Earth and Planetary Science Letters 203 (2002) 665^679 www.elsevier.com/locate/epsl The migration history of the Nazca Ridge along the Peruvian active margin: a re-evaluation Andrea Hampel à GEOMAR Research Center for Marine Geosciences, Wischhofstr. 1^3, 24148 Kiel, Germany Received 21 March 2002; received in revised form 9 July 2002; accepted 23 July 2002 Abstract The collision zone of the 200 km wide and 1.5 km high Nazca Ridge and the Peruvian segment of the convergent South American margin between 14‡S and 17‡S is characterized by deformation of the upper plate and several hundred meters of uplift of the forearc. This is evident by a narrowing of the shelf, a westward shift of the coastline and the presence of marine terraces. As the Nazca Ridge is oblique with respect to both trench and convergence direction of the Nazca Plate, it migrates southward along the active plate boundary. For reconstructing the migration history of the Nazca Ridge, this study uses updated plate motion data, resulting from a revision of the geomagnetic time scale. The new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity of V75 mm/a (before 10.8 Ma), V61 mm/a (10.8^4.9 Ma), and V43 mm/a (4.9 Ma to present). Intra-plate deformation associated with mountain building in the Peruvian Andes since the Miocene reduces the relative convergence rate between Nazca Plate and Peruvian forearc. Taking an intra-plate deformation at a rate of V10mm/a, estimated from space-geodetic and geological data, into account, does not significantly reduce these lateral migration velocities. Constraining the length of the original Nazca Ridge by its conjugate feature on the Pacific Plate yields a length of 900 km for the subducted portion of the ridge. Using this constraint, ridge subduction began V11.2 Ma ago at 11‡S. Therefore, the Nazca Ridge did not affect the northern sites of Ocean Drilling Program (ODP) Leg 112 located at 9‡S. This is supported by benthic foraminiferal assemblages in ODP Leg 112 cores, indicating more than 1000 m of subsidence since at least Middle Miocene time, and by continuous shale deposition on the shelf from 18 to 7 Ma, recorded in the Ballena industrial well. At 11.5‡S, the model predicts the passage of the ridge crest V9.5 Ma ago. This agrees with the sedimentary facies and benthic foraminiferal stratigraphy of ODP Leg 112 cores, which argue for deposition on the shelf in the Middle and Late Miocene with subsequent subsidence of a minimum of several hundred meters. Onshore at 12‡S, the sedimentary record shows at least 500 m uplift prior to the end of the Miocene, also in agreement with the model. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: Nazca Ridge; oblique subduction; plate reconstruction; forearc; Peru 1. Introduction Seamount chains, submarine ridges and other * Present address: GeoForschungsZentrum Potsdam, Tele- grafenberg, 14473 Potsdam, Germany. bathymetric highs on oceanic plates entering sub- Tel.: +49-331-288-1376; Fax: +49-331-288-1370. duction zones will, in general, laterally migrate E-mail address: [email protected] (A. Hampel). along the active margin, unless they are parallel 0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S0012-821X(02)00859-2 EPSL 6378 7-10-02 Cyaan Magenta Geel Zwart 666 A. Hampel / Earth and Planetary Science Letters 203 (2002) 665^679 to the convergence direction (e.g. [1,2]), and may would result in a variable lateral migration veloc- a¡ect the sedimentological and tectonic evolution ity. of the forearc system signi¢cantly. The lateral mo- The fate of bathymetric highs during subduc- tion of such features can lead to a temporal se- tion to greater depth has long been subject to quence of uplift and subsidence of the forearc, controversy. While some authors note the tempo- frequently accompanied by enhanced surface and rally irregular occurrence and reduced number of tectonic erosion as well as steepening of the inner large earthquakes in the vicinity of such features trench wall and faulting in the upper plate (e.g. (e.g. [10]), others argue that subducting sea- [3^8]). These e¡ects are generally recorded in the mounts and ridges form asperities, at which earth- morphology and sedimentary facies of the forearc quakes may nucleate [11] and increase seismic and in uplifted coastal shorelines. As a conse- coupling [12]. In addition, the buoyancy of sub- quence, models resolving the history of forearc ducted bathymetric highs may decrease the dip of and arc systems must account for these three-di- the subducting slab and thus may terminate the mensional e¡ects and their development through magmatic activity in the overriding plate [7,10,13^ time. 15]. The velocity at which a bathymetric high moves An outstanding example of a subducting bathy- along an active margin is controlled by three pa- metric high migrating along an active plate rameters: the convergence velocity vc and the two boundary is the Nazca Ridge, which has a¡ected angles a and P, de¢ned by the orientation of the the Peruvian portion of the long-lived Andean bathymetric high relative to convergence direction subduction zone. Due to southward migration of and trench, respectively (Fig. 1). The lateral ve- the ridge, the Peruvian margin displays, from locity vlat of a bathymetric high parallel to the south to north, di¡erent stages of its tectonic evo- plate boundary is then: lution during and after ridge passage. Various v sina features in the o¡shore and onshore geology of v ¼ c lat sinP the Peruvian margin, such as uplift and subsi- dence of forearc basins, tectonic erosion of the lower continental slope and uplift of marine ter- Even if the convergence velocity is constant, a races have been attributed to ridge subduction curvature of the trench line, i.e. a variable angle P, [16^21]. Moreover, the coastal area above the subducting ridge was ruptured by two shallow thrust earthquakes with magnitudes of Mw = 8.1 and Mw = 7.7 in 1942 and 1996, respectively [22]. The downward continuation of the ridge has been related to a zone of reduced intermediate depth seismicity and to the southern boundary of the low-angle subduction segment beneath Southern Peru [23^25], which coincides with the terminus of the Quaternary volcanic arc [14,26]. To corre- late these di¡erent observations with the subduc- tion of the Nazca Ridge, it is crucial to constrain both the rate of its lateral movement along the margin and the original length of this feature. The ¢rst part of this study calculates the migra- tion velocity of the Nazca Ridge and yields a sig- Fig. 1. Geometric relations between the lateral migration ve- ni¢cantly slower lateral motion than previously locity v of a bathymetric high parallel to an active plate lat inferred [16,18^21,25,27,28], with the consequence boundary, the plate convergence velocity vc, and the orienta- tion of the bathymetric high relative to convergence direction that ages at which the ridge passed speci¢c sites and trench [9]. increase signi¢cantly. The second part speci¢es EPSL 6378 7-10-02 Cyaan Magenta Geel Zwart A. Hampel / Earth and Planetary Science Letters 203 (2002) 665^679 667 the onset of ridge subduction, assuming that the ward shift of the trench and the magmatic arc original length of the Nazca Ridge approximates [27]. However, interpretations of seismic data that of its conjugate feature on the Paci¢c Plate and ODP cores, in particular in the Lima Basin [18,25,27,28]. at 11.5‡S, indicate that during some periods, the forearc subsided at a lower rate than during times of prevailing long-term tectonic erosion or has 2. Geodynamic setting even been uplifted [17,37]. Regarding the temporal evolution of the colli- The Nazca Ridge is a more than 1000 km long sion zone between the Nazca Ridge and the Pe- and 200 km wide aseismic submarine ridge, which ruvian margin, current models di¡er in the lateral formed at the Paci¢c^Farallon/Nazca spreading migration velocities, in the ages of ridge passage center in the early Cenozoic [25,29,30] (Fig. 2). assigned to di¡erent latitudes and in the predicted The linear crest of the ridge is elevated 1500 m length of the original Nazca Ridge. The following above the surrounding sea £oor and trends reconstructions cover the migration history of the N42‡E. The average crustal thickness of the ridge Nazca Ridge along the entire Peruvian margin: derived from the analysis of Rayleigh waves is Pilger ([25]; his ¢gure 4) shows that the ridge ¢rst 18 þ 3 km [30]. Where the ridge descends beneath came in contact with the Peruvian trench at 5‡S in the South American Plate, the trench does not the Middle Miocene and later passed 10‡S at V9 show a pronounced deviation from its linear Ma. Other studies [16,18,27], based on plate re- trend, but the water depth along the trench line constructions [28] and the NUVEL-1A conver- shoals from 6500 m south of the ridge to 4000 m gence rate [38], inferred that the Nazca Ridge be- at the ridge crest. In bathymetry and side-scan gan to subduct 8 Ma ago at 8‡S and was located sonar images, features indicating ongoing surface at 9‡S and 11.5‡S at 6^7 Ma and 4^5 Ma, respec- erosion and faulting have been identi¢ed on the tively.