Jean-Luc Epard

KINEMATIC MODEL OF DETACHMENT FOLDING INCLUDING LIMB ROTATION, FIXED HINGES AND LAYER-PARALLEL STRAIN

Abstract Detachment anticlines formed above a stratigraphically fixed detachment horizon and without thinning in the syncline will, in general, require layer-parallel shortening. A kinematic model for detachment folds is presented that has planar limbs and is area-balanced but requires bed-length changes that can be expressed as layer-parallel strain. The fold grows primarily by limb rotation, not by limb lengthening at constant dip. The kinematic evolution of the geometry and strain is examined. Fold geometry is well predicted and the strain estimated in applications of the model to two map-scale detachment folds; a triangle zone from the front of the Wyoming thrust belt and a duplex-cored anticline from the Appalachian Plateau.

The cover-basement contact beneath the Rawil axial depression (western Alps): true amplitude seismic processing, petrophysical properties, and modelling

Abstract Since 1986, several near-vertical seismic reflection profiles have been recorded in Switzerland in order to map the deep geologic structure of the Alps. One objective of this endeavour has been to determine the geometries of the autochthonous basement and of the external crystalline massifs, important elements for understanding the geodynamics of the Alpine orogeny. The PNR-20 seismic line W1, located in the Rawil depression of the western Swiss Alps, provides important information on this subject. It extends northward from the “Penninic front” across the Helvetic nappes to the Prealps. The crystalline massifs do not outcrop along this profile. Thus, the interpretation of “near-basement” reflections has to be constrained by down-dip projections of surface geology, “true amplitude” processing, rock physical property studies and modelling. 3-D seismic modelling has been used to evaluate the seismic response of two alternative down-dip projection models. To constrain the interpretation in the southern part of the profile, “true amplitude” processing has provided information on the strength of the reflections. Density and velocity measurements on core samples collected up-dip from the region of the seismic line have been used to evaluate reflection coefficients of typical lithologic boundaries in the region. The cover-basement contact itself is not a source of strong reflections, but strong reflections arise from within the overlaying metasedimentary cover sequence, allowing the geometry of the top of the basement to be determined on the basis of “near-basement” reflections. The front of the external crystalline massifs is shown to extend beneath the Prealps, about 6 km north of the expected position. A 2-D model whose seismic response shows reflection patterns very similar to the observed is proposed.

Structure, geometry and kinematics of the northern Adula nappe (Central Alps)

The eclogitic Adula nappe of the Central Alps (cantons Graubünden and Ticino, Switzerland) displays an exceptionally complex internal structure with the particularity of enclosing numerous slices of Mesozoic cover rocks (Internal Mesozoic) within the Palaeozoic gneiss basement. This study is principally based on detailed lithological and structural mapping of selected areas of the northern Adula nappe. Specific focus was placed on the Mesozoic slivers embedded in pre-Mesozoic basement (Internal Mesozoic). The most pervasive structures are related to the Zapport deformation phase that is responsible for the development of a fold-nappe and ubiquitous north-directed shear. Locally, the structures in the upper and frontal part of the nappe can be assigned to the older ductile Ursprung phase. These earlier structures are only compatible with top-to-S shear movement. The superposition of the Ursprung and Zapport phases is responsible for the north-dipping internal duplex-like structure and the sliced aspect of the Northern and Central Adula nappe. We conclude that the Adula nappe represents a major shear zone involving the entire nappe and responsible for the emplacement of the Lower Penninic sediments and the Middle Penninic nappes in the eastern part of the Lepontine Dome.

Computerized cross section balance and restoration

Abstract We describe how to make the necessary measurements and manipulations to balance and restore cross sections using the widely available programs Canvas ™, Photoshop ™, and Excel ™. Data or cross sections are input to Canvas from a flatbed scanner or a video camera, or sections are constructed using program drafting tools. Scanned photos and captured video are significantly improved using Photoshop filters. Lengths and areas are automatically measured with line-drawing tools in Canvas . Automation of measurement makes areadepth-strain relationship a powerful and practical tool for creating and testing balanced cross sections independent of kinematic model. An Excel procedure is given for calculating layer-parallel strain, fitting least-squares lines to area-depth data to find depth to detachment, and plotting the results. Restorations can be made in Canvas by rigid-block translation, rotation, and simple-shear distortion of images or drawings. We outline techniques for constant bed-length and oblique-shear restorations.

Spatial relation of surface faults and crustal seismicity: a first comparison in the region of Switzerland

The deformation pattern in active orogens is in general diffuse and distributed, and is expressed by spatially scattered seismicity and fault network. We select two relating datasets in the region encompassing Switzerland and analyse how they compare with each other. The datasets are not complete but are the best datasets currently available which fully cover the investigated area at a uniform scale. The distribution of distances from each earthquake to the nearest fault suggests that about two-thirds of the seismicity occurs near faults, yet about 10% occurs far from known faults. These numbers are stable for various selections of earthquakes and even when considering location uncertainties. Earthquake magnitudes in the catalogue are smaller than what could be expected from faults lengths. This suggests that the deep fracture pattern is more segmented than the superficial one, or mostly partial rupture during earthquakes, and (partly) the impropriety of the scaling law. Statistics on the distances from each fault to the nearest earthquake reveal that all supposedly-active faults in Switzerland have experienced a typically felt (magnitude 2.5 or larger) event, and only one out of six has not done so in the past four decades. Future applications of the presented approach to more complete or comprehensive fault databases may result in revised numbers regarding the connection between deep and superficial fracture patterns, representative of the stress regime of the region. The public and educational message: (1) in the region of Switzerland, earthquakes can happen in areas without known or mapped faults; (2) not all faults produce earthquakes within a human lifetime, but they seem to do so over long times.

Inventory of Rock Slope Deformations Affecting Folded Sedimentary Layers in Moderate Relief Context: The Case of the Livingstone Range Anticlinorium, AB, Canada

This contribution investigates the distribution of rock slope deformations (RSD) and their relationships with tectonic structures, lithological and topographic contexts. This is performed in a moderate relief context (1,100–2,500 masl.) affected by thin-skin tectonic. The proposed inventory, along the Livingstone range anticlinorium (LRA), contains 160 gravitational events that are spatially clustered in four areas. The thrust-folds properties (density, geometry and lithology) and the local relief appear to primary influence the development of RSD in this context.