Long-wavelength subsidence in the Andean broken foreland: Sublithospheric controls on the sedimentation and topography of the Sierras Pampeanas?

DÁVILA, F.M.; JORDAN, T.E.; ASTINI, R.A.

Barcelona

Congreso; 6 International Symposium of Andean Geodynamics; 2005

Considering that the SP was located above a flat slab segment at least since the Early Miocene (Dávila et al, 2004), the long-wavelength stratigraphic record and the uncommon thicknesses might be due to shallow subduction. Laramide subsidence has been partially explained by dynamic topography, when flat subduction occurred in western North America. Theoretically, during slab flattening the corner mantle wedge tends to shift toward the continent (eastward in the Andes), hence increasing the mantle flow in that direction, and creating an extra downwarping force that favors regional subsidence, not accounted for in previous flexural analysis in the SP and the Argentine Precordillera (eg., Cardozo and Jordan, 2001). Besides providing a logical interpretation for the stratigraphic record and deformation, dynamic topography would also explain elevations in the SP. The northern and southern SP differ, based on geological, geophysical and topographic characteristics. The northern SP has thicker Neogene units involved in deformation than the southern regions, and possesses higher altitudes of both peaks and valleys (Fig. 4). Both regions are isostatically unbalanced, but the northern SP presents evidence of isostatic “rebound” (Martínez and Giménez, 2003), with peaks >4-5 km and crustal thicknesses ~45 km. In contrast the southern SP seems to be pulled downward (Fromm et al., 2004), possessing altitudes that barely exceed 2 km, but crustal thickness that locally reaches 55 km (Fig. 3). The mean altitude in the valleys contrasts as well, >650 m in the northern SP and <500 m in the southern SP (Fig. 4). Dynamic topography modeling predicts that when subsidence diminishes the zone will recover progressively, favoring surface uplift. If we presume that flat subduction migrated southward as a propagation wave through the SP (cf. Yañez et al., 2001; Kay and Mpodozis, 2002), then during the early Miocene dynamic subsidence would have affected the northern portion of the SP producing extra subsidence and a great thickness of alluvial sequences. These sequences were exposed later, when the dynamic wave migrated to its present position in the southern SP, provoking the regional subsidence pattern recorded by seismic velocity studies (Fromm et al., 2004). This progression may also explain the differences in altitudes, not easily accounted for basement deformation. southern SP (Fig. 4). Dynamic topography modeling predicts that when subsidence diminishes the zone will recover progressively, favoring surface uplift. If we presume that flat subduction migrated southward as a propagation wave through the SP (cf. Yañez et al., 2001; Kay and Mpodozis, 2002), then during the early Miocene dynamic subsidence would have affected the northern portion of the SP producing extra subsidence and a great thickness of alluvial sequences. These sequences were exposed later, when the dynamic wave migrated to its present position in the southern SP, provoking the regional subsidence pattern recorded by seismic velocity studies (Fromm et al., 2004). This progression may also explain the differences in altitudes, not easily accounted for basement deformation.