f Title: 3D Gravity Inversion Supports the Guyana-Suriname Rifted-Passage Margin of Jurassic Age: But Where Is Its Conjugate?

The Guyana margin along the northeastern coast of South America represents a 700-km-wide, tectonic transition from a 40–50 km thick and elevated Archean greenstone belt of the Guiana shield and its cover of Precambrian to Paleozoic sediment to normal or thin oceanic crust that underlies the deep Guyana basin of Guyana and Suriname. Suriname is partially fronted by a volcanic margin composed of Middle Jurassic volcanic flows and overlying sedimentary cover that is 25 km thick and underlies the Demerara plateau. Two tectonic models have been proposed for the Mesozoic tectonic origin of the Guyana-Suriname margin: 1) the margin formed as a Jurassic northwest-oriented transform-passive margin during the breakup of North America and northwestern Africa; and 2) the margin formed as a Jurassic rifted-passive margin and was modified by oblique, Aptian opening of the Equatorial Atlantic Ocean. Using GPlates, both tectonic models have been reconstructed to demonstrate a transform-passive margin with northwest extension or an earlier Jurassic rifted-passive margin with initial northeast extension. We perform a 3D gravity inversion of marine satellite gravity constrained by previous marine refraction surveys to investigate both the transform and rift hypotheses for the earlier, Jurassic phase of the Guyana-Suriname margin formation. The inversion reveals the following characteristics supportive of its origin as a rifted-passive margin: 1) margin-perpendicular transects from the 3D volume show that the margin exhibits a necked zone with an average top basement gradient of 4 degrees; there is no evidence for an abrupt truncation and rapid change in crustal thickness that is a defining characteristic of a transform margin; 2) margin-perpendicular transects show crustal thinning beneath the Nickerie rift; and 3) the seaward limit of the Guyana rifted-passive margin is abruptly truncated by the Doldrums fracture zone formed during the Aptian transtensional opening of the Equatorial Atlantic Ocean and as seen in the rifted tectonic reconstruction. This revised interpretation or a Jurassic rifted-passive margin leads to the question of where is its conjugate on the West African margin? We provide a revised plate reconstruction to show a possible early opening rifted-passive margin scenario that was truncated by the later Aptian oblique opening of the Equatorial Atlantic Ocean.