Sallares & Charvis

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Earth and Planetary Science Letters 214 (2003) 545^559 www.elsevier.com/locate/epsl

Crustal thickness constraints on the geodynamic evolution of the Galapagos Volcanic Province

Valent|¤ Sallare's , Philippe Charvis

Institut de Recherche pour le De¤veloppement, Ge¤osciences Azur, Observatoire Oce¤anologique de Villefranche, B.P. 48, 06235 Villefranche-sur-mer, France

Received 31March 2003; received in revised form 3 July 2003; accepted 3 July 2003

Abstract

We developed a simple quantitative framework based on crustal thickness estimations along the Carnegie, Cocos and Malpelo ridges, to place first-order constraints on the tectonic evolution of the Galapagos Volcanic Province and on the along-axis intensity of the Galapagos melt anomaly during the last 20 m.y. Our results suggest that the Cocos^ Nazca spreading centre has migrated northwards at 26 þ 4 km/m.y. with respect to the Galapagos hotspot (GHS) during this period of time. At V20 m.y., the GHS was approximately ridge-centered, and thus the along-axis intensity of the melt anomaly at this time was the maximum. At V11.5 m.y. the hotspot was located 106 þ 27 km north of the spreading center, and the along-axis intensity of the melt anomaly was 0.54 þ 0.04 of that estimated at 20 Ma. At present day it is located at V190 km south of the spreading center and the along-axis intensity is only 0.19 þ 0.03 of that estimated at 20 Ma. These results are used to reconstruct the relative position between the GHS and the Cocos^ Nazca Spreading Center. The spreading center passed over the GHS for the last time at 7.4 þ 1.3 Ma. The Panama fracture zone was initiated at 8.9 þ 1.6 Ma, leading to the separation between the Cocos and Malpelo ridges. The present configuration of the Galapagos Volcanic Province and the plate velocities are consistent with symmetric spreading with a mean full spreading rate of V60 km/m.y. along the CNSC during the last 20 m.y. A melt flux for excess crustal production of 9.4 þ 5.1m 3/s is obtained for the Galapagos melt anomaly at 20 Ma, implying that the maximum potential intensity of the Galapagos plume is similar to that of the Icelandic plume and twice smaller than the Hawaiian one. ß 2003 Elsevier B.V. All rights reserved.

Keywords: hotspot^ridge interaction; ridge migration; melt £ux; excess crustal production

1. Introduction canic ridges, and seamount chains, are segments of thickened oceanic crust originated in regions Large igneous provinces, oceanic plateaus, vol- showing increased melt production within the mantle source (e.g. [1]). The excess of melting has been typically associated to the presence of thermal anomalies induced by hot plumes rising * Corresponding author. Tel.: +33-493763749; Fax: +33-493763766. from the deep mantle [2^4]. Alternatively, recent E-mail addresses: [email protected] (V. Sallare's), geophysical and geochemical studies seem to in- [email protected] (P. Charvis). dicate that, in some cases, the e¡ect of other pa-

EPSL 6754 5-9-03 546 V.Sallare's, P.Charvis / Earth and Planetary Science Letters 214 (2003) 545^559 rameters such as active upwelling [5,6] or mantle- source compositional heterogeneities (e.g. [7]) can be even more signi¢cant than mantle temperatures to account for the excess of melting. The existence of these anomalous regions is re£ected by broad swells showing striking topography and gravity anomalies (e.g. [8,9]), which are supported by a combination of crustal thickening (e.g. [10]) and sub-lithospheric mantle density anomalies [9,11^ 14]. It is widely accepted that normal oceanic crust (6^7 km thick and MORB-like composition) is generated by decompression melting of normal temperature (V1350‡C) pyrolitic mantle, welling up beneath diverging plates at spreading centers (Fig. 1a) [15]. Higher mantle temperatures (i.e. a thermal anomaly), or a more fertile mantle source (i.e. a geochemical anomaly), may enhance mantle melting beneath the spreading center, generating, in turn, a thicker igneous crust (Fig. 1b) (e.g. [16^ 18]). When the melt anomaly is located at mid- plate, far away from spreading centers, the excess of melting is likely to thicken the crust mainly by crustal underplating [19^23]. In this case, the total melt production is limited by the presence of a mechanical boundary (i.e. a cold lithospheric lid) restricting the minimum depth at which melt can be generated (Fig. 1c) (e.g. [18,24]). Between these two end-member cases, there is a range of inter- mediate possibilities in which the melt anomaly is located at increasing distances from the spreading center. In these cases, part of the hotspot-related melt feeds the spreading center (Fig. 1c), and spreads laterally along the ridge axis [25^27], while the other part is underplated beneath, or injected in, the pre-existing crust (e.g. [28]). Di¡erent geochemical and geophysical observations in- Fig. 1. Sketch summarizing melting processes for crustal generation considering three di¡erent situations. (a) Normal dicate that plumes can interact with oceanic ridges spreading center, (b) ridge-centered melt anomaly, (c) o¡- V up to distances of 1400 km [27^30], generating ridge melt anomaly. Zmin and Zmax indicate the minimum along-axis geophysical anomalies of widths ex- and maximum depth of melting, which depend primarily on ceeding 2000 km (e.g. [31]). the temperature and composition of the mantle source. The Galapagos Volcanic Province (GVP) con- stitutes an excellent example to investigate hot- m.y. (Fig. 2). Two of the ridges, Cocos and Car- spot^ridge interactions. It is constituted by several negie, are interpreted to be the tracks of the GHS ridges showing thickened oceanic crust, which over the north-eastward trending Cocos plate and likely resulted from the interaction between the the eastward trending Nazca plate, respectively. Galapagos hotspot (GHS) and the Cocos^Nazca These ridges show signi¢cant along-strike topo- spreading center (CNSC) during the last V20 graphic variations from the currently active

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CNSC to the subduction zones, which seem to be lution of the GVP was established by Hey [32] related to temporal variations in the relative hot- and Lonsdale and Klitgord [33] based on mag- spot^ridge location [32^35]. However, quantita- netic and bathymetric data. This model suggests tive comprehensive analysis of the GHS^CNSC that major plate reorganization occurred in the relative motion is lacking to date. region at V23 Ma, breaking the ancient Farallon The global tectonic evolution of the GVP is plate along a pre-existing ENE trending Paci¢c^ relatively well constrained from plate reconstruc- Farallon fracture zone. Subsequently, oceanic tion based on magnetic and bathymetric data spreading initiated along the CNSC originating [32,33,36,37], and from recent Global Positioning the Cocos and Nazca plates. The interaction be- System (GPS) measurements of the present-day tween the GHS and the CNSC originated the motion of Cocos, Nazca, and South America north-eastward trending Cocos Ridge and the plates [38^40]. In addition, several seismic experi- eastward trending Carnegie Ridge, which outline ments were carried out recently in the GVP. These the motion of the Cocos and Nazca plates relative studies have given accurate information on the to the GHS, respectively [48,49]. Based on the crustal thickness along the CNSC and at di¡erent analysis of magnetic anomalies and dredge sam- parts of the hotspot-related volcanic ridges. First, ples, a three stage development of the CNSC has during the PAGANINI-1999 cruise (R/V Sonne, been proposed, suggesting that the orientation of [41]) three wide-angle pro¢les were acquired the spreading center varied from the original ENE across the Cocos and Malpelo ridges [42,43]. Sec- (V23^19.5 Ma), to N75‡E (19.5^14.5 Ma), and ond, the PRIME-2000 cruise (R/V Maurice Ew- ¢nally to the present E^W orientation (14.5 Ma) ing) provided a number of wide-angle and multi- [35]. These data also evidenced that the history of channel seismic data along the present-day axis of rift propagation along the CNSC is complex, the CNSC [44] and the Galapagos platform [45]. being characterized by series of major southward Third, during the SALIERI-2001cruise (R/V ridge jumps between V20 and 14.5 Ma which Sonne, [46]) two wide-angle pro¢les were acquired kept the CNSC not far from the GHS during across the Carnegie Ridge [47] (Fig. 2). this period of time. The activation of the main In the present work, we have developed a sim- tectonic event along the CNSC, the Panama frac- ple quantitative framework to place ¢rst-order ture zone (PFZ), occurred between 13 Ma [32] constraints on the relative motion of the GHS and 8^11 Ma [33] (Fig. 2). This event caused the and the CNSC based on crustal thicknesses of end of sea£oor spreading east of the PFZ, where- the main volcanic features computed from avail- as it continued west of it until present day [32,48]. able seismic data. First, we describe the method The Malpelo Ridge is thought to be the former used to derive the along-axis intensity of the melt continuation of the Cocos Ridge, drifted away by anomaly from crustal thickness estimations. Sec- the dextral strike-slip motion along the PFZ [33]. ond, we estimate the relative hotspot^ridge loca- Recent GPS measurements indicate that the tion at di¡erent periods of time based on the cal- Cocos plate is moving at a rate of V83 km/m.y. culations of the along-axis intensity of the melt with a trend of VN41‡E, and that the Nazca anomaly. Third, we estimate the volumetric melt plate subducts almost perpendicularly beneath £ux of the GHS and we compare the results with South America at V58 km/m.y. with respect to that obtained for the Hawaiian and Icelandic hot- the stable South American craton [38,40]. The age spots. Finally, we show a tectonic reconstruction of the Cocos Ridge subducting at the trench is of the GVP for the last 20 m.y. which summarizes v 14.5 m.y. based on radiometric data [50], and all the results. the age of Carnegie Ridge at the trench and the conjugate Malpelo Ridge is estimated to be V20 m.y. from reconstruction of magnetic anomalies 2. Tectonic setting [32]. If we assume that the GHS is currently centered The ¢rst consistent model of the tectonic evo- beneath the most active island of the Galapagos

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Fig. 2. (a) Bathymetric map of the GVP showing the main tectonic and geologic features of the area. Large arrows display plate motions relative to the stable South American craton [40]. Black lines show location of all the wide-angle seismic pro¢les used in this work, and numbers indicate the maximum crustal thickness obtained along these pro¢les [43^45,47]. Boxes outline the di¡erent seismic experiments recently performed in the area (PAGANINI-1999, G-PRIME-2000, SALIERI-2001). CNSC: Cocos^Naz- ca spreading center, GHS: Galapagos hotspot, PFZ: Panama fracture zone. (b) Residual bathymetry of the GVP. It is the bathymetry derived from the sea£oor age [58], based on the plate cooling model of Parsons and Sclater [59] for oceanic crust younger than 70 m.y. (d = 2500+350t1=2), minus the bathymetry shown in Fig. 2a. Note that in this case depth anomalies mainly re£ect crustal thickness variations. Numbers show crustal ages of oceanic plates at 5 Ma interval [58].

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Archipelago (Isabela), the distance between the negligible, and (2) the along-axis width of plume CNSC and the hotspot is V190 km. The full material £ow, W, and the full spreading rate, U, spreading rate at 90‡W is 60 þ 4 km/m.y., using are constant over time. The validity of these two the global plate model NUVEL-1A [51],or assumptions is discussed in the next section. 66 þ 4 km/m.y., from GPS measurements [38]. In the following sections, we ¢rst describe and Magnetic anomalies suggest that sea£oor spread- then apply a method based on this idea to esti- ing along the CNSC has been roughly symmetric mate the ¢rst-order temporal variations in the in- though slightly variable during the last 20 m.y. tensity of the Galapagos melt anomaly along the [36,35]. The CNSC has moved toward the north axis of the CNSC, which is subsequently used to with respect to the hotspot at a velocity of V27 approximate the relative location of the GHS with km/m.y. [52], similar to the half-spreading rate, respect to the spreading center at di¡erent periods which could explain the approximate E^W trend of time. All those calculations are based on sev- of the Carnegie Ridge. The migration of the eral crustal thickness estimations at the GVP. CNSC, combined with prominent ridge jumps and temporal variations in the full spreading 3.1. Data set rate, have resulted in temporal variations in the relative location of the hotspot with respect to the The data set is constituted by a number of crus- spreading center. Although the details of the rel- tal thickness estimations along eight wide-angle ative motion are not fully understood, bathymet- seismic pro¢les and some multichannel seismic ric and magnetic data seem to indicate that be- lines acquired along the CNSC and across the tween V20 Ma and 15 Ma the hotspot was volcanic ridges of the GVP (Fig. 2). approximately ridge-centered, then at V14.5 Ma Three of the wide-angle pro¢les and all the a southward ridge jump placed the hotspot be- MCS data were acquired along the present-day neath the Cocos plate [35]. Eventually, at 7^9 axis of the CNSC (between 97‡W and 91‡20PW) Ma, the CNSC passed over the GHS, and from in 2000, during the G-PRIME experiment. Based this moment to present day the hotspot has been on part of these data, Canales et al. [44] con- located beneath the Nazca plate [36]. strained the crustal thickness variations along the western CNSC. A maximum crustal thickness of 8.1þ 0.2 km is obtained at 91‡25 PW, V290 km 3. Volumetric melt £ux provided by a melt north of the inferred location of the GHS. A min- anomaly imum crustal thickness of 5.6 þ 0.2 km is obtained at V97‡W, V700 km away from the point where If we assume that all mantle melts feeding a the crust is the thickest. At roughly mid-distance, ridge are emplaced as seismically observable igne- V94‡W, the crust is only 5% thicker with ous crust (e.g. [15]), the excess of crustal thickness 5.9 þ 0.3 km (Fig. 2). Thus, the rate of crustal along the ridge axis (compared to normal oceanic thickening is around ¢ve times higher east of crust) can be used to estimate the additional melt 94‡W than west of it, suggesting that the lateral £ux provided by a melt anomaly [29,44]. This val- extent of the along-axis migration of GHS melts ue is called the volumetric melt £ux for extra is con¢ned primarily to within V350 km from the crustal production, and can be taken as a measure point where the crust is thickest [44]. Therefore, of the along-axis intensity of the melt anomaly. the crust generated at 97‡W is considered to be The temporal variations of along-axis crustal outside of the zone of hotspot in£uence, and can thickness may be in turn interpreted to be the be taken as a reference for normal CNSC oceanic result of variations in the relative distance be- crust. tween the melt anomaly and the spreading center. Two wide-angle pro¢les were acquired across Basic assumptions made with this inference in- the Cocos Ridge at the subduction zone and at clude: (1) the temporal variations in the maxi- V6‡N, and another one across the Malpelo mum potential intensity of the melt anomaly are Ridge, during the PAGANINI-1999 experiment.

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Other pro¢les were collected across the Carnegie thicker than that generated at V11.5 Ma (Fig. 2), Ridge immediately east of the trench and at suggesting that at V15.5 Ma the GHS was quite V85‡W, during the SALIERI-2001experiment. closer to the spreading center than at V11.5 Ma. These two pro¢les are located approximately at None of these pro¢les is parallel to the paleo- the conjugate segments of the Malpelo pro¢le axis of the CNSC. However, the excess of crustal and the southern Cocos pro¢le, respectively. Seis- thickness along the thinnest of two conjugate seg- mic velocity models with the Moho geometry ments can be used as a proxy to estimate the along those ¢ve pro¢les were obtained using a along-axis volumetric melt £ux provided by the joint refraction and re£ection travel time inver- GHS. The thickest of both conjugate segments sion method [5]. Uncertainties of the model pa- is thus the result of the along-axis crustal produc- rameters (i.e. seismic velocities and Moho depth) tion and additional o¡-ridge crustal thickening by were estimated by performing a Monte Carlo-type intra-plate volcanism (Fig. 1c). analysis. Results indicate that maximum crustal thickness is 12.9 þ 0.3 km in western Carnegie, 3.2. Method 19.0 þ 0.4 km in eastern Carnegie [43], 16.7 þ 0.3 in southern Cocos, 18.4 þ 0.5 km in northern Co- The method that we describe here allows calcu- cos, and 19.1 þ 0.3 km in Malpelo [47] (Fig. 2). lating the relative distance between a melt anom- The age of the sea£oor at the location of the aly and a spreading center from crustal thickness di¡erent pro¢les can be inferred from plate veloc- estimations along the ridge axis. To do that, we ities derived from GPS measurements [40], and ¢rst calculate the along-axis volumetric melt £ux magnetic data [35]. For Malpelo Ridge and its for extra crustal production, Qv. Second, we de- conjugate eastern Carnegie Ridge, an age of termine the maximum along-axis potential inten- 20.0 þ 0.5 m.y. is obtained. For southern Cocos sity of the melt anomaly, QM, and we estimate its Ridge and its conjugate western Carnegie Ridge, relative intensity at di¡erent periods of time. Fi- the age is V11.5 þ 0.5 m.y., whereas for the nally, we link the temporal variations of Qv with northern Cocos pro¢le it is V15.5 þ 0.5 m.y. the relative distance of the melt anomaly center [35]. The later estimation is consistent with that and the ridge axis. 40 39 obtained from Ar/ Ar dating of volcanic sam- Qv can be calculated from crustal thickness ples o¡shore southern Costa Rica, which indicates measurements along the axis of a spreading center that the age of the segment of the Cocos plate as follows (e.g. [28]): currently subducting beneath Middle America Z just west of the Cocos Ridge is V14.5 m.y. [50]. Qv ¼ U ðhcðxÞ3hÞdx ð1Þ The comparison of isochronous crustal thick- W ness di¡erences at both sides of the CNSC allows where U is the full spreading rate, W is the along- inferring the relative location of the GHS with axis width of the overthickened crustal segment, x respect to the spreading center at the di¡erent is along-axis distance, hc(x) is the crustal thickness periods of time. At V11.5 Ma a thicker crust is measured at x, and h represents normal crustal obtained north of the CNSC than south of it, thickness. indicating that the GHS was probably located A ¢rst issue with this inference is that we rarely beneath the Cocos plate, north of the spreading know the Moho geometry along the axis of a center. At V20 Ma, the crustal thickness is al- spreading center. As we stated in the previous most the same at both sides of the CNSC. This section, the Moho geometry is well constrained suggests that the GHS was approximately ridge- along the present-day axis of the CNSC, but not centered at this time, thus providing a useful con- along the paleo-ridge axis. Nevertheless, the com- straint on the maximum potential intensity of the parison of the conjugate pro¢les at V11.5 Ma Galapagos melt anomaly. At V15.5 Ma, the and V20 Ma allows to estimate, at least, which ridge’s crust north of CNSC is slightly thinner was the maximum crustal thickness generated than that generated at V20 Ma, but signi¢cantly along the ridge axis at both periods of time

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(Fig. 1). Then, we can approximate in ¢rst order from Eq. 2. In a previous work based on the the along-axis Moho geometry, hc(x), with a func- analysis of along-isochron bathymetric (vRB) tion decaying linearly from H at x =0, to h at and gravity (vMBA) anomalies along the present- x =þW/2. In this case Eq. 1 is reduced to: and paleo-ridge axis for a number of hotspot^ ridge systems, Ito and Lin [34] showed that ðH3hÞUW Qv ¼ ð2Þ whereas the amplitudes of vRB and vMBA are 2 functions of ridge^hotspot distance, W depends where H is the maximum crustal thickness along primarily on U at the time of crustal accretion, the ridge axis at a given period of time. in agreement with previous notions of along-axis In a second approach, we assume that the tem- plume material £ow [26,28]. For the GHS^CNSC poral variations of Qv are primarily the result of system, it has been shown that the along-isochron variations in the distance between the melt anom- width of vRB has been roughly constant between aly and the spreading center. This hypothesis im- 7.7 Ma and 2.5 Ma (1250^1300 km) [29], despite plies that the temporal variations of the maximum the signi¢cant variations on the relative hotspot^ potential intensity of the melt anomaly are negli- ridge distance (V0^100 km). In addition, a recent gible. We assumed that to perform our analysis, tectonic reconstruction of the GVP based on mag- because it is the simplest hypothesis and there is netic data [35] suggests that the spreading rate not any evidence against it. The comparison of along the CNSC east of 90‡W has been nearly our results with those obtained in other works symmetric and uniform (UV60^70 km/m.y.) be- using independent data and methods will allow tween V19.5 m.y. and present day. Therefore, we to see if the assumption is plausible. assume, as a crude approximation, that both U Therefore, if the temporal variations of Qv are and W did not vary signi¢cantly during the last the result of the hotspot^ridge distance, Qv must V20 m.y. Then, substituting Eq. 2 into Eq. 4 we be maximum when the melt anomaly is ridge-cen- obtain: tered, and asymptotic to zero for increasing hot- H3h spot^ridge distance. This tendency can be repre- M ¼ ð6Þ H 3h sented by the following function: M In conclusion, if we know the maximum poten- Q ¼ Q expð3KyÞð3Þ v M tial crustal thickness that can be generated by a where QM represents the maximum potential melt anomaly, HM, and the hotspot^ridge dis- volumetric melt £ux (i.e. for a ridge-centered tance, y0, and maximum along-axis crustal thick- anomaly), y is the relative hotspot^ridge distance, ness, H0, at a given period of time, Eqs. 5 and 6 and K is a factor which determines the shape of provide an easy way to place ¢rst-order con- the function. This function does not represent a straints on the relative intensity of the along-axis rigorously based physical model but it is only an melt anomaly, M, and thus on the hotspot^ridge assumed idealization. Then, de¢ning: distance, y, at any other time at which we know Q the maximum along-axis crustal thickness, H. M ¼ v ð4Þ QM and comparing this value for two di¡erent pro- 4. Results ¢les, we obtain from Eq. 3: In the following sections, we ¢rst estimate the y logM ¼ y logM ð5Þ 0 0 relative intensity of the along-axis melt anomaly where M (M0) represents the relative intensity of the at present day, at V11.5 Ma, and at V15.5 Ma along-axis melt anomaly when it is located at y with respect to its maximum potential intensity, (y0) from the spreading center. which is obtained at V20 Ma. The results are A second issue with this approach concerns the used to infer the relative hotspot distance at these estimate of U and W in order to determine Qv periods of time, and thus the relative motion of

EPSL 6754 5-9-03 552 V.Sallare's, P.Charvis / Earth and Planetary Science Letters 214 (2003) 545^559 the CNSC with respect to the GHS during the last slightly south of the CNSC, we would obtain 20 m.y. The CNSC migration rate is then used to vH = 12.8 þ 0.7 km and M = 0.95 þ 0.04. determine the age of the activation of the PFZ The relative distance between the hotspot and and the time at which the CNSC passed over the ridge at the di¡erent periods of time can be the GHS from south to north. Finally, we esti- then estimated from Eq. 5 using as reference the mate the volumetric melt £ux for excess crustal distance from the GHS to the point of the CNSC production at the GHS and we compare the re- where the thickest crust is generated at present sults with those obtained at the Hawaiian and day, y0 = 290 þ 10 km (a in Fig. 3). The relative Icelandic plumes. distance was obviously yV0atV20 Ma. At We have also performed a formal error propa- V11.5 Ma we obtain y = 106 þ 27 km, and at gation analysis using the expression of the total V15.5 Ma it would be only y = 9 þ 9 km. di¡erential (Eq. 7), in order to estimate the order of uncertainty of the results: 4.2. Regional implications of ridge migration XN Df vz ¼ vxi ð7Þ The N^S component of the mean relative veloc- Dx i¼1 i ity between the GHS and the CNSC, VNS, can be where z = f(xi) is a function of the independent estimated by comparing their relative distances at variables xi (i =1,T, N), and vxi is the uncertainty present day (190 þ 10 km, b in Fig. 3), and at of these variables. V11.5 Ma. From the results described in the pre- Since many steps of our model are not general vious section we obtain VNS = 26 þ 4 km/m.y. but strongly assumption-dependent, this uncer- (Fig. 3). Assuming that the GHS remains ¢xed tainty does not represent the real error of the in the deep mantle reference frame, this result results, but only that associated to the propaga- denotes the northward migration of the CNSC tion of the errors of the basic variables, assuming with respect to the GHS. This is in good agree- a given functional relationship. ment with the V27 km/m.y. migration rate derived from the NUVEL-1global plate motion 4.1. Relative intensity of the melt anomaly and model in the hotspot reference frame [52]. hotspot^ridge distance The GVP is moving eastwards at a mean rate, VEW = 58 þ 2 km/m.y., measured from GPS data As we stated above, the similar crustal thick- [40] (Fig. 3). This motion is essentially the result ness obtained along the Malpelo and eastern Car- of the E^W sea£oor spreading along the East Pa- negie Ridge indicate that at V20 Ma the GHS ci¢c Rise. Using VNS and VEW, we have inferred was approximately ridge-centered. Then, we can the location of the GHS in the CNSC reference assert that at this time the intensity of the along- frame at V11.5 Ma, V15.5 Ma, and V20 Ma, in axis melt anomaly was the maximum (MV1). By order to compare the results with those predicted comparing this crustal thickness with that esti- by our model (Fig. 3). At 15.5 Ma, the GHS mated away from the zone of hotspot in£uence would be V30 km south of the CNSC; and at (Fig. 2) we obtain that the maximum potential 20 Ma, it would be approximately ridge-centered along-axis crustal thickening (vH) associated (assuming that the CNSC was continuous before with the presence of the GHS is 13.4 þ 0.5 km. the opening of the PFZ). The result at 20 Ma is Along the present-day axis, vH is 2.5 þ 0.4 km, therefore consistent with the predictions of our and from Eq. 6 we obtain M = 0.19 þ 0.03. At hotspot^ridge interaction model. At 15.5 Ma the V11.5 Ma, we obtain vH = 7.3 þ 0.5 km and model predicts that the GHS would have to be M = 0.54 þ 0.04. The lack of crustal thickness infor- V20 km northward from the inferred location. mation at the conjugate segment of the northern It is not possible, however, to say whether this Cocos pro¢le (Fig. 2) makes it impossible to infer discrepancy is real or not, because the inferred the relative hotspot^ridge location at V15.5 Ma. location is well within the limits of uncertainty However, if we assume that the GHS was located of the model.

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Fig. 3. Residual bathymetry map of the GVP. White disks with error bars indicate the location of the Galapagos melt anomaly center in the CNSC reference frame, as inferred from the results of this work: (1) present day, (2) 11.5 þ 0.5 Ma, (3) 15.5 þ 0.5 Ma, (4) 20.0 þ 0.5 Ma. The solid line indicates the inferred trajectory of the GHS in the CNSC reference frame. The thin line shows the current location of the CNSC. The active segment is located west from 83‡W, and the fossil segment east of 83‡W. Dashed line displays the inferred location of the CNSC before the opening of the PFZ. Relative distances: (a) GHS-nearest along-axis wide-angle seismic pro¢le, (b) GHS^CNSC, (c) lateral step between active and fossil segments of the CNSC, (d) separation between Cocos and Malpelo ridges, (e) scarp of the Carnegie Ridge. Full explanation of their respective signi¢cance is provided in the text. CNSC: Cocos^Nazca spreading center; IFZ: Inca fracture zone; PFZ: Panama fracture zone.

Using the CNSC migration rate, VNS, and the and 8^11 Ma [33]. Our result is thus more in present-day GHS^CNSC distance (b in Fig. 3), agreement with the later value. we can infer that the spreading center passed It is widely accepted that the Malpelo Ridge is over the GHS at 7.4 þ 1.3 Ma, which is in agree- a former continuation of the Cocos Ridge, drifted ment with the 7^9 m.y. estimated by Wilson and southward along the dextral strike-slip PFZ. Con- Hey [36] from the interpretation of magnetic data. sequently, the lateral N^S shift of 510 þ 10 km In addition, it is possible to constrain the time of between Cocos and Malpelo ridges (d in Fig. 3) opening of the PFZ, tPFZ, in view of the 230 þ 10 must be the result of the northward CNSC migra- km lateral step between the fossil segment of the tion, VNS, in addition to the mean northward CNSC east of the PFZ and the western (active) half-spreading rate, UN, along the CNSC, after segment (c in Fig. 3). West of the PFZ, the Cocos the opening of the PFZ. Taking the values ob- plate is subducting beneath Middle America, but tained above, we obtain UN = d/tPFZ3VNS =31þ east of PFZ the northward subduction is currently 13 km/m.y. (Fig. 3). blocked (Fig. 3). Assuming that c is the result of South of the CNSC, the di¡erential motion of the northward migration of the CNSC after the the oceanic basins west and east of the PFZ is opening of the PFZ, which did not occur east of much less marked than north of it, because in the PFZ, we obtain tPFZ = 8.9 þ 1.6 Ma. Previous this part the northward migration of the CNSC estimations of tPFZ range between V13 Ma [32] is mostly compensated by the southward sea£oor

EPSL 6754 5-9-03 554 V.Sallare's, P.Charvis / Earth and Planetary Science Letters 214 (2003) 545^559 spreading along the CNSC, Us. The only feature tained across the western Carnegie Ridge that is observed in the Nazca plate is a lateral step (325 þ 25 km) [47], and the E^W motion rate of of 50 þ 10 km in the northern £ank of the Carne- the Nazca plate, VEW. This gives QGP = 2.8 þ 1.6 gie Ridge (e in Fig. 3). Considering that this step m3/s, which is near the upper bound of the 0.3^3.0 is the result of the di¡erential motion we obtain m3/s previously reported by Ito et al. [29] from the Us = e/tPFZ+VNS = 31þ 6 km/m.y. ( Fig. 3). analysis of gravity and topographic anomalies Therefore, we conclude from our basic evalua- along the Carnegie Ridge between 85‡W and tion that roughly symmetric spreading with a 92‡W. This means that the total excess volume mean full spreading rate of 62 þ 19 km/m.y. has £ux currently supplied by the GHS is QT = 4.4 þ taken place along the CNSC during at least the 3.1m 3/s. last V10 m.y. (between V91‡W and the PFZ). At V20 Ma, an excess volume £ux of 3 As stated above, the global plate motion model Qv = 9.4 þ 5.1m /s is obtained considering the NUVEL-1A predicts a full spreading rate varying same values of W and U that we used to calculate from 58 þ 3 km/m.y. at 90‡W to 63 þ 4 km/m.y. at QCNSC. This value is more than twice larger than 87‡W [51], whereas a rate of 66 þ 4 km/m.y. is the total excess volume £ux estimated at present obtained from GPS data [38]. day, QT. Assuming that the maximum potential The north-eastward motion of the Cocos plate intensity of the Galapagos melt anomaly has re- with respect to the stable South American craton mained constant during this period of time, the must be then the result of the northward compo- di¡erence in Qv must be explained by the presence nent of N^S oceanic spreading along the CNSC, of a mechanical boundary (i.e. the lithosphere UN, the northward ridge migration, VNS, and the originated at the spreading axis), which restricts eastward motion of the GVP towards South the extent of the mantle melting zone and, in turn, America, VEW (Fig. 3). By combining the values the total amount of melt generated beneath the that we obtained above, we infer that the Cocos Galapagos platform (e.g. [52])(Fig. 1c). plate has been moving at a mean rate of 79 þ 14 We have performed a similar analysis for two km/m.y. with a mean trend of N44 þ 11‡E. This is end-member examples of purely intra-plate (Ha- also in agreement with the V83 km/m.y. and waii) and ridge-centered (Iceland) plumes, in or- VN41‡E measured from GPS data [40]. der to compare the excess volume £ux provided by these plumes with those obtained in the GVP 4.3. Volumetric melt £ux for excess crustal at present day (intraplate case) and at V20 Ma production (ridge-centered case). Maximum crustal thickness across the swell of the Hawaiian hotspot near The total volumetric melt £ux for excess crustal Oahu and Kauai islands is 14^15 km, whereas production currently supplied by the GHS, QT,is normal crustal thickness of the Paci¢c plate is the aggregate of excess volume £ux at the present- V7 km. The across-ridge width of the overthick- day axis of the CNSC, QCNSC, and at the Gala- ened crustal section is V400 þ 25 km [53]. Based pagos platform, QGP (Fig. 1c). QCNSC can be ap- on the model NUVEL-1A, the Paci¢c plate is proximated using an estimate of W (700 þ 50 km, moving at approximately 105 þ 5 km/m.y. with [44]) and the full spreading rate, U, which we respect to the Hawaiian hotspot [54]. We thus 3 obtained in the previous section. Then, from Eq. obtain an excess volume £ux, QH = 5.0 þ 3.7 m /s 3 2 we obtain QCNSC = 1.6 þ 1.5 m /s, in agreement for the Hawaiian hotspot, in agreement with the with the V1.5 m3/s determined by Canales et al. V5.1m 3/s estimated by Watson and McKenzie [44]. Maximum crustal thickness beneath the Ga- [24] from numerical modelling. By comparing this lapagos platform has been reported to be V15km value with that obtained beneath the Galapagos from wide-angle data acquired during the G- platform, QGP, we can infer that the Hawaiian PRIME experiment (Fig. 2) [45]. Then, we have melt anomaly is, potentially, almost twice stron- determined QGP from Eq. 2 considering the N^S ger than the Galapagos one, which is consistent extent of the overthickened crustal section ob- with the estimates of Phipps Morgan [52]. In Ice-

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Fig. 4. Cartoon of the GVP tectonic evolution during the last V20 m.y. based on the results obtained in this work. Thin arrows in panel a indicate the trend of oceanic spreading along the CNSC (UN and US), the migration of the CNSC with respect to the hotspot (VNS), and the E^W motion of the GVP relative to the stable South American craton, resulting from the oceanic spreading in the East Paci¢c Rise (VEW). Thick arrows correspond to the aggregate of these three components (the motion of Cocos and Nazca plates relative to South America), which is outlined by the trends of Cocos and Carnegie ridges. The motion of the continents with respect to the GHS has been reconstructed assuming 6 10 km/m.y. E^W convergence for South America and NE^SW convergence for Central America during the last 20 m.y. [60]. PFZ: Panama fracture zone, IFZ: Inca fracture zone, YG: Yaquina graben, Ca: Carnegie Ridge, Co: Cocos Ridge, M: Malpelo Ridge.

EPSL 6754 5-9-03 556 V.Sallare's, P.Charvis / Earth and Planetary Science Letters 214 (2003) 545^559 land, maximum crustal thickness is 30^35 km, thus the oldest aseismic ridge segments resulting whereas normal thickness of crust generated at from the GHS^CNSC interaction which are cur- the Reykjanes ridge is V7km[55]. The full rently preserved within the GVP. At V20 Ma, the spreading rate based on the model NUVEL-1A CNSC was already migrating to the north with is 19 þ 2 km/m.y. [51], and the along-axis width respect to the GHS, with a velocity slightly lower of the plume in£uence, derived from the gradient than the half-spreading rate. The combination of of bathymetric and Sr isotopes concentration CNSC migration, N^S spreading along the anomalies along the ridge, is V1200 km [56,57]. CNSC, and E^W spreading along the East Paci¢c This gives an along-axis excess melt £ux for the Rise, resulted in the SW^NE motion of the Cocos 3 Icelandic hotspot, QI = 9.2 þ 6.7 m /s, which is plate and the E^W motion of the Nazca plate, as similar to that estimated for the GHS at V20 outlined by the trends of Cocos and Carnegie. Ma. This value is about 20% larger than that The northward migration of the CNSC was previously estimated by Phipps Morgan [52], but compensated by a series of major southward ridge he just considered the excess of melt production jumps between V19.5 Ma and V14.5 Ma, and immediately beneath the island platform and not consequently the active spreading center was stay- along the Reykjanes ridge axis. Thus, as previ- ing near the GHS ( 6 50 km) during this period of ously noted by Canales et al. [44], the thicker time. The last ridge jump (at V14.5 Ma), which crust and the wider along-axis dispersal of plume probably corresponds to the Inca fracture zone material observed at the Reykjanes ridge are (IFZ), reshaped the CNSC into its present con¢g- mostly compensated by the three-fold increase of uration (Fig. 4). Barckhausen et al. [35] suggested the spreading rate along the CNSC. that the GHS was the driving force producing southward ridge jumps and resulting in an asym- metric crustal accretion. Nevertheless, no major 5. Tectonic evolution ofthe GVP ridge jumps occurred since 14.5 m.y., which is not in agreement with ridge jumps only related In this section we show a plausible tectonic sce- to the in£uence of the presence of the GHS. Al- nario for the GVP evolution from 20 Ma to ternatively, we suggest that ridge jumps could also present day based on the results described in the re£ect a global tectonic reorganization related to previous section (Fig. 4). Most tectonic recon- di¡erential forces between the north-eastward structions suggest that the GHS began interacting subducting Cocos plate and the eastward subduct- with the CNSC at V23 Ma [32,33,35]. The mag- ing Nazca plate, and thus they are not only re- netic anomalies of the oldest sea£oor created at lated with the GHS. the CNSC indicate that between V23 Ma and At V12 Ma, the IFZ passed over the GHS V20 Ma the spreading center was orientated from west to east, locating the hotspot beneath VN45‡E, and its full spreading rate was signi¢- the Cocos plate far away from the spreading cen- cantly higher than that observed at present day ter (V150 km). After that, the CNSC kept mi- (e.g. [37]). At 19^20 Ma, a major ridge jump re- grating northward, towards the GHS. shaped its geometry, changing also the ridge ori- At about 9 Ma, the subduction of the eastern entation to VN75‡E and diminishing the spread- part of the Cocos plate beneath Middle America ing rate. In the previous section we showed that probably blocked, leading to the opening of the the GHS was approximately ridge-centered at this PFZ. West of it, the subduction of the Cocos time. The interaction between the hotspot and the plate and the sea£oor spreading at the CNSC spreading center produced a thick oceanic plateau kept on, whereas the eastern segment was incor- centered on the CNSC, similar to Iceland today porated to the Nazca plate after the cessation of (Fig. 4). Subsequently, the northern (Malpelo) the sea£oor spreading and of the northward mi- and southern (Carnegie) segments of the plateau gration of the CNSC (Fig. 4). This produced a are split apart by sea£oor spreading along the di¡erential motion along the dextral strike-slip CNSC. Malpelo and easternmost Carnegie are PFZ which resulted in a southward drift of the

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Malpelo Ridge relative to the Cocos Ridge. South remained in the vicinity of the spreading center of the spreading center the PFZ continued as a between V20 Ma and V12 Ma. less active transform fault because most of the 3. Between V12 Ma and V7.5 Ma the GHS was southward sea£oor spreading was compensated located beneath the Cocos plate. by the northward migration of the CNSC (Fig. 4. The CNSC passed over the GHS at 7.4 þ 1.3 4). The small di¡erence between both motions Ma, and thus the GHS has been located be- generated the shift observed along the northern neath the Nazca plate since this time. £ank of Carnegie Ridge at V83‡W. The CNSC 5. The maximum potential excess volume £ux of moved toward the GHS between V12 Ma and the Galapagos melt anomaly is 9.4 þ 5.1m 3/s, V7.5 Ma. At about this time, the CNSC passed similar to that of the Icelandic one. The Ha- over the GHS, and from then to now it has been waiian melt anomaly is, potentially, almost moving away from the GHS to reach the location twice stronger than the Galapagos and Icelan- observed today (Fig. 4). dic ones. The subduction of the Cocos Ridge beneath 6. The PFZ opened at 8.9 þ 1.6 Ma, triggered by Middle America is therefore very recent, starting the locking of the subduction of the eastern- probably after V2 Ma. The ridge fragments orig- most Cocos plate beneath Middle America. inated at the CNSC between V23 Ma and 20 Sea£oor spreading and northward migration Ma, if they once existed, should have eventually of the CNSC stopped east of the PFZ leading subducted beneath the South American Margin, to the separation between the Cocos and Mal- between 0‡ and 2‡S (the possible continuation of pelo ridges. the Carnegie Ridge), and between 2‡N and 4‡N (a 7. The current separation between Malpelo and block similar to the Malpelo Ridge) (Fig. 4). This Cocos ridges, and the step observed in the would imply that the Yaquina graben once played northern £ank of the Carnegie Ridge at a role similar to that of the PFZ (Fig. 4). V83‡W, are well explained if symmetric spreading with a mean full spreading rate of 62 þ 19 km/m.y. occurred along the CNSC 6. Conclusions the last V9 m.y. 8. This model predicts that the Cocos plate is The joint interpretation of crustal thickness de- subducting beneath Middle America at a rate termined along the present-day axis of the CNSC of 79 þ 14 km/m.y. with a trend of N44‡ þ and across the di¡erent aseismic ridges allows us 11‡E, in agreement with the results of recent to constrain the relative motion of the CNSC with GPS measurements. respect to the GHS and, in turn, the tectonic evolution of the GVP during the last V20 m.y. Our estimations are only approximate and depend Acknowledgements strongly on several assumptions. Nevertheless, the remarkable agreement between our results We thank A. Bonneville and J. Phipps Morgan and those obtained in a number of previous for their constructive reviews, which helped to works using independent geophysical data and improve substantially the original manuscript. V. methods suggests that our approach is valid and Sallare's was supported by a Marie Curie Fellow- suitable to place ¢rst-order constraints on the ship (contract HPMF-CT-1999-0039) during the geodynamic evolution of the GVP. time this work was conducted. The study was The main conclusions of this study are the fol- also supported by Institut de Recherche pour le lowing: De¤veloppement (IRD). We acknowledge the cap- 1. The CNSC has been moving northwards with tain of the R/V Sonne, Henning Pepenhagen, the respect to the GHS at a mean rate of 26 þ 4 ship’s crew, and all the scienti¢c sta¡ that partici- km/m.y. from V20 Ma to present. pated in the PAGANINI-1999 and SALIERI- 2. The GHS was ridge-centered at V20 Ma and 2001seismic experiments. [VC]

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