The crustal structure of the southern Argentine margin

Abstract

SUMMARY Multichannel reflection seismic profiles, combined with gravimetric and magnetic data provide insight into the crustal structure of the southernmost Argentine margin, at the transition from a rifted to a transform margin and outline the extent of the North Falkland Graben. Based on these data, we establish a regional stratigraphic model for the post-rift sediments, comprising six marker horizons with a new formation in the Barremian/Lower Cretaceous. Our observations support that a N–S trending subsidiary branch of the North Falkland Graben continues along the continental shelf and slope to the Argentine basin. During the rift phase, a wide shelf area was affected by the E–W extension, subsequently forming the North Falkland Graben and the subsidiary branch along which finally breakup occurred. We propose the division of the margin in two segments: a N–S trending rifted margin and an E–W trending transform margin. This is further underpinned by crustal scale gravity modelling. Three different tectono-dynamic processes shaped the study area. (1) The Triassic/Early Jurassic extensional phase resulting in the formation of the North Falkland Graben and additional narrower rift grabens ended synchronously with the breakup of the South Atlantic in the early Valanginian. (2) Extensional phase related to the opening of the South Atlantic. (3) The transform margin was active in the study area from about Hauterivian times and activity lasted until late Cretaceous/early Cenozoic. Both, the rifted margin and the transform margin are magma-poor. Very limited structures may have a volcanic origin but are suggested to be post-rift. The oceanic crust was found to be unusually thin, indicating a deficit in magma supply during formation. These findings in combination with the proposed breakup age in the early Valanginian that considerably predates the formation of the Parana–Etendeka continental flood basalt provinces in Brazil and Namibia question the influence of the Tristan da Cunha hotspot during the initial formation of the South Atlantic.