The structural and stratigraphic expression of continental breakup
Creators: name: Reeve, Matthew Thomas
Publisher: name: Imperial College London
Year: 2017

description: The structural and stratigraphic record of passive margins documents the upper crustal expression of the large-scale geodynamic processes affecting the lithosphere during continental rifting and breakup. Classic models of rifting and breakup predict an initial period of fault-controlled extension, followed by post-rift thermal subsidence; however passive margins worldwide express periods of uplift and erosion during rifting. In particular, the development of breakup unconformities, documenting uplift at the time of lithospheric rupture, remain an enigmatic feature of rifted margins. Relating the timing of these uplift events to the processes of breakup and onset of seafloor spreading is often challenging, due to uncertainty in the timing of seafloor spreading, which is often assumed to relate to the first linear magnetic anomaly adjacent to the rifted margin. This study uses an extensive database of 2D and 3D seismic reflection and borehole data from the North Carnarvon Basin, offshore NW Australia, to constrain the timing and distribution of uplift and subsidence during the Early Cretaceous breakup of Gondwana, and the processes associated with development of the continent-ocean transition zone. The &lt;3 km thick pre-breakup Tithonian – Valanginian Barrow Group delta provides important insights into uplift and subsidence during the final stages of continental rifting. Compaction-based analyses of wells from the onshore South Carnarvon Basin indicate a period of uplift and erosion of the flanks of the Cuvier rift (&lt;1.5 km of erosion) over an area of ~400 km, which provided a major source of sediment for the offshore delta complex. Backstripping of wells from the southern Exmouth Plateau indicates a period of contemporaneous rapid subsidence (1.3 km of subsidence at a rate of &lt;0.24 mm yr-1) over an area of ~300 km, despite minimal upper crustal extension (β &lt; 1.1). This anomalous history of uplift and subsidence is attributed to the effects of depth-dependent lithospheric extension, lower crustal flow driven by sediment loading, or dynamic topography. This study also examines seismic reflection data from the Cuvier Abyssal Plain, which reveals the presence of a &gt;500 km wide previously unrecognised region of heavily intruded and thinned continental crust, overlain by seaward-dipping lava flows deposited during breakup. This area is characterised by well-developed linear magnetic anomalies M10N – M5r (135.3 – 131.4 Ma), which indicate a ~4 Ma transitional period between continental rifting and the onset of seafloor spreading. The record of breakup unconformities in the North Carnarvon Basin is also examined, with three major, regionally extensive unconformities identified (IVUC, TVUC and IHUC). Calcareous nannofossil occurrences from boreholes on the Exmouth Plateau suggest ages of ~135 Ma (IVUC), ~134 Ma (TVUC) and 132.5 – 131.4 Ma (IHUC), indicating that these unconformities represent punctuated periods of uplift during the transition period between rifting and seafloor spreading. This study suggest that timing of the IVUC appears to relate to the onset of transitional crust development, and the IHUC relates to full lithospheric rupture. This study suggests that breakup unconformity development may have been driven primarily by depth-dependent thinning and migration of strain during breakup, with possible influence locally by cratonic underplating and thermal uplift. In summary, the results of this study provide new insights into the temporal and spatial evolution of uplift, subsidence and erosion, and the contemporaneous geodynamic processes operating during the transition from rifting to seafloor spreading.