It is generally acknowledged that mantle plumes played a significant role in the process of fragmentation of the Pangaea supercontinent during the late Triassic – early Cretaceous time interval, although the importance of their contribution is often underestimated. I show that long-lived thermal anomalies in the mantle not only triggered the break-up of central Pangaea but also influenced considerably the tectonic evolution of the Atlantic Ocean from early Jurassic onward. A major point of controversy in modern geology is the role of far-field and “active” forces for rift initiation. Most authors distinguish two end-members in modelling of continental break-up: active rifting, associated with asthenosphere upwelling due to thermal or compositional anomalies (mantle plumes), or passive rifting associated with a regional tensional stress field. Although people generally agree that both mechanisms may contribute to the onset of rifting, it is still unclear if active rifting by itself can drive the rupture of continents. A common line of thought assumes that the lithosphere only breaks passively, although this process may in turn destabilize the lower lithosphere, leading to small-scale convective upwelling of asthenosphere, thereby active rifting may only take place in the late syn-rift or post-rift phase. The possibility that active rifting represents a viable mechanism for the break-up of a continent is suggested from the observation that active spreading pulses along mid-ocean ridges often cause deformation in the nearby continental margins. These phenomena were first described by Bott (1991), who showed that anomalous hot and low-density upper mantle zones beneath oceanic ridges, forming ocean floor swells, determine a substantial increase in the compressional deviatoric stress field in the oceanic area and in the adjacent continental margins (up to 90 MPa). The existence of post-break-up contractional deformation along passive margins, associated with horizontal compression, has been discussed in many papers, in particular for the northern and central Atlantic regions during the Cenozoic. For instance, I have recently showed that the inversion of the Mesozoic rift structures of the Atlas region, leading to the formation of the Atlas orogen, occurred during the Oligocene – early Miocene and was accompanied by higher spreading rates in the central Atlantic, possibly driven by the Azores mantle plume. Similarly, it was suggested that the rapid northward motion of India during the late Cretaceous – Eocene was driven by the force exerted by the Reunion plume head. I will outline a new paradigm for the driving forces of plate tectonics, in which divergent flows in the asthenosphere, associated with mantle heterogeneities, are driving mechanisms as well. In this model plates are moved by the combined action of drag stresses exerted on the base of the lithosphere and far-field boundary forces (slab pull and ridge push). Bott, M.H.P., 1991. Tectonophysics, 200(1-3), 17-32.

The role of hot spots in the break-up of Pangaea and the opening of the Atlantic ocean

SCHETTINO, Antonio
2012-01-01

Abstract

It is generally acknowledged that mantle plumes played a significant role in the process of fragmentation of the Pangaea supercontinent during the late Triassic – early Cretaceous time interval, although the importance of their contribution is often underestimated. I show that long-lived thermal anomalies in the mantle not only triggered the break-up of central Pangaea but also influenced considerably the tectonic evolution of the Atlantic Ocean from early Jurassic onward. A major point of controversy in modern geology is the role of far-field and “active” forces for rift initiation. Most authors distinguish two end-members in modelling of continental break-up: active rifting, associated with asthenosphere upwelling due to thermal or compositional anomalies (mantle plumes), or passive rifting associated with a regional tensional stress field. Although people generally agree that both mechanisms may contribute to the onset of rifting, it is still unclear if active rifting by itself can drive the rupture of continents. A common line of thought assumes that the lithosphere only breaks passively, although this process may in turn destabilize the lower lithosphere, leading to small-scale convective upwelling of asthenosphere, thereby active rifting may only take place in the late syn-rift or post-rift phase. The possibility that active rifting represents a viable mechanism for the break-up of a continent is suggested from the observation that active spreading pulses along mid-ocean ridges often cause deformation in the nearby continental margins. These phenomena were first described by Bott (1991), who showed that anomalous hot and low-density upper mantle zones beneath oceanic ridges, forming ocean floor swells, determine a substantial increase in the compressional deviatoric stress field in the oceanic area and in the adjacent continental margins (up to 90 MPa). The existence of post-break-up contractional deformation along passive margins, associated with horizontal compression, has been discussed in many papers, in particular for the northern and central Atlantic regions during the Cenozoic. For instance, I have recently showed that the inversion of the Mesozoic rift structures of the Atlas region, leading to the formation of the Atlas orogen, occurred during the Oligocene – early Miocene and was accompanied by higher spreading rates in the central Atlantic, possibly driven by the Azores mantle plume. Similarly, it was suggested that the rapid northward motion of India during the late Cretaceous – Eocene was driven by the force exerted by the Reunion plume head. I will outline a new paradigm for the driving forces of plate tectonics, in which divergent flows in the asthenosphere, associated with mantle heterogeneities, are driving mechanisms as well. In this model plates are moved by the combined action of drag stresses exerted on the base of the lithosphere and far-field boundary forces (slab pull and ridge push). Bott, M.H.P., 1991. Tectonophysics, 200(1-3), 17-32.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11581/265396
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