Donald U. Wise

Tharsis Province of Mars - Geologic sequence, geometry, and a deformation mechanism

Abstract The early history of Mars included two large-scale events of great significance: (1) the lowering and resurfacing of one-third of the crust, followed closely by (2) evolution of the Tharsis bulge. Tharsis development apparently involved two stages: (1) an initial rapid topographic rise accompanied by the development of a vast radial fault system, and (2) an extremely long-lived volcanic stage apparently continuing to the geologic present. A deformational model is proposed whereby a first-order mantle convection cell caused early subcrustal erosion and foundering of the low third of the planet. Underplating and deep intrusion by the eroded materials beneath Tharsis caused isostatic doming. Minor radial gravity motions of surficial layers off the dome produced the radial fault system. The hot underplate eventually affected the surface to cause the very long-lived volcanic second stage. Deep crustal anisotropy associated with the locally NE-trending boundary between the highland two-thirds and the lowland one-third caused the NE elongation of many features of Tharsis.

Fault-related rocks: Suggestions for terminology

Many traditional terms for fault-related rocks have undergone recent dynamic metamorphism under high-pressure discussions by various groups of specialists. A generally acceptable simplified framework encompassing these and associated structural terms is now needed for many geologic, engineering, and legal purposes. Such a framework is proposed here, focusing on a rate-of-strain versus rate-of-recovery diagram and relating this framework to the products of brittle and ductile deformation along faults.

Strike-slip fault-propagation cleavage in carbonate rocks: the Mattinata Fault Zone, Southern Apennines, Italy

Disjunctive, spaced solution cleavage in carbonate rocks is genetically associated with the propagation of the left-lateral, strike-slip Mattinata Fault in the Gargano Promontory, Italy. Typical cleavage development is restricted within the 200‐300-mwide fault zone, which is bounded by virtually unfractured wall rocks. The cleavage consists of sub-parallel solution surfaces, which are often reactivated as sheared solution planes. Geometrical and kinematic relationships exist between the fault and the associated cleavage planes, thus: (1) cleavage‐fault intersection lines lie parallel to the fault and the sheared cleavage rotational axes and (2) the cleavage‐fault angle is almost constantly equal to 408. A model for the development of the Mattinata Fault is proposed in which the cleavage surfaces are interpreted as fault-propagation deformations. Cleavage nucleates as solution planes at the front of the advancing fault as the result of stress concentration in this region. Two distinct, time-sequential processes are shown to operate during the fault propagation: (1) typical millimetre- to centimetre-spaced solution surfaces form in the distal tip zone of the advancing fault plane; (2) as the tip advances, the fault plane breaks through the cleavage as minor shear displacements reactivate some of these nascent surfaces. These observations may prove useful in understanding mechanisms for fault-controlled enhanced/reduced permeability and fluid pathways. # 1999 Elsevier Science Ltd. All rights reserved.

Topographic lineament swarms: Clues to their origin from domain analysis of Italy

Regional-scale, subparallel linear topographic features characterize almost all planetary surfaces. This experiment concerns their tectonic meaning by focusing on map limits or domains through which individual swarms of these linear features are developed. These domain limits are contrasted with map boundaries of better-known structural features in a tectonically active region (Italy) to seek clues to stress environments and times of origin of the lineaments. The study uses regional raised relief maps to filter the lineament data to a very simple subset: those strictly topographic features of a length and prominence capable of retaining detectability through generalizations required by relief-map production. A total of 5,372 lineaments were drawn using 4 differently lighted images of 1:1,000,000 scale relief maps of Italy. Seven different tests were used for reliability and reproducibility of the data. Rose diagrams were prepared for 86 subareas by computer fitting of gaussians to azimuth-frequency histograms. Individual azimuthal “petals” of these roses were then correlated to delimit the general area over which a given azimuthal swarm is developed. The precise swarm boundaries were then located by computer contouring the population density of lines of each swarm on the original data set. In this way, 48 local swarms were mapped. Boundaries of these swarms correlate poorly with traditional litho-tectonic provinces. Instead, they seem to be associated with basin axes, broad arches, coastal flexures, areas of normal fault swarms, and projections of structural grain from adjacent sea floors. The 48 domains may be grouped into 8 noncontiguous but azimuthally compatible super-swarms covering much of Italy. The most prominent of the super-swarms are greatly expanded versions of regional structural grains: trend of the Po Basin, axis of the upper Adriatic Basin, zone of south Alpine underthrusting, and landward extensions of structural grain of the Tyrrhenian Sea. From these relationships, a model for lineament-swarm origins is proposed, involving very minor regional stretching of the thin, brittle carapace of extremely large, sometimes subtle, structures deforming by ductile mechanisms at depth.

AN OUTRAGEOUS HYPOTHESIS FOR THE TECTONIC PATTERN OF THE NORTH AMERICAN CORDILLERA

Cordilleran tectonic trends diverge within the United States to create several distinctive structural provinces. It is proposed that the spreading pattern results from right-lateral distortion of several hundred miles from Paleozoic through modern times across a 300 mile-wide zone from the Colorado Plateau to the Pacific Northwest. Suitable lineaments are indicated on land and sea. Stress-strain patterns within individual structural provinces can be related to this deformation system. The hypothesis is admittedly outrageous and some of its difficulties are pointed out. Nevertheless, it explains enough of the tectonic pattern to merit inclusion among our working hypotheses of causes of Cordilleran spread.

Microjointing in Basement, Middle Rocky Mountains of Montana and Wyoming

Microjoints, defined as four or more macroscopic subparallel fractures spaced closer than 3 mm, appear as pervasive fracture systems in about half the Precambrian basement outcrops of Montana and Wyoming. Six thousand five hundred orientation measurements of the microjoints indicate that the common local pattern is two nearly vertical, perpendicular sets. Persistent vertical orientation at most, but not all, mountain fronts which have undergone demonstrable Laramide rotation of basement indicates microjoints developed subsequent to the bulk of Laramide deformation. The microjoints are interpreted as expansion features in Laramide block mountains, created as the blocks were lifted free of the confines of the adjacent basin floors. Microjoint systems simulate the orientations of ordinary joints, of Precambrian dikelets, and of most planes of microscopic fluid inclusions but are independent in degree of development. In thin section the microjoints are represented by one class of very tiny fluid inclusions paralleling and locally reopening older planes of inclusions. The microscopic fluid inclusion planes are interpreted as a system of Precambrian weakness directions partially controlling the orientations of all subsequent fracture systems including Laramide microjoints. On a regional scale the microjoints have a number of recurring directions similar to Spencer9s (1959) common joint directions in the Beartooth Mountains, Montana. Thus, the available measurements suggest a regional system of eight recurring fracture directions.

Linesmanship and the practice of Linear Geo-art

The theory of Linesmanship and the practice of Linear Geo-art are explained as critiques of methods observed in commonly published classic to recent papers devoted to fracture analysis and related forms of self-delusion.

Structural Geology of the Beartooth Mountains, Montana and Wyoming

The Beartooth Mountains—an 80 by 40 mile elevated crystal block of Precambrian crystalline rocks—are in the Middle Rocky Mountains east of the great north-south belt of overthrusts. The Beartooth Mountains are bounded on the east by the Bighorn Basin (average structural relief, 15,000 feet); on the northeast and north by the Nye-Bowler lineament (along which movement has been dominantly left-lateral); and on the north by the Crazy Mountain syncline (average structural relief of at least 10,000 feet). The southwest side (Yellowstone-Absaroka Mountains) is mostly covered by thick Tertiary volcanic rocks. Detailed mapping, supplemented by results of earlier workers, provides an integrated geologic picture of the tectonic development of the block. Much of the tectonic history is recorded in the structures developed in the Paleozoic and Mesozoic sedimentary rocks exposed around the edge of the mountains. The sedimentary rocks played a relatively passive role in the dynamic rise of the block during Laramide time, as they draped, folded, broke, and slid along the edge of the block. A thrust plane(s) dipping west-southwest is the major structural feature between the Clarks Fork of the Yellowstone and Nye, Montana. Near Red Lodge numerous tear faults displace the upper plate of the thrust as much as 10,000 feet. From Nye to Livingston, Montana, the major bounding structure is a north-northeast-dipping thrust plane(s) terminating in several left-lateral tear faults. Near Gardiner a high-angle thrust plane dipping northeast bounds part of the southwest Beartooth. Several structural mechanisms apparently operated during deformation: (1) Uplifting, tilting, and depressing of large crustal blocks. These commonly form ramps from the Beartooth massif to adjacent basins; during uplift smaller blocks were rotated in the opposite sense. (2) Vertical raising of the northwest and southeast corners. Here, the general lack of horizontal displacement relative to the adjacent basins limits the total horizontal displacement of the block. (3) Thrust faulting. Most of the frontal thrusts show moderate dips. Imbrication is developed particularly at the unconfined northeast corner. The major thrust steepens with structural depth to the south. The horizontal thrust displacement is not directly measurable at most places but is limited by the lack of horizontal movement at the two fixed corners of the block. (4) Tear faulting. Several major tear faults (displacements up to 10,000 feet) bound large keystone-shaped blocks near Red Lodge. (5) Lateral shearing. During the late stage of uplift, horizontal movement toward the unconfined Red Lodge corner created major lateral shears, associated with imbrication, along the north and east sides, with folding along vertical axes and consequent shortening of the sedimentary section. Tectonic development is attributed to (1) horizontal compressive forces, responsible for early crustal block movements and subcrustal plastic shifting, and (2) vertical forces of fluid pressures from the continued transfer of subcrustal material which probably helped raise the block. Uplift ceased when one or both of these forces terminated.

A new method of fracture analysis: Azimuth versus traverse distance plots

Traditional fracture analysis emphasizes the statistics of fracture orientations at individual stations. A new method, the azimuth versus traverse distance plot (AVTD), is proposed, whereby map patterns and fracture domain boundaries receive much greater emphasis. The method can be applied to any area where outcrop is nearly continuous and map data can be represented by strike alone. Ideal features for the method are nearly vertical joint sets which can be plotted on the proposed AVTD diagrams and contoured with concentration factor statistics somewhat analogous to those used for equal-area plots. Examples of AVTD plots are given for jointing in the southern Beartooth Mountains, Wyoming, illustrating ways in which generally overlooked joint set patterns may be identified and ways in which joint domain boundaries may be integrated in geologic mapping using the plots. The examples show the utility of the method in assigning relative ages to fracture domains, in relating the domain distributions to other structures, and in testing for tectonic heredity of older fractures by upward propagation through an unconformity.

Paleostress adjacent to the Alpine fault: Broader implications from fault analysis near Nelson, South Island, New Zealand

Stress tensor determinations from probable Mio-Pliocene age minor faults along the edge of the Richmond block, northwest of the Alpine fault system of South Island, show pervasive thrusting rather than strike-slip motion as proposed in some analyses. This study, with results almost identical to that of Nicol and Wise (1992) on the other side of the Alpine/Marlborough fault system, shows that the present pattern of regional compression at high angles to the Alpine fault has persisted through much of the Neogene over a width of about 150 km normal to the Alpine fault zone. One explanation might be San Andreas-type models invoking strike-slip motion concentrated in fault zones by highly lubricating gouge coupled with strain partitioning to allow compression in adjacent regions to act at high angles to strike. Such models are difficult to apply to the New Zealand example which now must include deformed forelands on either side of the zone but which shows little or no strike-slip motion. Instead, a model is proposed for a crustal scale “flower structure” to accommodate the along-strike convergence of opposite dipping, subducting plates. A regional surface slab comprising Marlborough and its adjacent forelands is undergoing compression normal to the strike of the system while semidetached, deeper plate motion accommodates most of the fundamental, converging, oblique, strike-slip displacements.