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We used optical experiments and high-speed photography to interpret the origins of tensile fractures that form during dynamic shear rupture in laboratory experiments. Sub-Rayleigh (slower than the Rayleigh wave speed, [c.sub.R]) shear ruptures in Homalite-100 produce damage zones consisting of an array of tensile cracks. These cracks nucleate and grow within cohesive zones behind the tips of shear ruptures that propagate dynamically along interfaces with frictional and cohesive strength, simulating a 'strong' fault. The tensile cracks are produced only along one side of the interface where transient, fault-parallel, tensile stress perturbations are associated with the growing shear rupture tip. Results of this study represent an important potential bridge between geological observations of structures preserved along exhumed faults and theoretical models of earthquake propagation, potentially leading to diagnostic criteria for interpreting velocity, directivity, and static prestress states associated with past earthquakes on exhumed faults.

Coupled climate-biome model simulations of the late Paleozoic were developed to determine the response of Pangean tropical climate to Gondwanan deglaciation. The model simulations predict substantial changes over equatorial Pangea including continental drying, a reversal of equatorial winds, warming, heavier [[delta].sup.18]O values of meteoric precipitation, and the expansion of deserts and the contraction of forests. The magnitude of these tropical responses is sensitive to the extent of Gondwana continental ice and the deglacial rise in atmospheric pC[O.sub.2], boundary conditions that are not well known for the late Paleozoic. Nonetheless the model predictions are consistent with climatic and environmental trends determined from terrestrial proxy data, implying that the deglaciation of Gondwana was a transformational climate event in tropical Pangea. Keywords: paleoclimate, Carboniferous, Permian, ice age, general circulation model.

We use the GENESIS atmospheric general circulation model (GCM) with water isotopic transport and fractionation capabilities to quantify the influence of atmospheric C[O.sub.2], sea level, and elevation of the Western Cordillera on the [[delta].sup.18]O of middle Cretaceous precipitation. The model predicts a systematic increase of nearly 3 [per thousand] in the [[delta].sup.18]O of North American precipitation due to warming associated with an increase in C[O.sub.2] from 2 to 12 times pre-industrial levels. In contrast, the specification of lowstand conditions and a high ancestral Western Cordillera reduces the [[delta].sup.18]O of North American precipitation locally by as much as 6%0 and 8 [per thousand]. We compare the simulated [[delta].sup.18]O of precipitation with the [[delta].sup.18]O of paleosol siderite spherules and find good agreement only when the model includes lowstand conditions and a high ancestral Western Cordillera. Our results imply either that Cretaceous high-latitude paleosol [[delta].sup.18]O was influenced by orographic precipitation and the Western Interior Seaway, or that the GCM's hydrological cycle is deficient at high pC[O.sub.2]. Additional paleosol data are needed to resolve this issue. Keywords: paleoclimate, Cretaceous, siderite, [[delta].sup.18]O, general circulation model.

Reconstructions of Phanerozoic C[O.sub.2] levels have generally relied on geochemical modeling or proxy data. Because the uncertainty inherent in such reconstructions is large enough to be climatically significant, inverse climate modeling may help to constrain paleo-C[O.sub.2] estimates. In particular, we test the plausibility of this technique by focusing on the climate from 360 to 260 Ma, a time in which the Siberian landmass was in middle to high latitudes, yet had little or no permanent land ice. Our climate model simulations predict a lower limit for C[O.sub.2]--the value beneath which Siberia acquires 'excess' ice. Simulations provide little new information for the period in which Siberia was at a relatively low paleoaltitude (360-340 Ma), but model results imply that paleo-C[O.sub.2] levels had to be greater than 2-4x modern values to be consistent with an apparently ice-free Siberia in the late Permian. These results for the later period in general agree with soil C[O.sub.2] proxies and the timing of Gondwanan deglaciation, thus providing support for a significant C[O.sub.2] increase before the end-Permian boundary event. Our technique may be applicable to other time intervals of unipolar glaciation. Keywords: paleoclimate, paleoatmosphere, carbon dioxide, glaciation.

General circulation model experiments test the geologically based correlation of high monsoon rainfall over interior Australia with Northern Hemisphere insolation and evaluate the sensitivity of the Australian Monsoon to ecosystem change. Our results suggest that Northern Hemisphere insolation control on the intensity of the Siberian High, rather than summer insolation over the Australian continent, determines the strength of the Australian Monsoon on millennial time scales, unlike a classic monsoon regime. Additional simulations show that the penetration of monsoon moisture into the interior is sensitive to biosphere-atmosphere feedbacks linked to vegetation type and soil properties. This sensitivity offers a resolution to the observed failure of the Australian Monsoon to penetrate the interior in the Holocene. Postulated regular burning practiced by early humans may have converted a tree-shrub-grassland mosaic across the semiarid zone to the modern desert scrub, thereby weakening biospheric feedbacks and resulting in long-term desertification of the continent. Keywords: Australia, monsoon, human, paleoclimate, climate models, biomass burning.

Results from coupled ice-sheet and atmospheric general circulation models show that the waxing and waning of ice sheets during the Late Ordovician were very sensitive to changes in atmospheric pC[O.sub.2] and orbital forcing at the obliquity time scale (30-40 k.y.). Without orbital forcing, ice sheets can grow with pC[O.sub.2] level as high as 10 times preindustrial atmospheric level (PAL). However, with orbital forcing, ice sheets can grow only with pC[O.sub.2] levels of 8 times PAL or lower. These results indicate that the threshold of pC[O.sub.2] for the initiation of glaciation is on the lower end of previously published estimates of 8-20 times PAL. The ice-sheet model results further indicate that during exceptionally long periods of low summer insolation and low pC[O.sub.2] levels (8-10 times PAL), large ice sheets could have formed that were able to sustain permanent glaciation under subsequently higher pC[O.sub.2] values. This finding suggests that in order to end the Late Ordovician glaciation with a rise in pC[O.sub.2], atmospheric pC[O.sub.2] must have risen to at least 12 times PAL. Ice sheets therefore introduce nonlinearities and hysteresis effects to the Ordovician climate system. These nonlinearities might have also played a role in the initiation and termination of other glaciations in Earth history. Keywords: Late Ordovician, climate model, ice sheet model, obliquity, glaciation.

Left-lateral strike-slip faults in the Lake Edison granodiorite (central Sierra Nevada, California) are composed of en echelon segments. Relative displacements across the faults apparently are transferred between segments by ductile shearing at right steps, and by extensional fracturing at left steps. The granodiorite within right steps displays mylonitic foliation, and thin sections show textures in quartz associated with dislocation glide, recovery processes, and dynamic recrystallization, whereas textures in feldspar are related to fracturing. Only centimetres outside the right steps, the rock fabric is approximately isotropic and deformation is accommodated by mineralized opening-mode fractures. The stress field calculated for the right-step geometry, when a boundary element model is used, shows an increase in mean compressive stress of up to 25 MPa within the step relative to that outside. This difference in stress apparently produced the contrasting behaviors of the granitic rock. Experimentally derived power-law flow laws do not predict these behaviors.

New structural data from the Novarupta basin, Katmai Natinal Park, Alaska, site of the largest volcanic eruption of this century (1912), provide limits for the location of magma chambers associated with this eruption. To investigate the subsurface structure of the 1912 vents, and to support an interdisciplinary study of this young volcaninc system, a project of geologic mapping of surficial and bedrock structures in the vent region of the 1912 eruption has been undertaken. Landslide scarps, arcuate grabens, a monoclinal fold, and truncated ridges circumscribe the Novarupta basin, marking the inferred outer rim of the vent. A set of radial fissures crosses the southern margin of the basin, striking 140[degrees], subparallel to the dominant bedrock joint set. The fissures and joints, along with the local plate-motion vector and the inferred regional stress orientation, are consistent with a feeder dike propagating from a reservoir beneath Trident volcano to the eruptive vent.

We introduce a method to infer the remote differential stress magnitude from the curvature of overlapping echelon fracture traces. The curving paths of overlapping echelon cracks imply the predominance of local crack-induced stresses over remote stresses during propagation. Nearly straight crack paths imply the controlling influence of a remote compressive crack-parallel differential stress. This method is used to interpret complex joint patterns mapped in sedimentary rock. It is also applied to the problem of fracture-pattern generation using computer models.

Reconstructions of Phanerozoic CO2 levels have generally relied on geochemical modeling or proxy data. Because the uncertainty inherent in such reconstructions is large enough to be climatically significant, inverse climate modeling may help to constrain paleo-CO2 estimates. In particular, we test the plausibility of this technique by focusing on the climate from 360 to 260 Ma, a time in which the Siberian landmass was in middle to high latitudes, yet had little or no permanent land ice. Our climate model simulations predict a lower limit for CO2-the value beneath which Siberia acquires "excess" ice. Simulations provide little new information for the period in which Siberia was at a relatively low paleoaltitude (360-340 Ma), but model results imply that paleo-CO2 levels had to be greater than 2-4x modern values to be consistent with an apparently ice-free Siberia in the late Permian. These results for the later period in general agree with soil CO2 proxies and the timing of Gondwanan deglaciation, thus providing support for a significant CO2 increase before the end-Permian boundary event. Our technique may be applicable to other time intervals of unipolar glaciation. [ABSTRACT FROM AUTHOR]