15.1 6.0 6.3 I43/45/ 151 349/14/47 ST Sbhd 40 IO 06'10'95 1027 -2.69 -77.89 33.6 5.3 5.4 230/80/- 1 148/80/-10 SS Sb.4nd IO.ì -11 .;I '83 13:13 2.41 -16.63 19.3 5.4 5.6 16,'76,'175 117/85/14 ss Col 42 oh 'Oh '94 1041 2.79 -75.91 5.4 6.4 6.8 206/76!110 799/60/14 Col 10 ss 43 /o1'80 1126 -2.58 -78.52 101.1 5.1 5.' 151 p5i-90 31/55/-93 dp.EcPcr -u 0s,'10:'s0 2201 - 1.40 -77.63 199.5 S.!i 6.0 1 IS/ 161- 1 17 323/76/-82 dp.EcPer 07:O' 45 03.11 1!S1 -1.87 -78.36 131.8 5.5 5.9 133;3/-99 313/62/-85 dp.EcPer 46 18'11:81 1.157 - 1.77 -76.65 193.1 6.0 6.5 163/"9/-51 301 /68/-109 dp.EcPer 41 11'0-1 '83 1207 -1.Sh -78.10 90.3 6.5 7.0 339/35/-86 1531551-93 dp.EcPrr 4s 11 '09!?3 06:14 -4.7 1 -76.27 120.2 5.6 5.7 154:u/ -109 359/49/-73 dp.EcPer 49 '79 '9 105 1259 5.14 -75.71 116.9 -1.9 5.0 174,t'Uj- 100 8/47/-80 dp.Col 50 3:11 '79 3-10 4.78 -76.11 107.8 6.4 7.2 157!4 1 /- 16.3 35/19/-51 dp.Col 'Ob '80 ;i4 51 25 11:O.F 4.4s -75.11 165.4 5.7 h.0 2.31 'I4 l37/16/164 dp.Col 52 IS 'OS 91 19:02 5.11 -75.60 116.8 5.7 5.9 22S,22,'-7 1 18/69/-91 dp.Col

Hypocmirr locxions ïmm Enpdahl ei al. 1181. M.-A. Gutscher cl al. /Eurrh und Plunctary Science Letters i68 (1999)255-270 26 1

the continuation of Carnegie Ridge supports a lo- base level in the submarine regions at the northern ca] ‘flat slab segment’ at ca. 100-140 km depth, end of the profile. If recent sedimentary fill is taken bounded to the south (and possibly to the north) by a into account the difference is again 2-2.5 To , ci&- km. lithospheric tear. ’ the south the picture is obscured by the dramatic A total of 189 fault plane solutions for the study variations in basement level in and around the Gulf area were available from the Harvard centroid mo-- of Guayaquil. Here the 8 km deep Progreso Basin ment tensor CMT) catalog [30] for the period Jan- and the 10 km deep Jambeli and Esperanza basins in uary 1976December 1997. A representative subset the Gulf of Guayaquil are believed to have formed , of 52 of these events is listed in Table 1 and shown in as pull-apart basins within the strike-slip regime of Fig. 3 using the more reliable EHB hypocenter loca- the Dolores-Guayaquil Megashear @GM) [3 1-33]. tions [ 181 to highlight areas of particular interest and The signal of the Carnegie Ridge induced basement more complicated smcture, not readily explained by uplift is less clear in the Inland Valley profile 180 km current models. east of the Colombia Trench (Fig. 4d). The basement In general, the mechanisms are consistent with is substantially higher in this region, but poorly con- the plate.tectonic model of the study area. Several strained sediment thicknesses to the north, the 10 km principal types can be distinguished: (1) shallow deep Jambeli Basin to the south and the overlying dextral strike-slip in the active Panama FZ (events 1- Manabi Basin, render interpretation uncertain. 5); (2) shàllow normal faulting in the active Panama Pliocene marine terríiees of the Tablazo Forma- Rift (events 6, 7); (3) shallow underthrusting in the tion [10,1 l] exposed at 200-300 m elevation in the subduction zone (events 8-1 6); (4) shallow normal Coastal Range and the regional morphology are con- faulting in the flexural bulge east of Yaquina Graben sistent with the southward mi-gation of Carnegie (events 17-21); (5) shallow thrusting and dextral Ridge according to the relative plate motions ob- strike-slip events in the upper plate (events 27-42); , tained by GPS measurements [7]. The young topog- and (6) deeper. commonly normal faulting events in raphy (in excess of 400 m) of the Coastal Range lies the downgoing slab (events 43-52). directly across the trench from Carnegie Ridge. To the north, in S. Colombia, the coastal block is currently subsiding as shown by vertical motions during 3. Neotectonics and recent vertical motions the 1979 earthquake [34] and by the coastal man- grove swamps [13]. The margin here is also marked The ‘basement uplift signal’ of Carnegie Ridge by large forearc basins [35.36] indicating subsidence was determined along a reference profile 100 km of the former continental shelf. This pattern of recent west of the trench and compared to profiles along a uplift across from a subducting high and subsidence curved uack parallel to the course of the Colombia to the north indicates collision since at least 8 Ma. Trench (Fig. 3). This was done to establish how far arc-ward the ’basement uplift signal’ can be identi- fied. Carnegie Ridge. together with the step from the 4. Geodym”c model Grijalva scarp. has a 400 km width and a height of ca. 2 km (Fig. 4a). The axial trench profile clearly Based on four principal pieces of evidence we shows the subducting ridge where the trench floor propose that a prolongation of Carnegie Ridge has shallows by 1-15 km with respect to regions north been subducting for at least 2 Ma and most likely 8 and south (Fig. 4bl. -4fter adding the 1-1.5 km sed- Ma to a position beneath the Andes. 400 km from imentary fill at the trench north of the ridge, the full the trench. 2-25 km amplitude basement uplift si,onal is ob- (1) The regional bathymetric/topographic profiles lained. Beneath the Coastal Range 110 km from the indicate a basement uplift signal comparable in size mnch, the regional bathymetric/topographic profile to Carnegie RidSe extending at least 110 km east again shows a pronounced uplift in the continuation from the trench. A N-S drainage divide 180 km of the Carnegie Ridge (Fig. 3c). The elevations of east of the trench lies along the prolongation of the the Coastal Range are ca. 1.5 knl higher than the CarneSie Rid_ce crest. 262 M.-A. Gutscher el ul. /Eanh und Planetary Science Letters 168 (19991 255-270 .-

5'5

- -.. . ~ - . 85'W V' 75'W X' Y' Z' Fig. 3. E3nhquAe fault plane solutions from the Harvard CMT (centroid moment-tensor) [30] catalog. Solutions for shallow events (40 kml are shaded black and deeper events (>SO h)gray. for 51representative events listed in Table 1 ushg I3-B 1181 hypocenter locations. Sevenl principal types can be distinguished (1) shallow dexd strike-slip in the Panama FZ (eVents 1-5): (7)shallow normal 6. faulting in the Pmma Rift (events 7): (3) shallow underthrusting in the subduction zone (events 8-16): (4) shallow normal fad~gin 17-21): the the flexural bulge east of Yaquina Graben (events (5) shallow thrusting and dextral smke-slip in upper plate (events 27-42) and deeper. conunonly normal faulting events in the downgoing slah (events 43-52). Candidate tearing events in the north (events 22, 23 and south (events X2S)are indicated. Bathymetry (1000 m contours) (511 and location of regional topo,mpGc profiles (Fig. 4) also a shown. with inferred positions of Camegie Ridge prolongation and lithospheric tears dashed.

(?I The coastal morphology [Coastal Ranse uplift, Carnegie Ridge according to relative plate motions subsidence to north in the Colombian coastal and indicate collision since 8 Ma. the S. (3) (70- block) are consistent jyith the southward migation of A pronounced gap in intermediate depth Gutscher ul. /Earth and Planetary Letters 168 íl999) I M.-A. er Science 255-270 263

2 i Undisturbed oceanic crust 0- pelagic seds. r--'--J Grijalva FZ -2 ' - /.-rJ " 4 Carnegie Ridge -4- Malpelo- -m vws/\"

-2 - trench fill Yaq. Gr.I- Ridge Grijalva FZ lower stope / - --/-+ '- %-'\ 4------1_ 4- / irench fill Carnegie Ridge y L :.:--_-_..:'. -- .__._. .

Tablazos o:. P --- -/S/\r J--J Carnegie Ridge P -2- ___c--_--c 5- - --- uplift 4 : forearc basin fill

c 2 :Inland valley profile Manabi Basin .-. - o: ------.--.------7-__-- .__-- -2 ------forearc basin fill (?) ? Carnegie Ridge '! -4- ? uplift ?

Fig. 4. Four regional topo,mphic profiles parallel to the Colombia Trench axis (locations in Fig. 3). Basin and basement stmcture (for 61 place names see Fig. adopted from earlier studies [31-331. (a) Section V-V, undishd oceanic crust profile, 110 h west of the the the trench. Note Malpelo Ridge to north and Grijalva FZ and two adjacent ridges in the Farallon Ocean Crust to south. (b) Section x-x", rrench axial profile. Note ca. 1-1.5 km thick trench fill north of Carnegie Ridge 135.361 and south of Grijalva FZ j311. The full x the 400 h 2 h sied of the Carnegie Ridge is visible. (c) Section l'-Y. Coastal Range profile. 110 h east of trench. Note 15km the Z-2'. difference in the base level berween Coastal Range and the submerged coastal block in S. Colombia. (d) Section Inland Valley Profile. 180 h east of uench. Kote the 8-10 km thick sediments in the Gulf of Guayaquil arca at the southern end of the profile.

200 h)seismicity is observed in the prolongation of below). Therefore, some major geodynamic element Carnegie Ridge. while normal seismicity continues is perturbing the system at depth. in a steep (ca. 30") Wadati-Benioff zone both north The simple model of a continuation of Carnegie and south of this reson. Seismic saps are commonly Ridge meets with two principal objections: 6: obmiedwhere an oceanic plateau or other structural (11 It conflicts with the widely accepted view that high (e.g. seamount chain. spreading center) collides the oceanic plateau terminates at the fossil Yaquina with the trench [:.;i']. FZ and that collision with the trench began recently (4) The active \volcanic arc is broad (spread out (I Ma) [l3.33]. over 150 km) and geochemically anomalous (see (7)If Carnegie Ridge has been subducting for 8 16& 264 M.-A. Gurscher er al. /Earlh and Planetary Science Letters (1999)255-270

Ma, why does arc volcanism continue in contradic- We propose a new model that combines the two tion to the model of a ridge-induced volcanic gap ideas of a lithospheric tear and a continuation of [37]? And why is steep subduction [15,38] observed Carnegie Ridge (Fig. 5). We suggest that two litho- directly to the north and to the south? spheric tears bound the Carnegie Ridge collision The opinion that Carnegie Ridge does not con- zone. Melting of the oceanic lithosphere at the edges tinue east of the Yaquina Graben is based on ge- of the tears can thus explain the adakitic magmas ometric considerations, on poorly constrained mag- observed within the broad volcanic arc of Ecuador. netic anomaly data and on the presumption that the The southern tear, imaged in Fig. 2, occurs along the hotspot trace began at 25 Ma [13,17]. The 'termi- Grijalva FZ separating the Oligocene age Farallon nation model' further proposed that both Carnegie crust to the southeast from the Cocos-Nazca crust Ridge and Cocos Ridge (Costa Rica) entered their (t25 Ma) to the northwest as has been suggested respective trenches at 1 Ma [13]. However, geolog- by previous workers [15,20]. The older, denser Far- ical data on uplift rates in Costa Rica, indicating allon crust between 2"s and 0.53 plunges steeply to strong uplift as early as 3 Ma [16], contradict this the northeast. The northern tear in the lithosphere is , interpretation for the Cocos Ridge collision and call tentatively located just north of the Carnegie Ridge into question its validity for the collision of its mirror along the extension of the Malpelo Rift fossil spread- image, Carnegie Ridge. ing center. To the north octhis region the Cocos- Recent studies suggest that the Caribbean Plateau Nazca crust (possibly attached to older Farallon crust is a large igneous province (LIP)formed as the plume at depth) subducts steeply to the east-southeast. Be- head of the Galapagos hotspot rose to the surface, pro- tween these two tears we propose a fiat slab seegent ducing immense quantities of flood basalts in the Late with a configuration similar to that observed in Peru Cretaceous ca. 90 Ma, [39,40]. Later Caribbean flood and central Chile. The buoyant prolongation of the basalt-type magmatic events occurred from 76 Ma to Carnege Ridge is interpreted to continue at least 63 Ma [40]. Thus, if the hotspot was active in the Cre- 110 km and probably up to 500 km from the trench ' taceous and from late Tertiary to Quaternary times (Figs. 3 and 5). An analysis of density and buoyancy (35 Ma to present), it seems likely that the plume and in subduction zones suggests that oceanic plateaus associated hotspot trace existed between 60 Ma and must have a basaltic crustal thickness in excess of 17 35 Ma. But if Carnegie Ridge has a subducted contin- h in order to resist subduction [45]. The western uation as suggested by some workers [15,31,41], why portion of Carnegie Ridge beneath the Galapagos does arc volcanism continue? Islands reaches a maximum crustal thickness of 18 Ecuador arc ma-gmas have an anomalous 'adaki- km 1461 and. though no geophysical data are avail- tic' geochemical signature requiring either unusually able on the crustal structure at the eastern end of the high temperatures allowing fusion of oceanic crust ridge, it is presumably similar. at shallow depths (

normal faults

steep NE dipping Fig. 5. 3-D view of the IWO-tearmodel for the Carnegie Ridge collision featuring: a step ESE-dipping slab beneath central Colombia: 2"s: a smp NE-dipping slab from 1"s to 2"s; the Peru Rat slab segment south of a northem tear along the prolongation of the by Malpelo fossil spreading center; a southm tear along the Grijalva Fz: a proposed Carnegie flat slab segment (C.ES.) supported the prolongation of Carnegie Ridge.

Ridge with the Caribbean-Galapagos Plume trace Another supporting observation is the substantial and thus succeeds in explaining the coastal pattern seismicity in the flexural bulge (five normal events of subsidence to the north and uplift in the coastal Mb = 5.1-6.0 between 1981 and 1995), limited to range in accordance with a southward-sweeping of a ca. 100 km long segment of Nazca Plate east of Carnegie Ridse. the Yaquina Graben (Fig. 3). This area corresponds The proposed lithospheric tears can be expected to to Regon 1 where the oceanic lithosphere subducts produce earthquakes with a steep E-W-striking focal most steeply and thus flexure is the greatest. Yet seis- plane and a steep slip vector. Such 'tearing events' mic activity appears to terminate southwards near the would be the only definitive validation of the model. proposed lithospheric tear. We interpret this pattern While no such events have been reported in earlier and the normal mechanisms as indications that the studies, two candidate events occurred in N. Ecuador lithosphere just north of the tear is subject to the in January 19S1. (Mh = 5.6-5.7) with a focal plane greatest flexural stresses causing surficial extension 60" striking N70"E and steep dip (events 22, 23 in in the bulge. Further south the buoyant Carnegie Table 1and Fig. 31. They occurred in the S. Colom- Ridge causes the subducting slab to bend at a re- bian forearc at depths of 14 and 43 km in the vicin- duced angle of curvature, thus lowering the flexural ity of subduction zone thrust earthquakes (Fig. 3). stress in the bulge (Fig. 5). b They cannot be extensional events due to flexure of There are candidate 'tearing events' to the south the oceanic lithosphere because (a) the flexural bulge (events 24-26 in Table 1 and Fig. 3) with a NE- is located 100 km further west (see events 17-21 in SW-smking focal plane (parallel to the Grijalva EL Fig. 31, and (b) the fault plane solutions are distinctly trend) and down to the south sense of motion. They different (Fig. 3).Since extension along the conjugate are located at 53-56 km depth and have sub-vertical N30"E focal plane seems unlikely given the MW- fault planes dipping 75-80". The substantial depth, ESE compressional stress field, down to the north mo- and their position well west of the Andes indicate tion along the N70"E-suiking fault is more plausible. that they are within the downgoing oceanic litho-

---- .._...-_ - 266 M.-A. Gutscher e? al. /Earth and Planetary Science Letters 168 (1999)255-270 sphere. Additionally, there is a band of seismicity Gulf of Guayaquil segment of the subduction zone paralleling the G~ijalvaFZ and extending from the (from 2.2"s to 3.8"s) has seen no earthquakes kfb > Gulf of Guayaquil down to the 200 km deep clus- 5.0 in the past 30 years. Either this region is largely ter seismicity (from event 26 to 44 in Fig. 3). This aseismic, due perhaps to the propagating lithospheric suggests that the opening of the Gulf of Guayaquil - tear here or, here too, seismic coupling may be quite and its 30-12 km deep basins may be a surface high and possibly building up to a magnitude 7 expression of the lithospheric tear at depth. or greater event. The SISRA catalog [19] indicates two that very large, shallow earthquakes, M 2 7.4 occurred near 3.5"s in 1953 (Fig. 1) and 1959 and 5. Interplate coupling and impact on the upper thus tends to support the latter interpretation. plate Interplate coupling at a much deeper and larger scale may also be affected by the Carnegie Ridge The Carnegie Ridge collision appears to have af- collision. In the region facing Carnegie Ridge, in- fected the coupling between the Nazca and South creased upper plate seismicity and deformation ex- American plates. Four great earthquakes have oc- tending 500-600 km inland to beyond the volcanic curred on the northern flank of the collision (1906, arc suggest the collision to be the driving mecha- 1942, 1958 and 1979), and one has occurred along nism behind the NE movement of the North Andean the southern flank (1901) (though the magnitude and block This movement 6 distributed along a net- locarion for the latter are uncertain). None of these work of transpressional faults at the southern edge events appears to have ruptured across the ridge of the N. Andes block [32,48,49] (Fig. 6). Individual itself. This pattern is similar to the Nazca Ridge fault splays visible in the morphology commonly collision in S. Peru. During the last 60 years four correlate with bands of seismicity. Yet, some faults great earthquakes M, 2 7.8 occurred north of Nazca include segments of ca. 100 km length without re- Ridge and two to the south, but again no shock cent activity (Fig. 6). These segments can also be appears to have ruptured across the ridge-trench considered potentially high risk areas. intersection [1.4]. Overall, the level of seismicity is comparable The recent shallow (t70 km) seismicity in to that of the San Andreas fault. For a 500 km Ecuador suggests a gap between 1"N and 0.8"s long segnent of the DGM, 41 Mb 2 5.0 and 5 (Fig. 3). This appears to confirm the model that Mh 2 6.0 non-subduction related, shallow events the subduction/collision of bathymetric highs lo- (t70 h depth) occurred during the period Jan- cally increases coupling [6] and causes short-term uary 1964-December 1995. Along the 800 km long seismic gaps. Recurrence intervals in such zones of se,gnent of the San Andreas fault system from San increased coupling may be as much as twice as long Francisco to the Mexican border 80 Mb 2 5.0 and and thus exceed 100 years, leading to gaps in recent 9 Mh 2 6.0 events were recorded during the same seismicity maps. Greater stress accumulation during 32 year period [18]. Historical intraplate seismicity an aseismic period implies accordingly greater stress in the Ecuadorian Andes is very high as well. Four release during rupture [6]. Indeed the August 4,1998 earthquakes of intensity 7-8, five of intensity 9, three (M,,= 7.1) event at 0.5"s occurred in an apparently events of intensity 10 and one of intensity 11 struck quiet region, but which had been correctly identi- the Andean region between O" and 4"s from 1541 to fied hy Nishenko in 1991 [47] as a region with a 1906 [1.19] pig. 6). a 66% probability of larse or great earthquake before Unfortunately, little is known about the slip rates 1999. along indilidual faults and even less about the sys- On the other hand. the regions between 1" and tem as a whole. Two studies have attempted to 9"s and between 3" and 6"s appear to be failing consuain slip rates along these faults. The first one, frequently hy moderate earthquakes (10 events and along a splay of the Pallatanga fault in the central 13 el'ents. respectively. Mh > 5.0 within 100 km Ecuador Andes, is based on displacement of mor- from the trench from 1964 to 1995) and do not phological features and arrived at a slip rate of 3-4.5 appear to present a ma-jor seismic risk. However, the mm/a [19]. The second study was performed along M.-A. Gutscher et al. /firth and Planetary Science Letters 168 IlUUU) 255-270 267

1"S

2"s

3"s

4"s

5% 80'W 81'W 79-w 78'W 77.W Fig. 6. &railed topography and shallou (e70 km depth) seismiciy of the Ecuadorian Andes showing known (solid) and inferred (hhcd) faulrs. Dolores-Guayaquil Megashear (DGM) shown bold. Pall. F = Pallatanga Fault. Topography from GTOPO-30 digiral databa. Filled circles from the EHB data set (1964-199s) [IS]. M,,2 3.0 scaled by magnitude. Filled squares (1900-1963). Mh 2 5.0 from the SISRA Catalog 1191 scaled similarly. Unfilled black circles are epicenters of 13 large historical earthquakes from 1541 to 1906 7 from the SlSRA C31dOg 1191 scaled according to magnitude and intensity from Ml = 5.7. I = small circles 10 Mt = 8.3. I = 11 (lqcst circle - 17971. 268 M.-A. Guischer EI ul. /Eunh und Plunetury Science Letters 168 (1999)255-270 the Rio Chimgual-La Sofia fault at the Ecuador- local flat slab segment with melting of the oceanic Colombia border based on displacement of dated plate at the edges of the tears producing adakitic pyroclastic flows and arrjved at a slip rate of 7 A 3 magmas. mm/a 1481. However, in each case these are indi- (4) Modem and historical intraplate seismicity vidual fault traces within a network of several faults is very high in the Ecuadorian Andes and in the and if all are active to a comparable degree then network of NE-striking transpressive faults at the total displacement could approach 2 cm/a. The most southern edge of North Andes block (five earth- reliable method to ascertain total strain rates would quakes Mb > 6.0 from 1964 to 1995). The transfer be a geodetic (GPS) study from the Brazilian craton of deformation inland and the NE movement of the across all the DGM fault splays. Given the high level N. Andean block appear to be consequences of in- of modem and historical intraplate seismicity in the creased coupling from the Carnegie Ridge collision. Ecuadorian Andes such a study is warranted and In view of this significant activity, the deformation at necessarp. the southern limit of the North Andes block should be quantified and the seismic risk along these faults reassessed. 6. Conclusions

(1) The seismicity and active volcanism along the Acknowledgements North Andean mar@n between 6"N and 6"s define four distinct regions. M.-A. Gutscher's post-doctoral research was - Region 1: from 6"N to 2.5"N, steep ESE-dipping funded by a European Union TMR (Training and subduction, narrow volcanic arc. Mobility of Researchers) Stipend. Many thanks to - Regon 2: from 2.5% to los, intermediate depth J.-P. Eissen and E. Bourdon for stimulating discus- seismic gap, broad volcanic arc. sions. Fi,wes 2,3,4,6 were produced using Wessel - Region 3: from 1"s to 2"S, steep NE-dipping and Smith's GNT software. [AC] subduction, narrow volcanic arc. - Region 4: south of 25, Peru flat slab segment, no volcanic arc. References (2) A prolongation of Carnege Ridge is proposed to continue >110 km beyond the trench. Four pieces [I]J.L. Swcnson. S.L. Beck. Historical 1942 Ecuador and 1942 of evidence support this conclusion. Peru subduction earthquakes, and earthquake cycles along Colombia-Ecuador and Peru subduction segments, hue (a) morphology: uplift in Coastal Range, Coastal Appl. Geophys. 146 (1996) 67-101. subsidence in S. Colombian coastal block, consis- [?I S.L. Beck LJ. Ruff. The rupture process of the great 1979 tent with southward migration of Carnegie Ridge Colombia earthquake: evidence for the asperity model, J. according to GPS motions. Geophys. Res. 89 (1984) 931-9291. x [3]W.R. McCann, S.P. Nishenko, L.R. Sykes. J. Krause, Seis- (b) Regional topography: a 400 km 2 km mic gaps and plate tectonics: seismic potential for major basement uplift signal at least 110 east of the km boundaries. Pure Appl. Geophys. 117 (1979) lOS3-1147. trench. [a] LJ. Ruff. Large earthquakes in subduction zones: sepent (c) Seismicity pattern: intermediate depth seismic interaction and recurrence times. in: G.E. Bebout, D.W. gap. Scholl, S.H. Kirby. J.P. Platt (Eds.), Subduction: Top to (d) Volcanic arc: a broad arc spread out over 150 Bonom. .4merican Geophysical Union, Washington. DC, 1996. pp. 91-104. the km. the anomalous geochemistry of the arc ma-mas. [SI hl. Cloos. R.L. Shreve. Shear-zone thickness and seis- (3) The geodynamic segmentation and anomalous micity of Chilean and Marianas-type subduction zones. 21 geochemistry (adakitic signature) of the arc magmas Geology (1996) 107-1 10. implies one or more lithospheric tears. The favored [6] C.H. Scholz. C. Small. The effect of seamount subduction model involves tears. at the boundaries between on seismic coupling. Geology 75 (1997) 467-490. d two [7] J.N. Kellogg. \'. Vega. Tectonic development of Panama. regions 1-7 and regions 2-3. The bowknot contin- Costa Rica. and the Colombian Andes: constraints from uation of Carnegie Ridse is interpreted to support a Glohal Positioning System geodetic studies and gravity. in: M.-A. Gurschrr cf al. /Ear?h and Plunelav Sciencp Letten 168 (1999) 255-270 269

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