Reconciling Extension from Brittle Faulting, Subsidence, and Kinematic Reconstructions: Lessons from the Woodlark Basin

Andrew M. Goodliffe¹, Joe Kington², and Brian Taylor^3
1Geological Sciences, University of Alabama, Tuscaloosa, AL
²Dept. of Geology & Geophysics, University of Wisconsin-Madison, Madison, WI
³SOEST, University of Hawaii, Honolulu, HI

In contrast to ancient rift margins where many of the mechanisms vital to the formation of the margin have long since been hidden, the Woodlark Basin of Papua New Guinea offers the opportunity to study active rift processes. Near the rifting-to-seafloor spreading transition the asymmetric rift system comprises large tilted fault blocks on the southern margin and a principally unfaulted northern margin that has subsided more than 3 km. As is often the case, estimates of extension derived by examining brittle faulting fall short of those calculated through subsidence. However, by including multiple phases of faulting and sub-resolution faulting, this gap can be closed - resulting in a total of 111+/-23km of extension. Assuming Airy isostasy, the extension calculated from subsidence along the same profile is 115+/-45 km. Though these estimates are in close agreement, it remains that locally a mechanism such as lower-crustal flow must be important. Extension can also be estimated by fitting Euler poles to fracture zones and magnetic chrons in the oceanic lithosphere. This gives an estimate of more than 200 km of extension since 6 Ma. Given that the basin has been opening since at least 8.4 Ma, this estimate far exceeds those predicted by brittle extension and subsidence. Can these extension estimates be reconciled? Estimates of brittle extension have so far ignored the potential role of metamorphic core complexes (MCC). At an MCC the upper crust has been removed - a 30 km wide MCC represents 30 km of extension. MCCs have not yet been identified along the study profile, but an MCC that has been dissected by normal faults may not be visible. The extension discrepancy may also be explained by a detachment between the mantle lithosphere and the upper crust. In this case estimates of extension from Euler pole kinematics should not agree with other estimates.

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