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Pangaea Plate Reconstructions: Using 2-D Gravity Modelling to Constrain Pre-Rift Continental Boundaries

Whittaker, Joanne 1; Mazur, Stansilaw 1; Stewart, Matthew 1; Markwick, Paul 1; Green, Christopher 1
1 GETECH, Leeds, United Kingdom.

Accurate full-fit Pangaean plate reconstructions are crucial for understanding syn-rift basin geometries, hinterland evolution and sediment supply relationships. However, determining precise pre-rift fits for Pangaean continental blocks is problematic due to the lack of conjugate, linear features on extended continental crust that can be used as ‘isochrons’ for plate modelling purposes. Coastlines, isobaths, magnetic and gravity lineations, and interpretations of the continent-ocean boundary (COB) have all been utilised as proxies for pre-rift ‘isochrons’ in plate tectonic models. However, these approaches do not adequately account for continental stretching during break-up. Continental stretching can be considerable (i.e. more than 200km) and may vary along individual margins, resulting in changing margin widths, beta factors and basin evolution. In order to accurately unravel relative plate motions occurring during the initial continental-rift stage of break-up the amount of continental extension that occurred must be calculated and incorporated into plate reconstruction models.

To restore the extended continental crust to its original pre-rift position, we use a set of 2D gravity forward profile models at key locations along conjugate passive margins. The 2D gravity models constrain the present-day location of the continent-ocean boundary, limit of undeformed crust and provide estimates of the cross-sectional area of the deformed continental crust. This allows the position of the pre-rift continental boundary to be determined. The 2D models make use of the highest resolution satellite gravity data in marine areas and gravity data from GETECH’s continental scale compilations.

The set of markers representing the pre-rift continental boundary quantifies the area of variable margin deformation key to correctly defining deformable plate geometries. These markers have been used to calculate relative plate motions and pre-rift plate fits for key plate-pairs in our global plate tectonic model. Reconstructions obtained using this methodology result in tighter fits between continental plates than can be achieved by reconstructions using magnetic anomalies or estimates of the continent-ocean boundary. Also, the beta factors and amounts of continental stretching implied by our model compare extremely well with estimates of continental stretching from sources, such as crustal thickness maps and seismic data.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009