Unthinkable Physical Analogs for the Modern Concepts on Continental Stretching and Rupturing
Zalán, Pedro V.
In the last five years geoscientists became familiar with very deep seismic sections at continental margins that display the complete structure of the crust and image further deep into the mantle. By interpreting these recent data obtained mostly in passive margins it was possible to recognize some tectonic features in the continental crust like the Moho (compositional) and Conrad (rheological) discontinuities, and, an overall tapering profile going from the original crust, through stretched, then thinned, and, finally, into hyper-extended crust. This taper profile may sometimes show irregularities due to necking of the crust adjacent to strong resistant continental ribbons or to mantle diapirs. A boudinage crustal profile is frequently observed. The continental-oceanic boundary (COB) may be represented by exhumed mantle in magma-poor margins or by transitional crust and seaward-dipping reflectors (SDR's) in volcanic margins. All these structures have been reported in recent models dealing with new ideas on how mega continents rupture and break apart. Well sampled homologous continental margins such as Iberia-Newfoundland and intensively studied exhumed tethyan continental margins high in the Alps provided the basis for the development of such revolutionary new models. Recently, studies performed on the continental margins of the South Atlantic confirmed the main aspects outlined in these models.
As these studies progressed, geologists started to physically model the rupturing and breaking of materials possessing a rheological behavior close to the upper lithosphere; that is, the jelly sandwich model constituted by an upper brittle crust, a lower ductile crust and an upper rigid mantle. Some of these physical models employed very common substances, easily found on our everyday routine and that would normally neither be considered for a scientific experiment nor be expected to replicate the deformation of mega continents. This work presents some of the modeling performed with such unthinkable physical analogs. But not all analogs are made up of such scientifically unexpected components. "Normal" rocks may display mesoscale deformations that closely resemble the deep structure of magma-poor passive margins. All these analogs will be compared with ultra-deep seismic lines shot on both sides of the South Atlantic Ocean.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013