New age constraints for a short pulse in Ross orogen deformation triggered by East–West Gondwana suturing

Abstract

Constraints on the timing of deformation within the Antarctic Ross orogenic belt are important for understanding early Paleozoic tectonic activity accompanying the assembly of Gondwana. One of the best areas for constraining the tectonic evolution of the Ross orogenic belt is the Holyoake Range in the central Transantarctic Mountains where previous work shows an abrupt change in the stratigraphic succession of the Cambrian Byrd Group possibly related to the inception of tectonism within the orogen at ∼ 515 Ma. To further constrain deformational timing, we conducted ⁴⁰Ar–³⁹Ar analyses on magmatic phases of the Holyoake Gabbro, which cross-cuts folded Lower Cambrian Shackleton Limestone of the Byrd Group. Our analyses yield average ages of 506.7 ± 3.6 Ma (2 sigma; MSWD = 0.9) and 504.1 ± 2.4 Ma (2 sigma; MSWD = 1.3) for hornblende from the early and late magmatic phases, respectively. Deformation of the Shackleton Limestone therefore occurred prior to 506.7 ± 3.6 Ma, which is ∼ 12 m.y. after the siliciclastic drowning of the carbonate platform inferred to record the start of Ross tectonism in the central Transantarctic Mountains. On a regional scale, the data are consistent with a short pulse of deformation, which was probably related to global plate motion changes following final suturing of East and West Gondwana.

Introduction

Isotopic and biostratigraphic age analyses provide important constraints on the timing and rates of structural, depositional, and magmatic processes within orogenic belts and their relationships to plate movements and past continental configurations (e.g., Dalziel, 1991, Moores, 1991, Goodge et al., 2004a, Goodge et al., 2004b, Boger and Miller, 2004, Squire and Wilson, 2005, Squire et al., 2006). Providing high precision isotopic constraints on deformational episodes is a frequent challenge in orogenic belts because appropriate mineral chronometers are not always present. Deformational events are commonly dated using cross-cutting relationships, which can result in a time window (sometimes large), rather than a precise age for deformation. As illustrated in Fig. 1, the Ross orogenic belt in Antarctica is no exception.

The Ross orogen represents a major continental arc, which formed at the inception of subduction along the Neoproterozoic to early Paleozoic passive margin of Gondwana (Borg and DePaolo, 1991, Stump, 1995, Encarnación and Grunow, 1996, Rocchi et al., 1997, Cox et al., 2000, Allibone and Wysocazanski, 2002, Goodge et al., 2002, Myrow et al., 2002, Goodge et al., 2004a). Early work suggested that the Ross orogen might have experienced two distinct deformational events (Grindley and McDougall, 1969, Goodge et al., 1991). The first of these occurred in the earliest part of the Early Cambrian and was associated with granitic magmatism and high grade metamorphism in the central Transantarctic Mountains (Goodge et al., 1993a, Goodge et al., 1993b, Stump et al., 2002a). One of the hallmark signatures of this earlier deformation event is a purported angular unconformity between the Shackleton Limestone and the underlying Goldie Formation at Cotton Plateau in the Nimrod Glacier region (Stump et al., 2002a). Although detrital zircons from the Goldie Formation allow it to be older and thus potentially deformed prior to deformation of the Shackleton Limestone (Goodge et al., 2004a), the contact between the Goldie Formation and the Shackleton Limestone is faulted (Goodge et al., 1999), and therefore cannot be used as a reliable indicator of the timing of possible older deformation at this locality. Regardless, deformational events are also documented to have occurred in the Neoproterozoic to Early Cambrian elsewhere in the Transantarctic Mountains in southern Victoria Land and the southern Transantarctic Mountains (Stump et al., 1986, Rowell et al., 1993, Stump et al., 1999). This deformational event was called the Beardmore or Nimrod orogeny (Grindley and McDougall, 1969, Goodge et al., 1991). After this earlier deformational event, Early Cambrian granitic batholiths, arc-related volcanism and carbonate sequences were emplaced along the Transantarctic Mountains. By the Middle to Late Cambrian, magmatism continued but the style of sedimentation changed (Myrow et al., 2002). Associated with this change in sedimentation style was the second deformational pulse, often considered the main Ross event (Gunn and Warren, 1962, Stump et al., 1986, Rowell et al., 1992).

The concept of two discrete orogenies, one in the earliest Early Cambrian and one in the Middle to Late Cambrian was reconsidered by Goodge (1997) and Rowell et al. (2001). They suggest that the two deformational events of the Beardmore and Ross orogenies represent just one protracted event, which they call the Ross orogeny.

Since the start of the last century geologists have been working to date rocks and associated tectonism within the Ross orogen (Laird, 1991). Although biostratigraphic and isotopic geochronologic studies have significantly advanced our understanding of the timing of deformation, sedimentation, and magmatism (e.g., Rowell et al., 1992, Goodge et al., 1993a, Encarnación and Grunow, 1996, Rowell et al., 2001, Goodge et al., 2002, Myrow et al., 2002, Goodge et al., 2004a, Goodge et al., 2004b, Paulsen et al., 2004), precise age constraints on deformation are still needed to provide a broader picture of deformation along this extensive orogen. For over 2000 km, from southern Victoria Land to the Pensacola Mountains, there are few localities where biostratigraphy and/or isotope chronology place constraints on deformation timing of the Ross orogen (Fig. 1). Upper age-limits on deformation timing are typically provided by dated volcanic or igneous rocks, whereas the ‘lower’ limits on deformation are provided by the youngest dated detrital minerals (zircons or mica) or volcanic rocks that are deformed. In some cases, the only upper limits on the age of deformation are the Devonian through Triassic Beacon Supergroup, which unconformably overlies metasedimentary and plutonic rocks of the Ross orogenic belt. Of course, the ‘lower’ age-limit on deformation does not preclude earlier deformational phases. Earlier phases are difficult to document in many areas, e.g. the Shackleton Glacier region, where there is a paucity of known cross-cutting relationships or isotopic analyses on syn-tectonic intrusions. In the Pensacola Mountains, the Miller and Geologists ranges, and southern Victoria Land, the age of deformation is well constrained by the ages of syn-tectonic intrusions and metamorphic rocks (Goodge et al., 1993a, Encarnación and Grunow, 1996, Jones, 1997, Curtis et al., 2004).