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Active crustal fragmentation along the Scotia–Antarctic plate boundary east of the South Orkney Microcontinent (Antarctica)
Earth and Planetary Science Letters 204 (2002) 33^46 www.elsevier.com/locate/epsl
Active crustal fragmentation along the Scotia^Antarctic plate boundary east of the South Orkney Microcontinent (Antarctica) J. Galindo-Zald|¤var a; , J.C. Balanya¤ b;c , F. Bohoyo c , A. Jabaloy a , A. Maldonado c , J.M. Mart|¤nez-Mart|¤nez a;c , J. Rodr|¤guez-Ferna¤ndez c , E. Surin‹ach d a Departamento de Geodina¤mica, Universidad de Granada, 18071 Granada, Spain Departamento de Ciencias Experimentales, Universidad Pablo de Olavide, Sevilla, Spain Instituto Andaluz Ciencias de la Tierra, CSIC/Universidad Granada, Facultad de Ciencias, 18002 Granada, Spain d Departament de Geodinamica i Geof|¤sica, Universidad de Barcelona, Barcelona, Spain b
c
Received 20 March 2002; received in revised form 31 May 2002; accepted 11 September 2002
Abstract The structure of the Scotia^Antarctic plate boundary is poorly known east of the South Orkney Microcontinent. New multichannel seismic profiles, together with magnetic, gravity and swath bathymetry data obtained during the SCAN97 cruise, show a complex relief of raised blocks and elongated depressions that may reach more than 6000 m in depth. These depressions develop in relation with extensional active structures and constitute an uncommon feature in the oceans, where most of the trenches are formed in subduction contexts. The main crustal elements of the area include the oceanic crust of the Scotia Plate, the Discovery Bank composed of continental crust, a tectonic domain with intermediate features between continental and oceanic crusts that includes the Southern Bank, and the oceanic crust of the northern Weddell Sea, representing the Antarctic Plate. The Intermediate Domain was probably developed during the Late Cenozoic subduction of the Weddell Sea oceanic crust below the Discovery Bank. The fault zone associated with the plate boundary is characterized at present by sinistral transcurrent and transtensional slips, which develop a NE^SW elongated deep pull-apart basin with extreme crustal thinning and mantle uplift. The complex bathymetry and structure of the plate boundary are consequences of the presence of continental and intermediate crusts ^ where the deformations are concentrated ^ between the two stable oceanic domains. The location of a major part of the plate boundaries around the Scotia Arc is probably determined by the position of the continental and intermediate crustal fragments surrounded by oceanic crust, due to the differential behavior experienced during deformation. ? 2002 Elsevier Science B.V. All rights reserved. Keywords: Scotia^Antarctic plate boundary; multichannel seismic pro¢les; gravity; crustal deformation; deep basins
* Corresponding author. Tel.: +34-958-243349; Fax: +34-958-248527. E-mail address: [email protected] (J. Galindo-Zald|¤var). 0012-821X / 02 / $ ^ see front matter ? 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 9 5 9 - 7
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Study area
Fig. 1. Geological setting of the study area in the frame of the Scotia Arc. Modi¢ed from Galindo-Zald|¤var et al. [8]. 1, Inactive fracture zone; 2, active fracture zone; 3, transform or transcurrent fault; 4, inactive subduction or reverse fault; 5, active subduction; 6, extensional fault; 7, active spreading axis; 8, inactive spreading axis; 9, continental-oceanic crustal boundary. FZ, fracture zone; APR, Antarctic-Phoenix Ridge; WSR, West Scotia Ridge; PB, Powell Basin; SOM, South Orkney Microcontinent; JB, Jane Basin; DB, Discovery Bank; HB, Herdman Bank.
1. Introduction The development of the Scotia Sea since the Oligocene is one of the main tectonic features of the recent evolution of the southern Atlantic [1^ 3]. This region shows a host of medium and small tectonic plates, essentially the Scotia and Sandwich plates, which accommodate the sinistral transcurrent motion between the major South America and Antarctic plates (Fig. 1). The Scotia Plate, mainly oceanic in nature, was formed by the activity of several spreading ridges [1,3]. The Sandwich Plate overthrusts the South America Plate, developing a subduction zone at the eastern end of the Scotia Sea, which is one of the most active structures of the region [2]. Jane Basin was developed in the Middle Miocene as a back-arc basin related to the subduction of the oceanic
crust of the Weddell Sea below the SE margin of the South Orkney Microcontinent [4,5]. One important geological feature observed in the region is the presence of a linear batholith of basic rocks intruded in continental crust during Cretaceous along the Paci¢c margin of the Antarctic Peninsula and South America and that is now well recognized around the Scotia Arc [6,7]. This batholith has associated a band of long wavelength, high-intensity magnetic anomalies named the Paci¢c Margin Anomaly or the West Coast Magnetic Anomaly [6,7]. The boundary of the Scotia and Sandwich plates with the Antarctic Plate is located within the complex South Scotia Ridge (Fig. 1), which includes blocks of continental crust and deep basins [5,8,9]. This boundary shows a moderately distributed seismicity along an E^W band charac-
C Fig. 2. GEOSAT free-air gravity anomaly map [20] including location chart of the SCAN 97 cruise with B/O Hespe¤rides track lines (A) and multibeam bathymetry plotted over satellite predicted bathymetry (B). A: Contour lines at 10 mGal intervals. Stars: earthquake epicenters (1973^2002) [17]. Earthquake focal mechanisms from Pelayo and Wiens [2]. Thick line, SM12 MCS pro¢le on Fig. 3. BB, Bruce Bank; JBs, Jane Basin; JBn, Jane Bank; SB, Southern Bank; DB, Discovery Bank; HB, Herdman Bank. B: Deep areas, blue; shallow areas, red.
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terized by extensional, transtensional and transcurrent sinistral earthquake focal mechanisms [2,8,9]. This is an area of intricate relief, with highs separated by steep slopes from elongated deep basins that reach more than 6000 m in depth (Fig. 2). The western part of this plate boundary has been studied in detail by multichannel seismic re£ection, gravity, magnetic and swath bathymetry surveys [5^10]. The eastern segment of the plate boundary, however, is poorly known, owing to the tectonic complexity of the region and to the logistic and weather-related di⁄culties. Some previous general surveys of the Scotia Arc region suggest that the ridge^trench collision eastwards of the South Orkney Microcontinent triggered changes in the Scotia Sea evolution [1,11^13]. A tectonic evolution with an active oceanic spreading axis located at the southeastwards margin of Discovery Bank is also proposed for the area [12]. In contrast, the regions located near the Scotia^ Sandwich spreading ridge and Sandwich trench have been studied more extensively [14,15]. The seismicity band in the region studied is oblique to the bathymetric features previously recognized in the area [1] and does not clearly delineate active structures as in other sectors of the Scotia Arc region. Moreover, the pole of rotation between the Scotia and Antarctic plates shows the sinistral transtensional character along the South Scotia Ridge [2]. The objectives of this study are oriented to establish the main active tectonic features of the eastern sector of the Scotia^Antarctic plate boundary, in a segment situated between the Discovery and Herdman banks (Figs. 1 and 2). We also discuss the origin of deep oceanic basins formed in extensional tectonics. Although in subduction zones the development of deep trenches is a common feature, the elongated basins described in this contribution are formed in extensional tectonics and are an uncommon bathymetric feature in the sea £oors. The study of this region will contribute to an understanding of the behavior of di¡erent types of crusts involved in complex plate boundaries that include local extensional and compressional stresses producing folds and faults with variable regimes. More speci¢cally,
this area may o¡er insights to the structures implicated in the development of tectonic arcs. This evolution involved the dispersion and deformation of crustal elements, essentially in the Caribbean and Scotia arc type of settings, but also in other tectonic arcs like in the eastern Mediterranean, Japan, Kuril and Banda among others. All these arcs are characterized by continental elements surrounded by oceanic crust. From a regional point of view, the new data show the main structures implicated in the active deformation zone between the Antarctic and Scotia plates, only recognized to present by a broad seismicity band, and the tectonic processes involved in the transference of the deformation along the southern boundary of the Scotia Plate.
2. Data and methodology Previous bathymetric data of the region were compiled from scarce ship tracks along the boundary between the Scotia and Weddell seas on the Tectonic Map of the Scotia Arc [1]. Free-air gravity and bathymetric data are also predicted from the altimetry data of the GEOSAT (Fig. 2A) [16], which, while representing a signi¢cant aid in the study of this remote region, does not provide detailed resolution for areas of complicated relief (Figs. 2B and 3). The seismicity data include both the earthquake location and the focal mechanism of some events [1,2,17]. The absence of permanent seismic stations in this area, however, increases the errors of location of the hypocenters [2] and inhibits to recognize precisely the location of single active structures. During the SCAN97 cruise aboard the R/V Hespe¤rides (January^February 1997), geophysical data were recorded along several pro¢les east of the South Orkney Microcontinent (Figs. 1 and 2). Most of the pro¢les are orthogonal to the trend of topographic features of the plate boundary (SM08, SM12, SM14 and SM17). In addition, we recorded a pro¢le parallel to this boundary (SM18). Gravity, magnetic, swath bathymetry and multichannel seismic data were acquired along these pro¢les. Gravity data were obtained
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Fig. 3. SM12 multichannel seismic pro¢le and interpretation. Location in Fig. 2. OC, oceanic crust; CC, continental crust; ID, Intermediate Domain; NDB, northward dipping band of re£ectors. The positions of blow-ups in Figs. 4, 5 and 6 are indicated.
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with a Bell Aerospace TEXTRON BGM-3 marine gravimeter. The free-air anomaly was determined with Lanzada software (A. Carbo¤, personal communication) and modelled with the GRAVMAG program [18]. Ship bathymetry (Fig. 2B) and gravity data ¢t in general with the available data from GEOSAT. Total intensity magnetic ¢eld data were recorded using a Geometrics G876 proton precession magnetometer. The IGRF 1995 [19] was used to calculate the magnetic anomalies. The multichannel re£ection seismic pro¢les were obtained with a tuned array of ¢ve BOLT air guns, and a streamer with a total length of 2.4 km and 96 channels. The shot interval was 50 m. Data in a frequency range of 8^128 Hz were recorded with a DFS V digital system and a sampling record interval of 2 ms and 10 s record lengths. The data were processed with a sequence, including migration using a DISCO/FOCUS system.
3. Crustal structure The main physiographic features of the study region are characterized by the presence of the Discovery and the Herdman banks and numerous minor elevations and intervening depressions recognized on the free-air gravity anomaly map (Fig. 2A). The Discovery Bank has a NE^SW elongation and an asymmetrical pro¢le, with a wide irregular slope to the Scotia Sea and a sharp rectilinear SE margin, where several perched elongated basins are located, the deeper one over 5600 m deep in the analyzed cross-section (Fig. 3). Toward the south, several minor and discontinuous NE^SW elongated highs with a very irregular bathymetry occur in the transition to the Weddell Sea. The Herdman Bank is located in the eastern sector (Fig. 2). This bank shows high seismic activity and it is separated from the Discovery Bank by the NE^SW depressions previously described. To the west, seismic activity is located between the Bruce Bank and the eastern end of Jane Basin. Pro¢le SM12 (Figs. 2 and 3) crosses the region from the Scotia to the Weddell seas, and it is representative of the structures observed in this sector of the Scotia^Antarctic plate boundary.
The MCS pro¢le shows a progressive transition from the crusts of the Scotia Sea to the Discovery Bank (Figs. 3 and 4). In the Scotia Sea, sea bottom is located at about 4 s (twt) and the sediment layer is up to 1.2 s (twt) thick. There is a band of high-amplitude re£ectors below the sediment layer, characteristic of the top of layer 2 of the oceanic crust [5]. These features disappear towards the Discovery Bank, which may correspond to continental crust. The northwestern margin of Discovery Bank exhibits features typical of a complex tectonic history. In the margin of the oceanic crust of the Scotia Sea reverse faults are recognized at depth, probably related to an early stage of compression that may have developed during the thrusting of the Discovery Bank over the Scotia Sea crust. Several subvertical and normal faults that a¡ect the sea bottom are seen in the transition to the Discovery Bank, where perched basins ¢lled by a thin sedimentary cover separate basement highs. The present structure of the southeastern margin of Discovery Bank is determined, however, by conjugated normal faults, mainly dipping southeastwards, with scarps greater than 3 s (twt), which bound small perched basins and highs (Figs. 3 and 5). The faults ¢nally develop a NE^ SW elongated depression reaching 7.7 s (twt) depth ^ the so-called Deep Basin ^ which is ¢lled by a layer of sediments of about 0.5 s (twt) that generally dips towards the NW. Northwestwards of the depression, it is possible to identify a northward low dipping re£ectors band (NDB, Figs. 3 and 5) that may be related to a former fault of unknown sense of motion, cut by the most recent main normal fault, and that does not a¡ect the sedimentary ¢lling of the basin. Toward the SE, an asymmetric elevation bounded by normal faults known as the Southern Bank is identi¢ed (Figs. 2, 3 and 6). The northwestern margin of the bank, with a scarp of 5 s (twt) is steeper than the southeastern margin, where several small basins are bounded by normal faults. The Weddell Sea crust is recognized in the southern segment of the pro¢le and seems to be oceanic in character because an intense band of re£ectors that may be related to the top of the igneous basement is recognized below a thin layer of sediments (Figs. 3
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and 6). The re£ectors seem to extend below the Southern Bank, suggesting the location of a former trench related to a subduction zone (Figs. 3 and 6). Above the igneous basement, a discontinuous depositional sequence with a maximum thickness of 1 s (twt) can be identi¢ed. The contact between the oceanic crust and the Southern Bank is covered by undeformed sediments, therefore it is not active. This contact is well expressed in the free-air gravity map (Fig. 2) because there is an abrupt change in the orientation between the NW^SE elongated anomalies related to the fracture zones of the oceanic crust of the Weddell Sea, and the NE^SW anomalies corresponding to the Southern and Discovery banks. In the Weddell Sea, some deep re£ections have been inferred to
represent the Moho on the basis of their depth and seismic expression (Figs. 3 and 6). The normal and vertical faults that deform Discovery Bank, the Deep Basin and the Southern Bank have scarps that are not covered with sediments and some of them, in addition, have related seismicity, that indicates an active or recently active character. One earthquake focal mechanism of transcurrent faulting is located in the southwestern central sector of Discovery Bank [2], probably related to an E^W oriented fault plane. NE^SW oriented normal fault mechanisms are situated one in the southeastern margin, near the Deep Basin, and the other near the northwestern border of the Discovery Bank (Fig. 2). The reverse faults located in the oceanic crusts of the
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Weddell and the Scotia seas and north of the Deep Basin, however, are inactive and covered by sediments (Figs. 3^6). We also analyzed free-air anomalies obtained along the pro¢les in order to study the deep structure. The ship gravity anomalies approximately coincide with the free-air anomalies determined by GEOSAT [20]. The di¡erences mainly concern the high-frequency anomalies and can probably be attributed to the di¡erent resolution of the two acquisition methods. The geometry of the gravity model is based on the bathymetry of the sea £oor and the sediment thickness derived from pro¢le SM12 (Figs. 3 and 7). In determining depths and thicknesses, we considered velocities of 1500 m/s, 2200 m/s and 6340 m/s respectively for the sea water, the sediments and the igneous oceanic crust, taking into account the velocities of the seismic refraction pro¢les in Powell Basin [21]. These data come from a nearby region with sim-
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ilar features. The Moho was attributed to a band of deep re£ectors located in the two pro¢le extremities (Figs. 4 and 6), between 7 and 8 s (twt). We considered densities of 3.35 g/cm3 for the standard mantle and of 1.03 g/cm3 for the sea water. The location of continental, intermediate and oceanic crusts was based on the seismic characteristics of the di¡erent units in the pro¢le, described before, and the signatures of the magnetic anomalies, that are described for each sector. The northern part of pro¢le SM12 represents the oceanic crust of the Scotia Sea. There is a progressive transition to the continental crust of Discovery Bank. Although the seismic data do not allow us to establish precisely the continental^oceanic crustal boundary, this boundary has been established with the gravity model and magnetic data (Figs. 2^7). The Scotia Sea probably was formed about anomaly Chron 5 in this sector, and although really there are no data located precisely
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Fig. 7. Free-air gravity pro¢le and model, and total ¢eld magnetic anomaly pro¢le. PMA, anomaly resembling the Paci¢c Margin Anomaly. See text for discussion. Location in Fig. 2.
in this area, anomaly Chrons 4 and 6 have been identi¢ed in adjacent sectors [1]. Taking into account these data and in order to justify the gravity anomalies, in gravity modelling we considered a density of 2.88 g/cm3 for the layers 2 and 3 of the oceanic crust and a density of 2.3 g/cm3 for the sediments, taking into account that they may be mostly consolidated and including diamictites. Discovery Bank is made up of continental crust (2.67 g/cm3 ), as con¢rmed by the shallow bathymetry, the seismic character of the basement, with discontinuous re£ectors of low amplitude, and the presence of magnetic anomalies characteristic of continental crust, similarly to the Paci¢c Margin Anomaly [6,7]. There is a progressive change in the nature of the crust and the relief becomes very irregular southeast of Discovery Bank. The Deep Basin and the Southern Bank correspond to an Intermediate Domain between the continental crust of Discovery Bank and the oceanic crust of the Weddell Sea, as shown by the intermediate densities (2.8 g/cm3 ) in the gravity model. In addition the magnetic anomalies do not show the high intensities of the Discovery
Bank or the pattern of ocean sea £oor spreading anomalies (Fig. 7). In this domain the Moho depth has been constrained by the gravity modelling. The geometry of the Moho in the Intermediate Domain was determined by modelling on the basis of the densities attributed to the tectonic elements located along the pro¢le (Fig. 7) and the Moho depths established with the seismic data observed in the extremities of pro¢le SM12. Moho in the oceanic crust ranges between 10 and 11 km depth. Finally, a typical oceanic crust corresponding to the Weddell Sea is seen in the southern segment of the pro¢le. For the Weddell Sea oceanic crust, a density of 3 g/cm3 was assigned to layers 2 and 3 of the oceanic crust and a density of 2.5 g/cm3 to the sediments by modelling constraints. The density considered for the oceanic crust in the Weddell Sea is higher than in the Scotia Sea. This is a constraint in the model imposed by the gravity and seismic data and also agrees with the high density determined during gravity modelling in the southern Weddell Sea [22]. It was also taken into account that this oceanic crust may be
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about 20 Ma old (anomaly Chron C6, [23]), and most probably older than in the Scotia Sea. Most of the di¡erence in thickness of the oceanic crusts of Weddell and Scotia seas is a consequence of the sedimentary layer thickness (Figs. 3 and 7). Discovery Bank is characterized by a small crustal thickening. The most interesting feature corresponds to a marked crustal thinning that is located in the Deep Basin, southward from Discovery Bank. The crustal thickness varies asymmetrically with respect to the center of the basin, showing sharper thinning in the northern area than in the southern segment. The Southern Bank, which belongs to the Intermediate Domain, reveals a crustal thickness similar to that of the oceanic crust.
4. Discussion Previous works indicated tentatively the location of the Scotia^Antarctic plate boundary east of the South Orkney Microcontinent on the basis of the distribution of seismicity [1^3,13]. However, no de¢nite boundary has been drawn due to the complex geological structure and the absence of detailed geophysical data of the region (Figs. 1 and 2). This region shows a di¡use seismicity that may be locally concentrated, whereas the three focal mechanisms of the area point to transcurrent and extensional tectonics (Fig. 2). The geodynamics of the area also indicates that the plate boundary acts as a transtensional sinistral fault zone [1^3]. This sector of the Scotia^Antarctic plate boundary is formed by a host of continental and intermediate crustal elements isolated by deep basins, which are located between the oceanic crusts of the Scotia and Antarctic plates (Figs. 2, 3 and 7). In the study area, however, there is not any E^ W trending tectonic lineation, parallel to the main trend of the seismicity band. This setting suggests that the plate boundary deformation should be distributed along oblique structures. Discovery Bank corresponds to a thinned continental crustal element. Southward, around the Deep Basin and the Southern Bank, the change in the seismic character of the crust and in the magnetic and
gravity anomaly features suggests that this area probably corresponds to a domain of intermediate crust, the Intermediate Domain. The age of the oceanic crust in the Weddell Sea decreases toward the NW [23], and almost all the elongated gravity anomalies of the fracture zones stop abruptly along the southern margin of the Intermediate Domain (Figs. 2A and 8). This contact is attributed to the relict subduction zone of the Weddell Sea oceanic crust below the southern Scotia Arc (Figs. 8 and 9), that was probably active until Late Cenozoic times. The contact between the oceanic crust of the Weddell Sea and the Intermediate Domain is covered by the more recent deposits in pro¢le SM12, which indicate the relict nature of the subduction, analogous to the western segments of this contact in the northern Weddell Sea [5]. The most reasonable origin for the Intermediate Domain is its development in the southeast margin of Discovery Bank during the northwestward subduction of the oceanic crust of the Weddell Sea (Fig. 9). In this tectonic setting probably a dextral transform fault was active at the SW border of the Discovery Bank (Fig. 9). However, we have not detailed data for the age of the oceanic crusts in this sector, in order to assess the age of the subduction, although it is inactive at present. The Intermediate Domain is probably formed by continental fragments, sediments and magmatic intrusions that may justify the intermediate crustal density determined in gravity modelling, the seismic and the magnetic characters. These characteristics are di¡erent from those of the surrounding oceanic and continental crusts. Other compressive structures probably were developed in this tectonic setting, such as the reverse fault observed at the northwestern margin of Discovery Bank (Figs. 3, 4 and 8). Moreover, there is a northward dipping band of re£ectors observed at the northern boundary of the Deep Basin (NDB, Figs. 3 and 5), that may be possibly a reverse fault, but this attribution is not totally univocal. All these structures are at present inactive and covered by sediments. The earthquake focal mechanisms and active faults, evidenced by recent scarps and associated seismicity, suggest the sinistral and transtensional
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character of the plate boundary at present (Figs. 2, 8 and 9). The northern margin of Discovery Bank is deformed by high-angle normal faults with associated transcurrent and normal earthquake focal mechanisms and producing perched basins (Figs. 2^4 and 8). The oblique orientation of the southeastern margin of Discovery Bank with respect to the E^W oriented seismicity band related to the plate boundary may have allowed the development of normal faults in this margin and also facilitated the growth of the elongated Deep Basin under extensional tectonics (Figs. 3, 5 and 8). Similar deep basins, which may be considered as pull-apart structures, have been described in the South Scotia Ridge, west of the South Orkney Microcontinent [8,9]. The gravity models clearly indicate extreme crustal thinning, but the sediment cover observed in the seismic pro¢le (Figs. 3 and 5) and the absence of sea £oor spreading anomalies in magnetic pro¢les (Fig. 7) show that oceanic spreading has not yet started. These observations contradict some previous interpretations of this boundary in regional models [1,11,15]. The geological and geophysical transect between the Scotia and Weddell seas across the South Orkney Microcontinent, Jane Basin backarc and Jane Bank volcanic arc [4,5] has similar features to the one crossing the continental Discovery Bank, the Deep Basin and the Southern Bank. In both cases the Weddell Sea oceanic crust subduction is inactive. A possible model for the development of the studied structures is to consider that the Deep Basin formed as a back-arc basin, like the Jane Basin, and was displaced eastwards together with the Discovery Bank and later reactivated as a pull-apart basin (Fig. 9). In this case, the Southern Bank would represent a volcanic arc similar to the Jane Bank. At present, however, there are not enough data to con¢rm univocally all the aspects of this model. The structures recognized in this transect of the plate boundary end toward the NE in Herdman Bank and toward the SW in Jane Bank^Jane Basin. These limits are evidenced by NW^SE elongated minima of free-air anomalies located to the SW of Discovery and Herdman banks. They probably correspond to transform faults, neces-
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sary to transfer the extension observed in the Deep Basin, located at the extremities of fracture zones of the Weddell Sea oceanic crust (Figs. 8 and 9). The seismicity continues westward along the southern margin of Bruce Bank, whereas eastward the seismicity is concentrated southeast of Herdman Bank (Fig. 2). This seismicity may re£ect the presence of an active spreading center or an axis of crustal thinning, within the same transtensional regime, and with a NE^SW orientation located along the southeastern border of the Herdman Bank (Figs. 8 and 9). This tectonic setting is compatible with the sinistral transtensional character along the South Scotia Ridge, determined from the pole of rotation between the Scotia and Antarctic plates [2]. Most of the sinistral transcurrent and transtensional deformations related to the plate boundary are distributed within the continental crust of the Discovery Bank, and the Intermediate Domain, as shown by the location of epicenters (Figs. 2 and 8). This setting is similar to that of the South Scotia Ridge, west of the South Orkney Microcontinent, where most of the active faults are located within the continental blocks, resulting in large elongated depressions inside the ridge, while the activity in the continent^ocean boundary is scarce [8]. This may be a consequence of the rheology of the lithospheric plates and suggests that plate boundaries in the Scotia Sea are preferentially located in areas of continental crust, which may be more easily deformed than the rigid oceanic crust. In this aspect, the continental crust has a rheology mainly determined by the behavior of quartz, while the oceanic crust is characterized by that of the olivine. Oceanic lithosphere is thinner than the continental, but quartz is weaker than olivine and the lithosphere is more easily deformed in continental crust than in oceanic crust under the same stress setting [24,25].
5. Conclusions The sector of the Scotia^Antarctic plate boundary located east of the South Orkney Microcontinent comprises a complex pattern of continental, intermediate and oceanic crustal elements, sepa-
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Fig. 8. Geological sketch of the main tectonic structures. 1, Oceanic crust; 2, continental crust; 3, intermediate crust; 4, volcanic arc; 5, active fault; 6, active normal fault; 7, active maximum crustal thinning axis, the stretched region covers a broad band; 8, scarp; 9, ridge; 10, elongated gravity minima; 11, elongated gravity maxima; 12, inactive fracture zone; 13, inactive subduction or reverse fault; 14, inactive normal fault; 15, 2000 m depth bathymetry contour line; 16, earthquake epicenters (1973^2002) [17]. JBs, Jane Basin; JBn, Jane Bank; DB, Discovery Bank; HB, Herdman Bank; SB, Southern Bank.
rated by deep extensional basins. Between the oceanic crusts of the Scotia and Weddell seas it is possible to di¡erentiate two main tectonic elements deformed by an active sinistral fault zone that accommodates the relative motion of the Scotia^Antarctic plate boundary. To the north, Discovery Bank is an element of thinned continental crust, whereas to the south, the Intermediate Domain is made up by crust of intermediate features between continental and oceanic and comprises the Southern Bank and the Deep Basin. The deformation along this segment of the South Scotia Ridge extends eastward across Herdman Bank, and carries westward between Bruce Bank and Jane Basin. The complex structure of the area re£ects the evolution of the Scotia Arc and the tectonics of
the northern Weddell Sea (Fig. 9). Since Oligocene times, the continental blocks that formed the connection between South America and the Antarctic Peninsula were dispersed around the Scotia Arc [3]. During an initial deformation stage, probably in Early or Middle Miocene, Discovery Bank underwent contractional deformations related to the subduction of the oceanic crust of the northern Weddell Sea at its southeastern border, and also related to its thrusting over the Scotia Sea oceanic crust at the northern border. The development of these structures is related to the eastwards migration of the Discovery Bank in respect to the Jane Bank, determining the presence of a dextral fault zone (Fig. 9). In this setting, the Intermediate Domain most likely developed. The Southern Bank and the Deep Basin of
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45º W
40º W
35º W
Present
Scotia Sea PB BB
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HB ? 60º S
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sin JB Ba
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in JB as
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? HB
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2
3
4
Fig. 9. Paleogeographic reconstruction of main tectonic elements. Based in previous reconstructions of the Scotia Arc [12,13] and modi¢ed with own data. 1, Transform fault; 2, subduction zone; 3, oceanic spreading axis; 4, active maximum crustal thinning axis. BB, Bruce Bank; DB, Discovery Bank; HB, Herdman Bank; JB, Jane Bank; PB, Pirie Bank; SB, Southern Bank; SOM, South Orkney Microcontinent.
45
this domain probably correspond respectively to an island arc and a back-arc basin. The reverse faults in the northern border of the Deep Basin would also have been active in such a tectonic setting. All of these compressional structures are covered by sediments and are inactive at present. The sinistral transcurrent motion between the Scotia and Antarctic plates determines the recent evolution of the area. Most of the active structures with associated seismicity are concentrated in the wide section of continental and intermediate blocks between the oceanic crusts of the Scotia and Weddell seas. Transcurrent sinistral and normal faults are located in the northwestern margin of Discovery Bank, whereas extensional faults determine the recent development of the Deep Basin and the Southern Bank. The Deep Basin may be the result of the reactivation of a former back-arc basin in a tectonic setting that caused it to become a pull-apart basin. The Deep Basin has an ENE^WSW oblique orientation with respect to the sinistral transcurrent fault zone related to the plate boundary, and is located between two NW^SE overlapping transcurrent faults (Figs. 8 and 9). This basin is characterized by extreme crustal thinning, but there is no clear evidence of sea £oor spreading, and constitutes one of the deepest examples of oceanic basins developed in extensional tectonics. The tectonics of the eastern South Scotia Ridge suggest that, in the case of complex plate boundaries with continental and oceanic crustal elements, the deformation related to faults tends to be concentrated within the continental and intermediate crusts, not along the boundaries between the oceanic and continental crusts. Thus, structures that deform the continental and intermediate crust may re£ect the nature of the behavior of lithospheric plates rather than the continental^ oceanic crust boundary. This is a consequence of the weakness of continental crust, whose rheology is determined by the presence of quartz, by comparison with the oceanic crust, whose behavior is controlled by the more resistant olivine. The present location around the Scotia Arc of continental fragments may be the most signi¢cant factor of control for the location of the active faults,
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plate boundaries and tectonic activity of the region.
Acknowledgements We thank the Commander, o⁄cers and crew of the B/O Hespe¤rides for their support and cooperation in obtaining these data under sometimes severe sea conditions. The diligence and expertise of engineers E. Litcheva and J. Maldonado who processed the MCS data and swath bathymetry is appreciated and Dr. A. Carbo¤ for the program to calculate gravity anomalies. The comments of Dr. J. Woodside, Dr. G. Eagles and an anonymous reviewer have greatly improved this contribution. Spain’s CICYT supported this research through Project ANT99-0817 and REN2001-2143/ANT. [BW]
References [1] Tectonic Map of the Scotia Arc, Sheet BAS (Misc) 3, Scale 1:3000000, British Antarctic Survey, Cambridge, 1985. [2] A.M. Pelayo, D.A. Wiens, Seismotectonics and relative plate motions in the Scotia Sea Region, J. Geophys. Res. 94 (1989) 7293^7320. [3] P.F. Barker, I.W.D. Dalziel, B.C. Storey, Tectonic development of the Scotia Arc region, in: R.J. Tingey (Ed.), Antarctic Geology, Oxford University Press, Oxford, 1991, pp. 215^248. [4] L.A. Lawver, B. Della Vedova, R.P. Von Herzen, Heat £ow in Jane Basin, northwest Weddell Sea, J. Geophys. Res. 96 (1991) 2019^2038. [5] A. Maldonado, N. Zitellini, G. Leitchenkov, J.C. Balanya¤, F. Coren, J. Galindo-Zald|¤var, A. Jabaloy, E. Lodolo, J. Rodr|¤guez-Ferna¤ndez, C. Zanolla, Small ocean basin development along the Scotia^Antarctica plate boundary and in the northern Weddell Sea, Tectonophysics 296 (1998) 371^401. [6] E. Surin‹ach, J. Galindo-Zald|¤var, A. Maldonado, R.A. Livermore, Large amplitude magnetic anomalies in the northern sector of the Powell Basin, NE Antarctic Peninsula, Mar. Geophys. Res. 19 (1997) 65^80. [7] S.W. Garrett, Aeromagnetic studies of crustal blocks and basins in West Antarctica: a review, in: M.R.A. Thomson, J.A. Crame, J.W. Thomson (Eds.), Geological Evolution of the Antarctica, Geol. Soc. London Spec. Publ. 28, London, 1991, pp. 419^431.
[8] J. Galindo-Zald|¤var, A. Jabaloy, A. Maldonado, C. Sanz de Galdeano, Continental fragmentation along the South Scotia Ridge transcurrent plate boundary (NE Antarctic Peninsula), Tectonophysics 242 (1996) 275^301. [9] J. Acosta, E. Uchupi, Transtensional tectonics along the South Scotia Ridge, Antarctica, Tectonophysics 267 (1996) 31^56. [10] K.A. Klepeis, L.A. Lawver, Tectonics of the Antarctic^ Scotia plate boundary near Elephant and Clarence Islands, west Antarctica, J. Geophys. Res. 101 (1996) 20211^20231. [11] P.F. Barker, A spreading centre in the East Scotia Sea, Earth Planet. Sci. Lett. 15 (1972) 123^132. [12] P.F. Barker, P.G. Barber, E.C. King, An Early Miocene ridge crest-trench collision on the South Scotia Ridge Near 36‡W, Tectonophysics 102 (1984) 315^332. [13] P.F. Barker, Scotia Sea regional tectonic evolution: implications for mantle £ow and palaeocirculation, Earth Sci. Rev. 55 (2001) 1^39. [14] R.D. Larter, E.C. King, P.T. Leat, A.M. Reading, J.L. Smellie, D.K. Smythe, South Sandwich slices reveal much about arc structure geodynamics and composition, EOS 79 (1998) 281^285. [15] N.J. Bruguier, R.A. Livermore, Enhanced magma supply at the southern East Scotia Ridge: evidence for mantle £ow around the subducting slab?, Earth Planet. Sci. Lett. 191 (2001) 129^144. [16] D.T. Sandwell, W.H.F. Smith, Marine gravity anomaly from Geosat and ERS-1 satellite altimetry, J. Geophys. Res. 102 (1997) 10039^10054. [17] USGS, National Earthquake Information Center, 2002, Web page URL, http://neic.usgs.gov/neis/epic/epic.html. [18] R.C. Pedley, J.P. Bubsby, Z.K. Dabek, GRAVMAG v1.5, British Geological Survey, 1993. [19] I.A.G.A., Division V, Group 8, International Geomagnetic Reference Field, 1995 revision, Geophys. J. Int. (1996) 125^318. [20] D.C. McAdoo, K.M. Marks, Gravity ¢elds of the Southern Ocean from Geosat data, J. Geophys. Res. 97 (1992) 3247^3260. [21] E. King, G. Leitchenkov, J. Galindo-Zald|¤var, A. Maldonado, E. Lodolo, Crustal structure and sedimentation in Powell Basin, in: P.F. Barker, A. Cooper (Eds.), Geology and Seismic Stratigraphy of the Antarctic Margin, Part 2, Antarctic Research Series, American Geophysical Union 71, Washington DC, 1997, pp. 75^93. [22] J.K. Ferris, A.P.M. Vaughan, B.C. Storey, Relics of a complex triple junction in the Weddell Sea embayment, Antarctica, Earth Planet. Sci. Lett. 178 (2000) 215^230. [23] R.A. Livermore, R.W. Woollet, Sea£oor spreading in the Weddell Sea and Southwest Atlantic since the Late Cretaceous, Earth Planet. Sci. Lett. 117 (1993) 475^495. [24] R.G. Park, Geological Structures and Moving Plates, Blackie, Glasgow, 1988, 337 pp. [25] E.M. Moores, R.J. Twiss, Tectonics, W.H. Freeman and Company, New York, 1995, 415 pp.
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Active crustal fragmentation along the Scotia^Antarctic plate boundary east of the South Orkney Microcontinent (Antarctica) J. Galindo-Zald|¤var a; , J.C. Balanya¤ b;c , F. Bohoyo c , A. Jabaloy a , A. Maldonado c , J.M. Mart|¤nez-Mart|¤nez a;c , J. Rodr|¤guez-Ferna¤ndez c , E. Surin‹ach d a Departamento de Geodina¤mica, Universidad de Granada, 18071 Granada, Spain Departamento de Ciencias Experimentales, Universidad Pablo de Olavide, Sevilla, Spain Instituto Andaluz Ciencias de la Tierra, CSIC/Universidad Granada, Facultad de Ciencias, 18002 Granada, Spain d Departament de Geodinamica i Geof|¤sica, Universidad de Barcelona, Barcelona, Spain b
c
Received 20 March 2002; received in revised form 31 May 2002; accepted 11 September 2002
Abstract The structure of the Scotia^Antarctic plate boundary is poorly known east of the South Orkney Microcontinent. New multichannel seismic profiles, together with magnetic, gravity and swath bathymetry data obtained during the SCAN97 cruise, show a complex relief of raised blocks and elongated depressions that may reach more than 6000 m in depth. These depressions develop in relation with extensional active structures and constitute an uncommon feature in the oceans, where most of the trenches are formed in subduction contexts. The main crustal elements of the area include the oceanic crust of the Scotia Plate, the Discovery Bank composed of continental crust, a tectonic domain with intermediate features between continental and oceanic crusts that includes the Southern Bank, and the oceanic crust of the northern Weddell Sea, representing the Antarctic Plate. The Intermediate Domain was probably developed during the Late Cenozoic subduction of the Weddell Sea oceanic crust below the Discovery Bank. The fault zone associated with the plate boundary is characterized at present by sinistral transcurrent and transtensional slips, which develop a NE^SW elongated deep pull-apart basin with extreme crustal thinning and mantle uplift. The complex bathymetry and structure of the plate boundary are consequences of the presence of continental and intermediate crusts ^ where the deformations are concentrated ^ between the two stable oceanic domains. The location of a major part of the plate boundaries around the Scotia Arc is probably determined by the position of the continental and intermediate crustal fragments surrounded by oceanic crust, due to the differential behavior experienced during deformation. ? 2002 Elsevier Science B.V. All rights reserved. Keywords: Scotia^Antarctic plate boundary; multichannel seismic pro¢les; gravity; crustal deformation; deep basins
* Corresponding author. Tel.: +34-958-243349; Fax: +34-958-248527. E-mail address: [email protected] (J. Galindo-Zald|¤var). 0012-821X / 02 / $ ^ see front matter ? 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 9 5 9 - 7
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Study area
Fig. 1. Geological setting of the study area in the frame of the Scotia Arc. Modi¢ed from Galindo-Zald|¤var et al. [8]. 1, Inactive fracture zone; 2, active fracture zone; 3, transform or transcurrent fault; 4, inactive subduction or reverse fault; 5, active subduction; 6, extensional fault; 7, active spreading axis; 8, inactive spreading axis; 9, continental-oceanic crustal boundary. FZ, fracture zone; APR, Antarctic-Phoenix Ridge; WSR, West Scotia Ridge; PB, Powell Basin; SOM, South Orkney Microcontinent; JB, Jane Basin; DB, Discovery Bank; HB, Herdman Bank.
1. Introduction The development of the Scotia Sea since the Oligocene is one of the main tectonic features of the recent evolution of the southern Atlantic [1^ 3]. This region shows a host of medium and small tectonic plates, essentially the Scotia and Sandwich plates, which accommodate the sinistral transcurrent motion between the major South America and Antarctic plates (Fig. 1). The Scotia Plate, mainly oceanic in nature, was formed by the activity of several spreading ridges [1,3]. The Sandwich Plate overthrusts the South America Plate, developing a subduction zone at the eastern end of the Scotia Sea, which is one of the most active structures of the region [2]. Jane Basin was developed in the Middle Miocene as a back-arc basin related to the subduction of the oceanic
crust of the Weddell Sea below the SE margin of the South Orkney Microcontinent [4,5]. One important geological feature observed in the region is the presence of a linear batholith of basic rocks intruded in continental crust during Cretaceous along the Paci¢c margin of the Antarctic Peninsula and South America and that is now well recognized around the Scotia Arc [6,7]. This batholith has associated a band of long wavelength, high-intensity magnetic anomalies named the Paci¢c Margin Anomaly or the West Coast Magnetic Anomaly [6,7]. The boundary of the Scotia and Sandwich plates with the Antarctic Plate is located within the complex South Scotia Ridge (Fig. 1), which includes blocks of continental crust and deep basins [5,8,9]. This boundary shows a moderately distributed seismicity along an E^W band charac-
C Fig. 2. GEOSAT free-air gravity anomaly map [20] including location chart of the SCAN 97 cruise with B/O Hespe¤rides track lines (A) and multibeam bathymetry plotted over satellite predicted bathymetry (B). A: Contour lines at 10 mGal intervals. Stars: earthquake epicenters (1973^2002) [17]. Earthquake focal mechanisms from Pelayo and Wiens [2]. Thick line, SM12 MCS pro¢le on Fig. 3. BB, Bruce Bank; JBs, Jane Basin; JBn, Jane Bank; SB, Southern Bank; DB, Discovery Bank; HB, Herdman Bank. B: Deep areas, blue; shallow areas, red.
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terized by extensional, transtensional and transcurrent sinistral earthquake focal mechanisms [2,8,9]. This is an area of intricate relief, with highs separated by steep slopes from elongated deep basins that reach more than 6000 m in depth (Fig. 2). The western part of this plate boundary has been studied in detail by multichannel seismic re£ection, gravity, magnetic and swath bathymetry surveys [5^10]. The eastern segment of the plate boundary, however, is poorly known, owing to the tectonic complexity of the region and to the logistic and weather-related di⁄culties. Some previous general surveys of the Scotia Arc region suggest that the ridge^trench collision eastwards of the South Orkney Microcontinent triggered changes in the Scotia Sea evolution [1,11^13]. A tectonic evolution with an active oceanic spreading axis located at the southeastwards margin of Discovery Bank is also proposed for the area [12]. In contrast, the regions located near the Scotia^ Sandwich spreading ridge and Sandwich trench have been studied more extensively [14,15]. The seismicity band in the region studied is oblique to the bathymetric features previously recognized in the area [1] and does not clearly delineate active structures as in other sectors of the Scotia Arc region. Moreover, the pole of rotation between the Scotia and Antarctic plates shows the sinistral transtensional character along the South Scotia Ridge [2]. The objectives of this study are oriented to establish the main active tectonic features of the eastern sector of the Scotia^Antarctic plate boundary, in a segment situated between the Discovery and Herdman banks (Figs. 1 and 2). We also discuss the origin of deep oceanic basins formed in extensional tectonics. Although in subduction zones the development of deep trenches is a common feature, the elongated basins described in this contribution are formed in extensional tectonics and are an uncommon bathymetric feature in the sea £oors. The study of this region will contribute to an understanding of the behavior of di¡erent types of crusts involved in complex plate boundaries that include local extensional and compressional stresses producing folds and faults with variable regimes. More speci¢cally,
this area may o¡er insights to the structures implicated in the development of tectonic arcs. This evolution involved the dispersion and deformation of crustal elements, essentially in the Caribbean and Scotia arc type of settings, but also in other tectonic arcs like in the eastern Mediterranean, Japan, Kuril and Banda among others. All these arcs are characterized by continental elements surrounded by oceanic crust. From a regional point of view, the new data show the main structures implicated in the active deformation zone between the Antarctic and Scotia plates, only recognized to present by a broad seismicity band, and the tectonic processes involved in the transference of the deformation along the southern boundary of the Scotia Plate.
2. Data and methodology Previous bathymetric data of the region were compiled from scarce ship tracks along the boundary between the Scotia and Weddell seas on the Tectonic Map of the Scotia Arc [1]. Free-air gravity and bathymetric data are also predicted from the altimetry data of the GEOSAT (Fig. 2A) [16], which, while representing a signi¢cant aid in the study of this remote region, does not provide detailed resolution for areas of complicated relief (Figs. 2B and 3). The seismicity data include both the earthquake location and the focal mechanism of some events [1,2,17]. The absence of permanent seismic stations in this area, however, increases the errors of location of the hypocenters [2] and inhibits to recognize precisely the location of single active structures. During the SCAN97 cruise aboard the R/V Hespe¤rides (January^February 1997), geophysical data were recorded along several pro¢les east of the South Orkney Microcontinent (Figs. 1 and 2). Most of the pro¢les are orthogonal to the trend of topographic features of the plate boundary (SM08, SM12, SM14 and SM17). In addition, we recorded a pro¢le parallel to this boundary (SM18). Gravity, magnetic, swath bathymetry and multichannel seismic data were acquired along these pro¢les. Gravity data were obtained
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Figure 4
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Figure 6
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1
Fig. 3. SM12 multichannel seismic pro¢le and interpretation. Location in Fig. 2. OC, oceanic crust; CC, continental crust; ID, Intermediate Domain; NDB, northward dipping band of re£ectors. The positions of blow-ups in Figs. 4, 5 and 6 are indicated.
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with a Bell Aerospace TEXTRON BGM-3 marine gravimeter. The free-air anomaly was determined with Lanzada software (A. Carbo¤, personal communication) and modelled with the GRAVMAG program [18]. Ship bathymetry (Fig. 2B) and gravity data ¢t in general with the available data from GEOSAT. Total intensity magnetic ¢eld data were recorded using a Geometrics G876 proton precession magnetometer. The IGRF 1995 [19] was used to calculate the magnetic anomalies. The multichannel re£ection seismic pro¢les were obtained with a tuned array of ¢ve BOLT air guns, and a streamer with a total length of 2.4 km and 96 channels. The shot interval was 50 m. Data in a frequency range of 8^128 Hz were recorded with a DFS V digital system and a sampling record interval of 2 ms and 10 s record lengths. The data were processed with a sequence, including migration using a DISCO/FOCUS system.
3. Crustal structure The main physiographic features of the study region are characterized by the presence of the Discovery and the Herdman banks and numerous minor elevations and intervening depressions recognized on the free-air gravity anomaly map (Fig. 2A). The Discovery Bank has a NE^SW elongation and an asymmetrical pro¢le, with a wide irregular slope to the Scotia Sea and a sharp rectilinear SE margin, where several perched elongated basins are located, the deeper one over 5600 m deep in the analyzed cross-section (Fig. 3). Toward the south, several minor and discontinuous NE^SW elongated highs with a very irregular bathymetry occur in the transition to the Weddell Sea. The Herdman Bank is located in the eastern sector (Fig. 2). This bank shows high seismic activity and it is separated from the Discovery Bank by the NE^SW depressions previously described. To the west, seismic activity is located between the Bruce Bank and the eastern end of Jane Basin. Pro¢le SM12 (Figs. 2 and 3) crosses the region from the Scotia to the Weddell seas, and it is representative of the structures observed in this sector of the Scotia^Antarctic plate boundary.
The MCS pro¢le shows a progressive transition from the crusts of the Scotia Sea to the Discovery Bank (Figs. 3 and 4). In the Scotia Sea, sea bottom is located at about 4 s (twt) and the sediment layer is up to 1.2 s (twt) thick. There is a band of high-amplitude re£ectors below the sediment layer, characteristic of the top of layer 2 of the oceanic crust [5]. These features disappear towards the Discovery Bank, which may correspond to continental crust. The northwestern margin of Discovery Bank exhibits features typical of a complex tectonic history. In the margin of the oceanic crust of the Scotia Sea reverse faults are recognized at depth, probably related to an early stage of compression that may have developed during the thrusting of the Discovery Bank over the Scotia Sea crust. Several subvertical and normal faults that a¡ect the sea bottom are seen in the transition to the Discovery Bank, where perched basins ¢lled by a thin sedimentary cover separate basement highs. The present structure of the southeastern margin of Discovery Bank is determined, however, by conjugated normal faults, mainly dipping southeastwards, with scarps greater than 3 s (twt), which bound small perched basins and highs (Figs. 3 and 5). The faults ¢nally develop a NE^ SW elongated depression reaching 7.7 s (twt) depth ^ the so-called Deep Basin ^ which is ¢lled by a layer of sediments of about 0.5 s (twt) that generally dips towards the NW. Northwestwards of the depression, it is possible to identify a northward low dipping re£ectors band (NDB, Figs. 3 and 5) that may be related to a former fault of unknown sense of motion, cut by the most recent main normal fault, and that does not a¡ect the sedimentary ¢lling of the basin. Toward the SE, an asymmetric elevation bounded by normal faults known as the Southern Bank is identi¢ed (Figs. 2, 3 and 6). The northwestern margin of the bank, with a scarp of 5 s (twt) is steeper than the southeastern margin, where several small basins are bounded by normal faults. The Weddell Sea crust is recognized in the southern segment of the pro¢le and seems to be oceanic in character because an intense band of re£ectors that may be related to the top of the igneous basement is recognized below a thin layer of sediments (Figs. 3
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39
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6 7 8
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Fig. 4. Enlarged view of SM12 multichannel seismic pro¢le showing the northern border of Discovery Bank and the Scotia Sea. Location in Fig. 3. OC, oceanic crust; CC, continental crust.
and 6). The re£ectors seem to extend below the Southern Bank, suggesting the location of a former trench related to a subduction zone (Figs. 3 and 6). Above the igneous basement, a discontinuous depositional sequence with a maximum thickness of 1 s (twt) can be identi¢ed. The contact between the oceanic crust and the Southern Bank is covered by undeformed sediments, therefore it is not active. This contact is well expressed in the free-air gravity map (Fig. 2) because there is an abrupt change in the orientation between the NW^SE elongated anomalies related to the fracture zones of the oceanic crust of the Weddell Sea, and the NE^SW anomalies corresponding to the Southern and Discovery banks. In the Weddell Sea, some deep re£ections have been inferred to
represent the Moho on the basis of their depth and seismic expression (Figs. 3 and 6). The normal and vertical faults that deform Discovery Bank, the Deep Basin and the Southern Bank have scarps that are not covered with sediments and some of them, in addition, have related seismicity, that indicates an active or recently active character. One earthquake focal mechanism of transcurrent faulting is located in the southwestern central sector of Discovery Bank [2], probably related to an E^W oriented fault plane. NE^SW oriented normal fault mechanisms are situated one in the southeastern margin, near the Deep Basin, and the other near the northwestern border of the Discovery Bank (Fig. 2). The reverse faults located in the oceanic crusts of the
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3 4 TWT (s)
Weddell and the Scotia seas and north of the Deep Basin, however, are inactive and covered by sediments (Figs. 3^6). We also analyzed free-air anomalies obtained along the pro¢les in order to study the deep structure. The ship gravity anomalies approximately coincide with the free-air anomalies determined by GEOSAT [20]. The di¡erences mainly concern the high-frequency anomalies and can probably be attributed to the di¡erent resolution of the two acquisition methods. The geometry of the gravity model is based on the bathymetry of the sea £oor and the sediment thickness derived from pro¢le SM12 (Figs. 3 and 7). In determining depths and thicknesses, we considered velocities of 1500 m/s, 2200 m/s and 6340 m/s respectively for the sea water, the sediments and the igneous oceanic crust, taking into account the velocities of the seismic refraction pro¢les in Powell Basin [21]. These data come from a nearby region with sim-
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Fig. 5. Enlarged view of SM12 multichannel seismic pro¢le showing the Deep Basin in the Intermediate Domain (ID). Location in Fig. 3.
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Fig. 6. Enlarged view of SM12 multichannel seismic pro¢le showing the Southern Bank and Weddell Sea. Location in Fig. 3. OC, oceanic crust; ID, Intermediate Domain.
ilar features. The Moho was attributed to a band of deep re£ectors located in the two pro¢le extremities (Figs. 4 and 6), between 7 and 8 s (twt). We considered densities of 3.35 g/cm3 for the standard mantle and of 1.03 g/cm3 for the sea water. The location of continental, intermediate and oceanic crusts was based on the seismic characteristics of the di¡erent units in the pro¢le, described before, and the signatures of the magnetic anomalies, that are described for each sector. The northern part of pro¢le SM12 represents the oceanic crust of the Scotia Sea. There is a progressive transition to the continental crust of Discovery Bank. Although the seismic data do not allow us to establish precisely the continental^oceanic crustal boundary, this boundary has been established with the gravity model and magnetic data (Figs. 2^7). The Scotia Sea probably was formed about anomaly Chron 5 in this sector, and although really there are no data located precisely
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Fig. 7. Free-air gravity pro¢le and model, and total ¢eld magnetic anomaly pro¢le. PMA, anomaly resembling the Paci¢c Margin Anomaly. See text for discussion. Location in Fig. 2.
in this area, anomaly Chrons 4 and 6 have been identi¢ed in adjacent sectors [1]. Taking into account these data and in order to justify the gravity anomalies, in gravity modelling we considered a density of 2.88 g/cm3 for the layers 2 and 3 of the oceanic crust and a density of 2.3 g/cm3 for the sediments, taking into account that they may be mostly consolidated and including diamictites. Discovery Bank is made up of continental crust (2.67 g/cm3 ), as con¢rmed by the shallow bathymetry, the seismic character of the basement, with discontinuous re£ectors of low amplitude, and the presence of magnetic anomalies characteristic of continental crust, similarly to the Paci¢c Margin Anomaly [6,7]. There is a progressive change in the nature of the crust and the relief becomes very irregular southeast of Discovery Bank. The Deep Basin and the Southern Bank correspond to an Intermediate Domain between the continental crust of Discovery Bank and the oceanic crust of the Weddell Sea, as shown by the intermediate densities (2.8 g/cm3 ) in the gravity model. In addition the magnetic anomalies do not show the high intensities of the Discovery
Bank or the pattern of ocean sea £oor spreading anomalies (Fig. 7). In this domain the Moho depth has been constrained by the gravity modelling. The geometry of the Moho in the Intermediate Domain was determined by modelling on the basis of the densities attributed to the tectonic elements located along the pro¢le (Fig. 7) and the Moho depths established with the seismic data observed in the extremities of pro¢le SM12. Moho in the oceanic crust ranges between 10 and 11 km depth. Finally, a typical oceanic crust corresponding to the Weddell Sea is seen in the southern segment of the pro¢le. For the Weddell Sea oceanic crust, a density of 3 g/cm3 was assigned to layers 2 and 3 of the oceanic crust and a density of 2.5 g/cm3 to the sediments by modelling constraints. The density considered for the oceanic crust in the Weddell Sea is higher than in the Scotia Sea. This is a constraint in the model imposed by the gravity and seismic data and also agrees with the high density determined during gravity modelling in the southern Weddell Sea [22]. It was also taken into account that this oceanic crust may be
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about 20 Ma old (anomaly Chron C6, [23]), and most probably older than in the Scotia Sea. Most of the di¡erence in thickness of the oceanic crusts of Weddell and Scotia seas is a consequence of the sedimentary layer thickness (Figs. 3 and 7). Discovery Bank is characterized by a small crustal thickening. The most interesting feature corresponds to a marked crustal thinning that is located in the Deep Basin, southward from Discovery Bank. The crustal thickness varies asymmetrically with respect to the center of the basin, showing sharper thinning in the northern area than in the southern segment. The Southern Bank, which belongs to the Intermediate Domain, reveals a crustal thickness similar to that of the oceanic crust.
4. Discussion Previous works indicated tentatively the location of the Scotia^Antarctic plate boundary east of the South Orkney Microcontinent on the basis of the distribution of seismicity [1^3,13]. However, no de¢nite boundary has been drawn due to the complex geological structure and the absence of detailed geophysical data of the region (Figs. 1 and 2). This region shows a di¡use seismicity that may be locally concentrated, whereas the three focal mechanisms of the area point to transcurrent and extensional tectonics (Fig. 2). The geodynamics of the area also indicates that the plate boundary acts as a transtensional sinistral fault zone [1^3]. This sector of the Scotia^Antarctic plate boundary is formed by a host of continental and intermediate crustal elements isolated by deep basins, which are located between the oceanic crusts of the Scotia and Antarctic plates (Figs. 2, 3 and 7). In the study area, however, there is not any E^ W trending tectonic lineation, parallel to the main trend of the seismicity band. This setting suggests that the plate boundary deformation should be distributed along oblique structures. Discovery Bank corresponds to a thinned continental crustal element. Southward, around the Deep Basin and the Southern Bank, the change in the seismic character of the crust and in the magnetic and
gravity anomaly features suggests that this area probably corresponds to a domain of intermediate crust, the Intermediate Domain. The age of the oceanic crust in the Weddell Sea decreases toward the NW [23], and almost all the elongated gravity anomalies of the fracture zones stop abruptly along the southern margin of the Intermediate Domain (Figs. 2A and 8). This contact is attributed to the relict subduction zone of the Weddell Sea oceanic crust below the southern Scotia Arc (Figs. 8 and 9), that was probably active until Late Cenozoic times. The contact between the oceanic crust of the Weddell Sea and the Intermediate Domain is covered by the more recent deposits in pro¢le SM12, which indicate the relict nature of the subduction, analogous to the western segments of this contact in the northern Weddell Sea [5]. The most reasonable origin for the Intermediate Domain is its development in the southeast margin of Discovery Bank during the northwestward subduction of the oceanic crust of the Weddell Sea (Fig. 9). In this tectonic setting probably a dextral transform fault was active at the SW border of the Discovery Bank (Fig. 9). However, we have not detailed data for the age of the oceanic crusts in this sector, in order to assess the age of the subduction, although it is inactive at present. The Intermediate Domain is probably formed by continental fragments, sediments and magmatic intrusions that may justify the intermediate crustal density determined in gravity modelling, the seismic and the magnetic characters. These characteristics are di¡erent from those of the surrounding oceanic and continental crusts. Other compressive structures probably were developed in this tectonic setting, such as the reverse fault observed at the northwestern margin of Discovery Bank (Figs. 3, 4 and 8). Moreover, there is a northward dipping band of re£ectors observed at the northern boundary of the Deep Basin (NDB, Figs. 3 and 5), that may be possibly a reverse fault, but this attribution is not totally univocal. All these structures are at present inactive and covered by sediments. The earthquake focal mechanisms and active faults, evidenced by recent scarps and associated seismicity, suggest the sinistral and transtensional
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character of the plate boundary at present (Figs. 2, 8 and 9). The northern margin of Discovery Bank is deformed by high-angle normal faults with associated transcurrent and normal earthquake focal mechanisms and producing perched basins (Figs. 2^4 and 8). The oblique orientation of the southeastern margin of Discovery Bank with respect to the E^W oriented seismicity band related to the plate boundary may have allowed the development of normal faults in this margin and also facilitated the growth of the elongated Deep Basin under extensional tectonics (Figs. 3, 5 and 8). Similar deep basins, which may be considered as pull-apart structures, have been described in the South Scotia Ridge, west of the South Orkney Microcontinent [8,9]. The gravity models clearly indicate extreme crustal thinning, but the sediment cover observed in the seismic pro¢le (Figs. 3 and 5) and the absence of sea £oor spreading anomalies in magnetic pro¢les (Fig. 7) show that oceanic spreading has not yet started. These observations contradict some previous interpretations of this boundary in regional models [1,11,15]. The geological and geophysical transect between the Scotia and Weddell seas across the South Orkney Microcontinent, Jane Basin backarc and Jane Bank volcanic arc [4,5] has similar features to the one crossing the continental Discovery Bank, the Deep Basin and the Southern Bank. In both cases the Weddell Sea oceanic crust subduction is inactive. A possible model for the development of the studied structures is to consider that the Deep Basin formed as a back-arc basin, like the Jane Basin, and was displaced eastwards together with the Discovery Bank and later reactivated as a pull-apart basin (Fig. 9). In this case, the Southern Bank would represent a volcanic arc similar to the Jane Bank. At present, however, there are not enough data to con¢rm univocally all the aspects of this model. The structures recognized in this transect of the plate boundary end toward the NE in Herdman Bank and toward the SW in Jane Bank^Jane Basin. These limits are evidenced by NW^SE elongated minima of free-air anomalies located to the SW of Discovery and Herdman banks. They probably correspond to transform faults, neces-
43
sary to transfer the extension observed in the Deep Basin, located at the extremities of fracture zones of the Weddell Sea oceanic crust (Figs. 8 and 9). The seismicity continues westward along the southern margin of Bruce Bank, whereas eastward the seismicity is concentrated southeast of Herdman Bank (Fig. 2). This seismicity may re£ect the presence of an active spreading center or an axis of crustal thinning, within the same transtensional regime, and with a NE^SW orientation located along the southeastern border of the Herdman Bank (Figs. 8 and 9). This tectonic setting is compatible with the sinistral transtensional character along the South Scotia Ridge, determined from the pole of rotation between the Scotia and Antarctic plates [2]. Most of the sinistral transcurrent and transtensional deformations related to the plate boundary are distributed within the continental crust of the Discovery Bank, and the Intermediate Domain, as shown by the location of epicenters (Figs. 2 and 8). This setting is similar to that of the South Scotia Ridge, west of the South Orkney Microcontinent, where most of the active faults are located within the continental blocks, resulting in large elongated depressions inside the ridge, while the activity in the continent^ocean boundary is scarce [8]. This may be a consequence of the rheology of the lithospheric plates and suggests that plate boundaries in the Scotia Sea are preferentially located in areas of continental crust, which may be more easily deformed than the rigid oceanic crust. In this aspect, the continental crust has a rheology mainly determined by the behavior of quartz, while the oceanic crust is characterized by that of the olivine. Oceanic lithosphere is thinner than the continental, but quartz is weaker than olivine and the lithosphere is more easily deformed in continental crust than in oceanic crust under the same stress setting [24,25].
5. Conclusions The sector of the Scotia^Antarctic plate boundary located east of the South Orkney Microcontinent comprises a complex pattern of continental, intermediate and oceanic crustal elements, sepa-
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Fig. 8. Geological sketch of the main tectonic structures. 1, Oceanic crust; 2, continental crust; 3, intermediate crust; 4, volcanic arc; 5, active fault; 6, active normal fault; 7, active maximum crustal thinning axis, the stretched region covers a broad band; 8, scarp; 9, ridge; 10, elongated gravity minima; 11, elongated gravity maxima; 12, inactive fracture zone; 13, inactive subduction or reverse fault; 14, inactive normal fault; 15, 2000 m depth bathymetry contour line; 16, earthquake epicenters (1973^2002) [17]. JBs, Jane Basin; JBn, Jane Bank; DB, Discovery Bank; HB, Herdman Bank; SB, Southern Bank.
rated by deep extensional basins. Between the oceanic crusts of the Scotia and Weddell seas it is possible to di¡erentiate two main tectonic elements deformed by an active sinistral fault zone that accommodates the relative motion of the Scotia^Antarctic plate boundary. To the north, Discovery Bank is an element of thinned continental crust, whereas to the south, the Intermediate Domain is made up by crust of intermediate features between continental and oceanic and comprises the Southern Bank and the Deep Basin. The deformation along this segment of the South Scotia Ridge extends eastward across Herdman Bank, and carries westward between Bruce Bank and Jane Basin. The complex structure of the area re£ects the evolution of the Scotia Arc and the tectonics of
the northern Weddell Sea (Fig. 9). Since Oligocene times, the continental blocks that formed the connection between South America and the Antarctic Peninsula were dispersed around the Scotia Arc [3]. During an initial deformation stage, probably in Early or Middle Miocene, Discovery Bank underwent contractional deformations related to the subduction of the oceanic crust of the northern Weddell Sea at its southeastern border, and also related to its thrusting over the Scotia Sea oceanic crust at the northern border. The development of these structures is related to the eastwards migration of the Discovery Bank in respect to the Jane Bank, determining the presence of a dextral fault zone (Fig. 9). In this setting, the Intermediate Domain most likely developed. The Southern Bank and the Deep Basin of
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45º W
40º W
35º W
Present
Scotia Sea PB BB
DB
SOM
Ja n e
HB ? 60º S
SB
sin JB Ba
Weddell Sea
Scotia Sea
10 Ma
PB HB ?
BB DB SB
SOM
Ja n e B
in JB as
Weddell Sea
20 Ma
Scotia Sea PB
? HB
BB DB SB
SOM JB
Weddell Sea
1
2
3
4
Fig. 9. Paleogeographic reconstruction of main tectonic elements. Based in previous reconstructions of the Scotia Arc [12,13] and modi¢ed with own data. 1, Transform fault; 2, subduction zone; 3, oceanic spreading axis; 4, active maximum crustal thinning axis. BB, Bruce Bank; DB, Discovery Bank; HB, Herdman Bank; JB, Jane Bank; PB, Pirie Bank; SB, Southern Bank; SOM, South Orkney Microcontinent.
45
this domain probably correspond respectively to an island arc and a back-arc basin. The reverse faults in the northern border of the Deep Basin would also have been active in such a tectonic setting. All of these compressional structures are covered by sediments and are inactive at present. The sinistral transcurrent motion between the Scotia and Antarctic plates determines the recent evolution of the area. Most of the active structures with associated seismicity are concentrated in the wide section of continental and intermediate blocks between the oceanic crusts of the Scotia and Weddell seas. Transcurrent sinistral and normal faults are located in the northwestern margin of Discovery Bank, whereas extensional faults determine the recent development of the Deep Basin and the Southern Bank. The Deep Basin may be the result of the reactivation of a former back-arc basin in a tectonic setting that caused it to become a pull-apart basin. The Deep Basin has an ENE^WSW oblique orientation with respect to the sinistral transcurrent fault zone related to the plate boundary, and is located between two NW^SE overlapping transcurrent faults (Figs. 8 and 9). This basin is characterized by extreme crustal thinning, but there is no clear evidence of sea £oor spreading, and constitutes one of the deepest examples of oceanic basins developed in extensional tectonics. The tectonics of the eastern South Scotia Ridge suggest that, in the case of complex plate boundaries with continental and oceanic crustal elements, the deformation related to faults tends to be concentrated within the continental and intermediate crusts, not along the boundaries between the oceanic and continental crusts. Thus, structures that deform the continental and intermediate crust may re£ect the nature of the behavior of lithospheric plates rather than the continental^ oceanic crust boundary. This is a consequence of the weakness of continental crust, whose rheology is determined by the presence of quartz, by comparison with the oceanic crust, whose behavior is controlled by the more resistant olivine. The present location around the Scotia Arc of continental fragments may be the most signi¢cant factor of control for the location of the active faults,
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plate boundaries and tectonic activity of the region.
Acknowledgements We thank the Commander, o⁄cers and crew of the B/O Hespe¤rides for their support and cooperation in obtaining these data under sometimes severe sea conditions. The diligence and expertise of engineers E. Litcheva and J. Maldonado who processed the MCS data and swath bathymetry is appreciated and Dr. A. Carbo¤ for the program to calculate gravity anomalies. The comments of Dr. J. Woodside, Dr. G. Eagles and an anonymous reviewer have greatly improved this contribution. Spain’s CICYT supported this research through Project ANT99-0817 and REN2001-2143/ANT. [BW]
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