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SCAR Report No 16

Appendix 7

THE ROSS SEA REGION: GEOLOGY OF THE LAST 100 M.Y.
F. J. Davey
Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand

Introduction

The Ross Sea lies along the Pacific margin of the Jurassic rift [Schmidt and Rowley, 1986] that is delineated by the extensive dolerite sills of the Ferrar Group in the Transantarctic Mountains, and coincides in part with the active, Cenozoic, West Antarctic Rift System[LeMasurier, 1978; Behrendt et al., 1991], considered to be caused by a mantle plume [Hole and LeMasurier, 1994; Behrendt et al., 1992]. The Transantarctic Mountains, one of the world's great mountain chains, forms, for a large part of its length, the rift shoulder of the West Antarctic Rift System. It is over 4000 km long, reaches elevations of over 4000 m, and is block faulted and backtilted to the west (craton). The complementary rift shoulder in Edward VII Peninsula and western Marie Byrd Land is far more subdued and has more of a horst and graben (basin and range) style of morphology [LeMasurier and Rex, 1983] that is mostly ice covered and extends about 1000 m above and below sea level [Drewry, 1983]. The different flexural rigidity for the lithosphere of the two rift margins, an old craton and a younger orogenic belt [e.g., Stern and ten Brink, 1989], may give rise to this difference in character [ e.g., Behrendt et al., 1991].

The Ross Sea and its southern continuation under the Ross Ice Shelf form the Ross Embayment, which is about 500 m deep and generally has a gentle ridge and valley morphology with a maximum depths (1200 m) are along the western margin, adjacent to the Transantarctic Mountains. The depth of the Ross Sea is significantly deeper than is normal for a continental shelf (50-200 m). This is considered to arise largely by crustal thinning [Fitzgerald et al., 1986; Stern et al., 1992] but glacial erosion, which over deepened the inner shelf, and sediment loading on the continental rise during the time of a more extensive ice sheet, may also contribute, as proposed by ten Brink and Cooper [1992] for the Antarctic continental shelf in general.

The sedimentary basins in the Ross Sea were probably formed largely by rifting processes during and since the break-up of this part of Gondwana in the Late Cretaceous [e.g., Davey, 1981]. Cooper and Davey [1985] and Cooper et al. [1987, 1991] noted two phases to the basin formation: i) an initial regional extension phase related to the Gondwana break-up episode and ii) a possibly mid-Cenozoic phase, which was localized in the western Ross Sea. Elliot [1992] suggested that the earliest extensional event in the formation of the basins and the morphologically depressed Ross Sea - Ross Ice Shelf region was the poorly defined rifting episode associated with the intrusion of the Ferrar dolerites at about 180 Ma. The subsequent tectonic development of the region resulted from the development of the West Antarctic Rift System in the late Mesozoic [Behrendt et al., 1991], associated with the initial sea-floor spreading between New Zealand, Australia and Antarctica [e.g., Weissel et al., 1977] and with the presumed concurrent development of a mantle plume [Weaver et al., 1994]. Hole and LeMasurier [1994], however, associate the beginning of plume activity with the onset of active volcanism in the region and uplift in western Marie Byrd Land at about 30 Ma. Uplift of the adjacent Transantarctic Mountains appears to have occurred in several phases commencing about 115 Ma [Fitzgerald, 1994].

Tectonic Events In The Ross Sea Region

The eastern margin of Ross Sea (western Marie Byrd Land and Edward VII Peninsula) forms the present continental margin of western Antarctica. In general terms, the limited geological outcrop in Marie Byrd Land shows a Paleozoic crystalline basement overlain by young, Cenozoic basaltic volcanics with a major erosion surface of Late Cretaceous to early Cenozoic age (80 Ma to 28 Ma) separating the two [LeMasurier and Rex, 1994]. Mid-Cretaceous to Late Cretaceous mafic dikes occur parallel to the coast in Marie Byrd Land, and granitoids of same age have been intruded [LeMasurier and Rex, 1994]. Extensive basaltic hyaloclastite deposits, less then 28 Ma, overlie the erosion surface and are overlain by felsic shield volcanoes of less than 16 Ma [LeMasurier, 1990]. The uniformity of alkaline basalt composition throughout the West Antarctic rift is interpreted to indicate a mantle plume and uniformly minimal crustal extension during the late Cenozoic by Hole and LeMasurier [1994].

In the Ford Ranges of western Marie Byrd Land, Ar-Ar, U-Pb and apatite fission track data show several episodes of heating and denudation [Luyendyk et al., 1992; Luyendyk, 1993; Richard et al., 1994]. The subduction of the Phoenix-Antarctic spreading center beneath Marie Byrd Land resulted in the elevation of the geothermal gradient by magmatic advection, and high-temperature low-pressure metamorphism of the middle crustal rocks by about 104 Ma. Between about 104 and 100 Ma, granites were intruded into the metamorphic rocks (the present Fosdick metamorphic complex), and was followed by rapid exhumation (about 1.5 mm/yr) and cooling over 6 m.y. (100-94 Ma), [Richard et al., 1994]. This uplift, possibly involving removal of up to 15 km of overburden, was accompanied by N to NNE extension, consistent with dextral transcurrent rifting at the coast [Luyendyk et al., 1992]. The geology of the Alexandra Mountains in the Edward VII Peninsula is similar, Weaver et al. [1992] give an age of 95-100Ma for extensional processes (anorogenic granites), with the commencement of regional uplift at about 100 Ma, immediately after granite emplacement. I type granites dated at 108-124 Ma and A-type granites dated at 95-120 Ma, indicate a rapid change from subduction related magmatism (I type) to rift related magmatism (A-type).

Between about 94 Ma and 80 Ma, the cooling and denudation rate in the Ford Ranges slowed significantly. A period of rapid cooling of crustal rocks from 80 to 70 Ma, nearly coeval with the initiation of sea-floor spreading between New Zealand and West Antarctica, can be related to further exhumation, possibly associated with faulting and N-S extension [Richard et al., 1994]. Since 70 Ma, slow cooling and exhumation (about 3 km), with possibly minor faulting, has occurred [Richard et al., 1994]. This prolonged stability led to the the development of a widespread Late Cretaceous to early Tertiary erosion surface of very low relief over most of West Antarctica [LeMasurier and Rex, 1994]. Post 50 Ma changes in the trend of the glacial striations have been interpreted by Luyendyk [1993] to indicate a late Cenozoic trend of extension of about N-S to NE-SW (Figure 4). At 28-30 Ma, volcanic activity started in the Marie Byrd Land volcanic province [LeMasurier, 1990], with peaks of activity at 8-12 Ma and 0-1 Ma, contemporaneous with block faulting and uplift. Significant post-Eocene vertical tectonics have occurred, similar to basin and range tectonics [Luyendyk et al., 1992] with up to 1.5 km vertical movement in western Marie Byrd Land. Uplift rates, derived from displacements of the erosion surface associated with volcanic activity, average about 100 m/m.y. for the past 25 Ma [LeMasurier and Rex, 1989].

The Transantarctic Mountains form the western margin of the Ross Sea. Indicators of horizontal deformation since Jurassic time are few. Wilson [1992], from a study of fracture and dike orientations, has derived a NE trend for Jurassic extension and a SE trend for Cenozoic extension in South Victoria Land. Fission track data for the Transantarctic Mountains show an onset of rapid denudation beginning at about 50-55 Ma with an indication of rapid denudation in the early Cretaceous (115 Ma) at Scott Glacier and in the Beardmore Glacier region [Fitzgerald, 1994]. Rapid denudation in the Late Cretaceous (about 85 Ma) is suggested at Scott Glacier and possibly at Admiralty Mountains in North Victoria Land and in South Victoria Land, and in the latest Cenozoic in lower Tucker Glacier region of North Victoria Land [Fitzgerald and Gleadow, 1988]. Uplift rates reach at least 200 m/m.y. for about 10 to 15 m.y. after uplift commenced at 55 Ma in South Victoria Land [Fitzgerald, 1992]. The present elevation of sub-aerial volcanics indicate a maximum uplift of 209 m since 2.57 Ma in the Dry Valleys of South Victoria Land [Wilch et al., 1993]. Uplift rates derived from drill hole information in the Dry Valleys and McMurdo Sound region, based on microfossils, give 150 m/m.y. [Wrenn and Webb, 1982], and Ishman and Webb [1988] derived a rate of 125 m/m.y. since 3 Ma. McKelvey et al. [1991] and Webb et al. [1994] have demonstrated that, at the Beardmore Glacier, the Sirius Group is essentially terrestrial strandline deposits which have been uplifted by about 1300 m to 1700 m since Pliocene time. A detailed study of the Cenozoic glacial geology of North Victoria Land [van der Wateren and Verbers, 1992] has suggested uplift of the Transantarctic Mountains with rates of about 100 m/m.y. in the early Pliocene rising to about 1000 m/m.y. in the Pleistocene to present. Behrendt and Cooper [1991] also suggest that high uplift at a rate of about 1000 m/m.y. since mid-Pliocene is possible.

The segmentation of the Transantarctic Mountains into several crustal blocks is indicated by the differing amounts of uplift between major crustal blocks [Fitzgerald, 1994] and within the same crustal block (e.g., within South Victoria Land and the Scott Glacier region) [Fitzgerald, 1992; Stump and Fitzgerald, 1992]. Uplifts of 6 km are inferred for South Victoria Land and 10 km for the lower Tucker Glacier of North Victoria Land during the mid-Cenozoic. Differential uplift of North Victoria Land is suggested by Cenozoic glacial geology [van der Wateren and Verbers, 1992], and inferred major cross range faults along the major outlet glaciers through the Transantarctic Mountains also supports segmentation of the Transantarctic Mountains [Tessensohn, 1994; Tessensohn and Woerner, 1991; Redfield and Behrendt, 1992; van der Wateren et al., 1994].

The tectonic history of the Transantarctic Mountains is also reflected in its Cenozoic igneous history [LeMasurier and Thomson, 1990]. Alkali basaltic volcanism of the McMurdo Volcanic Group commenced between 25 and 18 Ma in three elongate (N-S) provinces along the Ross Sea margin: Hallett (Cape Adare to Coulman Island, 0 to 13 Ma), Melbourne (0 to 26 Ma) and Erebus (0 to 19 Ma). Most of the volcanics are younger than 10 Ma. Volcanic sediments, interpreted to be derived from the McMurdo Volcanic Group, were sampled throughout the CIROS-1 drillhole in western McMurdo Sound [George, 1989]. The interpretation of the age of the oldest sediments in CIROS-1 as middle Eocene [Hannah, 1994] suggested that late Cenozoic volcanism commenced in this region about middle Eocene times (about 45 Ma).

The structural and depositional framework of the Ross Sea is formed by four main depocenters, trending approximately north-south across the continental shelf: the Victoria Land basin, the Northern basin, the Central trough and the Eastern basin, [Houtz and Davey, 1973; Davey, 1981, 1983; Hinz and Block, 1984; Cooper et al., 1987, 1994].

The crustal thinning processes in the Ross Sea that formed these major basins are probably related to the separation of Antarctica, Australia and New Zealand [Davey, 1981; Cooper et al., 1987, 1991; Behrendt et al., 1991]. Two main rifting episodes have been proposed: i) an early, essentially non-magmatic, rifting event throughout the Ross Sea which formed all the main depocenters, and ii) a late rifting event, with associated bimodal alkali basalt volcanics, which was localised in the western sectors and formed the Terror Rift in the Victoria Land basin in western Ross Sea. The ages of these two events are not well constrained. A late Mesozoic age has been suggested for the early rifting, associated with Gondwana break-up, and an Eocene and younger age for the later rifting event [Cooper et al., 1987, 1991].

Hinz and Kristofferson, 1987 delineated the major features of the main basins of the Ross Sea and identified a structural trend across the western Ross Sea aligned with the onshore Bowers structure of North Victoria Land Davey [1981] proposed that a major transform fault zone divided the Ross Sea into eastern and western parts, linking up with Late Cretaceous and post-Cretaceous spreading on the Pacific-Antarctic ridge. Cooper at al. [1987] noted major normal faulting in the western Ross Sea of two ages: faults forming basement half grabens, which frequently terminated within the sedimentary section, and more recent Cenozoic faulting which, in places, reached the sea floor. Some of the recent faulting is associated with Cenozoic volcanism. The trend of faulting is largely north-south. This style of faulting is documented in more detail for the whole of the Ross Sea and transverse trends (NW-SE and NE-SW) are defined by Cooper et al..

Aeromagnetic data over the western Ross Sea margin show a north-south fabric in the magnetic anomalies over the western Ross Sea and a major magnetic anomaly, the Polar 3 anomaly, between Coulman Island and Mount Melbourne [Bosum et al., 1989]. Bosum et al. [1989] interpreted the magnetic data in terms of basic igneous bodies and inferred an extensional direction for the formation of these features. Recent magnetic data [Damaske et al., 1994] delineate a highly magnetic province east of Ross Island. Damaske et al. [1994] inferred that this anomaly group and the Polar 3 anomaly correspond to transfer faults between the extensional systems giving rise to the volcanic provinces along the western Ross Sea margin and the Victoria Land basin.

Crustal structure studies of the Ross Sea [Smithson, 1972; Davey and Cooper, 1987; McGinnis et al., 1985; Behrendt et al., 1991; Trehu et al., 1993] indicate a crustal thinning from between 30 and 40 km to about 20 km, indicating 100% extension. Under the Central trough, the extension is far higher with the crystalline crust thinned to about 5 km in places[Trehu et al., 1993]. Here the stretching has been modelled for associated crustal/mantle decompression melting, and is consistent with thin (1 km) volcanics at the base of the basin and a high velocity wedge of mantle melt, now inferred to be incorporated into the lower crust under the basin [Trehu et al., 1993]. Crustal thicknesses modelled from gravity and seismic data vary from 4 km (under Victoria Land basin) to 20 km (under Central high), with the crust underlying the Eastern basin showing as a distinct unit at 12-16 km thick. Assuming a normal crustal thickness of 30-40 km, these crustal thicknesses indicate an average extension of about 100 to 140% (beta = 2-3) over the width (900 km) of the Ross Sea, or 350 to 450 km of extension. This compares with previous estimates of 350 km by Behrendt and Cooper [1991] and the post 100 Ma extension of 1130±690 km derived by DiVenere et al. [1994] from paleomagnetic data in Marie Byrd Land.

Seismic data linked to drillhole information has shown that some thousands of meters of sediments are present in the deepest part of the Ross Sea depocenters [ANTOSTRAT, 1996]. The sediments form two major sequences. In the upper sequence, 6 major unconformities, U1-U6, have been identified [Hinz and Block, 1984] and the intervening units mapped [Busetti and Zayatz, 1994; ANTOSTRAT, this volume]. Unconformity U6 separates these sediments from the underlying sediments, which are probably early Eocene and Paleocene to early Mesozoic in age, and perhaps older [Cooper et al., 1987]. However the history in the basins is not well defined because of the lack of age control.

The significance of unconformities recognized on the seismic data in terms of vertical deformation may vary considerably. Unconformities may arise from tectonic or sea-level changes. They may also be caused by the action of ice erosion associated with an expansion of the Antarctic Ice Sheet that can erode to depths several hundred meters below sea level [Barnes and Lien, 1988; Bartek et al., 1991]. Differential erosion may result from the differential flexural response of the continental shelf to ice loading or unloading [ten Brink and Schneider, 1994].

The identification of significant tectonic events or eustatic events, and estimates of the rates of subsidence for the Eocene and younger sediments, may be deduced at the drill sites in the region. The correlation of the drillcore ages with the identified unconformities is not well constrained, and extrapolation of these ages to other areas depends on the unconformities not being time transgressive with respect to the sediments above or below [ANTOSTRAT, 1996]. Subsidence rates for the older sedimentary section (Eocene and older) depend on the assumption of the age of these sediments. Drill hole data are available from the eastern and central Ross Sea: on the western margin of the Eastern basin (DSDP site 270, 271 and 272 [Hayes et al., 1975]), from the central Ross Sea - over the Central trough (DSDP site 273 [Hayes et al 1975]) and from the western Ross Sea (McMurdo Sound), where the section sampled may be affected by local tectonic and glacial events (MSSTS-1 and CIROS-1 & -2 [Barrett, 1986, 1989] and DVDP sites 8 to 15 [McGinnis, 1981]).

Major changes in sedimentation rate may result from tectonic events. Rates of several tens of meters per million years are indicated by the drillhole data. Savage and Ciesielski [1983] recognise an extremely high sedimentation rate (over 150 m/m.y.) at site 272 and 273 suggesting an increase in subsidence rate for the early and middle Miocene. Cores from CIROS-1, initially, were also interpreted to show sedimentation rates which differed greatly downhole: 40 m/m.y. for the upper sequence and 200 m/m.y. for the lower sequence [Barrett, 1989]. However, the recent reassessment of the age of the deepest sediments in CIROS-1 as middle Eocene [Hannah, 1994], suggests a rate of about 40 m/m.y. or less for the whole sequence, and reassessment, based on better age control on diatom biostratigraphic events from recent Southern Ocean drilling, indicates lower sedimentation (and subsidence) rates for the DSDP sites than previous published [D. Harwood, pers. comm.]. The very high sedimentation rate derived for site 273 is critically dependant on the short time range inferred for the lower sedimentary unit.

The outstanding problems of the Ross Sea region are related to timing - of events and processes. The timing (and amount) of the main extensional events in the Ross sea, and associated with the major depositional (subsidence) episodes in the Ross Sea sedimentary basins, is only inferred. The timing of the uplift episodes in the TAM is poorly constrained and it is not known whether uplift over periods of several tens of millions of years was episodic or continuous. The relationship of extension and subsidence in the Ross Sea to TAM uplift and to Marie Byrd Land is unknown.

Additional problems to be studied include

References

Provided in:
Davey, F.J. and G.Brancolini, 1995, The late Mesozoic and Cenozoic structural setting of the Ross Sea Region, IN Cooper, A. K., Barker, P. F., and Brancolini, G., editors, Geology and Seismic Stratigraphy of the Antarctic Margin (Antarctic Research Series 68): American Geophysical Union, p. 167-182.