Josef Jezek

THERMAL EVOLUTION AND EXHUMATION IN OBLIQUELY CONVERGENT (TRANSPRESSIVE) OROGENS

Most P-T-t path models to date have considered a linear erosion rate for exhumation from burial depth, related to isostatic readjustment of crustal thickness. A few have discussed extension-enhanced exhumation. Erosional exhumation can only restore lower crustal rocks from the thickened mountain root to their previous original depth in the pre-collisional crust. One major assumption of all models to date is that the compressive forces responsible for crustal thickening cease before elevation and erosion begins. However, compression is often still active, even if the crustal thickening has stopped. Further compression of the thickened and strongly deformed orogenic root is responsible for forceful exhumation-extrusion of softened rocks upwards. Hence, the rate of exhumation is related to the rate of convergence of colliding plates. Extrusional exhumation can elevate buried rocks to any depth depending on the action of fault-shear systems. In the extrusional exhumation models examined here, the ascending rocks cool faster because they approach the zero temperature surface conditions more rapidly than by isostatic erosion. The exhumation rate is also governed by the angle between the plate boundary and the displacement vector (α), implying that the convergent plate boundaries are regarded as complex transpressive systems in which the degree of obliquity can be expressed by the ratio of pure to simple shear components. A low ratio of pure/simple shear typical for wrench-dominated plate boundaries, implies long-distance horizontal transport from the buried position in the original orogen, and longer periods during which metamorphic heating occurs. A high ratio of pure/simple shear characteristic for frontal-like convergence implies a rapid exhumation without significant heating. Thus granulites, Barrovian-type, and blueschist facies metamorphism are characterised by increasing angle of obliquity (α∼10°, α∼30°, α∼90°), respectively. The high-temperature limit at low ratio of pure/simple shear is the geotherm T∞ and lies at temperatures that would commonly be taken to indicate that addition of mantle heat was required to generate hot geotherms and P-T-t paths. Wrench faulting over 100′s of kilometres at reasonable slow convergence rates could lead to extensive dehydration melting and granulite formation in large-scale collisional orogens.

Thermally softened continental extensional zones (arcs and rifts) as precursors to thickened orogenic belts

Abstract Intra-continental deformation of soft zones during continental collision requires weak continental lithosphere which is able to be shortened across considerable width during later convergence. This enables significant thickening with formation of an orogenic root. We have examined models with a history of lithospheric thinning by pure shear during an earlier phase of intra-continental extension with associated heating. Geologically this situation is appropriate to intra-continental rifts and back-arc basins. If thinning of elevated thermal structure is decoupled from the thinning of lithology then a weak (soft) lower crust and sub-arc/rift mantle result. This weak structure has a favoured rheology for subsequent convergent thickening while the lithosphere is still hot. These regions are associated with formation of granulites and metamorphic assemblages typical of high-temperature/low-pressure (HT/LP). If convergence starts while the heat input is still active then the failed rifts and arcs are shut off by lateral wedging of the hard lower crust and upper mantle of shoulder regions into the softened arc/rift domain. Such sites are ideal for the formation and for the exhumation of metamorphic core complexes. Subsequent thickening during convergence leads to HT eclogites when the previous arc/rift was hot and to medium-T eclogites for a thickened “standard” geotherm. These P–T paths are counterclockwise and their shapes are strongly dependent on the amount of previous thinning and type of initial geotherm. If the compression starts long after cessation of the extensional event and associated thermal anomaly, then the geotherm of the extended area relaxes and the whole region hardens. In this case, no homogeneous thickening occurs and deep continental roots cannot form.

Extrusion tectonics and elevation of lower crustal metamorphic rocks in convergent orogens

In the proposed model, weak zones of rheologically homogeneous crust, continually compressed between rigid lithospheric indenting plates, can be exhumed upward by extrusion. The rate of exhumation is governed by the rate of convergence of approaching plates and by the width of the weak deformable zone. Modeled extrusion results in near-isothermal decompression during the rapid elevation history in narrow orogenic belts for realistic plate velocities. Such pressure-temperature-time paths exhibit distinct collapsed geochronologies and record maximum metamorphic temperatures ( T max ) at the maximum burial depth ( P max ).

Perpendicular Linear Fabrics in Granite: Markers of Combined Simple Shear and Pure Shear Flows?

Recent results of numerical modelling of crystallographic fabrics of biotite and feldspar found in the porphyritic granite of the borehole EPS-1 at Soultz-sous-Foret, France, are described and interpreted. Preferred orientations of biotite (001) planes and feldspar (010) and (100) cleavages, were measured by means of an optical goniometer. The poles of (001) biotites show stable orientations throughout the core with maximum eigenvector v oriented vertically, and minimum eigenvector v3, or biotite-lineation, constantly oriented in the NE-SW direction. Within the subhorizontal magmatic foliation plane, also defined by feldspar (010) planes, two mutually orthogonal linear fabrics were found to be NE and NW trending, defined by the feldspar long axes. Variations in biotite fabric intensity and symmetry in different sections of the vertical profile were used to determine a plausible flow geometry in these sections. Numerical modelling involving a component of coaxial flattening, with various amounts of extension parallel and perpendicular to stretch, in addition to simple shearing, agrees qualitatively with the observed data. The models demonstrate that, under combined coaxial and simple shear flow regimes, mutually orthogonal linear fabrics may result from progressive rotation of the shortest crystals.

Numerical simulation of inclination shallowing by rolling and slipping of spherical particles

We report algorithms for two- and three-dimensional numerical simulations of settling spherical magnetic particles with prescribed size-distributions. Particles roll, or roll and slip, on the substrate, which causes their magnetic moments to rotate. These models are applied to the problem of inclination shallowing, which is repeatedly encountered in paleomagnetic studies of sedimentary rocks, where the recorded inclination is less than the expected field inclination. Simulations of equal-sized assemblages of magnetic spheres yield shallowing factors of 0.6, similar to that found in nature and in laboratory redeposition experiments. Comparable results are obtained when varying the size distributions of the spheres. Inclination shallowing is more pronounced when the smaller particles are magnetic and the larger ones are non-magnetic. Our study shows that rolling and/or slip (translation) of spherical particles can significantly contribute to inclination shallowing.

Software for modeling the magnetic anisotropy of strained rocks

The anisotropy of magnetic susceptibility is controlled by the preferred orientation of magnetic minerals in rocks and may be significantly influenced by deformation. This allows us to replace difficult estimation of strain parameters by indirect anisotropy of magnetic susceptibility (AMS) measurements. The interpretation of AMS is based on observation experience and modeling the AMS to strain relationship. The modeling is complex due to the large number of input parameters: different carriers of magnetism and their magnetic properties, their initial orientation distribution, the manner in which the deformation influences the reorientation of magnetic carriers, the character and duration of deformation. These parameters are taken into account in the presented package of Matlab functions for modeling AMS of strained rocks. The functions can be freely combined by the user to cover all basic types of homogeneous deformation (simple shear, pure shear, plane strain, coaxial deformation, transpression). A procedure is presented to treat the example of inhomogeneous deformation.

Determination of the orientation of magnetic minerals from the anisotropy of magnetic susceptibility

Abstract The anisotropy of magnetic susceptibility (AMS) of rocks is controlled by preferentially oriented magnetic mineral grains that carry AMS and, therefore, it contains information about both the grain susceptibilities and the grain orientations. Under certain conditions, information about the grain orientations can be deduced from the AMS. For a multigrain system composed of identical grains that are magnetically uniaxial (for the grain principal susceptibilities it holds that K1 > K2 = K3, or K1 = K2 > K3), an exact relationship exists between the AMS and the orientation tensor. We investigate the extent to which the theoretical relationships can be used when grains are generally triaxial. The parallelism of the principal directions of the susceptibility tensor and those of the orientation tensor are well preserved in all basic grain configurations. If grain leading axes have polar or girdle distributions and the two other axes have balanced distributions (similar orientation tensors), the parameters of intensity I and shape T based on the eigenvalues of the orientation tensor are well estimated. For unbalanced distributions, formulas are found for possible errors of I and T estimates.

A program for magnetic susceptibility–equivalent pore conversion

Pore magnetic anisotropy can be used to estimate the average geometry of void spaces in rocks in the form of the equivalent pore (EP) ellipsoid. Direct computation of EP from measured magnetic susceptibility is impossible. We present a method and a Matlab program for automatic magnetic susceptibility–equivalent pore conversion. Input data are the magnetic parameters (P and T, or L and F) representing the bulk magnetic anisotropy, and the intrinsic susceptibility of the fluid used in the measurement. EP is estimated iteratively by a repeated look-up table procedure using P and T values computed in a coarse grid of EP axial ratios. The program may be downloaded from the EarthRef.org Digital Archive.