Giuliano Brancolini

Cenozoic geodynamics of the Ross Sea region, Antarctica : Crustal extension, intraplate strike-slip faulting, and tectonic inheritance

An integrated study of onshore and offshore geology of the Ross Sea region (namely, Victoria Land, north of Ross Island, and the Ross Sea, Antarctica) has revealed a complex, post-Eocene tectonic framework. Regional NW-SE right-lateral, strike-slip faults are the outstanding feature of this framework and overprint an older Mesozoic extensional event, responsible for formation of N-S basins in the Ross Sea. The Cenozoic framework includes kinematic deformation and reactivation along the NW-SE faults, including formation of pull-apart basins, both positive and negative flower structures, and push-up ridges. N-S extensional faults are well developed between NW-SE faults and indicate E-W extension during the Cenozoic, produced by the NW-SE right-lateral strike-slip motion together with regional crustal extension. NNW-SSE compression, induced by the right-lateral, strike-slip kinematics, is indicated by locally inverted NE-SW faults and basins. The evolution, geometry, and location of the Rennick Graben and the Lanterman Range fit well into this model. Variations in the deformational style across the region can be linked to corresponding variations in the bulk crustal rheology, from brittle behavior in the west, to ductile deformation (at subseismic-scale resolution) near the Eastern Basin. A semibrittle region that favors N-S clustering of Cenozoic magmatic activity lies in between. In this region, Cenozoic volcanoes develop at the intersections of the NW-SE and the major N-S faults. The NW-SE faults cut almost continually from the Ross Sea to East Antarctica through lithospheric sectors with different rheology and thickness. At least two of the NW-SE faults correspond to older Paleozoic terrane boundaries in northern Victoria Land. The NW-SE faults link in the Southern Ocean with major transform faults related to the plate motions of Australia, New Zealand, and Antarctica.

Transect across the West Antarctic rift system in the Ross Sea, Antarctica

Abstract In 1994, the ACRUP (Antarctic Crustal Profile) project recorded a 670-km-long geophysical transect across the southern Ross Sea to study the velocity and density structure of the crust and uppermost mantle of the West Antarctic rift system. Ray-trace modeling of P- and S-waves recorded on 47 ocean bottom seismograph (OBS) records, with strong seismic arrivals from airgun shots to distances of up to 120 km, show that crustal velocities and geometries vary significantly along the transect. The three major sedimentary basins (early-rift grabens), the Victoria Land Basin, the Central Trough and the Eastern Basin are underlain by highly extended crust and shallow mantle (minimum depth of about 16 km). Beneath the adjacent basement highs, Coulman High and Central High, Moho deepens, and lies at a depth of 21 and 24 km, respectively. Crustal layers have P-wave velocities that range from 5.8 to 7.0 km/s and S-wave velocities from 3.6 to 4.2 km/s. A distinct reflection (PiP) is observed on numerous OBS from an intra-crustal boundary between the upper and lower crust at a depth of about 10 to 12 km. Local zones of high velocities and inferred high densities are observed and modeled in the crust under the axes of the three major sedimentary basins. These zones, which are also marked by positive gravity anomalies, may be places where mafic dikes and sills pervade the crust. We postulate that there has been differential crustal extension across the West Antarctic rift system, with greatest extension beneath the early-rift grabens. The large amount of crustal stretching below the major rift basins may reflect the existence of deep crustal suture zones which initiated in an early stage of the rifting, defined areas of crustal weakness and thereby enhanced stress focussing followed by intense crustal thinning in these areas. The ACRUP data are consistent with the prior concept that most extension and basin down-faulting occurred in the Ross Sea during late Mesozoic time, with relatively small extension, concentrated in the western half of the Ross Sea, during Cenozoic time.

The Eastern Ross Sea continental shelf during the Cenozoic: implications for the West Antarctic ice sheet development

Abstract The present-day bathymetric profile in the Ross Sea, as in other regions around the Antarctic margin, is deepening landward and shows unusually high water-depths: up to 1000 m in the inner shelf. These two features are the product of multiple ice sheet advances and retreats on the continental shelf. In this paper, we present a reconstruction of paleo-bathymetric profiles of the Eastern Ross Sea throughout the Cenozoic. The evolution of the sea-floor morphology from shallow and seaward dipping to the present-day configuration gives new insights into the understanding of the West Antarctic Ice Sheet (WAIS) history in this sector. Paleo-bathymetric profiles have been calculated by applying a reverse post-rift modelling, starting from a cross-section derived from multichannel seismic data. The post-rift reverse modelling includes: sediment decompaction, isostatic compensation after removing and recovering sediments of the post-rift thermal subsidence. The major uncertainty in our model is due to the paucity of stratigraphic constraints for the late Miocene and Pliocene sequences that prevents precise values of paleowater-depth being estimated. Nevertheless, major changes in the shape of the continental shelf and slope throughout the Cenozoic can be recognised, and mark some critical steps in the Ross Sea evolution. (1) Pre-Miocene: the Eastern Ross Sea was a deep structural basin bordered to the west by areas (e.g., the Central High) outcropping the sea level and hosting valley glaciers or small ice caps. A continental shelf edge was not clearly developed yet, the eastern flank of the Central High appeared as an inclined ramp, dipping towards the ocean. (2) Early to middle Miocene: tectonic subsidence gradually produced a marine flooding over most of the pre-Miocene sub-aerial areas. A continental shelf, slope and rise are gradually delineated. The shelf profile was seaward dipping and not yet overdeepened. The geometry of the depositional sequences is mainly determined by eustacy, tectonic and sediment supply. (3) Starting from Late Miocene (likely from 10 Ma to at least 4 Ma) the bathymetric profile evolved progressively from seaward to landward dipping and reached an overdeepened configuration, very similar to the present-day profile. Depositional and erosional processes over the continental shelf were largely controlled by ice streams. Outcropping of large parts of the continental shelf during the early Cenozoic has important implications on the volume of the West Antarctic Ice Sheet. At that time, the WAIS contribution to the eustatic fluctuations was most likely much larger than today.

Continental shelf drift deposit indicates non-steady state Antarctic bottom water production in the Holocene

A late Quaternary, current-lain sediment drift deposit over 30 m in thickness has been discovered on the continental shelf of East Antarctica in an 850 m deep glacial trough off George Vth Land. Radiocarbon dating indicates that a period of rapid deposition on the drift (averaging 290 cm/kyr) occurred in the mid-Holocene, between about 3000 and 5000 yr before present. Slower rates of around 10 cm/kyr, during the past 0-3000 yr and from 5000 to about 13000 yr BP, coincides with deposition of bioturbated, ice-rafted debris (IRD) rich, sandy mud under an energetic bottom current regime. In contrast, the mid-Holocene (3000-5000 yr BP) sediments are fine-grained, laminated to cross-laminated with minimal IRD content, and are contemporaneous with a period of warmer marine conditions with less sea ice production. This pattern suggests that bottom currents were weaker than present day in the mid-Holocene, and that the rate of dense bottom water production was reduced at that time. This scenario is consistent with the hypothesis of non-steady state rates of Antarctic bottom water production through the Holocene as recently proposed by Broecker and his colleagues.

Morphostratigraphic framework of the Venice Lagoon (Italy) by very shallow water VHRS surveys: Evidence of radical changes triggered by human‐induced river diversions

[1] This study is mainly based on a wide Very High Resolution Seismic (VHRS) survey that utilized an ad hoc technique designed for investigations in very shallow waters (about 1 m depth). This method allowed the acquisition of excellent images of the subsurface down to 15-20 m b.s.1. with a resolution of about 10 cm. Buried geomorphological features, such as fluvial channel-levee systems and tidal channels, were imaged for the first time in the shallows and provided new insight into the Holocene evolution of the southern lagoon basin. Furthermore, the new seismic data were used to reconstruct the morphostratigraphic framework of the Venice Lagoon. We provide an Upper Quaternary morphostratigraphic model of the Venice Lagoon and present some evidence of radical changes resulting from human-induced river diversion in the sedimentary regime and in the morphological setting of the southern basin that has occurred over the last millennium.

Cenozoic climate history from seismic reflection and drilling studies on the Antarctic continental margin

Seismic stratigraphic studies and scientific drilling of the Antarctic continental margin have yielded clues to the evolution of Cenozoic climates, depositional paleoenvironments and paleoceanographic conditions. This paper draws on studies of the former Antarctic Offshore Stratigraphy Project and others to review the geomorphic and lithostratigraphic offshore features that give insights into the long-duration (m.y.) and short-term (k.y.) changes that document the great variability of Cenozoic Antarctic paleoenvironments. The lithologic drilling record documents non-glacial (pre-early Eocene) to full-glacial (late Pliocene to Holocene) times, and documents times of cyclic ice-sheet fluctuations at k.y. scales (early Miocene to Pliocene and Holocene). Times of significant change in types and/or amounts of glaciation are also seen in the offshore lithologic record (early Oligocene, mid-Miocene, early Pliocene). Seismic data illustrate large-scale geomorphic features that point to massive sediment erosion and dispersal by ice sheets and paleoceanographic processes (e.g. cross-shelf troughs, slope-fans, rise-drifts). The commonality of these features to East and West Antarctica since late Eocene time points to a continent that has been intermittently covered, partially to completely, by glaciers and ice sheets. The greatest advances in our understanding of paleoenvironments and the processes that control them have been achieved from scientific drilling, and future progress depends on a continuation of such drilling.

Sedimentary processes in the Wilkes Land margin: a record of the Cenozoic East Antarctic Ice Sheet evolution

Multichannel seismic data collected off Wilkes Land (East Antarctica) reveal four main units that represent distinct phases in the evolution of the Cenozoic depositional environment. A Cretaceous synrift succession is overlain by hemipelagic and distal terrigenous sequences deposited during Phase 1. Sediment ridges and debris-flow deposits mark the transition to Phase 2. Unit 3 records the maximum sediment input from the continent and is characterized by the predominance of turbidite deposits. During Phase 4 the sediment supply from the continental margin was reduced, and draping and filling were the dominant processes on the continental rise. Unit 4 also contains the deposits of sediment wave fields and asymmetric channel-levee systems. These four units are a response to the Cenozoic evolution of the East Antarctic Ice Sheet. During Phase 1, small ice caps were formed in the innermost continental areas. The ice volume increased under temperate glacial regimes during Phases 2 and 3, when large volumes of melt-water production led to high sediment discharge to the continental rise. Change to a polar regime occurred through Phase 4, when a thick prograding wedge developed on the continental shelf and slope and the sediment transport to the rise diminished, producing general starvation conditions.

Three-dimensional analysis of the Plio-Pleistocene seismic sequences in the Venice Lagoon (Italy)

Integrated seismic and well data provide for the first time a picture of the geological evolution of the Venice area over the last 5 Ma and a 3D subsoil model, which is fundamental to prediction of the anthropogenic uplift of Venice by seawater injection. A Pliocene southward prograding complex formed a shelf–slope system, whereas subsidence related to the Apennine foredeep development led to the establishment of a Early Pleistocene deep-water environment, favouring the accumulation of a thick turbidite succession. The NE progradation of the palaeo-Po river delta during the Middle Pleistocene promoted a drastic environmental revolution, followed by the deposition of cyclothems linked to glacio-eustatic changes. Supplementary material: Details of the seismic data and their relationship to the boreholes are available at http://www.geolsoc.org.uk/SUP18537.

Glaciomarine Deposits on the Continental Shelf of Ross Sea, Antarctica

The Ross Sea is part of a large embayment of the west Antarctic coast, and is characterized by continental rift basins filled by thick sequences of Cenozoic glaciomarine sediments [Hayes and Frakes et al., 1975; Cooper et al., 1991; Anderson and Bartek, 1992; Hambrey and Barrett 1993; Brancolini et al., 1995a; De Santis et al., 1995].

Erratum Journal of the Geological Society, London, Vol. 164, 2007, pp. 243–256. Sedimentary processes in the Wilkes Land margin: a record of Cenozoic East Antarctic Ice Sheet evolution

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