Deep Australia - understanding plate architecture and evolution of the transition between Proterozoic Australia and the eastern margin of Gondwana

2017-03-01T23:49:58Z (GMT) by Spampinato, Giovanni Pietro Tommaso
The basement rocks in central Queensland are largely obscured by the Phanerozoic sedimentary succession and the basement geology is known from limited drill holes. Constrained regional potential field analysis is applied to unveil the crustal architecture of the Proterozoic southern Mount Isa terrane and the Phanerozoic Thomson Orogen in Queensland. The Mount Isa terrane forms part of the Palaeoproterozoic North Australian Craton. The exposed Mount Isa Inlier preserves geological record that spans more than 350 million years between ca. 1870 and 1500 Ma. An early cycle of a ca. 1870 Ma orogenesis and ca. 1870 Ma - 1850 Ma extensive batholith emplacement was succeeded by ca. 1800 - 1600 Ma superimposed rifting and subsidence. This was followed by major period of Mesoproterozoic orogeny. Constrained aeromagnetic and gravity data indicate that the depositional sequences and the regional architecture of the Mount Isa terrane extend for ~250 km south of the exposed Inlier, beneath Palaeozoic cover. In the southern parts of the Mount Isa terrane, Palaeoproterozoic sedimentation and volcanism were controlled by NNW-trending structures in half graben setting, resulting in complex superimposed and stacked basins prior to intense basin inversion associated with the Isan Orogeny. Petrophysically constrained geophysical interpretation indicates that prominent regional magnetic and gravity anomalies of the region reflect shallowing of the Pre-1800 Ma crystalline basement and the distribution of meta-sedimentary and meta-volcanic rocks deposited during the development of the ca. 1790 Ma to 1730 Ma Leichhardt Superbasin. Regional low magnetic and low gravity responses coincide with sedimentary successions deposited during the development of the Calvert and Isa superbasins. The ca. 1600 - 1500 Ma Isan Orogeny reactivated the existing extensional fault network and determined the current regional architecture. Extensive low density batholith emplacement occurred at that time. The Thomson Orogen forms the northen extent of the eastern Australian Tasmanides. The region records a protracted tectonic evolution that spans the Neoproterozoic to Triassic and is coincident with one of the largest period of continental growth of the Australian continent. Combined potential field and seismic interpretation indicates that NE- and NW-trending high angle reverse thrusts represent the main crust-scale structural elements in the mid- to lower crust. Gravity data indicate that the eastern and western portions of the Thomson Orogen have a different crustal architecture. The eastern Thomson Orogen is characterized by a regional NE trend and a network of orthogonal faults resulting in a series of troughs and highs. The negative gravity anomalies reflect mostly the distribution of the basinal sequences inferred from drill holes and deep seismic surveys. The western part appears as a series of NW-trending structures interpreted to reflect reverse thrust faults. The smooth magnetic signature of the Thomson Orogen is interpreted to represent source bodies at mid- to lower crustal level. Seismic profiles and forward models consistently indicate that the upper and lower crusts show different geophysical properties. The Thomson Orogen can be distinctly divided in a non-magnetic to weakly magnetic upper crust and a magnetic lower crust. Long wavelength magnetic anomalies are inferred to reflect the topography of the magnetic lower basement. Forward modelling indicates that the western and eastern parts of the Thomson Orogen are petrophysically indistinguishable and the region is interpreted to be a single terrane. The constrained geophysical interpretation indicates that lower basement consists of attenuated Precambrian and mafic enriched continental crust. The Cork Fault is a continental scale structure that defines the boundary between the Proterozoic Mount Isa terrane and the Phanerozoic Thomson Orogen. The Cork Fault coincides with one of the most distinguishable geophysical features of the Australian continent. The prominent geophysical signature of the Cork Fault reflects the abrupt termination of positive NNW-trending geophysical features of the Mount Isa terrane against NE-trending low gravity and low magnetic anomalies of the Thomson Orogen. 2D forward modelling indicates that the lower basement crust of the Thomson Orogen is petrophysically indistinguishable from the Proterozoic crust of the Mount Isa terrane. Best fit reconstruction suggests that high angle listric faults offset the magnetic crust which regionally deepens toward the Thomson Orogen. The prominent potential field gradients associated with the Cork Fault are interpreted to represent the displacement and burying of the Mount Isa terrane crust under the Thomson Orogen basement rocks. The geophysical interpretation indicates that the timing of initiation of the Cork Fault postdates the Isa Orogeny. Initiation of the Cork Fault may be consistently dated to the Mesoproterozoic and may be related to the continental re-organization of the Australian continent that led to the separation of the Mount Isa terrane from the Curnamona Province. The mechanism of separation envisages initial N-S- to NNW-directed extension and development of normal faults. The Cork Fault formed part of a network of major north-dipping and south-dipping normal faults active at that time. Constrained aeromagnetic and gravity data provide support for a Neoproterozoic extensional event in the Thomson Orogen which was controlled by NW-oriented rift segments and occurred in a continental setting as a distal response to the E-W to NE Rodinia break-up. A tectonic model is proposed in which the Thomson Orogen represents the interior extensional architecture during the Rodinia break-up, which is recorded further east in the Anakie Inlier and south in the Koonenberry Belt and the Adelaide Fold Belt in south Australia. This implies that the continental break-up did not occur along the Cork Fault. Instead during the Rodinia break-up, the Cork Fault may have been reactivated as a strike-slip fault, being in a favourable orientation, and formed part of the NE-striking strike-slip faults and NW-oriented normal faults that accommodated the deposition of the Late Neoproterozoic to Middle Cambrian stratigraphy in the Thomson Orogen. Alternatively, at ca. 580 Ma the Thomson Orogen may have switched to a continental back-arc setting driven by the roll-back of a NW-dipping subduction zone. Renewed deposition and volcanism occurred during the Ordovician that may have continued into the Late Silurian, resulting in thinned Proterozoic basement crust and extensive basin systems that formed in a distal continental back-arc environment. The Devonian extensional basinal system in the Thomson Orogen formed in a continental setting. At this time, the extension was controlled by the existing NE- and NW-trending fault architecture. The basin is interpreted to record a distal back-arc basin in the interior of the Australian continent. The Early Palaeozoic tectonic evolution of the continental crust of the Thomson Orogen differs from that of the oceanic crust of the Lachlan Orogen to the south and amalgamation of the two geological terranes might have occurred during Middle Palaeozoic. Thinned Precambrian crust of the Thomson Orogen may be representative of an Early Palaeozoic continental margin or back-arc setting whereas the Lachlan Orogen formed via incorporation of arc type, oceanic and continental rocks. These results demonstrate that regional potential field datasets are a valuable tool in the comprehension of regional geology of tectonic systems with little or no geological exposure.