Tim F. Wawrzyniec

High‐temperature embrittlement of extensional Alpine mylonite zones in the midcrustal ductile‐brittle transition

Structural and fluid inclusion analyses of two large-displacement extensional Alpine shear zones show that embrittlement occurred at anomalously high temperatures and indicate that factors other than gradually changing temperature and pressure can exert primary control on the ductile-brittle transition. Most rocks within the Brenner and Simplon mylonite zones, including abundant weak schists, failed brittlely by distributed faulting, fracturing, and brittle-ductile shearing at T = 450°–575°C and P = 400–750 MPa, conditions in which plastic or semibrittle flow rather than brittle deformation is expected, even in strong orthogneiss. Embrittlement was caused by transiently(?) high fluid pressure and local bending strain rather than by temperature or pressure decrease. Mylonitization shut off permanently in the embrittled parts of the shear zones despite continued high-T denudation of the footwalls. However, mylonitization apparently did continue in the structurally highest ∼50 m of the shear zones where brittle structures are absent or rare. A strength contrast evolved between these late, thin mylonite zones and the stronger, deeper parts where mylonitization ended. This contrast probably reflects both weakening of the late mylonite zones and strengthening of the deeper embrittled parts, although differential stress may have ultimately increased in the former due to strain rate increase as the shear zones thinned. The shear zones probably evolved to discrete frictional faults by T ≈ 450°C and P ≈ 400 MPa (∼15 km).

Chronotopographic analysis directly from point-cloud data: A method for detecting small, seasonal hillslope change, Black Mesa Escarpment, NE Arizona

The advent of high-resolution, precise, back-pack portable terrestrial lidar scanners (TLS) provides a revolutionary new tool for obtaining quantitative, high-resolution (2-mm to 30-mm point spacing) measurements of landscape surface features. Moreover, data collected using these instruments allow observation of geomorphic processes in systems that can experience change on a daily basis. We have introduced TLS techniques in ongoing investigations of semiarid landscapes associated with weakly cemented sandstones along part of the Black Mesa escarpment of NE Arizona. Clay-cemented, Jurassic sandstones exposed along this escarpment are sensitive to moisture, and thus climate, via hydration-expansion weathering of interstitial clay. Sediment shed from weathered slopes has caused locally rapid valley floor aggradation and upper basin slope vertical denudation rates of 2–3 mm/yr over 10- to 100-yr timescales, as indicated by dendrochronology coupled with soil geomorphic analysis. These rates suggest rapid hillslope denudation rates. Employing the University of New Mexico Lidar Laboratory Optech Ilris 3D TLS, we are constructing a high-resolution model of two major basins along the escarpment. Focusing on a single, small (30 × 60 m) area of a mostly non-vegetated, steep slope (>35°), we demonstrate in this paper a method of comparative analysis of point-cloud data sets that can detect subcentimeter change resulting from a single season of monsoon precipitation along the escarpment. Using repeat scans can provide an empirical evaluation of single season erosion rates in the study site, and because our observations are geospatial in nature, we can also document the parts of the slopes that make the greatest contribution to local valley floor aggradation. In demonstrating the utility of this method, we expect that continued investigation of this site will provide insight to the key processes associated with soil-mantled versus bedrock-dominated slopes during modern escarpment retreat and hillslope modification, which, in turn, may further elucidate the impacts of Holocene climate change on this rapidly evolving landscape.

Traps and turbidite reservoir characteristics from a complex and evolving tectonic setting, Veracruz Basin, southeastern Mexico

The Miocene and Pliocene interval of the Veracruz Basin, southeast Mexico, experienced an evolving array of shortening, strike-slip, and volcanic forces in response to plate-scale interactions. The basin is divided into six structural domains that define regions of comparable timing and type of structural deformation, and the basin fill is separated into two long-term depositional phases, each of which can be tied to a waning and then waxing of major basin-bounding tectonic events. The first phase of deposition took place from the early to late Miocene and is tied to the waning effects of the Laramide orogeny. The Miocene basin inherited a tectonically steepened basin margin, across which deep canyons were carved and variably filled with mudstone and thin remnants of coarse sandstone and conglomerate. This zone of erosion and bypass grades into thick, sandstone-rich basin-floor fans. Later in phase I, subaqueous volcanoes, tied to distant plate subduction, developed offshore and formed a bathymetric barrier that prevented turbidity currents from entering the ancestral Gulf of Mexico. The volcanoes also served as immovable buttresses, around which intrabasinal thrust belts developed in response to regional shortening.The second depositional phase is tied to the onset of internal basin shortening and uplift of the north basin margin known as the trans-Mexican volcanic belt. This uplift caused a dramatic reconfiguration of the sediment-dispersal system, whereby large shelf clinoforms prograded from north to south across the basin. In contrast to the onlapping stacking pattern of phase I units, phase II units stack in a strongly offlapping pattern.Proven and postulated reservoir-trap combinations, ranging from four-way to three-way combination (stratigraphic), to pure stratigraphic traps are common. Four-way closures mapped from the two-dimensional and three-dimensional seismic data are large (P50: 5000 km2) and are covered with thick, lower Miocene fan sandstones. Traps that depend on a stratigraphic component are thinner and smaller in size (P50: 1000 km2), but more numerous than the four-way closures. Because many structures have experienced prolonged pulses of compression, top seal is considered an important geologic risk to the retention of substantial gas-column heights.

Dextral Shear along the Eastern Margin of the Colorado Plateau: A Kinematic Link between Laramide Contraction and Rio Grande Rifting (Ca. 75–13 Ma)

Kinematic data associated with both Laramide‐age and ‐style and Rio Grande rift‐related structures show that the latest Cretaceous to Neogene interaction between the Colorado Plateau and the North American craton was dominantly coupled with a component of dextral shear. Consistent with earlier studies, minor‐fault data in this study yielded results of varied kinematics. Inverted to a common northeast‐oriented hemisphere, the mean trend of kinematic shortening associated with Laramide‐age structures is \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Correlations between fluid composition and deep-seated structural style in the footwall of the Simplon low-angle normal fault, Switzerland

056^{\circ }\pm 6

Describing the dimensionality of geospatial data in the earth sciences—Recommendations for nomenclature

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Paleomagnetic data bearing on style of Miocene deformation in the Lake Mead area, Southern Nevada

300^{\circ }\pm 34