Equatorial Atlantic Deep-Water OCT Structure and Crustal Type From Satellite Gravity Inversion

Abstract

Determining the structure of the ocean-continent-transition (OCT), the location of the continent-ocean-boundary (COB) and the composition of crustal basement are substantial challenges during deep-water exploration. Inversion of satellite-derived free-air gravity-anomaly data, using a technique which allows for (i) the thermal state of the lithosphere following rifting and breakup and (ii) magmatic addition to the crust during rifting and breakup, provides a useful method for mapping crustal structure and crustal type across global rifted margins. Maps of crustal thickness and continental-lithosphere thinning-factor may be used to determine COB location and the distribution of oceanic lithosphere. Crustal cross-sections using Moho depth from gravity inversion allow OCT structure and basement composition to be constrained. We have used this gravity inversion technique to investigate the crustal structure of the Equatorial Atlantic conjugate margins. Using maps of crustal thickness and continental-lithosphere thinning-factor, with the shaded-relief free-air gravity anomaly superimposed, we can improve the determination of pre-breakup rifted-margin conjugacy and the trajectory of sea-floor spreading during ocean basin formation. These maps illustrate how the Equatorial Atlantic opened as a set of oblique rift-transform segments. A set of cross-sections has been extracted from the results of the gravity inversion along both equatorial margins. These illustrate the crustal structure of both rifted-margin segments and transform-margin segments. The maps and cross-sections are used to delineate OCT structure, crustal type and their lateral variation. On both margins anomalously thick crust is resolved along a number of oceanic fracture zones. Three possible origins for this are discussed, (i) continental crust attenuated along the fracture zones, (ii) oceanic crust magmatically thickened at the fracture zones, (iii) oceanic crust thickened by transpression along the fracture zones. Gravity inversion alone cannot discriminate between these possibilities. Maps and cross-sections show the existence of both magma-poor and magma-rich margin segments, in addition to large areas of “normal” magmatic addition. Using maps of crustal-thickness and thinning-factor as input to plate reconstructions, the regional palaeogeography of the Equatorial Atlantic at breakup and the subsequent development of deep-ocean connectivity have been examined.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017