Are the mantle lithosphere and lower crust preferentially thinned during continental rifting?

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Worldwide, estimates of extension in rift zones vary greatly depending on the method used to calculate the extension. This variability is the result of the discrepancy between different methodologies and may be the result of polyphase faulting, subresolution faulting, and/or depth-dependent extension. Such inconsistency between estimates has been noted in the Woodlark Basin, an active transition zone between continental rifting and seafloor spreading. Previous work in the basin, where seafloor spreading has not initiated, calculated extension by summing fault heaves, calculating subsidence, and determining plate motion from Euler pole kinematics, yielding estimates of 111 km (23) from brittle extension, 115 km (47) from subsidence, and 200 km (40) from Euler pole kinematics (Kington and Goodliffe, 2008). By incorporating polyphase and subresolution faulting into the brittle extension estimate, Kington and Goodliffe (2008) resolved the discrepancy between estimates of extension derived from brittle faulting and subsidence. The third method used to estimate extension, Euler pole kinematics, produced a large discrepancy. Kington and Goodliffe (2008) interpreted this to be the result of preferential extension of the lower crust and mantle lithosphere during the rifting phase and proposed that uniform extension would occur throughout the lithosphere after seafloor spreading initiation. The current study explores potential errors in previous work in the basin and determines if the results are applicable to other portions of the basin. In contrast to Kington and Goodliffe (2008), the current study determines extension where seafloor spreading initiated at ~0.8 Ma. Using the methods and associated errors from Kington and Goodliffe (2008), Euler pole extension estimates (~202 to 238 km) are ~2 times higher than brittle (~69 to 90 km) and subsidence (~60 to 79 km) extension estimates, consistent with the previously seen discrepancy. When taking into account other sources of error not considered by Kington and Goodliffe (2008), the current study shows the previous methods lack the constraints necessary to produce conclusive results. This would also render the results of the previous study by Kington and Goodliffe (2008) inconclusive. Therefore, it is not necessary to invoke the Kington and Goodliffe (2008) model to explain rifting in the western Woodlark Basin.

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Geophysics, Geology