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Fault rocks from the classic Rocky Mountain foreland fold-and-thrust belt in southwestern Canada were dated by Ar analysis of clay grain-size fractions. Using X-ray diffraction quantification of the detrital and authigenic component of each fraction, these determinations give ages for individual faults in the area (illite age analysis). The resulting ages cluster around 72 and 52 Ma (here called the Rundle and McConnell pulses, respectively), challenging the traditional view of gradual forward progression of faulting and thrust-belt history of the area. The recognition of spatially and temporally restricted deformation episodes offers field support for theoretical models of critically stressed wedges, which result in geologically reasonable strain rates for the area. In addition to regional considerations, this study highlights the potential of direct dating of shallow fault rocks for our understanding of upper-crustal kinematics and regional tectonic analysis of ancient orogens. Keywords: Rocky Mountains, Canada, faults, orogeny, geochronology, gouge.

PolyQuant, a new method to quantify illite polytypes in physical mixtures, has been coupled with Illite Age Analysis (IAA, Pevear 1992) to determine accurate K-Ar ages of the individual polytypes. K-Ar radioisotope dates from illite in shales and sandstones are typically meaningless because these rocks are mixtures of diagenetic and detrital components in unknown proportions. IAA uses linear extrapolation to unmix measured K-Ar ages and establish the end-member ages. This procedure requires samples to have a diagenetic Illite-Smectite (I-S) component mixed with a detrital discrete illite component because they are separable and quantifiable using X-ray diffraction (XRD) of oriented aggregate samples. We extend IAA for use in mixtures of diagenetic and detrital illite with no I-S component by coupling a genetic algorithm (GA) with the WILDFIRE programs (1993) of Reynolds in a new computer program, PolyQuant, which quantifies illite polytypes in a natural sample. Based on first principles of XRD, WILDFIRE is a forward model that calculates hkl reflections by varying parameters such as rotational disorder, octahedral cation occupancy (i.e., cis- and trans-vacancies), chemical substitutions (K and Fe), orientation, and crystallite thickness. PolyQuant executes several forward models, using the GA to vary the parameters and weight percents of each polytype to automatically achieve a 'best fit' with an experimental XRD pattern. Sensitivity tests reveal that changes in the weight percent of an illite polytype, disorder ([P.sub.o], [P.sub.cv]), and the orientation function affect the pattern more than other parameters. In addition, PolyQuant successfully quantified illite components in prepared mixtures of 1M illite and 2[M.sub.1] muscovite and 1[M.sub.d] illite and 2[M.sub.1] muscovite. We show examples to validate IAA from prepared mixtures of end-members with known ages and also to obtain end-member ages of highly illitic clay fault gouge. Using K-Ar ages and PolyQuant to quantify the 1M and 2[M.sub.1] components from mixtures of RM-30 (25 [+ or -] 1 Ma) and a 2[M.sub.1] muscovite from Wards (428 [+ or -] 9 Ma), IAA determined end-member ages of 19.9 Ma (0-44 Ma) and 429 Ma (408-459 Ma)--in reasonable agreement with the measured results. In addition, we use clay gouge from the Canadian Rockies to date movement along thrust faults. Regional geologic data indicate movement along these faults occurred between 73 and 48 Ma. IAA determined the ages of faulting to be from 79.2 to 53.5 Ma, in good agreement with the known range of fault dates. Our results indicate that neoformation of illite in the fault zone must have modified the ages to younger dates.

Author(s): Ben A. van der Pluijm (corresponding author) [1]; Chris M. Hall [1]; Peter J. Vrolijk [2]; David R. Pevear [2]; Michael C. Covey [2] Direct dating of ductile shear [...]

Smectites, a group of hydrated phyllosilicate minerals, are often a main component of sediments entering a subduction zone. They are common in hemipelagic sediments and consist of an authigenetic component that arises from the alteration of volcanic ash intercalated in the sediment and a detrital component. Smectite-rich strata are structurally weak, retain water, and are highly porous. A review of sediment lithology in subduction zones around the world suggests that decollement zones may preferentially form in smectite-rich horizons. One consequence of this observation is that the assignment of high frictional resistance to materials in a decollement may be unwarranted. At deeper structural levels smectite is transformed to illite, a mechanically stronger mineral. Estimates of the depth of this transition coincide with the depth at which earthquakes are first detected along plate interfaces. I speculate that the smectite-illite transition in subduction zones may be an important component of sediment consolidation processes that trigger interplate seismicity.

Abstract: The oil and gas industry has a long history of evaluating the adequacy of subsurface traps for containing buoyant fluids and has developed methods for weighing the geologic probability of trap leakage when limited data are available. We have adapted these methods for evaluating the adequacy of sub-surface saline formation traps for CO2 storage. A principal difference in the evaluation for gas and oil exploration versus CO2 storage is that the probability of leakage is viewed differently. The oil and gas industry tolerates substantial leak probabilities in exploration if the potential gas or oil accumulation is sufficiently large, but the tolerance for CO2 leakage (i.e. the probability of injected CO2 returning to the surface) will likely be much lower, and that low tolerance has the potential to limit the amount of CO2 injected into a subsurface trap. We consider three types of saline formation traps: [•] Depleted gas or oil fields [•] Drilled trap structures that failed to discover commercial gas or oil volumes [•] Undrilled trap structures Each of these types has some probability of CO2 leakage, but the geologic probability is smaller and better constrained in depleted gas or oil fields and greater and more uncertain in untested structures. However, each of these trap types still possess some geologic probability of leakage. We evaluate the overall probability by considering for each trap element (e.g., capillary seal capacity) the likelihood that each element is adequately developed and the quality and quantity of data available to make that judgment. By following procedures applied to gas and oil trap evaluation, we arrive at probability distributions for a range of CO2 fill limits. What this analysis reveals, however, is that if we fill traps with CO2 to levels with minimal probability of CO2 leakage, the volume of CO2 that could be stored in subsurface traps is small. Moreover, all traps will, on average, have huge remaining pore volumes that could accommodate additional CO2 storage and could go unfilled (i.e. pore volumes with a higher probability of leakage). We propose an evaluation method based on a series of redundant traps in a fill-leak sequence. The purpose of secondary and tertiary traps arranged in series is that they allow for filling the primary trap to its uncertain fill limit while minimizing the chance of leakage to the surface. [Copyright &y& Elsevier]

The article reports on the study which found that fluid flow patterns through impermeable rocks where flow is controlled by faults and fractures can be complex. The principal driving mechanism for flow is rapid burial in accretionary prisms. Moreover, researchers suggested that there is another mechanism for deflecting flow from the overall, seaward-directed fluid potential gradient. They added that the placidly geometry of the scaly fabric in the décollement is a texture that is consistent with flow parallel to the deformation front.