"Collaborative Research: Milankovitch-Scale Time Resolution in Growth Strata to Determine Rates and Kinematics of Folding"

Lehigh University, Bethlehem PA

Investigators

Abstract

Using sedimentary cycles generated from orbitally forced climatic changes (~20,000-400,000 year duration Milankovitch rhythms), the PI's seek to elucidate the complex kinematics of decollement folds, refine methodological approaches for evaluating growth structures, and determine high-resolution deformation chronologies with a multidisciplinary structural, stratigraphic, and magnetic study. Decollement folds are an important structural style in most orogenic belts and dominate where strong competency contrasts and thick, weak layers characterize deformed stratigraphic sequences. The interpretation, evaluation, and reconstruction of tectonic structures requires an understanding of kinematics and geometry alone cannot constrain the distribution of deformation, fold kinematics, or fold mechanics. Growth structures provide a unique opportunity to deconvolute the deformational and depositional processes of orogenic landscapes. The PI's propose to evaluate spatial variations in deformation patterns as a function of structural geometry, lithology, and stratigraphic position throughout two well-exposed anticlinoria; one at Sierra del Fraile, northeastern Mexico and another in the central External Sierra, north central Spain. Both field areas contain large-scale, evaporite-cored, synsedimentary decollement folds. Prolonged fold growth occurred in varying water depths and depositional environments at each site and were recorded by a variety of unconformity geometries and changes in growth strata thickness. These relationships will allow comparison of the complex interaction between deformational topography and depositional processes and a means to evaluate kinematic controls on growth strata geometries. The PI's will map and sample rocks for magnetic susceptibility, anisotropy of magnetic susceptibility (AMS), and calcite twinning measurements. AMS measurements in pregrowth strata will be used as a proxy for deformation fabric and will be augmented by twinning studies to constrain fold kinematics. Growth strata geometry will be used to distinguish between fixed and migrating axial surfaces as a fully independent means of assessing fold kinematics. Milankovitch-scale correlation of magnetic susceptibility variations and absolute ages will be used to establish a chronostratigraphy allowing for ~20,000 year temporal control on depositional and deformational rates within both the Sierra del Fraile and External Sierra growth strata. Reconnaissance investigations in both field areas provided pilot data confirming unparalleled opportunities to generate high-resolution spatial and temporal analysis of fold kinematics. The combination of excellent exposures of climatically controlled cyclic stratigraphy and synsedimentary deformation in the two field areas provide an exceptional opportunity to determine the kinematics of large-scale decollement folding, an important structural style in most orogenic belts.