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Knowledge of the last reactivation age of faults in the western Basin and Range province (western North America) is key to understanding the recent evolution of the province in the context of the Pacific-North America plate boundary region. The Dixie Valley fault system (DVFS; Nevada, USA) is an important active normal fault in this region, but its slip history is not well known. Our goal is to determine the age of faulting initiation and the history of vertical displacement at the million-year time scale. We used well data from the Dixie Valley geothermal field and geologic maps to calculate offsets of geologic units. Tectonic events and their ages were determined from knickpoint analysis in the Stillwater Range. We then matched basin unconformities and knickpoints to obtain a vertical displacement history. The age of inception of the modern DVFS is likely between 13 and 7 Ma. The total vertical displacement of the geologic units is between 3 and 3.5 km, and most of it occurred in the last 2-3 Ma. Assuming an erosion rate between 0.03 and 0.07 m/ka, the maximum vertical displacement rate (~1-2 mm/a) occurred between 3 and 0.5 Ma, and resulted in ~2 km of vertical displacement. Finally, the age-versusdistance profile of the Stillwater Range crest is tapered at both ends, as expected for a propagating range-bounding normal fault, but it also has two maxima matching the deepest parts of the Dixie Valley basin. This indicates that the present-day DVFS may have started out as two separate strands that connected within 2-3 Ma of inception. Our work shows that the combination of stratigraphic and structural data from the basin with geomorphological data from the adjacent mountain range is suitable for placing constraints on the displacement history at the million-year time scale of an active range-bounding normal fault. The resulting displacement-time paths for normal faults may not be as uniform as commonly assumed. [ABSTRACT FROM AUTHOR]

Two outcrops in the Asturian Basin (northwest Iberian Peninsula) composed of Lower-Middle Jurassic alternations of limestones and marls, including black shales proven to be hydrocarbon source rocks, were analyzed from a structural point of view. In both outcrops, an inversion tectonic event of Cenozoic age caused contractional folds and faults, as well as a change in the structural style up section, superimposed on previous Mesozoic extensional structures. However, the mode and kinematic evolution of the inversion tectonics were very different; folding predominated in one of the outcrops, whereas faulting is the most important phenomenon in the other one. The characteristics of the two coastal outcrops (irregular surface, inaccessible portions, accessibility restricted to low tide periods, good quality of rock exposure and scale) led us to build virtual outcrop models using photogrammetry (structure from motion) and interpret them in a computer-assisted virtual environment (CAVE). The analysis of both field data and the virtual outcrop model-derived results (three-dimensional models and cross sections) allowed us to conclude that mechanical stratigraphy and the type, characteristics, and distribution of previous structures were the key factors that controlled the inversion tectonic mode. Awareness of their influence should help when trying to understand inversion structures in similar geological settings. [ABSTRACT FROM AUTHOR]

Various methods have been proposed to measure fracture aperture distributions, including X-ray computed tomography (CT) imaging, which has the advantage that it can be combined with dynamic flow experiments. In this paper, we present a calibration-free missing CT attenuation (CFMA) imaging method for measuring fracture apertures that avoids time-consuming calibration. In addition, this model does not assume a homogeneous matrix and thus provides a good estimate of fracture apertures even when rock properties are heterogeneous. The validity of the CFMA model is established by four approaches: comparing apertures calculated with the conventional calibration-based method; evaluating model predictability at different scanner voxel sizes; comparing with calibration coefficients in the literature from a number of experiments with different rocks and X-ray scanners; and comparing aperture measurements for dry and wet scans. We analyze the systematic error and the random error introduced by rock heterogeneities and CT scanning and show that by averaging 5 replicate scans, we reduce the aperture measurement error to ~22 µm. [ABSTRACT FROM AUTHOR]

A strike-slip fault is present outboard and subparallel to the Wassuk Range front within the central Walker Lane (Nevada, USA). Recessional shorelines of pluvial Lake Lahontan that reached its highstand ca. 15,475 ± 720 cal. yr B.P. are displaced ~14 m and yield a right-lateral slip-rate estimate approaching 1 mm/yr. The strike-slip fault trace projects southeastward toward the eastern margin of Walker Lake, which is ~15 km to the southeast. The trace is obscured in this region by recessional shorelines features that record the historical dessication of the lake caused by upstream water diversion and consumption. High-resolution seismic CHIRP (compressed high intensity radar pulse) profi les acquired in Walker Lake reveal ~20 k.y. of stratigraphy that is tilted westward ~20- 30 m to the Wassuk Range front, consistent with ~1.0-1.5 mm/yr (20-30 m/20 k.y.) of vertical displacement on the main rangebounding normal fault. Direct evidence of the northwest-trending right-lateral strikeslip fault is not observed, although a set of folds and faults trending N35°E, conjugate to the trend of the strike-slip fault observed to the north, is superimposed on the west-dipping strata. The pattern and trend of folding and faulting beneath the lake are not simply explained; they may record development of Riedel shears in a zone of northwest-directed strike slip. Regardless of their genesis, the faults and folds appear to have been inactive during the past ~10.5 k.y. These observations begin to reconcile what was a mismatch between geodetically predicted deformation rates and geological fault slip rate studies along the Wassuk Range front, and provide another example of strain partitioning between predominantly normal and strikeslip faults that occurs in regions of oblique extension such as the Walker Lane. [ABSTRACT FROM AUTHOR]

The retroarc fold-and-thrust belt of the Central Andes exhibits major along-strike variations in its pre-Cenozoic tectonic configuration. These variations have been proposed to explain the considerable southward decrease in the observed magnitude of Cenozoic shortening. Regional mapping, a cross section, and U-Pb and (U-Th)/He age dating of apatite and zircon presented here build upon the preexisting geological framework for the region. At the latitude of the regional transect (24-25°S), results demonstrate that the thrust belt propagated in an overall eastward direction in three distinct pulses during Cenozoic time. Each eastward jump in the deformation front was apparently followed by local westward deformation migration, likely reflecting a subcritically tapered orogenic wedge. The first eastward jump was at ca. 40 Ma, when deformation and exhumation were restricted to the western margin of the Eastern Cordillera and eastern margin of the Puna Plateau. At 12-10 Ma, the thrust front jumped ~75 km toward the east to bypass the central portion of a horst block of the Cretaceous Salta rift system, followed by initiation of new faults in a subsystem that propagated toward the west into this preexisting structural high. During Pliocene time, deformation again migrated >100 km eastward to a Cretaceous synrift depocenter in the Santa Bárbara Ranges. The sporadic foreland-ward propagation documented here may be common in basement-involved thrust systems where inherited weaknesses due to previous crustal deformation are preferentially reactivated during later shortening. The minimum estimate for the magnitude of shortening at this latitude is ~142 km, which is moderate in magnitude compared to the 250-350 km of shortening accommodated in the retroarc thrust belt of southern Bolivia to the north. This work supports previous hypotheses that the magnitude of shortening decreases significantly along strike away from a maximum in southern Bolivia, largely as a result of the distribution of pre-Cenozoic basins that are able to accommodate a large magnitude of thin-skinned shortening. A major implication is that variations in the pre-orogenic upper-crustal architecture can influence the behavior of the continental lithosphere during later orogenesis, a result that challenges geodynamic models that neglect upper-plate heterogeneities. [ABSTRACT FROM AUTHOR]

This paper presents a detailed investigation of the structure and evolution of the Precordillera southern sector (Argentina). We document the development and successive reactivation of regional and discrete structural grain through time, and discuss the existence of a large-scale mechanical anisotropy present in the lithosphere. Our kinematic studies indicate that the Permian orogeny generated a doubly vergent foldand- thrust belt of transpressive deformation, where strain was partitioned into two different types of deformation domains. The westvergent western domain was characterized by partitioned transpression with shortening dominating, and a strike-slip-dominated subdomain. The east-vergent eastern domain was characterized by pure contractional deformation. Our model for the Late Permian to Early Triassic evolution of the Pre cordillera involves a north-northwest-trending weakness zone affected by north-northeast- directed extension, generating an area with transtensional deformation during the Choiyoi volcanism development. Later, during the Triassic generation of the Cuyana rift basin, the northeast stretching direction was orthogonal to the rift trend, indicating pure extensional deformation. We propose a model where the clear parallelism between the distribution of an inferred early Paleozoic suture zone, a northnorthwest- trending late Paleozoic belt, and Permian-Triassic rift-related magmatism indicates the reactivation of a northnorthwest- trending long-lived lithospheric weakness zone. [ABSTRACT FROM AUTHOR]