Extensional stresses in the Colombian Eastern Cordillera fold-and-thrust belt (northern Andes): insights from 2D finite element modeling

Deformation and stress characteristics in the upper crust of the fold-and-thrust belt in the Colombian Eastern Cordillera were investigated by numerical analysis. The structural trend of Colombian Eastern Cordillera has long been considered a possible example of a true contraction orogen. The issue of the convergent displacement along an elastic structural body, which controls present-day deformation in this Cordillera is examined here. Modeling results are presented in terms of three parameters: 1) distributions, orientations, and magnitudes of principal stresses; 2) maximum shear stress (_max) contour; and 3) proximity to failure of elements within faults. Mohr-Coulomb failure criteria with bulk rock properties are applied to analyze the faults. The model shows extensional stresses in the crust at shallow crustal levels (from surface to about 6 km) despite overall contraction, and contraction at depth is confirmed. Measurement results indicate that, for homogeneous crustal thickening, extensional stresses are concentrated in the Servita half-gaben and Bucaramanga fault systems, where the vertical thrust faults and thickening processes are located. Our two-dimensional (2D) modeling results emphasize that extensional stresses are still active along the vertical to sub-vertical fault system in the fold-and-thrust belt of the Colombian Eastern Cordillera

Numerical fault simulation in Himalayas with 2 D finite element method

The nature of the stress field in the Himalaya is examined by the 2D finite element method where linear elastic rheology and plain strain condition are assumed. The Mohr-Coulomb failure criterion has been adopted to analyze the relationship between stress distribution and fault formation.Two profile models are prepared and convergent displacement is imposed on them along the NE-SW horizontal direction.The convergent displacement and physical properties of the rock layer control the distribution,orientation,magnitude and intensity of the stress and fault development.According to the calculated stress pattern,thrust faults are expected to develop in the central Himalaya (model A).Normal and some thrust faults take place in the north-western Himalaya (model B).The results from our numerical experiment are in agreement with those from the seismicity and focal mechanism solution of earthquakes and also with those of M.M.Alam and D.Hayashi (Bull.Fac.Sci.Univ. Ryukyus, 73, 15, 2002) in the central Himalaya

Geology of Western Nepal and a Comparison with Kumaun, India

Geologic structure of western Nepal has two tectonic systems, one being the Himalayan gneiss zone and its klippes of allochthonous nature and the other being the Midland meta-sediment zone of autochthonous or parautochthonous nature; the former is thrust over the latter along a salient tectonic zone called the Main Central Thrust zone. The Himalayan geneisses composed of high grade metamorphic rocks of the Barrovian type have a monotonous structure dipping gently northward. Meanwhile, the Chakhure-Mabu crystallines, which make up a klippe on the Midland meta-sediments, have been derived from the root of the Himalayan gneisses in the north. It is considered that the Jajarkot crystallines also form another klippe derived from the Main Central thrust zone in the north. The Midland meta-sediments show an anticlinorium structure composed of a thick sequence of platform-type sediments and are divided into five formations. The lower part of them is composed of the calcareous Ila Formation with stromatolite and the quartzite-rich Dali Formation, both of which are respectively correlative with the upper calcareous Subgroup and the middle siliceous Subgroup of the Midland meta-sediment Group of central Nepal. The Midland meta-sediments of western Nepal is considered to range in age from Riphean to Eocambrian despite of a possibility that the upper part would be of Palaeozoic in age. The Main Central Thrust zone is a profound tectonic zone characterized by intensely mylonitized rocks accompanied with characteristically blastomylonitic "augen" gneiss and garnet-mica-chlorite phyllitic schist. Geology and tectonics of western Nepal are correlated very well with those of eastern Kumaun of India immediately adjacent to the west

Extensional stresses in the Colombian Eastern Cordillera fold-and-thrust belt (northern Andes): insights from 2D finite element modeling

Deformation and stress characteristics in the upper crust of the fold-and-thrust belt in the Colombian Eastern Cordillera were investigated by numerical analysis. The structural trend of Colombian Eastern Cordillera has long been considered a possible example of a true contraction orogen. The issue of the convergent displacement along an elastic structural body, which controls present-day deformation in this Cordillera is examined here. Modeling results are presented in terms of three parameters: 1) distributions, orientations, and magnitudes of principal stresses; 2) maximum shear stress (_max) contour; and 3) proximity to failure of elements within faults. Mohr-Coulomb failure criteria with bulk rock properties are applied to analyze the faults. The model shows extensional stresses in the crust at shallow crustal levels (from surface to about 6 km) despite overall contraction, and contraction at depth is confirmed. Measurement results indicate that, for homogeneous crustal thickening, extensional stresses are concentrated in the Servita half-gaben and Bucaramanga fault systems, where the vertical thrust faults and thickening processes are located. Our two-dimensional (2D) modeling results emphasize that extensional stresses are still active along the vertical to sub-vertical fault system in the fold-and-thrust belt of the Colombian Eastern Cordillera

FEM simulation of fold-and-thrust belts in the South Central High Andes of Chile and Argentina

FEM analysis for stress determination of fold-and-thrust belts of the South Central High Andes (SCRA) was carried out on Paleozoic to Quaternary rocks from the Coastal Cordillera to Precordillera regions of the Chile and Argentinean Andes, between 30^o and 33^o south latitudes. A two-dimensional vertical cross-section including 8 layers through the Andean crust up to Moho has been represented by a finite element model composed of an assembly of 2699 elements and 1456 nodes in a state of plane strain conditions. The model is assumed to be multilayered lithospheric crustal block undergoing convergence. and we choose to demonstrate our results by applying horizontal displacement rate (average velocity of 6.50 cm/yr) of descending Nazca plate. The failure of elements is defined by adopting the concept of Mohr-Coulomb failure criterion. The assigned rock rheology and physical properties of each layer in simulation make the model behavior as elastic body in which well defined failure location gives a hint of thrust development. Studies of lithospheric deformation of the fold-and-thrust belts in this zone where structural style varies from thinskinned to thick-skinned fold-and-thrust belts have revealed various compressional states of stress. The maximum compressional stress (σ_1) was generally oriented in an E-W direction. Along the westernmost part of the Coastal Cordillera, the strain deformation is extensional in some elements. which can be explained by a co-seismic crustal bending readjustment. Overall study results imply that most of the basement-involved deep thrusts (about 20 km) and deformation. occurred within the upper crustal parts that lie in. Layers-4, 5 and 6, are more likely related to crustal anisotropy

Stress Distribution and Fault Development Around Nepal Himalaya by Means of Finite Element Method

There are a number of methods to analyse geological structures. Finite element method is one of them. Numerical modeling based on finite element analysis is an effective tool for studying the elastic behaviors of earth's crust due to tectonic movement. This study describes how to use an advanced numerical modeling technique, the finite element method, to compute rock deformation and to predict stress and fault development as a function of material properties, cohesion and friction angle. Stress distribution and fault development of 2 dimensional plane strain FEM models of four Himalayan cross sections are described. SW (south west) to NE (north east) horizontal shortening up to maximum 375 m (equivalent displacement at 7.5 cm/yr of 5000 yr) is applied at the southwestern end of the cross-sections. Proposed models show that the direction of maximum principal stresses (σ_1) are horizontal along the shallower part of all the models. Variation of the velocity boundary condition indicates the changes of direction of principal stresses along the deeper part while that along the shallower part remain unchanged. According to the Mohr - Coulomb criterion, failure is observed along the shallower part of Siwalik, Tethys and Granitic layer, and a very few near the surface of MBT, MCT and STDS. Failure has not occurred in the deeper part of Higher Himalaya and Lesser Himalaya, inspite of changing physical parameters of rock formations, because of the hydrostatic condition that is observed along the models. Previous studies on focal mechanism solutions of earthquakes in the Himalayan region provide the existence of thrust faults along its EW stretching with one plane dipping gently north beneath the Himalaya. Simulation shows the same distribution of thrust faults along the upper part of the models as shown by the focal mechanism solutions