Post-Miocene topographic rejuvenation in an elevated passive continental margin not characterized by a sharp escarpment (northern end of the Mantiqueira Range, Brazil)

Abstract

A sharp escarpment lying close to the coast or more in the continental interior defines the morphology of Elevated Passive Continental Margins (EPCMs). Most studies on the dynamics and evolution of EPCMs, including numerical modeling exercises, concentrate on the geometry and evolution of escarpments located at or near a continental drainage divide. However, topographic relief varies considerably along the length of an EPCM, as one can observe in southeastern Brazil and southeastern Australia, and in some cases, the steep, wall-like escarpment may not be present. Here we investigate the post-rift geomorphic evolution in an EPCM lacking a sharp escarpment (southeastern Brazil), focusing on exploring topographic rejuvenation and links with potential controls such as the arrangement of pre-existing structures and lithological variability. We present topographic data showing a regional zone of high channel steepness and local relief extending continuously through the middle part of all seaward-dipping catchments. In this regional “belt” of high topography lies many knickpoints, and all rivers crossing it are marked by non-linear shapes in elevation-χ space. The structural data show a large number of NE-SW and NW-SE oriented brittle structures with slickensides recorded on filling materials following the orientation of pre-existing basement structures, suggesting a multiphase brittle deformation with at least two main paleostress tectonic regimes between the Miocene and Pleistocene. Our results indicate post-Miocene topographic rejuvenation and brittle deformation, probably driven by the accumulation of far-field stresses in pre-existing weakness zones. Nevertheless, the origin of the regional drainage divide and adjacent areas, located considerably upstream of the regional knickzone, must predate the topographic resurgence we infer, implying that older and more recent topographic elements coexist in the landscape. Our study demonstrates that post-rift topographic rejuvenation (and brittle deformation) in an EPCM is not necessarily linked to the formation or persistence of a “Great Escarpment”.