Foliation Intersection Research Articles

Survival Conditions of Folding in Different Depth During Orogenesis‐Deformation in Texas Creek and Chester Dome, USA

AbstractA succession of 5 FIA trends (foliation intersection or inflection axes in porphyroblasts) preserved in high temperature‐low pressure regime PreCambrian rocks in the Texas Creek, Arkansas River region reflected by the fold axial plane traces and schistosity data in this region. Similar fold axial plane trace data measured in Palaeozoic rocks in Chester Dome, Vermont, which is high temperature to medium pressure regime, only preserve the effects of the youngest FIAs of the all 5 FIA sets that obtained in this region. The other three FIA sets have no equivalent fold axial planes. This difference from shallow to deeper orogenic regimes reflects decreasing competency at greater pressure with collapse and unfolding of earlier formed folds. The greater overlying load of rocks has tended to flatten all but the very largest early‐formed structures, preserving only those folds that were more recently developed.

Porphyroblast rotation versus nonrotation: Conflict resolution!

Data on foliation intersection and/or inflection axes preserved in porphyroblasts (FIAs) indicate that no rotation occurs during ductile deformation relative to spatial coordinates. This contrasts with 99% of investigations of rigid objects in non-coaxially deforming media where the objects rotate. When anastomosing shear zone formation around relatively strong objects in a weaker matrix is modeled, no porphyroblast rotation occurs. Formation of these anastomosing zones controls the development of this phenomenon, called gyrostasis. If such zones are absent, porphyroblasts rotate. In weak materials the gyrostatic situation arises because the superposition of simple shearing deformation normal to initial coaxial shortening results in only small rotations of principal axes of stress. Since shear zones are controlled by the orientations of principal axes of stress, initial anastomosing zones retain their orientations and positions during subsequent non-coaxial deformation. The porphyroblast is isolated from the embedding non-coaxially deforming matrix, but this material close to the continues to deform coaxially; no local rotation occurs. This has major tectonic significance because, allowing for the effects of rotation due to brittle deformation, porphyroblasts can now be routinely used to access lengthy structural and/or metamorphic histories destroyed in the matrix by reactivation such as movement directions, shear senses, and extended pressure-temperature-time paths.