Dating folding beyond folding, from layer-parallel shortening to fold tightening, using mesostructures: Lessons from the Apennines, Pyrenees and Rocky Mountains

Dating syntectonic sedimentary sequences is often seen as the unique way to constrain the initiation, kinematics and rate of folding and the sequence of deformation in the shallow crust. Beyond fold growth however, deformation mesostructures accommodate the internal shortening of pre-folding strata before, during and after strata tilting. Absolute dating of mesostructures developed during extension at fold hinge may help constrain the duration of fold growth in the absence of preserved growth strata, while dating of mesostructures related to layer-parallel shortening and late fold tightening provide a valuable access to the timing and duration of the entire folding event. We compile existing ages in the literature and provide new U-Pb ages of calcite cements from veins and faults from four folds (Apennines, Pyrenees, Rocky Mountains). Our results not only better constrain the timing of fold growth but also reveal a contraction preceding and following folding, the duration of which might be function of the tectonic style and regional sequence of deformation. This study paves the way for a better appraisal of folding lifetime and processes and of stress evolution in folded domains.

Strain variation associated with the development of a major recumbent curvilinear fold in the Dalradian rocks of NW Achill Island, western Ireland

ABSTRACTThe regionally metamorphosed, Riphean–Cambrian Argyll Group Dalradian rocks of NW Achill Island, western Ireland are disposed in a large-scale, regionally west-facing, tight, recumbent F2 curvilinear fold, with which two ductile shear zones are associated. Clasts in conglomerates within the Dalradian sequence that are deformed by the shear zones preserve evidence for a constrictional overprint of earlier plane strain as the fold became curvilinear, while stretched clasts maintained a constant orientation as the hinge curvilinearity developed. During the constrictional overprint a crenulation fabric, S2b, overprinted a penetrative foliation, S2a, in the shear zones. The S2b has an orientation that varies systematically with that of the fold hinge. It is inferred that, although the S2b surfaces initiated as a dip-slip fabric, there was an increasing degree of strike slip on these surfaces as the fold hinge approached parallelism with the direction of tectonic transport. It is possible that many curvilinear folds have an early history involving plane strain, but that increasing constrictional strain is intrinsic to the later stages of their development.

Influence of tectonic folding on rockfall susceptibility, American Fork Canyon, Utah, USA

We examine rockfall susceptibility of folded strata in the Sevier fold-thrust belt exposed in American Fork Canyon in north-central Utah. Large-scale geologic mapping, talus production data, rock-mass-quality measurements, and historical rockfall data indicate that rockfall susceptibility is correlated with limb dip and curvature of the folded, cliff-forming Mississippian limestones. On fold limbs, rockfall susceptibility increases as dip increases. This relation is controlled by several factors, including an increase in adverse dip conditions and apertures of discontinuities, and shearing by flexural slip during folding that has reduced the friction angles of discontinuities by smoothing surface asperities. Susceptibility is greater in fold hinge zones than on adjacent limbs primarily because there are greater numbers of discontinuities in hinge zones. We speculate that susceptibility increases in hinge zones as fold curvature becomes tighter.

Geology of slopes in the Crocker Range, Sabah, Malaysia

Slope failures are frequent occurrences along roads in Malaysia. Not until recently, geological inputs were rarely sought when designing and constructing roads on mountainous areas. This paper highlights the result of a geological study on selected slopes along a major road across Sabah's main mountain range, the Crocker Range, which is comprised mostly of folded Eocene sedimentary rocks. A total of 48 slopes facing potential failure problems were studied. The following four main potential sources of failures were recognised: 1) related to intensely sheared mudstones within a localised fault zone; 2) related to unfavourable orientation of discontinuity planes whereby bedding and joint planes of sandstone beds occur parallel or sub-parallel to the slope face; 3) related to the presence of intensely fractured and sheared sandstone and mudstone beds within a regional fold hinge; and 4) related to the presence of old landslide deposits. The recommendations to stabilise problematic slopes include covering the unstable slope face with concrete or vegetation and cutting back the slopes further.