Geology topics

J.D. Bradshaw

Publications and source records attributed to J.D. Bradshaw.

3 recordsLinked to original sources

USGS2007

The Ross Orogen and Lachlan Fold Belt in Marie Byrd Land, Northern Victoria Land and New Zealand: implication for the tectonic setting of the Lachlan Fold Belt in Antarctica

Correlation of the Cambrian Delamerian Orogen of Australia and Ross Orogen of the Transantarctic Mountains widely accepted but the extension of the adjacent Lachlan Orogen into Antarctica is controversial. Outside the main Ross-Delamerian belt, evidence of this orogeny is preserved at Mt Murphy in Marie Byrd Land and the in Takaka Terrane of New Zealand. In all pre-break- configurations of the SW Pacific, these two areas are far removed from the Ross-Delamerian belt. Evidence from conglomerates in the Takaka Terrane, however, shows that in Late Cambrian times it was adjacent to the Ross Orogen. This indicates major tectonic displacements within Gondwana after the Cambrian and before break-up. The Lachlan Orogen formed in an extensional belt in a supra-subduction zone setting and the Cambrian rocks of Marie Byrd Land and New Zealand are interpreted as parts of a rifted continental ribbon on the outboard side of the Lachlan belt.

Open-File Report↗

USGS2007

The Cambrian Ross Orogeny in northern Victoria Land (Antarctica) and New Zealand: A synthesis

In the Cambrian, the paleo-Pacific margin of the Gondwana supercontinent included East Antarctica, Australia, Tasmania and New Zealand and was affected by themajor Ross-Delamerian Orogeny. In Antarctica, evidence suggests that this resulted from oblique subduction and that in northern Victoria Land it was accompanied by the opening and subsequent closure of a back-arc basin. Comparison of the type and timing of sedimentary, magmatic and metamorphic events in areas noted above shows strong similarities between northern Victoria Land and New Zealand. In both regions Middle Cambrian volcanites are interpreted as arc/back-arc assemblages produced by west-directed subduction; sediments interbedded with the volcanites show provenance both from the arc and from the Gondwana margin and therefore place the basin close to the continent. Back-arc closure in the Late Cambrian was likely accomplished through a second subduction system

Open-File Report↗

USGS1988

Limestone and chert in tectonic blocks from the Esk Head subterrane, South Island, New Zealand

The Esk Head subterrane is a continuous belt, generally 10-20 km wide, of tectonic mélange and broken formation on the South Island of New Zealand. This subterrane separates older and younger parts of the Torlesse terrane which is an extensive accretionary prism composed mostly of quartzo-feldspathic, submarine-fan deposits ranging from Permian to Early Cretaceous in age. The Torlesse is the most Pacific-ward of several Permian and Mesozoic accreted terranes in New Zealand that record tectonic amalgamation and ultimate accretion against the Pacific-facing Gondwana margin. The Esk Head subterrane of the Torlesse is especially informative because it includes within it conspicuous tectonic blocks of submarine basalt and a variety of basalt-associated seamount and sea-floor limestones and cherty rocks thought to be representative of the subducted plate. Limestones in tectonic blocks are of Late Triassic and probably Jurassic ages and include (1) submarine-cemented, pelagic-bivalve, geopetal packstone-grainstone; (2) brachiopod-bryozoan encrinite; and (3) radiolarian, pelagic lime mudstone. Most of the Triassic blocks have been dated using conodonts which have remarkably low color alteration index (CAI) values (<1.5). An incomplete sampling of cherts in tectonic blocks and from Holocene gravels derived from the Esk Head subterrane yields radiolarian-based ages of Late Triassic, Early Jurassic, Middle Jurassic, and Late Jurassic. Paleogeographic inferences drawn from megafossils, bioclasts, and radiolarians, as well as from carbonate cements, indicate deposition of the oceanic sedimentary rocks at paleolatitudes somewhat lower than that of the New Zealand part of the Gondwana margin, but higher than paleoequatorial latitudes. These oceanic sediments and their basaltic substrates were evidently emplaced in the Torlesse accretionary prism following off-scraping from an extensive subducting oceanic plate, probably the Phoenix plate, which was obliquely convergent with the northwest-trending Gondwana margin during Late Jurassic and/or Early Cretaceous time.

Geological Society of America Bulletin↗