13th International Congress of the Brazilian Geophysical Society & EXPOGEF, Rio de Janeiro, Brazil, 26–29 August 2013, 2013
Ultra-deep reflection seismic lines (American COCORP, diverse European projects such as ECORS, so... more Ultra-deep reflection seismic lines (American COCORP, diverse European projects such as ECORS, some industrial surveys such as those from ION-GXT) have conspicuously shown that the lower part of the continental crust is highly reflective. Strong, short, discontinuous, sub-horizontal, wavy reflections are characteristic, imparting an undulating highly reflective pattern to the lower continental crust. Most of the times, the Moho is interpreted at the base of such reflections, at the boundary with the seismically transparent upper mantle. The other important crustal discontinuity, the Conrad, is usually interpreted at the top of such reflectors, at the boundary with the seismically transparent upper crust. In this manner, ultra deep seismic sections usually display the lower continental crust as a strongly reflective wavy layer of varying thickness sandwiched between the transparent upper crust and the transparent subcontinental upper mantle. The reasons for such high reflectivity include the development of abundant subhorizontal ductile shear zones and the dominant subhorizontal foliation so characteristic of exposed highgrade metamorphic rocks of the lower crust. Introduction and Discussion Since the start of the recording of ultra-deep reflection seismic lines, first in Europe, later in the USA, it has been recognized that the lower crust is characterized by an anomalous high reflectivity when compared to the seismically transparent upper continental crust and mantle. Strong, short, discontinuous and undulating reflections form a wavy band of variable thickness at the base of the continental crust in most ultra-deep seismic sections that imaged several locations around the globe (Figure 1). Interestingly, this is exceptionally visible at places where the crust has undergone stretching and thinning due to extensional stresses in a breaking continent. Examples of this are known from the Rhine Graben in Europe (Figure 2), in the Basin and Range Province of Western USA and in the passive margins of the South Atlantic Ocean (Figure 1). In most cases, the Moho does not appear as a single, strong reflector. Usually its position is inferred at the base of the lower crust short and strong reflections. Below them the mantle is practically devoid of seismic reflections (Figures 1 and 2). The Conrad discontinuity, that marks a significant increase in the velocity of the compressional seismic waves, broadly coincides with the top of the reflective lower crust (Figure 2); but, interestingly enough, it appears much more frequently as a well marked seismic reflection than the Moho (Zalan et al., 2009). On the other hand, the thinner oceanic crust does not show such reflectivity layering throughout its 7 to 11 kilometers of thickness. Well exposed outcrops of obducted oceanic crust, such as those in the Oman Mountains, and high resolution seismic sections, point to a tripartite division of this type of crust consisting of a thin, weakly reflective, lower layer of banded gabbros, a thick middle layer composed of criss-crossing sheeted dykes and an upper seismically transparent layer of pillow lavas (Zalan et al., 2011). The sub-oceanic Moho, contrary to the sub-continental Moho, is almost invariably displayed as a discrete strong continuous to discontinuous seismic reflection. These distinct seismic behaviors between the continental and oceanic crusts point to differences in composition and depths of occurrence; thus, differences in confining pressure and temperature. All these converge to suggest that strong differences in the rheology of both types of crust may be responsible for their characteristic seismic response. The reason for such strong, sub-horizontal reflectivity of the lower continental crust has been historically attributed to ductile sub-horizontal shear zones, developed in response to the change in the rheology of quartz and feldspar, the two most abundant minerals in the continental crust, from brittle to ductile behavior. Their brittle friction and plastic flow laws indicate a change in rheology below the depths of 10-12 km for quartz and 2030 km for feldspars (Figure 3). So, above such depths the continental rocks tend to behave in an elastic manner, displaying mostly brittle deformation. Below these depths, the behavior is predominantly plastic, dominated by ductile deformation. The concept of sub-horizontal shear zones dominating the lower continental crust was derived from the exposures of stretched lower crustal rocks exhumed in the metamorphic core complexes of the Basin and Range Province of the Western USA (Figure 4) (Wernicke and Burchfiel, 1982). Recent AFTA studies of uplifted passive continental margins have shown tremendous amounts of rising and
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