The Ross Sea region of the East Antarctic plate provides evidence for intraplate tectonic activity in Cenozoic times. Still unresolved are the cause, timing and kinematics of this intraplate tectonism. By integrating and discussing the different (kinematic and temporal) signals of Cenozoic tectonism, intraplate dextral shearing is recognized as the main tectonic regime controlling the structural architecture of the Ross Sea region from the Mid-Eocene (c. 40-50 Ma) onward. We speculate that propagation and persistence of this tectonic regime through time constitutes a feasible seismogenetic framework to explain past and current tectonism in the Ross Sea region.
The Mesozoic-Cenozoic tectonic evolution of the Ross Sea region (namely Victoria Land and the Ross Sea; Figs 1 and 2) is dominated by the separation of the Antarctic continent from Australia and greater New Zealand and the development of the Ross Sea embayment (e.g. Stock & Cande 2002, and references therein). These events led to the development of two roughly orthogonal passive margins in the Southern Ocean and the Ross Sea, bounding the East Antarctic craton to the north and to the east, respectively (e.g. Lawver & Gahagan 1994; Sutherland 1999; Mukasa & Dalziel 2000; Stock & Cande 2002) (Fig. 1).
The post-break-up Cenozoic separation history is particularly complex and includes the transfer of continental blocks originally belonging to the Antarctic plate (Tasmania and South Tasmania Rise) to the Australia plate, through a diffuse transform boundary active until about Oligocene time (Stock & Cande 2002, and references therein).
The occurrence of Cenozoic intraplate deformation in the Southern Ocean and the Ross Sea regions has been recently proposed to reconcile global plate tectonics based on plate closure calculations (Cande et al. 2000) and models of global plate motions in the Pacific region (Steinberger et al. 2004). In particular, Steinberger et al. (2004) advocated a pre-Mid-Eocene, major intraplate dextral motion at the northeastern edge of the East Antarctic plate, whereas Cande et al. (2000) reconstructed a Cenozoic (from 43 to 26 Ma) rigid rotation between East and West Antarctica during the opening of the Adare Trough in the northwestern Ross Sea (Fig. 2). An increasing body of evidence, based on different datasets, attests that Cenozoic dextral intraplate shearing has affected both the Ross Sea and the Southern Ocean shoulders along NW-SE dextral fault systems, striking oblique to both passive margins, which can be traced from one margin to the other across the intervening continental lithosphere of Victoria Land (Salvini et al. 1997; Rossetti et al. 2003; Fig. 2). This tectonic regime is interpreted as responsible for the post 32 Ma transition from orthogonal to oblique rifting in the western Ross Sea (Salvini et al. 1997), and for magma production and emplacement of the Cenozoic McMurdo Magmatic Province along the western Ross Sea shoulder (Salvini et al. 1997; Rocchi et al. 2002). Additional supporting evidence for recent tectonic activity in the region comes from the occurrence of a low but significant level of seismic activity at the northeastern edge of the East Antarctica plate (Reading 2002: Fig. 1). In north Victoria Land, earthquakes seem to be localized along the Cenozoic, NW-SE-striking intraplate dextral fault systems or along strands emanating from them (Fig. 2). Furthermore, fossil seismic activity is recorded along the western Ross Sea shoulder, as indicated by the occurrence of pseudotachylyte-bearing fault cores distributed along the western margin of the Ross Sea (Fig. 2). The discovery of pseudotachylyte-bearing fault rocks is of particular significance as it offers the possibility of directly sampling the products of dynamic ruptures produced during coseismic faulting at near-focal depths along exhumed fault zones (Sibson 1975). The recent dating at c. 34 Ma of the fault-zone hosted pseudotachylyte veins exposed at the tip of the NW-SE-striking, right-lateral Priestley Fault (Di Vincenzo et al. 2004; Fig. 2) opens new perspectives for the reconstruction of the tectonic and geodynamic evolution of the region, as the concomitant occurrence of pseudotachylytes and modern earthquakes along the same strike-slip fault systems may be diagnostic of long-lived fault activity in the Ross Sea region extending back into the Cenozoic.
The characteristics described above define the Ross Sea region as a key area for assessment of the history of intraplate faulting in the East Antarctic plate and, consequently, the tectonic regime(s) responsible for the past and present seismic activity. Still unresolved is the cause of Antarctic seismic intraplate activity in terms of plate tectonics, as plate closure calculations for magnetic anomalies younger than 26 Ma do not require any intra-Antarctic motions (e.g. Stock & Cande 2002) and the lithospheric stresses have been tentatively interpreted as the effect of competing tectonic forces and ice loading-unloading cycles (Reading 2002).
In this paper, after revisiting the available apatite fission-track (AFT) thermochronological ages from Victoria Land, we present results from a pilot study where kinematic and temporal data from the tip of the NW-SE-striking Priestley Fault are integrated with results obtained from the interpretation of a reprocessed offshore multichannel seismic reflection profile acquired across the southern termination of the Priestley Fault itself (IT90AR-61B; see Fig. 2 for location). Our purpose is twofold: (1) to provide an updated tectonic framework of the Ross Sea region, suitable for incorporation into refined plate tectonic reconstructions of the Australia-Antarctica-Pacific plate motions; (2) to provide support to the hypothesis that dextral intraplate faulting has been responsible for the distribution and origin of seismic activity in this region of the Antarctic plate, in a time span running from at least the Mid-Eocene to the present. Our results indicate that well after the onset of rifting between Antarctica and Australia, the near-perpendicular passive margins in the Southern Ocean and Ross Sea were tectonically connected and interacted during the late Cenozoic.
The Cenozoic tectonic architecture of the Ross Sea region
The Neoproterozoic to Early Palaeozoic basement rocks exposed in the Ross Sea region were exhumed since the Cretaceous, with a major phase of denudation starting in Cenozoic times, at c. 40-50 Ma (e.g. Fitzgerald 1992, 2002; Lisker 2002). This Cenozoic age coincides with a major tectonic change in the Ross Sea region, as attested by the opening of the Adare Trough (Cande et al. 2000), the onset of the Cenozoic McMurdo igneous activity (Rocchi et al. 2002), and the activation of dextral intraplate faulting (Salvini et al. 1997; Rossetti et al. 2003). The Cenozoic tectonic architecture in the Ross Sea region consists of NW-SE-striking right-lateral strike-slip to transpressional fault systems in Victoria Land, which are abutted by north-south-striking right-lateral transtensional basin boundary faults in the Ross Sea and onshore along the entire western Ross Sea margin (Wilson 1995; Salvini et al. 1997; Hamilton et al. 2001; Storti et al. 2001) (Fig. 2). The western boundary of the continental sector at the northeastern edge of Antarctica affected by important NW-SE-striking intraplate right-lateral strike-slip faulting can be traced approximately from the USARP Mountains area in the north to the David Glacier in the south (Fig. 2).
The Eocene-Oligocene fault history
The Cenozoic age of the dextral fault network in the Ross Sea region has been mainly established from interpretation of offshore reflection seismic profiles (e.g. Salvini et al. 1997; Hamilton et al. 2001). The recent availability of AFT thermochronological data along both the Ross Sea and the Southern Ocean margins (see Fitzgerald (2002) and Lisker (2002) for a review) and the discovery of pseudotachylyte-bearing fault cores along the western Ross Sea provides the opportunity to place new onshore kinematic and time constraints on the activity of this fault network.
For normal geothermal gradients, the low-temperature conditions for fission-track retention in apatite (partial annealing zone between 60° and 120 °C, with a mean effective closure temperature constrained at 110 ± 10 °C; Green & Duddy 1989), makes AFT thermochronology an efficient tool to establish the age of the late-stage deformation history affecting the upper 3-4 km of the crust, where brittle deformation predominates.
The pattern arising from over 300 sets of AFT data from north Victoria Land is characterized by a distinctive trend of increasing ages from the Ross Sea coast towards the East Antarctic Craton (Fig. 3a). In particular, the age pattern and the regional distribution of the track lengths indicate a markedly different cooling history for the tectonic blocks of northern Victoria Land, with a rather sudden transition west of the Rennick Graben region (Fig. 3a). Virtually all AFT sample ages from westernmost northern Victoria Land and Gates Land (Fig. 3a), as well as from the adjacent George V Land (Lisker & Olesch 2003) and Terre Adélie (Arné et al. 1993), are generally older than Cretaceous (between c. 300 and c. 100 Ma), showing an age pattern similar to that of the pre-Gondwana break-up conjugate margin of southeastern Australia (Lisker 2002, and references therein). In contrast, the vast majority of samples from the western shoulder of the Rennick Graben and the Admiralty Block have AFT ages younger than 100 Ma. Only a few older ages were obtained from the morphologically highest samples of the Transantarctic Mountains, representing a break in slope of vertical sample profiles (Fig. 3b).
Offsets in age-elevation slopes, indicating the onset of cooling-denudation stages, were recognized at various localities at c. 80 and at c. 50-40 Ma (e.g. Balestrieri et al. 1994; Lisker 2002). A comparison of the vertical AFT profiles indicates that the stage commencing at c. 40-50 Ma was the episode with the highest amount of denudation, as well as the only episode consistently recorded in northern Victoria Land (Fig. 3). The amount of denudation during the Cenozoic varies considerably throughout the Admiralty Block, and does not strictly correlate with the topographic elevation (or the amount of Cenozoic uplift), or with the distance to the Ross Sea margin (Fig. 3). Accordingly, the Cenozoic AFT age pattern does not fit into a classical passive continental margin configuration, where distinctive regional patterns of AFT data with younger ages at the coast monotonically increasing towards the continental interior are expected (e.g. Gallagher & Brown 1997). In particular, such a tendency is not exhibited across the Southern Ocean passive margin of northern Victoria Land, where the denudational response induced by activation and propagation of Cenozoic, partitioned strike-slip faulting at c. 40-50 Ma is documented by opening of the Rennick Graben (Rossetti et al. 2003).
Pseudotachylyte vein occurrence and age
Pseudotachylytes have been found along the western coast of the Ross Sea, along both NW-SE- and north-south-striking fault systems (Fig. 2). Typically, pseudotachylytes occur within zones of cataclastic deformation and are marked by dark, fine-grained, and locally foliated steeply dipping cataclasites, up to several metres thick. The fault rocks exposed at the tip of the Priestley Fault (Fig. 2) were selected as a key site to decipher the kinematics and timing of fossil coseismic faulting in north Victoria Land.
The studied pseudotachylyte-bearing fault zone occurs within Early Palaeozoic migmatite rocks, exposed close to the German Gondwana Station on the Ross Sea coast (Fig. 2). The fault trace consists of a broad (at least 50 m thick), NW-SE-striking cataclastic damage zone, consisting of high-angle anastomosing fault strands, preferentially dipping to the SW (Stereoplot I in Fig. 4). The direction and sense of shear on these fault surfaces was deduced from the orientation of corrugation, striations and grooves, and the arrangement of secondary Riedel shears (e.g. Petit 1987). Strike- and dip-slip slickenlines typically coexist on the fault surfaces (Stereoplots I and II in Fig. 4), with mutual overprinting relations. Kinematic indicators within the fault zone, mostly provided by the Riedel shears (lunate structures and extensional cracks), consistently record dominantly dextral strike slip to oblique reverse slip.
The fault core is several metres thick and contains cohesive foliated cataclasites, where the primary metamorphic structures of the host rock are completely obliterated (Fig. 4a). The fault core is also characterized by a higher degree of rock alteration and veining. Fine-grained ultracataclasites and pseudotachylyte veins occur in the central portion of the core and are typically bounded by foliated cataclasites. The orientation of the cataclastic foliation, subsidiary faults and extension cracks is consistent with distributed right-lateral shear (Fig. 4b). Pseudotachylytes consist of black to dark green aphanitic zones a few millimetres to 5-6 cm wide, and occur both as generation surfaces along the strike of the cataclastic zones and as injection veins roughly orthogonal to them (Fig. 4c and d, and Stereoplot III in Fig. 4). Details of petrological analysis, the dating procedure and interpretation of ^sup 40^Ar/^sup 39^Ar laserprobe analyses have been provided by Di Vincenzo et al. (2004). In summary, in situ laserprobe analyses on selected samples yielded concordant ages at c. 34 Ma for an injection vein, interpreted to date a major episode of coseismic faulting during dextral shearing at the Eocene-Oligocene boundary.
The post-Miocene to present fault history
The present-day intraplate stress field in Antarctica is largely unknown because of the scarcity of earthquakes with magnitudes yielding reliable focal mechanism solutions and localization problems (Reading 2002). Information on the intraplate stress field are consequently mostly derived from borehole data on Neogene-Quaternary cores samples drilled in the Cape Roberts area, where the fracture pattern documents a stress regime that is compatible with regional late Cenozoic dextral transtension along the Transantarctic Mountains front (e.g. Wilson & Paulsen 2000).
There is limited evidence for neotectonic activity in the region, mostly derived from faulting that affects the McMurdo volcanic products (e.g. Lanzafame & Villari 1991; Storti et al. 2001) and geomorphological features (e.g. Jones 1997). Furthermore, analysis of young volcanic cone alignments in south Victoria Land documents that magmatism emplacement occurred under a nonisotropic regional stress field (Paulsen & Wilson 2003). Nevertheless, seismic activity in the Ross Sea region has been recorded along the trace of the Cape Adare (Behrendt et al. 1996), Lanterman (Cattaneo et al. 2001) and David (Bannister & Kennett 2002) dextral fault systems of Salvini et al. (1997), all of which are argued to have had earlier Cenozoic activity (Fig. 2).
The IT90AR-61B multichannel seismic reflection profile
Interpretation of the multichannel seismic reflection profile IT90AR-61B (see Fig. 2 for location) provides important constraints on the recent tectonic activity in the adjacent offshore region of the Priestley Fault (Fig. 5). The profile intersects the prominent Cenozoic north-south-striking Terror Rift basin, a deep tectonic depression developed at the southern tip of the Priestley Fault to accommodate its residual dextral shear (Storti et al. 2001). The most significant feature is the occurrence of numerous fault strands post-dating the major Miocene glaciomarine erosional surface and also affecting the Pliocene-Quaternary sediments. Some of the faults are responsible for the sea-floor morphology, causing formation of small (about 15-20 m high) escarpments, mounds c. 1-2 km long, and sags of several hundred metre width. The majority of these fault strands are steeply dipping, and the vertical separation across the fault-bounded contacts attests to dominant extensional fault throw. Each fault strand shows up to 100 m of vertical offset in the buried sedimentary sequence. The steep and depth-convergent fault arrays as well as the presence of reverse throws along the same fault surface (see top right side of Fig. 5) also suggest the occurrence of deformation partitioning with a significant horizontal component of slip in addition to the dominant extensional one. This is a common feature all along the western part of the Ross Sea shoulder, which is dominated by north-south transtensional shearing (Salvini et al. 1997; Hamilton et al. 2001).
Tectonic synthesis: towards a model for the neotectonic evolution of the Ross Sea region
The integration of the information derived from structural data, AFT mermochronology (c. 40-50 Ma) and the ^sup 40^Ar/^sup 39^Ar dating of pseudotachylytes along the Priestley Fault (c. 34 Ma) supports the activity of dextral wrench faulting across Victoria Land at least since the Eocene-Oligocene boundary. This age overlaps with the timing of the rifting processes affecting the Ross Sea and resulting in the opening of the Adare Trough in the interval 43-26 Ma (Cande et al. 2000; Fig. 2). Nevertheless, the AFT age pattern in north Victoria Land (Fig. 3) does not conform to a regional denudational response of the Transantarctic Mountains acting as a rift-shoulder, as the denudation phase commencing at c. 40-50 Ma was systematically recorded far west in the continent interior and intimately linked with the activity of the NW-SE-striking dextral fault systems. Accordingly, the episode indicated at c. 40-50 Ma by the AFT ages can be tentatively interpreted as the denudational response to localized extension and uplift triggered by the activation and propagation of Cenozoic, partitioned strike-slip faulting within the continental crust of north Victoria Land from c. 40-50 Ma onward (Rossetti et al. 2003). The slip history is attested by fossil earthquake ruptures occurring in concomitance with the onset of the McMurdo magmatism along the western margin of the Ross Sea at about 48 Ma (Tonarini et al. 1997; Rocchi et al. 2002). This suggests that strike-slip deformations played an important role in controlling the development of the Cenozoic volcanic margin in Victoria Land. Continuing magmatic activity may in turn have enhanced the seismic activity in a feedback mechanism, by analogy to that proposed for the East Greenland volcanic rifted margin (Karson et al. 1998).
The presence of modern earthquakes along the NW-SE-striking fault networks and the offshore seismic evidence for neotectonic activity at the southern tip of the Priestley Fault support the Quaternary activity of the same strike-slip-related fault network. This suggests a long-lasting faulting regime, active at least from the Early Oligocene until the present in the Ross Sea region. If we assume that earthquake nucleation started in the early phases of dextral wrench faulting as documented along the Southern Ocean coast (i.e. at c. 40-50 Ma) and continued up to modern times, this implies that fossil and modern seismicity at the northeastern edge of East Antarctica may relate to the activation, propagation and linkage of major intraplate dextral fault systems from the Mid-Eocene onward. The kinematic scenario for the neotectonic evolution of the Ross Sea region is thus consistent with the progressive SE-directed extrusion of the continental lithosphere of north Victoria Land operated by dextral wrench faulting. Further support for this hypothesis derives from preliminary global positioning system (GPS) data showing the SE motion of the Terra Nova Bay permanent station (Negusini el al. 2005).
The proposal of active tectonism in the Ross Sea region made in this paper may contrast with plate closure calculations for magnetic anomalies in the Southern Ocean rifting zone, which do not necessitate significant intra-Antarctica motion younger than 26 Ma (Cande et al. 2000; Stock & Cande 2002). On the other hand, the strike-slip motion described in this paper may well fall within the confidence interval of magnetic anomaly data. Furthermore, in the proposed model of southeastward extrusion of north Victoria Land, accommodation and compensation of the continental dextral displacement occurs in the previously rifted offshore region within the north-south-trending transtensional belt of the western Ross Sea margin (see also Storti et al. 2001; Rossetti et al. 2003; Fig. 2). This is confirmed by the rift history reconstructed for the southwestern Ross Sea area, where a late Early Oligocene rift reorganization occurred, and a Miocene to recent continuum of deformation has been reconstructed (Wilson et al. 2003). We thus propose (at least from the Early Oligocene to the present) tectonic connection between the rifting processes occurring in the Southern Ocean and the Ross Sea neotectonics induced by the propagation of the dextral faulting through the continental crust of Victoria Land.
The tectonic scenario discussed above may also provide further constraints for the geodynamic reconstructions of the plate tectonic regime in the Pacific-Antarctic region during the Cenozoic. Our reconstruction, in particular, suggests that there are consistent kinematic and temporal relationships documenting changes in the tectonic regime of the northeastern edge of the East Antarctic plate, including the onset of diffuse intraplate dextral wrench faulting, that were roughly coeval with the Mid-Eocene major reorganization of spreading in the Pacific basin (e.g. Millier et al. 2000; Veevers 2000). We present here a reappraisal of the relevance of intraplate dextral shearing in controlling the post 40 Ma tectonic architecture of the Ross Sea region to indirectly contribute to a new generation of plate motion reconstructions among the Antarctic, Australian and Pacific plates.
The following key issues can be extracted from this study and should be carefully considered in any future tectonic model: (1) dextral wrench faulting dominates intraplate deformation at the northeastern edge of East Antarctica; (2) intraplate dextral wrench faulting commenced in the Mid-Eocene; (3) propagation of distributed and partitioned dextral shear in the continental crust of northern Victoria Land is indicated by a long-lasting seismogenetic fault network that is possibly responsible for the neotectonic activity in the Ross Sea region.
The complex picture of neotectonic activity in the area cannot be explained within a simple and rather uniform passive margin framework and/or by ice cap dynamics alone. Instead, it fits the framework of a diffuse intraplate dextral wrench-faulting scenario, where the western Ross Sea margin has to be regarded as a continental sheared margin.
This work was supported by the Italian Antarctic Research Program (PNRA) and by the German Research Foundation (DFG). A. Vaughan is thanked for constructive advice on an early version of the manuscript, and J. Gamble and R. Sutherland are thanked for their thoughtful and constructive reviews. R. Strachan is thanked for his editorial handling.
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Received 18 January 2005; revised typescript accepted 16 June 2005.
Scientific editing by Rob Strachan
FEDERICO ROSSETTI1, FABRIZIO STORTI1, MARTINA BUSETTI2, FRANK LISKER3, GIANFRANCO DI VINCENZO4, ANDREAS L. LÄUFER5, SERGIO ROCCHI6 & FRANCESCO SALVINI1
1 Dipartimento di Scienze Geologiche, Università Roma Tre, 00146 Roma, Italy (e-mail: firstname.lastname@example.org)
2 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, 34010 Trieste, Italy
3 FB Geowissenschaften, Universität Bremen, 28334 Bremen, Germany
4 Istituto di Geoscienze e Georisorse-CNR, 56124 Pisa, Italy
5 Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, 30655 Hannover, Germany
6 Dipartimento di Scienze della Terra, Università di Pisa, 56126 Pisa, Italy…