Caroline Mehl

Sub-seismic fractures in foreland fold and thrust belts: insight from the Lurestan Province, Zagros Mountains, Iran

ABSTRACT The Simply Folded Belt of the Zagros Mountains, Iran, is a spectacularly well-exposed example of a foreland fold and thrust belt. A regional analysis of the Cenomanian–Coniacian Sarvak and Ilam Formations, exposed in the southern Lurestan Province, is presented as a case study for sub-seismic fracture development in this type of compressive setting. The area is characterized by gentle to tight anticlines and synclines parallel to the NW–SE trend of the belt. In the Lurestan Province, the Cenomanian–Coniacian interval is exposed in the core of most of the outcropping anticlines. Fold style is intimately related to both vertical and lateral facies distribution. Geometry, kinematics and timing of sub-seismic fractures were characterized through extensive fieldwork, interpretation of orthorectified QuickBird imagery and interpretation of 3D photorealistic models derived from LiDAR. Data were collected from 12 anticlines covering an area of approximately 150 × 200 km. Key outcrops for understanding fracture geometry, kinematics and timing are presented. Field observations and interpretation of QuickBird and 3D photorealistic models reveal the complexity of fracture geometry and timing. Fractures record pre-, syn- and post-folding stages of deformation. Pre-folding structures include synsedimentary normal faults, and subsequent small-scale thrusts, systematic veins and stylolites. During folding, pre-existing fracture planes were re-activated and through-going fractures and reverse faults developed. Strike-slip faults typically postdate pre- and syn-folding structures and are probably related to the late stages of fold tightening. All structures are geometrically and kinematically consistent with the trend of the Arabian passive margin and its subsequent tectonic inversion.

Structural evolution of Andros (Cyclades, Greece): a key to the behaviour of a (flat) detachment within an extending continental crust

Abstract The continental crust extends in a brittle manner in its upper part and in more distributed (ductile) manner in its lower part. During exhumation of HP metamorphic rocks, brittle features superimpose on earlier ductile ones as a result of the progressive localization of deformation. The islands of Tinos and Andros are part of the numerous metamorphic core complexes exhumed in the Aegean domain. They illustrate two steps of a gradient of finite extension along a transect between Mt. Olympos and Naxos. This study confirms the main role of boudinage as an initial localizing factor at the brittle–ductile transition and emphasizes the continuum of strain from ductile to brittle during exhumation. Early low-angle semi-brittle shear planes superimpose onto precursory ductile shear bands, whereas steeply dipping late brittle planes develop by progressive steepening of structures or sliding across en echelon arrays of veins. The comparison between Tinos and Andros allows us to propose a complete dynamic section of the Aegean extending continental crust and emphasizes that the strain localization process depends on both its rheological stratification and its compositional heterogeneity.

Stress-strain rate history of a midcrustal shear zone and the onset of brittle deformation inferred from quartz recrystallized grain size

Abstract The quantification of quartz shear stress and strain rate within a midcrustal shear zone provides a mechanical frame to describe the evolution from penetrative ductile deformation to localized deformation and the onset of brittle deformation. The quantification is based on the relationships between the quartz recrystallized grain size, the quartz shear stress (piezometric relation) and the strain rate (dislocation creep flow law). Increasing strain is accompanied by a general decrease of quartz recrystallized grain size and a decrease in grain size scattering. These are interpreted as a result of a complex loading history. The evolution from penetrative ductile deformation toward strain localization, marked by an increase of the strain rate by one order of magnitude, is inferred from grain size memory. Brittle deformation is triggered for quartz shear stress of the order of 70 MPa and strain rate close to 10−12 s−1. This relative low value of the quartz shear stress necessary to trigger faulting implies a less important strength for the midcrust compared with strengths predicted by classical rheological envelopes.

Resistivity Modelling: Handling Source Singularity and Topography within a Single Step

The singularity of the potential occurring at the source location is a key point of electrical resistivity forward modelling because it might lead to large numerical errors. To tackle this problem a classical method consists of splitting the total potential into a primary part containing the singularity and a secondary part. The primary potential is defined analytically for flat topography but requires numerical computation in the presence of topography. In that case, an accurate solution happens to be computationally expensive. For any geometry we propose to keep for the primary potential the analytic solution defined for homogeneous models and flat topography, and to modify accordingly the free surface boundary conditions for the secondary potential. The primary potential still contains the singularity and new free surface conditions ensure that the total potential still satisfies the Poisson equation. The modified singularity removal technique thus remains fully efficient even in the presence of topography, without additional numerical computation. The modified secondary potential in a homogeneous model is not null in the case of topography as it would be in the classical approach. We implement the approach with a Finite Difference method. We present potential distributions computed with this technique to illustrate its versatility.

Resistivity Modelling with Topography

A major difficulty of electrical resistivity forward modelling is caused by the singularity of the potential occurring at the source location. To avoid large numerical errors, the total potential is split into a primary part containing the singularity and a secondary part. The primary potential is defined analytically for flat topography, but is classically computed numerically in the presence of topography: in that case, an accurate solution requires expensive computations. We propose to select for the primary potential the analytic solution defined for homogeneous models and flat topography, and to modify accordingly the free surface boundary condition for the secondary potential, such that the total potential still satisfies the Poisson equation. The modified singularity removal technique thus remains fully efficient even in the presence of topography, without any additional numerical computation. The modified secondary potential in a homogeneous model is not null in the case of topography as it would be in the classical approach. We implement the approach with the Generalized Finite Difference method. We present a 2.5D inversion example on a simple synthetic data set.