Evidence for Depth Dependent Lithosphere Thinning During Continental Breakup in the Woodlark Basin

Simon Gozzard¹, Nick J. Kusznir¹, Andrew M. Goodliffe², and Gianreto Manatschal^3
1Earth and Ocean Sciences, University of Liverpool, Liverpool, United Kingdom
²University of Alabama, Tuscaloosa, AL
³Université Louis Pasteur, Strasbourg, France

Understanding how the continental lithosphere thins during continental breakup and sea-floor spreading initiation is key to understanding the continental breakup process, ocean-continent transition structure, and rifted margin subsidence and heat flow history. We have determined thinning for the lithosphere, crust and crust in the region of propagating continental breakup and sea-floor spreading in the Woodlark Basin, western Pacific. Thinning factors have been determined for the whole lithosphere from subsidence analysis, whole continental crustal from gravity inversions, and upper crust from fault analysis. Subsidence analysis using flexural backstripping of 2D cross-sections near the Moresby Seamount in the region of pre-breakup continental lithosphere thinning gives whole lithosphere thinning factors 0.5 to 0.8 increasing eastwards towards the progagating tip pf sea-floor spreading, and indicates that subsidence requires substantial crustal thinning. Gravity inversion has been used to determine Moho depth, crustal thickness and thinning factors; predicted thinning factors from gravity inversion are similar to those obtained from flexural backstripping and Moho depths from gravity inversion are consistent with those from receiver function analysis. Fault analysis of seismic reflection data show upper crustal thinning factors of between 0.1 to 0.4 for the vicinity of the Moresby Seamount, substantially lower than thinning factors predicted for the whole lithosphere and continental crust, indicating depth-dependent lithosphere stretching and thinning. Crustal thicknesses predicted from gravity inversion immediately to the east of the Moresby Seamount are greater than expected for oceanic crust; it is inconclusive whether this region is thick oceanic crust, attenuated continental crust or thin continental crust with volcanics.

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