The nature and regional significance of structures in the Gala Group of the Southern Uplands terrane, Berwickshire coast, southeastern Scotland

Structures deforming Llandovery turbidites of the Gala Group in the Southern Uplands terrane are spectacularly exposed in the Berwickshire coastal section, southeastern Scotland. The upward-facing, upright to NW-vergent folds and associated structures appear to record a single regional phase of subhorizontal NW-SE contractional deformation, with a steeply dipping direction of bulk finite extension. These structures are markedly different from those developed in rocks correlated with the Upper Llandovery Hawick Group exposed some 5 km to the south in the Eyemouth-Burnmouth coastal section. Here a highly domainal system of sinistral transpressional strain occurs, with zones of steeply plunging curvilinear folds, clockwise cleavage transection and bedding-parallel sinistral detachment faults. The markedly different bulk strain patterns in the Berwickshire coastal sections are thought to reflect the regionally diachronous nature of transpressional deformation in the Southern Uplands terrane. There are striking similarities in the structures recognized in the Berwickshire coastal sections and those developed in stratigraphically equivalent units along strike in southwestern Scotland and Northern Ireland. This confirms the lateral structural continuity and correlation of tracts and tract boundaries along the entire length of the Southern Uplands terrane. The regional structure suggests that a phase of top-to-the-NW backtbrusting and backfolding associated with the southern margin of the Gala Group outcrop marks the transition from orthogonal contraction to sinistral transpression in the Southern Upland thrust wedge during late Llandovery times

Growth and dissection of a fold and thrust belt: the geological record of the High Agri Valley, Italy

We present a 130 km2 wide geological map for the NE side of the fault-bounded High Agri Valley Southern Italy, that formed in the Quaternary in response to extensional tectonics dissecting the folds and thrusts of the Lucanian Apennine. To prepare the map, at 1:25,000 scale, we integrated information obtained through field surveys and the review of pre-existing geological data. Our work describes a number of significant map-scale structures, which can be related to well-constrained tectonic episodes. The new geological map provides important constraints that can be used to distinguish ancient structures from those that were active during the Quaternary, allowing a more detailed reconstruction of the processes that operate during the development of a post-orogenic trough. We expect that the new map will be used for different types of geological investigations, including studies of inversion tectonics, active tectonics, geosite mapping, 3D modelling of geological structures. © 2020, © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of Journal of Maps

Fluid-assisted brecciation of Lower Cretaceous Maiolica limestone in the Umbria-Marche Apennines: Hydrodynamical implications

The formation of the “expansion breccia” observed in the Lower Cretaceous Maiolica limestone in the Umbria-Marches region of Italy is attributable to a fluidassisted brecciation process that occurred during the late Miocene exhumation of the Northern Apennines. The hydrothermal fluids probably originated as brine solutions trapped in the Burano anhydrite while it was in a plastic state. The migration of the Burano from the plastic to the brittle domain during unroofing resulted in liberation and injection of over-pressured hydrothermal fluids into the overlying limestone, causing hydraulic fracturing. Mapping of breccia morphology along a 400-m transect showed structures produced by different flow regimes, with chaotic and mosaic breccia characterizing the core parts of the section and mineral-filled fractures and veins in the margins. Based on the clast size in the chaotic breccia, the estimated velocities for fluidizing the aggregates of clasts and sustaining the clasts in suspension are, respectively, 15 cm/s and 65 cm/s. Crack growth was probably the main mechanism for the fragmentation of the limestone. Explosion fracturing patterns were only sporadically observed in the breccia, indicating substantial heat loss of the over-pressured fluids during their ascent to the Earth’s surface

The interdisciplinary use of “overpressure”

Overpressure is a polysemic word that has a variety of meanings within and across different disciplines. This is likely to be a particular problem in analysis of geothermal resources, where reservoir engineers, volcanologists and structural geologists may each confidently use overpressure but mean different things. We suggest that, to avoid confusion, the term should be carefully and accurately defined whenever used, and ideally only used to mean fluid pressure in excess of hydrostatic pressure

Stress rotations and the long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment

International audienceWe used a field analysis of rock deformation microstructures and mesostructures to reconstructthe long-term orientation of stresses around two major active fault systems in Japan, the Median TectonicLine and the Rokko-Awaji Segment. Our study reveals that the dextral slip of the two fault systems, activesince the Plio-Quaternary, was preceded by fault normal extension in the Miocene and sinistral wrenching inthe Paleogene. The two fault systems deviated the regional stress field at the kilometer scale in their vicinityduring each of the three tectonic regimes. The largest deviation, found in the Plio-Quaternary, is a more faultnormal rotation of the maximum horizontal stress to an angle of 79° with the fault strands, suggesting anextremely low shear stress on the Median Tectonic Line and the Rokko-Awaji Segment. Possible causes of thislong-term stress perturbation include a nearly total release of shear stress during earthquakes, a low staticfriction coefficient, or lowelastic properties of the fault zones comparedwith the country rock. Independently ofthe preferred interpretation, the nearly fault normal orientation of the direction of maximum compressionsuggests that the mechanical properties of the fault zones are inadequate for the buildup of a pore fluidpressure sufficiently elevated to activate slip. The long-term weakness of the Median Tectonic Line and theRokko-Awaji Segment may reside in low-friction/low-elasticity materials or dynamic weakening rather than inpreearthquake fluid overpressures

Distinguishing rift-related from inversion-related anticlines: Observations from the Abu Gharadig and Gindi Basins, Western Desert, Egypt

Distinguishing the tectonic origin of anticlinal structures is problematic in regions with a complex history of rifting and inversion. We present the results of seismic mapping, in the form of time-depth (isochron) and time-thickness maps to characterize how sedimentary thickness differentials evolved in response to normal faulting and to inversion events on faults within the Abu Gharadig and Gindi Basins in the Western Desert of Egypt. Late Cretaceous rift-related faults in the Abu Gharadig Basin strike NW-SE, W-E and SW-NE. In the eastern part of the basin, a prominent SW-NE trending interbasinal saddle formed in response to preferential subsidence forming half-grabens to its north-west and southeast, during the Mid-Turonian to Santonian interval. Santonian to Palaeogene inversion in the Abu Gharadig Basin developed on its northern basin margin, the absence of SW-NE striking faults in the eastern central basin resulting in any inversion effects being minor. In the central Gindi Basin, Upper Cenomanian to Lower Turonian SW-NE striking rift faults underwent inversion as early as the Mid-Turonian. The orientation of existing rift faults and modification of the local stress fields control the extent to which inversion was taken up in each basin trough time. The Abu Gharadig and Gindi Basins are two of the rift basins developed in West and Central Africa that underwent rifting, inversion and dextral shearing during the Late Cretaceous. We emphasize the value of high-resolution stratigraphic mapping to characterize short-lived and subtle pop-up events that may have gone unnoticed

Late Cretaceous–earliest Paleogene deformation in the Longmen Shan fold-and-thrust belt, eastern Tibetan Plateau margin: pre-Cenozoic thickened crust?

This study presents structural and 40Ar/39Ar geochronological data from the southern part of the Longmen Shan fold-and-thrust belt that forms the eastern margin of the Tibetan Plateau. Investigations focused on hinterland ductile top-to-the-WNW shear deformation, which has been linked previously to late Cenozoic lower crustal flow. Consistent with previous studies, the sense of deformation is mapped as top-to-the-WNW in the Longmen Shan hinterland. The timing of the deformation is constrained by 40Ar/39Ar geochronological data of recrystallized minerals aligned along the shear foliation as Late Cretaceous–earliest Paleogene, thus predating the inferred late Cenozoic crustal flow. This deformation is contemporaneous with SE verging thrusting and loading along the Longmen Shan front, which formed a coeval ~2–3 km thick foredeep sequence along the southwestern margin of the Sichuan Basin. In the context of the regional geology, this tectonic configuration could result from either extrusion of a crustal wedge or back thrust in a duplex. Compared to other orogens, where similar crustal configurations have been reported, it is speculated that the eastern Tibetan Plateau margin acquired thickened crust and highly elevated topography in Late Cretaceous–earliest Paleogene time