Constraining back-arc basin formation in the eastern Coral Sea: preliminary results from the ECOSAT voyage

The eastern Coral Sea region is an underexplored area at the northeastern corner of the Australian plate, where long-lived interaction between the Pacific and Australian plate boundaries has resulted in an intricate assemblage of deep oceanic basins and ridges, continental fragments and volcanic products. A paucity of marine geophysical and geological data from this complex region has resulted in the lack of a clear conceptual framework to describe its formation, ultimately affecting our understanding of the connection between the plate boundaries of the SW Pacific and SE Asia. In particular, the tectonic relationship between two back-arc basins, the Santa Cruz and d'Entrecasteaux Basins, and the South Rennell Trough, has yet to be resolved. In October-November, 2012, we collected 6,200 km of marine magnetic, 6,800 km of gravity and over 13,600 km2 of swath bathymetry data from the eastern Coral Sea onboard the RV Southern Surveyor. A complementary dredging program yielded useful samples from 14 seafloor sites. Our preliminary geochemical interpretation of the dredge samples obtained from the South Rennell Trough reveal volcanic rocks resembling MORB or BABB-type basalts, similar in composition to the recently re-analysed and dated ORSTOM dredges from the area that yielded ~28 Ma MORB-like basalts. Swath bathymetry profiles from the Santa Cruz Basin reveal that the South Rennell Trough extends into this basin, with seafloor spreading fabric being parallel to the trough. Preliminary analysis of the three full and four partial new magnetic anomaly profiles across the Santa Cruz Basin, coupled with limited existing profiles, reveals that the basin may have formed between Chrons 13-18 (~32-38 Ma), with an extinct spreading ridge along the inferred continuation of the South Rennell Trough, consistent with ORSTOM age dates. Our results suggest that the South Rennell Trough is an extinct southwestward propagating spreading ridge, which may have initiated along a pre-existing zone of weakness. A preliminary interpretation of the 4 magnetic profiles collected in the d'Entrecasteaux Basin and existing profiles of seafloor fabric shows that this basin does not share a common seafloor spreading history with the Santa Cruz Basin, as has been suggested previously. Our preliminary interpretation of the relationship between the Santa Cruz Basin, South Rennell Trough and d'Entrecasteaux Basin requires a re-interpretation of existing models of the SW Pacific to take into account a southwestward propagating spreading ridge between 38-32 Ma, contemporaneous with seafloor spreading further south in the North Loyalty Basin. Further work on age-dating and geochemical analysis of the newly collected dredge samples and an in-depth analysis of the magnetic anomalies in the d'Entrecasteaux Basin may further yield important information concerning the tectonic evolution of the area.