F. Rosas

Are subduction zones invading the Atlantic? Evidence from the southwest Iberia margin

Subduction initiation at passive margins plays a central role in the plate tectonics theory. However, the process by which a passive margin becomes active is not well understood. In this paper we use the southwest Iberia margin (SIM) in the Atlantic Ocean to study the process of passive margin reactivation. Currently there are two tectonic mechanisms operating in the SIM: migration of the Gibraltar Arc and Africa-Eurasia convergence. Based on a new tectonic map, we propose that a new subduction zone is forming at the SIM as a result of both propagation of compressive stresses from the Gibraltar Arc and stresses related to the large-scale Africa-Eurasia convergence. The Gibraltar Arc and the SIM appear to be connected and have the potential to develop into a new eastern Atlantic subduction system. Our work suggests that the formation of new subduction zones in Atlantic-type oceans may not require the spontaneous foundering of its passive margins. Instead, subduction can be seen as an invasive process that propagates from ocean to ocean.

Sheath folds formed by drag induced by rotation of rigid inclusions in viscous simple shear flow: nature and experiment

Abstract Natural examples of fold patterns associated with rotated boudins occur in the Ossa–Morena Zone (S Portugal) and triggered the present study. In ductile marbles with embedded competent mafic boudins, the folds seem to have originated by rotation of these bodies and show differential development, from gentle deflections to sheath folds. This suggests a dependence of the structure type on the distance of the metamorphic layering to the rigid body. The experimental work presented here was performed with rotating inclusions in bulk simple shear and introduces a new variable, the distance of the marker layer from the rigid inclusion (d), expressed in terms of E, the ratio between the spacing between marker layers (D) and the greater principal dimension of the inclusion (a). Contrary to previous experimental work in the literature, we used planar marker layers without concomitant boudinage. Our results show new processes of development of sheath folds from planar layers, and also that some non-cylindrical layer deflections are transient and, thus, that sheath folds do not originate from them. When d is small and the inclusion starts with its longest axis normal to the shear plane, sheath folds develop by drag pull processes on originally planar marker layers. Transient non-cylindrical deflections develop on originally planar marker layers when the distance between the marker layers is smaller than the longest principal dimension of the rigid particle. The monoclinic symmetry of the drag sheath fold patterns associated with rotating rigid inclusions indicate non-coaxial flow and can be used as a shear sense criterion (quarter structure).

Anatomy and tectonic significance of WNW-ESE and NE-SW lineaments at a transpressive plate boundary (Nubia-Iberia)

The Subandean Basins of South America extending from Trinidad to Tierra del Fuego have been the object of intensive exploratory activities (Fig. 1). The largest amount of hydrocarbons discovered during the last 30 years in these basins was found in complex structural terrains. A total of 59 Billion Barrels of Oil Equivalent (BBOE) have been discovered in areas affected by compressional tectonics. Of these basins, the largest discoveries are in the Furrial Trend of Venezuela (24 BBOE), followed by the Chaco area in Bolivia and Argentina (13 BBOE), the Llanos Foothills of Colombia (4.4 BBOE), and the Madre de Dios Basin of Peru (4.2 BBOE).

The future of Earth's oceans: consequences of subduction initiation in the Atlantic and implications for supercontinent formation

Subduction initiation is a cornerstone in the edifice of plate tectonics. It marks the turning point of the Earths Wilson cycles and ultimately the supercycles as well. In this paper, we explore the consequences of subduction zone invasion in the Atlantic Ocean, following recent discoveries at the SW Iberia margin. We discuss a buoyancy argument based on the premise that old oceanic lithosphere is unstable for supporting large basins, implying that it must be removed in subduction zones. As a consequence, we propose a new conceptual model in which both the Pacific and the Atlantic oceans close simultaneously, leading to the termination of the present Earths supercycle and to the formation of a new supercontinent, which we name Aurica . Our new conceptual model also provides insights into supercontinent formation and destruction (supercycles) proposed for past geological times (e.g. Pangaea, Rodinia, Columbia, Kenorland).