INTENSIVE FOLD FLATTENING DURING QUASI-FLEXURAL FOLDING OF LIMESTONE PSEUDO-BEDS

Shortening of late Cretaceous/early Cenozoic pseudo-bedded limestone beds in the Pindos Belt, Greece, is partitioned into 1/ pre-thrust layer-parallel shortening and 2/ in-sequence thrust faulting and associated macro-folding. Pre-thrust shortening at outcrop scale exploited dissolution-creep mechanisms as displayed in layer-perpendicular tectonic stylolites, stylo-thrusting, and stylo-interpenetrations. Folding is quasi-flexural and includes intensive fold flattening. For sedimentary sequences subjected to non-metamorphic tectonic loading, intensive flattening in competent layers is seldom observed: folds become tight but typically do not evolve into isoclines, because faulting intervenes. Here, however, the intensive fold flattening is locally achieved through aggressive pressure dissolution. The obvious signature for the intensive flattening is isoclinal fold geometry, but another is the presence of markedly irregularly curved axial surfaces. Intensive fold flattening appears to have caused tight folds with planar axial surfaces to evolve into tighter, even isoclinal, folds with tortuous axial surfaces. The markedly curviplanar axial surfaces appear to have evolved from differential resistance to pressure dissolution. Parts of limbs and hinge zones of starting folds are pressed into contact against limbs and hinge zones of adjacent folds. Dissolution-loss of core regions of individual upright folds brings about limb rotations to near isoclinal geometries. In extreme cases, individual anticlines or synclines largely disappear, in such ways that two synclines move into agency without an intervening anticline, or two anticlines become directly juxtaposed through corruption and removal of the once-intervening syncline. Isogon methods introduced by John Ramsay can be adapted to illuminate the properties of these intensively flattened quasi-flexural folds. My regional-structure speculation is that transitions from one in-sequence thrust ‘event’ to the next may have been marked by punctuated shortening intervals marked by intensive flattening of earlier-formed buckle folds. The flattening tightens thrust horses to full measure. Testing this notion will require innovations in absolute dating of products of dissolution creep.