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 Andrea Hampel Hampel
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 South Americ American an mar margin gin betwee between n 14‡S 14‡S and and 17‡S 17‡S is charac character terize ized d by deform deformati ation on of the upper upper plate plate and severa severall 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 75 mm mm/a /a (befor (beforee 10.8 10.8 Ma), Ma), 61 mm/a mm/a (10.8^ (10.8^4.9 4.9 Ma), Ma), and and 43 mm/a mm/a (4.9 (4.9 Ma to presen present). t). IntraIntra-pla plate te deform deformati ation on associ associate ated d with with mount mountain ain buildi building ng in the Peruvi Peruvian an Andes Andes since since the Miocen Miocenee reduc reduces es the relati relative ve convergence rate between Nazca Plate and Peruvian forearc. Taking an intra-plate deformation at a rate of 10 mm/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 11.2 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 9.5 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. 2002 Elsevier Science B.V. All rights reserved. V
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Keywords: Nazca Ridge; oblique subduction; plate reconstruction; forearc; Peru
1. Introduct Introduction ion * Present address: GeoForschungsZentrum GeoForschungsZentrum Potsdam, Potsdam, Telegrafenberg, 14473 Potsdam, Germany. Tel.: +49-331-288-1376; Fax: +49-331-288-1370. E-mail address: andrea@gfz-pots
[email protected] dam.de (A. Hampel).
Seamou Seamount nt chains, chains, submar submarine ine ridges ridges and other other bathymetric highs on oceanic plates entering subductio duction n zones zones will, will, in genera general, l, latera laterally lly mig migrat ratee along along the active active mar margin gin,, unless unless they they are parall parallel el
0012-821 0012-821X X / 02 / $ ^ see front matter 2002 Elsevier Elsevier Science Science B.V. All rights reserved reserved.. PII: PII: S 0 01 2 - 8 2 1X ( 02 ) 0 0 8 59 - 2
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to the convergence direction (e.g. [1,2]), [1,2]), and may a¡ect the sedimentological and tectonic evolution of the forearc system signi¢cantly. The lateral motion of such features can lead to a temporal sequen quence ce of uplif upliftt and and subs subsid iden ence ce of the the forea forearc rc,, frequently accompanied by enhanced surface and tectonic erosion as well as steepening of the inner trench wall and faulting in the upper plate (e.g. [3^8]). [3^8] ). These e¡ects are generally recorded in the morphology and sedimentary facies of the forearc and and in uplif uplifte ted d coas coasta tall shor shorel eline ines. s. As a cons conseequence quence,, models models resolv resolving ing the histor history y of forear forearcc and arc systems must account for these three-dimensional e¡ects and their development through time. The velocity at which a bathymetric high moves along an active margin is controlled by three parameters: the convergence velocity vc and the two angles a and P , de¢ned by the orientation of the bathymetric bathymetric high relative relative to convergence convergence direction and trench trench,, respec respectiv tively ely ( Fig. 1). 1). The lateral velocity vlat of a bath bathym ymet etric ric high high para paralle llell to the the plate boundary is then: vlat
¼
vc sina sin P
Even if the convergence velocity is constant, a curvature of the trench line, i.e. a variable angle P ,
Fig. 1. Geometric relations between the lateral migration velocity vlat of a bathym bathymetr etric ic high high parall parallel el to an active active plate boundary, the plate convergence velocity vc , and the orientation of the bathymetric high relative to convergence direction and trench [9] [9]..
would result in a variable lateral migration velocity. The fate fate of bathym bathymetr etric ic highs highs during during subduc subduc-tion tion to grea greate terr dept depth h has has long long been been subj subjec ectt to controversy. While some authors note the temporally irregular occurrence and reduced number of large earthquakes in the vicinity of such features (e.g. [10]), [10]), othe others rs argu arguee that that subd subduc ucti ting ng seaseamounts and ridges form asperities, at which earthquakes quakes may nucleat nucleatee [11] and and increas increasee seismic seismic coupling [12]. [12]. In addition, the buoyancy of subducted bathymetric highs may decrease the dip of the subduc subductin ting g slab slab and thus may terminate terminate the magmatic activity in the overriding plate [7,10,13^ 15].. 15] An outstandin outstanding g example example of a subducting subducting bathymetr metric ic high high migr migrat atin ing g alon along g an acti active ve plat platee boundary is the Nazca Ridge, which has a¡ected the Peruvi Peruvian an portio portion n of the long-l long-live ived d Andean Andean subduction zone. Due to southward migration of the the ridg ridge, e, the the Peru Peruvi vian an ma marg rgin in disp displa lays ys,, from from south to north, di¡erent stages of its tectonic evolution lution during during and after after ridge ridge passag passage. e. Variou Variouss featur features es in the o¡shor o¡shoree and onshore onshore geolog geology y of the the Peruv Peruvia ian n ma marg rgin in,, such such as uplif upliftt and and subs subsiidence dence of forear forearcc basins, basins, tectonic tectonic erosio erosion n of the lower continental slope and uplift of marine terraces races have have been attrib attribute uted d to ridge ridge subduc subductio tion n [16^21].. More [16^21] Moreov over, er, the the coas coasta tall area area abov abovee the the subd subduct uctin ing g ridge ridge was was rupt ruptur ured ed by two two shal shallo low w 8.1 thrust thrust earthquake earthquakess with magnitudes of M w = 8.1 and M w = 7.7 in 1942 1942 and 1996, 1996, respect respective ively ly [22] [22].. The downward continuation of the ridge has been related related to a zone zone of reduce reduced d interme intermedia diate te depth depth seismici seismicity ty and to the southe southern rn bounda boundary ry of the low-angle low-angle subduction subduction segment segment beneath beneath Southern Southern Peru [23^25], [23^25] , which which coinci coincides des with with the termin terminus us of the Quaternary volcanic arc [14,26] [14,26].. To correlate these di¡erent observations with the subduction of the Nazca Ridge, it is crucial to constrain both both the rate of its lateral lateral moveme movement nt along along the margin and the original length of this feature. The ¢rst part of this study calculates the migration velocity of the Nazca Ridge and yields a signi¢can ni¢cantly tly slower slower latera laterall motion motion than than previo previousl usly y inferred [16,18^21,25,27,28] inferred [16,18^21,25,27,28],, with the consequence that ages at which the ridge passed speci¢c sites increas increasee signi¢ signi¢can cantly tly.. The second second part part speci¢ speci¢es es
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the onset of ridge subduction, assuming that the original length of the Nazca Ridge approximates that of its conjugate feature on the Paci¢c Plate [18,25,27,28].. [18,25,27,28]
2. Geodynamic Geodynamic setting setting
The Nazca Ridge is a more than 1000 km long and 200 km wide aseismic submarine ridge, which formed at the Paci¢c^Fara Paci¢c^Farallon/Na llon/Nazca zca spreading spreading center center in the early early Cenozo Cenozoic ic [25,29,30] (Fi Fig. g. 2). The linear crest of the ridge is elevated 1500 m abov abovee the the surr surrou ound ndin ing g sea sea £oor £oor and and tren trends ds N42‡E. The average crustal thickness of the ridge deriv derived ed from from the the anal analys ysis is of Rayl Rayleig eigh h wave wavess is 18 3 km km [30] [30].. Where the ridge descends beneath the the Sout South h Am Amer eric ican an Plat Plate, e, the the tren trench ch does does not not show show a pron pronou ounc nced ed devi deviat atio ion n from from its its line linear ar trend, but the water depth along the trench line shoals from 6500 m south of the ridge to 4000 m at the ridge crest. crest. In bathym bathymetr etry y and side-scan side-scan sonar images, features indicating ongoing surface erosion and faulting have been identi¢ed on the continental continental slope [32,33]. [32,33] . Landwa Landwards rds,, the recent recent collision zone is expressed by a narrowing of the shelf shelf,, a seaw seawar ard d shif shiftt of the the coas coastl tlin inee and and the the pres presen ence ce of raise raised d ma marin rinee terr terrac aces es at the the coas coastt betw between een 13.5 13.5‡S ‡S and and 15.6 15.6‡S ‡S [19,20]. [19,20] . Abov Abovee the the northern £ank of the subducted ridge, the recent subsidence of the marine terraces, which had been uplifted earlier by the ridge until the passage of its crest, illustrates illustrates its southward southward movement [19,20]. [19,20] . Further inland, the Abancay De£ection (Fig. ( Fig. 2), 2), which marks the northern boundary of the zone of active arc volcanism and separates segments of continental crust di¡ering in geochemical composition, has been related to the continuation of the Nazca Ridge [34] [34].. North of the collision zone, a small accretionary wedge may have begun to grow in the wake of the ridge ridge [18]. [18]. Furt Furthe herr nort north, h, o¡ Cent Centra rall and and North Peru, the absence of a large accretionary prism and tectonic erosion as the dominant mass tran transf sfer er regi regime me have have been been reco recogn gnize ized d [35,36]. [35,36] . Along this part of the margin, long-term tectonic erosion since at least the Middle Miocene has led to rapid subsidence of the forearc and to an east-
ward ward shif shiftt of the the tren trench ch and and the the ma magm gmat atic ic arc arc [27].. Howe [27] Howeve ver, r, inte interp rpret retat atio ions ns of seism seismic ic data data and ODP cores, in particular in the Lima Basin at 11.5‡S, indicate that during some periods, the forearc subsided at a lower rate than during times of prevai prevailin ling g long-t long-term erm tecton tectonic ic erosio erosion n or has even been uplifted [17,37] [17,37].. Regarding the temporal evolution of the collision zone between the Nazca Ridge and the Peruvian margin, current models di¡er in the lateral migration velocities, in the ages of ridge passage assigned to di¡erent latitudes and in the predicted length of the original Nazca Ridge. The following reconstructions cover the migration history of the Nazca Nazca Ridge Ridge along along the entire entire Peruvi Peruvian an mar margin: gin: Pilger ([25] ([25] ; his ¢gure 4) shows that the ridge ¢rst came in contact with the Peruvian trench at 5‡S in the Middle Miocene and later passed 10‡S at 9 Ma. Other Other studies studies [16,18,27], [16,18,27] , based based on plate plate reconstructions [28] and and the NUVELNUVEL-1A 1A conver conver-gence rate [38] [38],, inferred that the Nazca Ridge began to subduct 8 Ma ago at 8‡S and was located at 9‡S and 11.5‡S at 6^7 Ma and 4^5 Ma, respectively. Three other reconstructions concentrate on the the migr migrat ation ion of the the ridge ridge from from the the end end of the the Miocene to the present: Based on the plate motion data by Pardo-Casas and Molnar [39], [39], Hsu [19] infer inferss a latera laterall mig migrat ration ion veloci velocity ty of 71 mm/a. mm/a. Machar Macharee¤ and Ortlieb Ortlieb [20] use use the the plat platee motion data by Pardo-Casas and Molnar [39] to dedu deduce ce a pass passag agee of the the ridg ridgee cres crestt at 13‡S 13‡S at 4 Ma, i.e. a lateral velocity of 64 mm/a. Le Roux et al. [21] suggest that the ridge crest was located at 13.5‡S at 5.3 Ma and thus o¡ Lima (12‡S) before the end of the Miocene, i.e. laterally moving at a velocity of 42 mm/a, derived from converg vergenc encee rate ratess give given n by Stein Stein et al. al. [40]. [40]. Thes Thesee di¡erences in the inferred migration rates of the Nazca Ridge underline the importance of the reevaluation presented here. V
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3. Reconstruct Reconstruction ion of the migration migration history
3.1. Lateral Lateral mig migratio ration n velocity velocity Unraveling the migration history of subducting ridg ridges es,, seam seamou ount nt chai chains ns and and othe otherr subm submar arine ine
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Fig. 2. Map [31] showing the location of the Nazca Ridge, the spatial distribution of seismicity and active volcanoes (black triangles; from the Smithsonian Global Volcanism Program). ODP Leg 112 sites and two industrial wells (Ballena, Del¢n) are marked by white circles. The Peruvian low-angle subduction segment is located between 5‡S and 14‡S. Note the gap in the intermediate depth seismicity seismicity (70^300 (70^300 km) (dotted line) and the presence presence of deep seismic events (500^650 (500^650 km) beneath Brazil (dashed line). (Earthquake data from 1973 to 2002; US Geological Survey^National Earthquake Information Center.)
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bath bathym ymet etric ric high highss requ require iress know knowled ledge ge of past past plat platee moti motion ons, s, whic which h can can be obta obtain ined ed by two two types types of data data sets. sets. Plate Plate motion motionss averag averaged ed over over the the last last 3 Ma are are prov provid ided ed by the the NU NUVE VELL-1A 1A model, model, based based on evalua evaluatio tion n of spread spreading ing rates, rates, transform fault azimuths and earthquake slip vectors [38] [38].. On longer time scales, paleo-plate positions tions and motions motions can be recons reconstru tructe cted d by anaanalyzin lyzing g the the ma magn gnet etic ic anom anomal alie iess of the the ocea oceani nicc crust. This method yields average velocity vectors for di¡erent time intervals (e.g. [28,41] [28,41]). ). This This stud study y uses uses upda update ted d Na Nazca zca (Far (Faral allo lon)^ n)^ South American relative motions [42] which take into account a revision of the global geomagnetic time time scale scale [43]. [43]. This This data data set provides provides consta constant nt convergence velocities and directions for di¡erent time intervals for the last 40 Ma at di¡erent latitudes, of which the values given at 12‡S are applied (Table (Table 1). 1). The convergence rate of 75 mm/a for the last 5 Ma [42] agrees well with the NUVEL-1A prediction [38] prediction [38].. Both estimates are higher than the current convergence rate determined by spacespace-geod geodetic etic measurem measurement ents, s, i.e. 61 3 mm/a at 12‡S [44,45]. [44,45] . Since Since the convergen convergence ce rate rate may be slowin slowing g with with time, time, the space-g space-geod eodeti eticc values values are less relevant for this reconstruction. Using Using the averag averagee conver convergen gence ce veloci velocities ties and directions for the three latest time intervals, three displa displaceme cement nt vector vectorss and respect respective ive paleopaleo-pos posiitions of the Nazca Ridge relative to a ¢xed South American Plate are constructed (Table ( Table 1 and Fig. and Fig. 3a). 3a ). The resulting time path allows one to determine when the ridge crest passed a speci¢c point on the trench line, assuming a linear continuation of the ridge towards the trench, as suggested by the shape of the present ridge, and a paleo-trench posi positi tion on sim simila ilarr to the the pres presen entt tren trench ch line line [18^ 21,25,27,28] (Fig. 3b). 3b). Uncertaintie Uncertaintiess in the convergence convergence velocities velocities are
not speci¢ed speci¢ed [42] [42],, but may be of the order of 10% [44].. Since the errors are likely to be smaller in the [44] latest time interval, as suggested by the errors of the NUVEL-1A convergence rates [38], [38], and may be larger in the earliest time interval, this study assigns uncertainties of 5%, 10% and 15% to the convergence convergence velocities velocities of the 0^4.9 Ma, 4.9^10.8 Ma and 10.8^1 10.8^16 6 Ma time time interv intervals als,, respect respective ively ly (Table 1). 1). Using these error limits, the uncertainties in the ages of ridge passage with respect to the conver convergen gence ce rates rates of the three three time intervals intervals are given in Table 1. 1. Potential errors of the geomagnetic time scale and of the convergence azimuths for the di¡erent time intervals have not been taken into account. An implicit assumption of this reconstruction is that that the decrea decreasin sing g relati relative ve conver convergen gence ce rate rate between the Nazca Plate and stable South America over over the the last last 15^2 15^20 0 Ma, Ma, as deri derive ved d from from plat platee reconstruct reconstructions, ions, equals the amount amount of relative motion tion betwee between n the Nazca Plate Plate and the Peruvi Peruvian an forearc. This assumption has also been the basis for for all all prev previo ious us reco recons nstr truc uctio tions ns of the the Na Nazca zca Ridge motion [16,18^21,25,27,28]. [16,18^21,25,27,28] . Howe However ver,, the the prese presenc ncee of the the Ande Andean an moun mounta tain in belt belt east east of the forear forearcc demons demonstra trates tes that, that, strict strictly ly speaki speaking, ng, this assumption is not correct, since some of the relativ relativee plate plate motion motion is taken taken up by intraintra-pla plate te deforma deformatio tion n within within the South South America American n Plate. Plate. Obviously, Obviously, this intra-plate intra-plate deformatio deformation n tends to redu reduce ce the the rela relati tive ve moti motion on betw betwee een n the the Na Nazca zca Rid Ridge and the the Peru Peruvi vian an fore forear arcc syst system em.. At prese present nt,, a rigor rigorou ouss asse assess ssmen mentt of the the am amou ount nt and the direct direction ion of shorte shortening ning accomm accommoda odated ted in the Peruvian Andes is di⁄cult due to the lack of su⁄cient geological data. Nevertheless, spacegeodetic measurements [44] measurements [44] and and geological pro¢les ([46^48] and references therein) across the Andes can be used to estimate the present-day and past
Table 1 Relative plate motion between Nazca and South American plates at 12‡S [42] Time interval [Ma]
Convergence velocitya [km/Ma]
Conver Converge gence nce direct direction ion
Length Length of displa displacem cement ent vector vector [km]
Age Age uncert uncertain ainty ty [Ma]
0^4.9 (chrons 0^3) 4.9^10.8 (chrons 3^5) 10.8^16 (chrons 5^5C)
75 4 106 11 123 18
77‡ 82‡ 84‡
368 20 625 65 640 94
0.3 0.6 0.8
a
Errors are assumed to be 5%, 10% and 15% for the latest, intermediate, and earliest time interval, respectively.
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Fig. 3. (a) Three paleo-positi paleo-positions ons of the Nazca Ridge and displacem displacement ent vectors for the present intersection intersection point of ridge ridge and trench. Gray lines represent the assumed linear continuation of the ridge. Inset (b) shows diagram in which the latitudinal position of the linearly continuing ridge crest on the trench line and the migration velocity of the ridge parallel to the plate boundary are plotted versus time. The two black lines are derived by using the relative plate motion data as given in [42]. [42]. The black arrow marks the onset of ridge subduction inferred by this study ( Section 3.2). 3.2). The two gray lines refer to a scenario in which a small amount of intra-plate deformation (10 mm/a) accommodated in the Peruvian Andes is subtracted from the convergence rates of [42].. [42]
shortening shortening rates across across the Eastern Cordillera Cordillera and the Suband Subandean ean belt [49]. [49]. These These data data show show that that geolog geologic ic and spacespace-geo geodet detic ic displa displaceme cement nt rates rates are are gene genera rally lly cons consist isten entt and and that that dire direct ctio ions ns of shor shorte tenin ning g in the the East Easter ern n Ande Andess are are appr approx oxiimately parallel to the Nazca^South America convergence vector. The data have been interpreted in terms terms of a two-st two-stage age model with with rates rates of shortshort-
ening across the Eastern Andes of 5^8 mm/a for the last 25^10 Ma and of 10^15 mm/a for the last 10 Ma [49]. [49]. In order to account for the Andean intra-plate deformation, the lateral migration velocity of the Nazca Ridge is also presented for a scenario in which an average Andean shortening rate of 10 mm/a for the last 16 Ma is subtracted from from the relativ relativee conver convergen gence ce veloci velocity ty betwee between n
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Nazca and South American plates (gray lines in Fig. 3b). 3b). Considering the intra-plate deformation tends to slightly increase the ages of ridge passage assigned to speci¢c latitudes, in other words, the lateral migration velocity of the ridge slightly decreases creases.. However However,, the geolog geologica icall implica implication tionss of the the model model (see (see below below)) rema remain in valid valid,, even even if the the intr intraa-pl plat atee defo deform rmat atio ion n is take taken n into into acco accoun unt. t. To allo allow w a stra straig ight htfo forw rwar ard d comp compar ariso ison n of the the model with previous reconstructions of the Nazca Ridge Ridge motion motion,, the follow following ing discus discussio sion n uses uses the modelmodel-curv curvee neglect neglecting ing intraintra-pla plate te deform deformati ation on (black lines in Fig. in Fig. 3b). 3b). Once more detailed information on Andean shortening shortening rates and directions in Peru becomes available, it should be incorporated into the model. In summary, the ¢rst part of the reconstruction demons demonstra trates tes that that the ridge ridge moved moved signi¢ signi¢cant cantly ly slowe slowerr para paralle llell to the the ma marg rgin in than than infer inferre red d by previous studies [ studies [16,18^21,25,27,28 16,18^21,25,27,28]] . In particular, a ridge of su⁄cient length would have passed the ODP Leg 112 sites in the Trujillo/Yaquina (9‡S) and Lima basins basins (11.5‡ (11.5‡S) S) at 14.5 14.5 Ma and and at 9.5 Ma, respectively. Apart from this migration histor history, y, deduci deducing ng the onset onset of ridge ridge subduc subductio tion n requires an estimate of the length of the original ridge. V
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3.2. Original Original length length of the Nazca Ridge Ridge and onset onset of ridge subduction The preservation of oceanic ridges and plateaus in the Southeastern Paci¢c o¡ers the possibility to constrain the shape of already subducted parts of bath bathym ymet etric ric high highss on the the Na Nazc zca a Plate Plate by their their mirro mir rorr ima image gess on the the Paci¢ Paci¢cc Plate Plate ( Fi Fig. g. 4). As these pairs of conjugate highs have formed simultaneously at the Paci¢c^Farallon/Nazca spreading center (e.g. [29,52]), [29,52] ), they are thought to have a simil sim ilar ar leng length th and and shap shapee assu assumi ming ng symm symmet etri ricc spreading [25,52] [25,52].. The Nazca Ridge has a common origin with the Tuamot Tuamotu u Platea Plateau u at the Paci¢c Paci¢c^Fa ^Faral rallon/ lon/Naz Nazca ca spreading center [25,29,30] and the pre-condition of symmetric spreading seems to be met, since the resp respec ecti tive ve segm segmen ents ts of the the Na Nazc zca a and and Paci Paci¢c ¢c plat plates es betw betwee een n chro chrons ns 13 and and 23 have have simil similar ar widths (see Figs. 4b and 5). 5 ).
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The N70‡W trending, elongated Tuamotu Plateau teau is a compos composite ite featur featuree consist consisting ing of island island chains and oceanic plateaus with volcanic edi¢ces that that once once were were subaer subaerial ial and today form form atolls atolls [54],, wher [54] whereas eas the the Na Nazc zca a Ridg Ridgee is char charac acte teriz rized ed by sma smaller ller,, but simi similar lar submar submarine ine volcan volcanic ic feafeatures [32] tures [32].. Despite these di¡erences in their topography, raphy, both both ridges ridges have have an overal overalll linear linear trend. trend. Therefo Therefore, re, the 4000 m water water depth depth contour contour line line of the Tuamotu Plateau has been used to approximate the outline and total length of the original Nazca Ridge [18,25,27,28] [18,25,27,28].. To estimate the length of the subducted part of the Nazca Ridge, however ever,, it has has to be take taken n into into acco accoun untt that that the the northw northwest estern ernmos mostt part part of the Tuamot Tuamotu u Platea Plateau u form formed ed on 10^2 10^20 0 Ma old old ocea oceani nicc crus crustt of the the Paci¢c Plate, indicating an origin 600 km o¡ the spreading spreading center [54] center [54].. The hotspot that generated the northw northwest estern ernmos mostt part part of the Tuamot Tuamotu u PlaPlateau [55] most likely had no e¡ect on the Nazca Plate [54]. [54]. For this reason, the northwestern end of the plateau probably does not have a counterpart part on the the Na Nazc zca a Plate Plate.. Anot Anothe herr assu assump mpti tion on made to specify the onset of ridge subduction is the the use use of the the pres presen entt tren trench ch line line as the the paleo paleo-trench position [18^21,25,27,28] [18^21,25,27,28].. To esti estima mate te the the lengt length h of the the orig origin inal al Na Nazca zca Ridge Ridge,, a mir mirro rorr ima image ge of the the Tuam Tuamot otu u Plate Plateau au is created using its 4000 m contour line. To ¢nd the the corr correc ectt posi positi tion on of the the mirro mirrorr ima image ge on the the Nazca Na zca Ridg Ridge, e, ma magn gnet etic ic anom anomal aly y linea lineati tion onss of the surrounding sea £oor are ¢tted, using a global data set [51,52] together with speci¢c data for the Tuamotu Tuamotu Plateau Plateau region [53]. [53]. Chrons Chrons 15^20 are the the olde oldest st ma magn gnet etic ic anom anomal alies ies comm common on to the the sea £oor close to both features ( Fig. 4b). 4b). To ¢t these chrons north and south of the Tuamotu Plateau to the ones on the Nazca Plate, no scaling of the mirror image is needed, which indica dicate tess symm symmet etri ricc sea sea £oor £oor spre spread adin ing. g. On the the Nazca Plate, the trends of the chrons are better cons constr trai aine ned d nort north h than than sout south h of the the ridge ridge and and appear to be roughly parallel to each other ( Fig. 4b). 4b ). In contrast, the same magnetic lineations are at an angle with each other north and south of the Tuamot Tuamotu u Plateau Plateau.. As a conseq consequen uence, ce, ¢tting ¢tting the chrons chrons leads leads to two endmember endmember positi positions ons ( Fig. 5). Matching chrons 19 and 20, located south of
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Fig. 4. (a) Bathymetric map [31] of the South Paci¢c showing the Paci¢c^Nazca spreading center and the conjugate features Nazca Ridge Ridge and and Tuamot Tuamotu u Platea Plateau. u. (b) Outlines Outlines of the Nazca Ridge Ridge and the Tuamot Tuamotu u Platea Plateau u are are shown shown by their their 4000 4000 m water water depth contour lines. The global age grid [50] of the oceanic crust interpolated from magnetic anomalies is shown by color code. Selected Selected magnetic magnetic anomaly anomaly lineations lineations are represente represented d by black black [51], [51], blue [52] and red [53] lines.
the ridges, with chrons 18^21 being parallel, leads to an abru abrupt pt bend bend of the the orig origin inal al Na Nazc zca a Ridg Ridgee which results in a N16‡W trend and a length of about 1000 km corresponding to an onset of subduction 10.0 Ma ago at 8.5‡S ( Fig. 5). 5). Adjusting chrons 16 and 18, located north of the bathymetric metric highs, highs, with with chrons chrons 15^20 15^20 being being paralle parallel, l, leads to the position of the mirror image preferred by this this stud study, y, beca becaus usee in that that case case the the Na Nazc zca a Ridge continues linearly beneath South America for 1100 km, suggesting that the ¢rst contact of ridge ridge and trench trench occurr occurred ed 12.5 Ma ago at a latitude of 10‡S (Fig. ( Fig. 5). 5). Regarding the location of chron 18, it should be noted that the spatial exte extent nt of its its ma magn gnet etic ic sign signal al allo allows ws di¡e di¡ere rent nt V
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phases to be picked as chron 18. Since the details of the picking procedures are not available for all publications ([51^53] ([51^53] and references therein), this study uses the locations of chron 18 as shown in the published maps. Given that the preferred reconstructio construction n is additionally additionally constrained constrained by chrons 15 and 16, the possible non-unique identi¢cation of chron 18 by di¡erent authors is considered to have have only only a mino minorr e¡ect e¡ect on the the reco recons nstr truc ucte ted d length of the Nazca Ridge. The values of 1000 km and 1100 km for the origin original al length length of the Nazca Ridge, Ridge, as inferr inferred ed above, are maximum values. Taking into account that the 200 km long northwesternmost part of the Tuamotu Plateau most likely does not have a V
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Fig. 5. Migration Migration history history of the Nazca Ridge Ridge on the assumptio assumption n that the mirrored mirrored Tuamotu Plateau Plateau resembles resembles the subducted subducted part part of the Nazca Nazca Ridge. Ridge. The The magne magnetic tic anoma anomalie liess on the Nazca Nazca Plate Plate are marked marked in black black and blue. The magnetic magnetic lineat lineation ionss north and south of the Tuamotu Plateau have red and green colors, respectively (see inset). At the present collision zone, two endmember endmember models for the continuation continuation of the Nazca Ridge are shown: Adjusting Adjusting chrons chrons 15^20, 15^20, located north of both features, features, yields the red mirror image of the plateau. Fitting chrons 18^21, located south of both ridges, leads to a position of the mirrored Tuamotu Plateau shown as the green mirror image. Both mirror images are plotted without consideration of the variable dip of the subducting plate. For both mirror images, the lighter colors at their northeasternmost ends mark the 200 km long part of the Tuamotu Plateau which most likely does not have a counterpart on the Nazca Plate (see text for details). Thus, the red mirror image with a linear continuation of 900 km is the preferred scenario of this study. Note the coincidence of the preferred red mirror image with the reduced intermediat intermediatee depth seismicity seismicity (dotted line) and with the presence presence of deep seismic events beneath Brazil (dashed line). For the onset of ridge subduction, three di¡erent scenarios are presented: Using the preferred con¢guration, the original Nazca Ridge entered the trench 11.2 Ma ago at 11‡S (red). If the original Nazca Ridge continues for 1100 km, its subduction began 12.5 Ma ago at 10‡S (light red). The northernmost possible contact of ridge and trench at 8.5‡S corresponds to a mirror image adjusted to chrons 19^21 (green). V
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coun counte terp rpar artt on the the Na Nazc zca a Plate Plate yield yieldss the the prepreferred scenario of this study, in which the original ridge ridge cont contin inue uess for for 900 900 km bene beneat ath h Sout South h America America and entere entered d the trench trench 11.2 Ma ago at 11‡S (Fig. ( Fig. 5). 5). V
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4. Discussion Discussion
The The new new mode modell pres presen ente ted d for for the the kine kinema mati ticc
evoluti evolution on of the Nazca Ridge Ridge predic predicts, ts, for both endmember positions described above (Fig. ( Fig. 5), 5), a lateral lateral migration migration history history that di¡ers signi¢cantly signi¢cantly from from previo previous us studie studiess [16,18^21,25,27,28]. [16,18^21,25,27,28] . With With respect to the two possible positions of the mirror image of the Tuamotu Plateau, this study prefers ¢tting the magnetic anomaly lineations 16 and 18 north of the plateau instead of chrons 19 and 20 south of it for the following reasons: First, with this ¢t, the straight Nazca Ridge continues with-
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out a bend. Second, the outline of the northern arm of the Tuamotu Plateau resembles the modern Nazca Nazca Ridge, Ridge, in agreeme agreement nt with with their their probprobable alignment during their common origin [30]. [30]. Apart from that, the uncertainties in the direction of the magnetic anomalies 16 and 18 are considerably smaller than those of the shorter chrons 19 and and 20, 20, which which,, like like in the the Tuam Tuamot otu u Plate Plateau au reregion, might not be parallel to chrons 16 and 18. Di¡er Di¡eren entt orie orient ntat atio ions ns of chro chrons ns 18 and and 19 are are also suggested by the magnetic anomalies of the Nazca Nazca Plate Plate south south of the Nazca Fractu Fracture re Zone Zone (Fig. 4). 4). Another argument is that a linear continuation of the Nazca Ridge coincides well with the zone of reduced intermediate depth seismicity and the southern boundary of the segment of lowangle subduction beneath South Peru ( Figs. 2, 5). 5). The predicted northeastern end of the ridge correla relate tess with with the the clust cluster er of deep deep seis seismic mic even events ts beneat beneath h Brazil Brazil betwee between n 8.5‡ 8.5‡S S and and 10.5‡S (compare Figs. 2 and 5). 5 ). This This agre agrees es well well with with interpretations of the deep seismicity that propose an association of the southern earthquake cluster with with the the subd subduc ucte ted d part part of the the Na Nazc zca a Ridg Ridgee [56,57].. Moreover, a [56,57] N42‡E trending ridge coincides with the northern boundary of active volcanism and the Abancay De£ection [34,15] [34,15].. Pilger [25],, howe [25] however ver,, argu argued ed for for the the posi positi tion on ¢tte ¢tted d to chron 19, because the ridge then extends farther to the the nort north h and and thus thus can can expla explain in the the £at £at slab slab beneat beneath h Northe Northern rn Peru. Peru. This northe northern rn £at slab, slab, howeve however, r, may be caused caused by the subducte subducted d part part of the Carneg Carnegie ie Ridge Ridge o¡ North North Peru/E Peru/Ecuad cuador or or by anot anothe her, r, comp complet letely ely subd subduc ucte ted d ocea oceani nicc plateau [58]. [58]. Take Taken n toge togeth ther, er, thes thesee argu argume ment ntss strongly strongly support a linear continuation continuation of the ridge of 900 900 km and and an onse onsett of ridg ridgee subd subduc ucti tion on 11.2 Ma ago at 11‡S. If the the Na Nazca zca Ridge Ridge,, cont contin inui uing ng with with a linea linearr tren trend, d, had had ente entere red d the the tren trench ch at 8‡S 8‡S [16,18,27, 28],, its subduc 28] subductio tion n would would have have begun begun 16 Ma ago ago and and the the orig origin inal al ridg ridgee woul would d have have to be at leas leastt 1500 1500 km long long.. Such Such a leng length th is not not supsupV
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port ported ed by the the conj conjug ugat atee feat featur uree of the the Na Nazca zca Ridge on the Paci¢c Plate, since the entire Tuamotu Plateau is at most 1100 km longer than the modern Nazca Ridge. Thus, although the trench has probably been shifted eastward for at least 20 Ma due to tectonic erosion [27] [27],, a linearly trending original Nazca Ridge could not have reached the trench north of 10‡S. The new reconstruction has signi¢cant implications for models of the tectonic, sedimentological and geomorphic evolution of the Peruvian forearc and arc system systems. s. In partic particular ular,, di¡eren di¡erentt seismic seismic data data sets sets (e.g. (e.g. [59,60]) [59,60] ) and and OD ODP P Leg Leg 112 112 core coress in the the Truj Trujill illo/ o/Ya Yaqui quina na (9‡S (9‡S)) and and Lima Lima basin basinss (11.5‡ (11.5‡S) S) have have been been interp interpret reted ed [17,18,60,61] in light of previous previous reconstruct reconstructions ions suggesting suggesting that the the ridg ridgee cres crestt pass passed ed thes thesee sites sites 6 Ma and 4 Ma ago, respectively [16,18,27,28] [16,18,27,28].. According to this this stud study, y, howe however ver,, the the ridg ridgee was was not not su⁄su⁄ciently long to in£uence the region at 9‡S, while at 11.5‡S, it already caused maximum uplift 9.5 Ma ago (Fig. ( Fig. 3). 3). The marine and subaerial sedimentol mentologi ogical cal record record of the forear forearc, c, the onshor onshoree tect tecton onic ic hist histor ory, y, and and the the temp tempor oral al and and spat spatia iall evolution of volcanism of the Andean magmatic arc in Peru support the new model and will be discussed in some detail. In marine sediments, uplift of the forearc region can, in general, be derived from a trend to coarser deposits, possibly accompanied by an increase in the number of unconformities, and from benthic foraminifera foraminiferall stratigrap stratigraphy hy that gives informatio information n on the the wate waterr dept depth h at whic which h the the sedime sediment nt was was deposi deposited ted.. At 9‡S, 9‡S, benthi benthicc forami foraminif nifera erall assemassemblages in ODP Leg 112 cores and dredge samples indicate that the continental slope and shelf subsided 1500 1500 m betw between een the the Middle Middle Eocen Eocenee to Middle Middle Miocen Miocenee and experie experience nced d furthe furtherr subsisubsidence of 1300 m since 12^13 Ma [37,62]. [37,62] . Apart Apart from that, cores recovered during ODP Leg 112 and two industrial wells are characterized by the deposi depositio tion n of ¢ne-gr ¢ne-grain ained ed materi material, al, while while sandy sandy depo deposi sits ts are are miss missin ing g in the the Mioc Miocen enee ( Fi Fig. g. 6a 6a)) V
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Fig. 6. (a) 9‡S: Lithology of ODP Leg 112 sites [60] and of the Ballena industrial well, located on the shelf, with ages of dated samples (black circles) [63]. [63]. (b) 11.5‡S: Lithology of ODP Leg 112 sites that drilled into Miocene strata [60]. [60]. Paleo-bathymetry is derived from benthic forminiferal assemblages of site 679, located on the outer shelf shelf [37]. [37].
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[60]. Especi [60]. Especial ally ly in the the Balle Ballena na indu indust stria riall well, well, loca locate ted d abov abovee the the crest crest of the the oute outerr shelf shelf high high,, cont contin inuo uous us shal shalee depo deposi siti tion on betw betwee een n 18 and and 7 Ma [63] argues argues agains againstt the distur disturban bance ce of the depo deposi siti tion on milie milieu u due due to the the pass passag agee of a ridg ridgee (Fig. 6a). 6a). A comparison with the recent collision zone shows that the shelf area is strongly a¡ected by the Nazca Ridge. Marine deposits of Eocene to Upper Pliocene age that correlate with equivalent strata in submerged o¡shore forearc basins o¡ Central Peru have been raised above sea level [19].. [19] At 11.5‡S 11.5‡S,, deposi deposits ts at ODP Leg 112 112 sites sites become coarser, with a decrease in mud and an increase in silt and sand during the Middle and Late Miocene Miocene (Fi Fig. g. 6b 6b). ). At site 679, a layer of conglomerates has been deposited before the end of the Miocene. In cores recovered at ODP site 679, Middle Middle and Late Late Miocen Miocenee benthi benthicc forami foraminif nifera erall assemb assemblag lages es re£ect re£ect deposit deposition ion on the inner inner shelf shelf in shallow water [37] [37].. Following the hiatus at the end of the Late Miocene, deposition resumed at the the oute outerr shelf shelf in the the early early Plio Pliocen cene. e. The The next next forami foraminif nifers ers-bea -bearin ring g strata strata are of Quater Quaternar nary y age, age, with with depo deposi sitio tion n dept depth h £uct £uctua uatin ting g arou around nd 400 m. At site 682, Middle to Late Miocene foraminiferal miniferal assemblages assemblages have been deposited at middle bathyal depths (500^1500 m), while the Late Pliocen Pliocenee paleopaleo-env enviro ironmen nmentt was lower lower bathya bathyall (2000^400 (2000^4000 0 m) [37]. [37] . Site Site 688 688 is barr barren en of Late Late Miocene foraminiferal assemblages, however, between Early Miocene and Quaternary, the paleobiotope biotope changed changed from upper middle bathyal bathyal (500^ (500^ 1500 1500 m) to lower lower bathya bathyall depth depth (2000^ (2000^400 4000 0 m) [37].. In additio [37] addition, n, earlier earlier invest investiga igatio tions ns based based on dredge samples indicate more than 2000 m subsidence for 6 Ma, since Late Miocene benthic foraminifers, aminifers, living at 500 500 m dept depth, h, were were reco recovvered ered in the Lima Lima Basi Basin n at a wate waterr dept depth h of more more than than 2600 2600 m [62]. [62]. Base Based d on thes thesee init initia iall ODP Leg 112 results, a phase of uplift and erosion at 11.5‡S was derived to begin at 11 Ma and last until 7 Ma, while 6 Ma ago, a transition from uplift uplift to subsid subsidence ence occurre occurred d [16]. [16]. The o¡shor o¡shoree geological record of ODP Leg 112 as summarized above shows uplift of the forearc during Middle and Late Miocene and subsidence since the end of the Miocen Miocene. e. This This correl correlate atess very very well well with with the V
age of 9.5 Ma derived from the new reconstruction for passage of the ridge crest. The new model is also compatible with the sedimentological record of the R|¤mac^Chillo¤ n rivers at 12‡S, which eroded deep valleys on the Lima coastal plain during the Miocene. The alluvial fan deposited by these rivers experienced uplift of at least 500 m, which is attributed to the passage of the Nazca Ridge [21]. [21]. Potential sea level changes during during the Quater Quaternar nary y and Pliocen Pliocenee are sma smaller ller than 125 125 m and and have have been been cons consid idere ered d [21]. [21]. The uplift maximum at 12‡S was attained before the end of the Miocene [21] [21].. Anot Anothe herr piece piece of evid eviden ence ce in supp suppor ortt of the the presented model may be inferred from the correlation of the Nazca Ridge with the associated segment of low-angle subduction and the cessation of magmat mag matic ic arc activi activity. ty. At presen present, t, the bounda boundary ry between active and ceased volcanism in the south and and in the the nort north, h, resp respec ecti tive vely ly,, is loca locate ted d in the the land landwa ward rd cont contin inua uati tion on of the the ridg ridge, e, but but ma may y have gradually propagated southward due to the lateral movement of the ridge. O¡shore, volcanic ash layers recovere recovered d during during ODP Leg 112 have been been inte interp rpre rete ted d to show show high higher er acti activi vity ty of the the Peru Peruvi vian an volca volcanic nic arc arc in the the Late Late Mioce Miocene ne for for 9‡S than for 12‡S [64] [64].. Onshore geochronological data throwing light on a possible southward propagatin agating g zone, zone, where where volcan volcanism ism has ceased ceased,, are, are, however, however, rather rather limited [65,66] limited [65,66].. Pulses of Miocene volcanic activity [65,66] have been interpreted in cont contex extt of the the Quec Quechu hua a tect tecton onic ic phas phases es of the the Ande Andean an orog orogen eny y in Peru Peru duri during ng the the Midd Middle le to Late Late Miocene Miocene [64,67]. [64,67] . The Quechua II ( 10 Ma) and Quechua III ( 5 Ma) tectonic phases, which seem to be related to changes in the relative plate motion of the Nazca and South American plates [65,39], [65,39] , have have been been correla correlated ted with with unconunconform formiti ities es in OD ODP P Leg Leg 112 112 core coress at 11.5 11.5‡S ‡S [16]. [16]. Accordi According ng to this this study, study, the Nazca Ridge Ridge in£uin£uenced this region 9.5 Ma ago, which seems to coincide with the Late Miocene Quechua II tectonic phase. Despite this apparent correlation, it should be noted that the concept of distinct tectoni tonicc phas phases es in Peru Peru has has been been crit critici icized zed,, as the the availa available ble tempor temporal al constr constrain aints ts argue argue in favor favor of prolonged periods of tectonic activity [68] [68].. Nevertheless theless,, in the Ecuado Ecuadoria rian n Andes, Andes, subduc subductio tion n of V
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the the Carn Carneg egie ie Ridg Ridgee sinc sincee the the Midd Middle le Mioc Miocen enee may be responsible for the development of a higher topography, a compressional stress regime, and increas increased ed crusta crustall coolin cooling g and exhuma exhumatio tion n rates, rates, deduce deduced d from from ¢ssion ¢ssion track track data data in the collision collision zone [69]. [69]. While While,, in summa summary ry,, no sing single le obse observ rvat atio ion n is conclu conclusiv sivee about about its relatio relation n to the subduc subductio tion n of the Nazca Ridge, the combination of the arguments raised above strongly suggests that the new model is more compatible with the existing geological and geomorphic data.
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passag passagee with with region regionss that that have have been been in£uen in£uenced ced by the ridge, but otherwise share similar boundary conditions. Such a comparison may enable a better quanti¢cation of the geodynamic in£uence of the Nazca Ridge on the Peruvian margin in future studies. The case of the Nazca Ridge emphasizes that models regarding regarding the geodynamic geodynamic evolution of active active margins margins have have to take into into accoun accountt the migrat mig ration ion histor history y and threethree-dim dimens ension ional al e¡ects e¡ects associa associated ted with with latera laterally lly mig migrat rating ing bathym bathymetr etric ic highs.
Acknowledgements 5. Conclusion Conclusionss
This new reconstruction of the migration history of the the Na Nazc zca a Ridg Ridgee alon along g the the Peru Peruvia vian n ma marg rgin in suggests that the lateral motion of the ridge has decelerated through time. Considering that a small amount amount of the relative convergen convergence ce rate between the Nazca and South American plates is taken up by intra-plate deformation in the Andean mountain belt results in slower lateral migration of the ridge. However, this has no e¡ect on the geological ical impl implic icat atio ions ns of the the new new mode model. l. On the the asassump sumpti tion on that that the the origi origina nall Na Nazc zca a Ridg Ridgee has has a length length similar similar to its mir mirror ror image on the Paci¢c Paci¢c Plate, Plate, it contin continues ues for for 900 km beneat beneath h South South America. America. Therefor Therefore, e, the northeastern northeastern end of the Nazca Nazca Ridge Ridge entere entered d the trenc trench h 11.2 Ma ago at 11‡S 11‡S.. As a cons conseq eque uenc nce, e, the the ridg ridgee did did not not have an impact on the region north of 10‡S, where the northern transect of ODP Leg 112 is located. The region at 11.5‡S o¡ Lima has been a¡ected by ridge subduction 9.5 Ma ago. Support for the model is provided by the sedimentological and paleo-bathymetric record in ODP Leg 112 and industrial trial well well cores. cores. At 9‡S, 9‡S, cores cores show show mostly mostly ¢negraine grained d sedime sediments nts on the the contin continent ental al slope slope and, and, on the the shel shelf, f, cont contin inuo uous us shal shalee depo deposi siti tion on.. At 11.5‡S, the predicted age of the new model correlates well with a Late Miocene period of uplift and erosion followed by subsidence since 6 Ma. In light of this study, seismic and drilling data sets sets acqu acquir ired ed alon along g the the Peruv Peruvia ian n ma marg rgin in in the the last last decade decadess o¡er o¡er the possibili possibility ty to compar comparee region gionss that that have have not not been been a¡ect a¡ected ed by the the ridg ridgee V
Helpful Helpful comment commentss and discus discussio sions ns with with Nina Kuko Kukows wski ki,, Onno Onno Onck Oncken en,, Ulri Ulrich ch Rill Riller er and and David Hindle are gratefully acknowledged. acknowledged. Udo Barck Barckha haus usen en and and Ga Garr rret ettt Ito Ito are are than thanke ked d for for their their help with the magnetic magnetic anomal anomaly y data data and usef useful ul comm commen ents ts.. Many Many than thanks ks to Edmu Edmund ndo o Norabuena for his helpful comments on the plate motion data. The GMT [70] [70] software software was used to create Figs. Figs. 2^ 2^5 5. I than thank k the the revi review ewer erss Emil Emilee Okal, Tim Dixon and Steven Cande for constructive comments that helped to improve the manuscrip script. t. Fund Funding ing was was prov provide ided d by the the Germ German an Minist Ministry ry of Educat Education ion,, Science Science and Technol Technology ogy (BMBF) (BMBF) within within the GEOPEC GEOPECO O project project (Grant (Grant no. 03G0146A). [AC]
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