Daniel M. Davis

Pressure-induced microcracking and grain crushing in berea and boise sandstones: acoustic emission and quantitative microscopy measurements

Abstract The micromechanics of hydrostatic compaction and porosity reduction was investigated by acoustic emission (AE) and microscopy measurements. The room temperature hydrostatic tests were conducted at confining pressures up to 550 MPa and constant pore pressures up to 10 MPa. Contrary to our expectation, the porosity reduction processes were very efficient in generating AE. The hydrostat of a porous sandstone usually has an inflection point which separates the compaction process into two stages. Distinctly different AE activity, AE efficiency and microstructure are associated with each of these two stages of compaction, implying a transition in micromechanism at the inflection point. Probably the dominant micromechanical processes for porosity reduction was grain slip and rotation in the first stage of compaction and grain crushing in the second stage. Stress-induced cracking associated with the grain crushing introduces relatively thin, intragranular cracks. At the same time, the grain crushing provides additional degrees of freedom for grain rotation allowing some of the crushed grains to move into the pore space. Both mechanisms modify the pore dimension as well as aspect ratio. We developed a stereological technique to characterize quantitatively the pore dimension distribution. We also used an indirect technique based on Waishs (1965) theoretical analysis to infer microcrack porosity. Grain crushing eliminated the relatively large pores (of dimension ranging from 200 μm to 700 μm) in a compacted Boise sandstone sample. Even though the overall porosity decreased with pressure, the microcrack porosity actually increased with compaction. Most of the pressure-induced microcracks were of relatively high aspect ratios. The Kaiser effect and compaction creep were also investigated by cyclic tests and time-dependent AE measurements.

Oblique convergence and the lobate mountain belts of western Pakistan

The thin-skinned structures of the Pakistani convergent margin have formed as a consequence of the relative motion between India and Eurasia. Most of the resultant motion is being accommodated along or near the current edge of the Eurasian plate: the southwest-northeast striking Chaman fault zone. It has been observed at oblique margins that the total plate motion is resolved into a component parallel to the margin, accommodated through strike-slip faulting, and a component normal to the margin taken up as contraction. However, the orientations of structures along the Pakistani convergent margin in and around the Sulaiman lobe and Sulaiman Range cannot be explained simply by resolving the plate motion vector into components normal and parallel to the plate boundary. Our modeling suggests that the complex juxtaposition of strike-slip faults with thrust faults of various orientations can be explained by the presence of a block centered upon the Katawaz basin that translates along the southwest-northeast structural barrier of the Chaman fault zone, moving with respect to both Eurasia and India. As this relatively rigid block moves northeastward relative to Asia, it causes deformation of the sedimentary cover and is responsible for much of the structural complexity in the Pakistani foreland. Our simple model explains several first-order features of this oblique margin, such as the eastward-facing Sulaiman Range, the strike-slip Kingri fault (located between the Sulaiman lobe and Sulaiman Range), and the reentrant at Sibi. This leads us to conclude that very complex structural and geometric relationships at oblique convergent plate boundaries can result from the accommodation of strain with simple initial geometric constraints.

The brittle-ductile transition in porous sedimentary rocks: geological implications for accretionary wedge aseismicity

Abstract Thrusting earthquakes in subduction zones generally occur along only part of the plate boundary, with motion along the shallowest part of the plate boundary occurring ascismically. The maximum size of subduction boundary thrust earthquakes depends strongly upon the down-dip width of the seismogenic zone. The single most uncertain factor in determining that width is the location of the up-dip limit of the zone (the seismic front), which depends upon the mechanical state of the sedimentary rocks in the plate boundary zone. In order to come to a better understanding of the seismic potential of sediments in a subduction zone, we carried out a series of triaxial experiments on Berea and Kayenta sandstones. Based on our experimental data, a brittle-ductile transition map was constructed showing that both porosity and effective pressure are important factors controlling the transition from brittle to macroscopically ductile behavior in porous rocks. In the brittle field, a sample fails by shear localization on one slip plane accompanied by strain softening and dilatancy, whereas in the ductile field, a sample deforms homogeneously with a constant yield stress or slight hardening. By comparing such a map with the estimated porosity profile of an accretionary wedge, the likely nature and rough location of the boundary between brittle and ductile behavior can be inferred. If the sediments along a plate boundary are too young and undercompacted to be capable of brittle shear localization, then their deformation is likely to be aseismic. In this way, it may be possible for even a very broad fore-arcs to produce no great earthquakes. However, great earthquakes are to be expected at margins that have large zones of plate contact along which many sediments are compacted and well lithified. Such rocks are expected to be capable of shear localization and brittle failure with the potential for stick-slip behavior.