F. Rummel

An experimental study into the rheology of synthetic polycrystalline coesite aggregates

Coesite has been found as a relic in ultrahigh pressure metamorphic (UHPM) crust worldwide and is expected to play a major role in the mechanical behavior of continental crust at UHPM conditions. We performed triaxial compression tests on synthetic polycrystalline coesitite in a solid medium apparatus at confining pressures of 3.1 to 3.7 GPa, temperatures of 700° to ∼1160°C, and strain rates betweeen 6×10−7 and 1×10−3 s−1. The problem of the limited stress resolution of the solid medium apparatus was addressed by applying two extreme friction corrections that yield lower and upper bounds to the differential stress. The correlation between the mechanical data and the microstructural record of the deformed samples, as a function of temperature and imposed strain rate, is consistent with deformation by dislocation creep. We deduced parameters of a power law as n ≈ 3±1 and Q ≈ 275±50 kJ mol−1. Extrapolation of the experimental data to natural conditions cannot be constrained by comparison with natural microstructures, due to the lack of preserved coesite other than as single crystal inclusions. Nevertheless, the extrapolation indicates a low strength (of order 10 MPa) for natural strain rates at typical UHPM conditions. Absent deformation of the UHPM Brossasco granite (Dora Maira Massif, Western Alps) thus implies low stresses; deformation must have been localized in very weak shear zones during burial and exhumation.

The effect of experimental and microstructural parameters on the transition from brittle failure to cataclastic flow of carbonate rocks

Abstract Triaxial compression tests were conducted on cold-pressed calcite, aragonite and limestone aggregates and on Solnhofen limestone specimens to study the effect of experimental and microstructural parameters on the transition from brittle failure to cataclastic flow. The tests were performed at confining pressures up to 195 MPa and at strain rates between 5 · 10−4 s−1 and 5 · 10−6 s−1. Axial as well as volumetric strain were measured. Samples were produced by cold-pressing powders of crushed calcite and aragonite crystals and of crushed Solnhofen limestone. Sample porosity ranged between 5 and 25% and the average grain size varied between 5 and 400 μm. For both the cold-pressed aggregates and the intact limestone specimens, the confining pressure at the transition from localized brittle failure to non-localized cataclastic flow decreases with increasing porosity and grain size. The transition is characterized by a zero work-hardening coefficient, by dilation for low porosity and compaction for high porosity rocks, by a constant ratio between axial stress and confining pressure, and by decreasing yield strength for increasing confining pressure. The experimental results disagree with the critical state concept over most of the porosity range investigated, and indicate non-associated material behaviour. These properties of the brittle-ductile transition are addressed on the basis of continuum mechanics or by models suggested for granular materials. The problems discussed and the results obtained are of fundamental interest to rock deformation and structural geology.

Stress constraints and hydrofracturing stress data for the continental crust

Faulting and seismicity in the upper continental crust require considerable differential stresses. Application of experimentally developed friction, fracture and flow laws shows that high differential stresses can only exist in the uppermost crust. Direct hydraulic fracturing measurements in deep boreholes seem to support this rock mechanics conclusion. The experimental data base presently consists of approximately 500 hydrofrac tests conducted in about 100 boreholes at about 30 different geographical locations. To illustrate the variation of measured stresses with depth, the data are expressed as dimensionless horizontal stresses in the formSH,h/Sv=(α/z)+β, whereSv=ρgz Extrapolation of the experimental data to greater depth shows that the minor horizontal stress approaches the valueSh/Sv=0.5 which limits friction on wet faults, and that the major horizontal stress approaches a value close toSH/Sv=1 at rather shallow depth (5 to 10 km.). This limits faulting and seismicity in most of the upper crust to either strike-slip or normal faults. The lower boundary for seismicity is mainly dependent on tectonic strain accumulation and rock creep at the environmental conditions at depth.

Hydraulic Fracturing Stress Measurements near the Hohenzollern-Graben-Structure, SW Germany

Hydraulic fracturing stress measurements have been performed in a limestone quarry near the Hohenzollern-Graben, a fault structure in SW Germany. The values of the two horizontal principal stresses were 24 and 15 bars at a depth of 25 m. The magnitude and the direction of the stresses agree with the results obtained by door-stopper measurements at the same location and the direction of the maximum horizontal principal stress derived from fault plane solutions of shallow earthquakes of this tectonic active area.