Abstract: The Tectonic Evolution of a Failed Rift System: Marajo Basin, Northern Brazil

Marie Ester Lara

The Marajo intracontinental rift underwent extensional stresses from the Early/Middle Jurassic to the Cenomanian, first related to the southward propagation of the southern North Atlantic, and second to the transform rifting of the equatorial Atlantic. Seismic data show that basin architecture is defined by asymmetric half grabens bounded by listric and growth normal faults, which seem to sole out at a detachment surface located at mid-crustal levels. The resulting asymmetric basin configuration is adequately predicted by the simple shear or detachment model of lithospheric extension.

The pattern in which half grabens link together in the Marajo rift yields increasingly symmetric cross sections toward the northern end of the rift. This northward "progression" toward symmetry is accompanied by an increase in faulting intensity and complexity, possibly reflecting different stages in the evolution of rifting. In addition, gravimetric and aeromagnetic data indicate that the southern subbasin is underlain by continental crust, whereas the central and northern subbasins are underlain by "transitional" crust, further supporting that the northern end of the rift underwent higher amounts of extension. This northward progression in rift morphologies may be associated with the southward "weakening" of rifting, as the opening of the Atlantic Ocean propagated southward. Later d velopment of the Equatorial fracture zones offset the locus of main extension and prevented further opening in Marajo'. Magmatism and associated uplift occurred in the late stages of rifting, as a result of the stretching and thinning of the continental lithosphere. Post-rift subsidence would be thermally induced.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994