Sedimentary and volcano-tectonic processes in the British Paleocene Igneous Province: a review

Research on the British Paleocene Igneous Province (BPIP) has historically focused on the emplacement, chemistry and chronology of its elaborate central intrusive complexes and lava fields. However, the BPIP has also been dramatically shaped by numerous erosion, sedimentation and volcano-tectonic events, the significance of which becomes ever clearer as localities in the BPIP are re-investigated and our understanding of volcano-sedimentary processes advances. The resultant deposits provide important palaeo-environmental, palaeo-geographical and stratigraphical information, and highlight the wide range of processes and events that occur in ancient volcanic settings such as the BPIP. In this paper we review the sedimentary and volcano-tectonic processes that can be distinguished in the BPIP, and conceptualize them within a generalized framework model. We identify, and describe, the sedimentary responses to four broadly chronological stages in the history of the BPIP volcanoes: (1) the development of the lava fields, (2) early intrusion-induced uplift, (3) caldera collapse and (4) post-volcano denudation and exhumation of central complexes. We highlight and illustrate the range of sedimentary processes that were active in the BPIP. These operated on and helped shape a dynamic landscape of uplands and lowlands, of alluvial fans, braided rivers, lakes and swamps, and of volcanoes torn apart by catastrophic mass wasting events and/or caldera collapse

PFC2D modelling of sinkhole cluster in karstic depressions

Djamil Al-Halbouni, Sacha Emam, Eoghan P. Holoan, Abbas Taheri, Martin P.J. Schöpfer & Torsten Dah

Mechanical properties of quartz sand and gypsum powder (plaster) mixtures: 25implications for laboratory model analoguesfor the Earth’s upper crust

Granular materials are a useful analogue for the Earth’s crust in laboratory models of deformation. Constraining their mechanical properties is critical for such model’s scaling and interpretation. Much information exists about monomineralic granular materials, such as quartz sand, but the mechanical characteristics of bimineralic mixtures, such as commonly-used quartz sand mixed with gypsum powder (i.e. plaster), are largely unconstrained. We used several mechanical tests (density, tensile, extension, shear) to constrain the failure envelope of various sand-plaster mixtures. We then fitted linear Coulomb and parabolic Griffith failure criteria to obtain cohesions and friction coefficients. Tests of the effects of emplacement technique, compaction and humidity demonstrated that the most reproducible rheology is given by oven-drying, pouring and mechanically compacting sand-plaster mixtures into their experimentation container. As plaster content increases, the tensile strength of dry sand-plaster mixtures increases from near zero (pure quartz sand) to 166±24 Pa (pure plaster). The cohesion increases from near zero to 250±21 Pa. The friction coefficient varies from 0.54±0.08 (sand) to 0.96±0.08 (20 weight% plaster). The mechanical behaviour of the resulting mixtures shifts at 20-35 weight% plaster from brittle Coulomb failure along a linear failure criterion, to more complex brittle-ductile Coulomb-Griffith failure along a non-linear failure criterion. With increasing plaster content, the brittle-ductile transition occurs at decreasing depth within a pile of sand-plaster mixture. We infer that the identified transitions in mechanical behaviour with increasing plaster content relate to (1) increasing porosities, (2) increasing grain size distributions, and (3) a decrease in sand-sand grain contacts and corresponding increase in gypsum-gypsum grain contacts. The presented characterisation enables a more quantitative scaling of the mechanical behaviour of sand-plaster mixtures, including of their tensile strength. Sand-plaster mixtures can thereby realistically simulate brittle-ductile properties of the Earth’s crust in scaled laboratory models

A sagging-spreading continuum of large volcano structure

International audienceGravitational deformation strongly infl uences the structure and eruptive behavior of large volcanoes. Using scaled analog models, we characterize a range of structural architectures produced by volcano sagging and volcano spreading. These arise from the interplay of variable basement rigidity and volcano-basement (de-)coupling. From comparison to volcanoes on Earth (La Réunion and Hawaii) and Mars (Elysium and Olympus Montes), the models highlight a structural continuum in which large volcanoes throughout the Solar System lie