Weathering history of an exposed bedrock fault surface interpreted from its topography

Morphologic features of bedrock fault scarps are underutilized in studying faulting and weathering history, partly because of a lack of accurate quantitative parameters for topography. The study employs ground-based LiDAR to measure five patches at different levels on the same fault surface and then calculates roughness in the form of power spectral density in directions parallel and perpendicular to the slip. The power spectral density and spatial frequency typically follow a power law for each fault patch, showing approximately linear relationships in a log–log plot. However, due to additional power introduced by weathering, all spectral curves, especially those parallel to the slip, can be divided into two segments, lower-frequency (wavelengths of several centimeters – several meters) and higher-frequency (wavelengths of several centimeters and below) domains. This shows that the topographic features at different spatial scales are dominated by different mechanical processes: faulting abrasion in the lower-frequency domain and the weathering process in the higher-frequency domain. Moreover, we develop two parameters to quantify the degree of weathering of a fault outcrop, which is significant to describe the evolution of the fault-scarp and infer the date of faulting under calibration.     

Growth, linkage, and termination processes of a 10-km-long strike-slip fault in jointed granite: the Gemini fault zone, Sierra Nevada, California

Field-based structural analysis of an exhumed, 10-km-long strike-slip fault zone elucidates processes of growth, linkage, and termination along moderately sized strike-slip fault zones in granitic rocks. The Gemini fault zone is a 9.3-km-long, left-lateral fault system that was active at depths of 8–11 km within the transpressive Late-Cretaceous Sierran magmatic arc. The fault zone cuts four granitic plutons and is composed of three steeply dipping northeast- and southwest-striking noncoplanar segments that nucleated and grew along preexisting cooling joints. The fault core is bounded by subparallel fault planes that separate highly fractured epidote-, chlorite-, and quartz-breccias from undeformed protolith. The slip profile along the Gemini fault zone shows that the fault zone consists of three 2–3-km-long segments separated by two ‘zones’ of local slip minima. Slip is highest (131 m) on the western third of the fault zone and tapers to zero at the eastern termination. Slip vectors plunge shallowly west-southwest and show significant variability along strike and across segment boundaries. Four types of microstructures reflect compositional changes in protolith along strike and show that deformation was concentrated on narrow slip surfaces at, or below, greenschist facies conditions. Taken together, we interpret the fault zone to be a segmented, linked fault zone in which geometrical complexities of the faults and compositional variations of protolith and fault rock resulted in nonuniform slip orientations, complex fault-segment interactions, and asymmetric slip-distance profiles.     

Fission-track dating of the western border of the Bohemian massif: thermochronology and tectonic implications

 Since 1985, apatite fission-track analysis was applied to more than 70 samples from surface outcrops and shallow boreholes at the western margin of the Bohemian massif. Apatite ages were determined by the grain-population method. Additional information from the frequency distributions of fully confined spontaneous tracks was used for modelling of t–T paths in the low-temperature range (<120 °C). Seven zircon samples were dated by the external detector method. Zircon ages between 283 and 215 Ma indicate unroofing during the Permian molasse stage and the Triassic. Tectonic quiescence and slow subsidence prevailed from the Jurassic until the middle Cretaceous. In the basement area south of Weiden, a Mesozoic partial annealing zone (for apatite fission tracks) is now exposed at the surface. Farther north, the basement was affected by stronger Cretaceous and Palaeogene erosion, which yielded cooling ages between 110 and 49 Ma. This second period of post-Variscan denudation was correlated to reverse faulting along the Franconian Line. Received: 30 June 1996 / Accepted: 24 October 1996     

Model for Paleoearthquake Recurrence Along Baying Segment of the North Margin Fault of Fanshan Basin in Hebei Province

Based on the measurement of faulted geomorphic surfaces,large trench logging and systematic chronostratigraphic sampling,we suggest that the total slip amount along the Baying segment of the north margin fault of Fanshan Basin since 90 ka B.P.is 34 m.Nine paleoseismic events occurred in the last 75 ka.Among them,the four events A,B,C,and D revealed by the large trench were dated at 28 ka,14 ka,7.5 ka,and 656 B.P.,respectiv eJy.The model for paleoearthquake recurrence along the segment may have two stages; in the stage from 90 ka to 14 ka B.P.,the "time-predictable model" is suitable,while in the stage from 14 ka B.P.to now,the "displacement-attenuation model" applies.     

Ground-penetrating radar detection of small-scale channels,joints and faults in the unconsolidated sediments of the Atlantic Coastal Plain

The geological characterization of the shallow subsurface in the unconsolidated sediments of the Atlantic Coastal Plain, and other unconsolidated sediment regimes, may involve jointing, faulting, and channeling not readily detectable by conventional drilling and mapping. A knowledge of these features is required in environmental, geotechnical, and geomorphological studies. Ground-penetrating radar (GPR) may be used to routinely map these structures. Three principal shallow subsurface features are readily detectable using GPR: paleochannels, joints or fractures, and faults. The detection of paleochannels is dependent on the scale of the GPR survey and the attitude of the channel within the survey area. Channel morphological features such as scour surfaces, point bars, and thalwegs are observable. Joints and fractures are more difficult to detect depending upon size, patterns, orientation, and fill material. Vertical joints may not be visible to radar unless they are wider than the sampling interval or are filled with radar-opaque materials such as limonite. Angled joints or fractures may be distinguished by an apparent continuous reflector on the radar profile. Faulting on radar profiles may be observed by the offset of reflectors, the image of the fault plane, or the coherent interpretation of a fault system.     

Damage formation associated with bending under a constant moment

Deformation within the Earth's lithosphere is largely controlled by the rheology of the rock. Fracture and faulting are characterized by elastic rheologies with brittle mechanisms, while folding and flow are characterized by plastic and/or viscous rheologies due to ductile mechanisms. However, it has been recognized that deformation that resembles ductile behavior can be produced within the confines of the brittle lithosphere. Specific examples are folds that form in the shallow crust, steep hinges at subduction zones that are accompanied by seismicity, and large-scale deformation at plate boundaries. In these cases, the brittle lithosphere behaves elastically with fracture and faulting yet produces ductile behavior. In this paper, we attempt to simulate such ductile behavior in elastic materials using continuum damage mechanics. Engineers utilize damage mechanics to model the continuum deformation of brittle materials. We utilize a modified form of damage mechanics that represents a reduction in frictional strength of preexisting fractures and faults. We use this empirical approach to simulate the bending of the lithosphere under the application of a constant moment.We use numerical simulations to obtain elastostatic solutions for plate bending and where the longitudinal stress at a particular node exceeds a yield stress, we apply damage to reduce Young's modulus at the node. Damage is calculated at each time step by a power-law relationship of the ratio of the yield stress to the longitudinal stress and the yield strain to the longitudinal strain. This results in the relaxation of the material due to increasing damage. To test our method, we apply our damage rheology to an infinite plate deforming under a constant bending moment. We simulate a wide range of behaviors from slow relaxation to instantaneous failure, over timescales that span six orders of magnitude. Using this method, stress relaxation produces elastic-perfectly plastic behavior in cases where failure does not occur. For cases of failure, we observe a rapid increase in damage leading to failure, analogous to the acceleration of microcrack formation and acoustic emissions prior to failure. The changes in the rate of damage accumulation in failure cases are similar to the changes in b-values of acoustic emissions observed in triaxial compression tests of fractured rock and b-value changes prior to some large earthquakes. Thus continuum damage mechanics can simulate the phenomenon of ductile behavior due to brittle mechanisms as well as observations of laboratory experiments and seismicity.     

Extensional neotectonics around the bend of the Western/Central Alps: an overview

The Western Alps’ active tectonics is characterized by ongoing widespread extension in the highest parts of the belt and transpressive/compressive tectonics along its borders. We examine these contrasting tectonic regimes using a multidisciplinary approach including seismotectonics, numerical modeling, GPS, morphotectonics, fieldwork, and brittle deformation analysis. Extension appears to be the dominant process in the present-day tectonic activity in the Western Alps, affecting its internal areas all along the arc. Shortening, in contrast, is limited to small areas located along at the outer borders of the chain. Strike-slip is observed throughout the Alpine realm and in the foreland. The stress-orientation pattern is radial for σ3 in the inner, extensional zones, and for σ1 in the outer, transcurrent/tranpressional ones. Extensional areas can be correlated with the parts of the belt with the thickest crust. Quantification of seismic strain in tectonically homogeneous areas shows that only 10–20% of the geodesy-documented deformation can be explained by the Alpine seismicity. We propose that, Alpine active tectonics are ruled by isostasy/buoyancy forces rather than the ongoing shortening along the Alpine Europe/Adria collision zone. This interpretation is corroborated by numerical modeling. The Neogene extensional structures in the Alps formed under increasingly brittle conditions. A synthesis of paleostress tensors for the internal parts of the West-Alpine Arc documents major orogen-parallel extension with a continuous change in σ3 directions from ENE–WSW in the Simplon area, to N–S in the Vanoise area and to NNW–SSE in the Briançon area. Minor orogen-perpendicular extension increases from N to S. This second signal correlates with the present-day geodynamics as revealed by focal-plane mechanisms analysis. The orogen-parallel extension could be related to the opening of the Ligurian Sea during the Early-Middle Miocene and to compression/rotation of the Adriatic indenter inducing lateral extrusion.     

Tertiary paleomagnetic results and structural analysis from the Transdanubian Range (Hungary): rotational disintegration of the Alcapa unit

We carried out an integrated paleomagnetic and structural study in the Transdanubian Range, western and central Hungary. As a result, the Tertiary tectonic history of this area can be characterized by three events of counterclockwise (CCW) rotation and four or five phases of brittle deformation. The change of the orientation of stress axes between phases is mainly apparent and reflects the rotation of the faults predating a particular rotation event. The first two rotation events (R1 and R2) were probably governed by the rollback mechanism of the subducting European plate. We suggest that these rotations were taking place from 18–17 and 16–14.5 Ma, respectively, i.e. simultaneously with the rotations of the North Hungarian Paleogene Basin and the main part of the Western Carpathians. However, the angle of both rotations was less in the Transdanubian Range due to increasing distance from the subduction front. The differential rotation was accommodated by extensional faulting by formation of a graben system. On the other hand, the youngest rotation event R3 seems to be connected to the renewed rotation of the Adriatic plate around 5 Ma. Our combined data set strongly supports earlier conclusions, namely, that the different subunits of the Eastern Alpine–Western Carpathian–Northern Pannonian unit (Alcapa) did not form a rigid unit, although they moved in similar manner.     

Variable post-Paleozoic deformation detected by seismic reflection profiling across the northwestern “prong” of New Madrid seismic zone

High-resolution shallow seismic reflection profiles across the northwesternmost part of the New Madrid seismic zone (NMSZ) and northwestern margin of the Reelfoot rift, near the confluence of the Ohio and Mississippi Rivers in the northern Mississippi embayment, reveal intense structural deformation that apparently took place during the late Paleozoic and/or Mesozoic up to near the end of the Cretaceous Period. The seismic profiles were sited on both sides of the northeast-trending Olmsted fault, defined by varying elevations of the top of Mississippian (locally base of Cretaceous) bedrock. The trend of this fault is close to and parallel with an unusually straight segment of the Ohio River and is approximately on trend with the westernmost of two groups of northeast-aligned epicenters (“prongs”) in the NMSZ. Initially suspected on the basis of pre-existing borehole data, the deformation along the fault has been confirmed by four seismic reflection profiles, combined with some new information from drilling. The new data reveal (1) many high-angle normal and reverse faults expressed as narrow grabens and anticlines (suggesting both extensional and compressional regimes) that involved the largest displacements during the late Cretaceous (McNairy); (2) a different style of deformation involving probably more horizontal displacements (i.e., thrusting) that occurred at the end of this phase near the end of McNairy deposition, with some fault offsets of Paleocene and younger units; (3) zones of steeply dipping faults that bound chaotic blocks similar to that observed previously from the nearby Commerce geophysical lineament (CGL); and (4) complex internal deformation stratigraphically restricted to the McNairy, suggestive of major sediment liquefaction or landsliding. Our results thus confirm the prevalence of complex Cretaceous deformations continuing up into Tertiary strata near the northern terminus of the NMSZ.     

新疆古牧地断裂全新世的断错活动

新疆古牧地断裂是一条全新世活动断裂,全新世以来有４ 次明显的断错活动,历次活动的ＴＬ时间分别为１１．０３±０．８９ ｋａＢ．Ｐ．,９．７３±０．７９ ｋａＢ．Ｐ．,８．３２±０．６７ ｋａＢ．Ｐ．和６．８２±０．５４～４．８７±０．３９ ｋａＢ．Ｐ．,断错活动间隔为１．３±０．１～２．０±０．１５ ｋａ。全新世时期累积垂直断距大于５．０３ｍ ,平均活动速率大于０．４１ ｍ ｍ ／ａ