Prominent Bend Research Articles

SE Asia as part of Gondwana: a 1960s regional mapping project over Southern Thailand that lay behind the hypothesis

Abstract In the 1960s, anticipating the award of concessions in the Gulf of Thailand, BP carried out reconnaissance geological mapping over all of Southern Thailand including its Gulf islands. The resulting eight geological maps at 1:250 000 scale provided a database that underpinned discussions of Thailand's place in global plate-tectonic reconstructions that were emerging at that time. In particular, new light was thrown on the Paleozoic succession, and important differences were found between the Upper Thai Peninsula and the Lower Peninsula (respectively, NW and SE of the prominent bend in the peninsula's outline). The Phuket Group of the Upper Peninsula is a very thick diamictite-bearing succession of possible Devonian to Early Permian age, apparently sourced in the west, a region now occupied by the Indian Ocean. That in turn suggested a possible Gondwana origin for SE Asia, a contentious concept at the time, but one that is now widely accepted. The emphasis of field mapping these days is on obtaining detailed data that necessarily means over areas of limited extent. But a very broad regional mapping project such as BP's in Thailand revealed stratigraphic and tectonic insights that could be revealed only from a wide-ranging regional study of this kind.

Polymer-chain configurations in active and passive baths.

The configurations taken by polymers embedded in out-of-equilibrium baths may have broad implications in a variety of biological systems. As such, they have attracted considerable interest, particularly in simulation studies. Here we analyze the distribution of configurations taken by a passive flexible chain in a bath of hard, self-propelled, vibrated disks and systematically compare it to that of the same flexible chain in a bath of hard, thermal-like, vibrated disks. We demonstrate experimentally that the mean length and mean radius of gyration of both chains agree with Flory's law. However, the Kuhn length associated with the number of correlated monomers is smaller in the case of the active bath, corresponding to a higher effective temperature. Importantly, the active bath does not just simply map on a hot equilibrium bath. Close examination of the chains' configurations indicates a marked bias, with the chain in the active bath more likely assuming configurations with a single prominent bend.

A cochlea with three parts? Evidence from otoacoustic emission phase in humans.

The apical and basal regions of the cochlea appear functionally distinct. In humans, compelling evidence for an apical-basal transition derives from the phase of otoacoustic emissions (OAEs), whose frequency dependence differs at low and high frequencies. Although OAEs arising from the two major source mechanisms (distortion and reflection) both support the existence of an apical-basal transition-as identified via a prominent bend (or "break") in OAE phase slope-the two OAE types disagree about its precise location along the cochlea. Whereas distortion OAEs at frequency 2f1-f2 suggest that the apical-basal transition occurs near the 2.5 kHz place, reflection OAEs locate the transition closer to 1 kHz. To address this discrepancy, distortion and reflection OAEs were measured and analyzed in 20 young human adults from 0.25-8 kHz and at eight primary-frequency ratios f2/f1 in the range 1-1.5. Break frequencies and OAE phase-gradient delays were estimated by fitting segmented linear models to the unwrapped phase. When distortion- and reflection-OAE phase are considered as functions of ln f2-that is, as linear functions of the location of their putative site of generation within the cochlea-the analysis identifies not just two but three main cochlear segments, meeting at transition frequencies of approximately 0.9 and 2.6 kHz, whose locations are largely independent both of primary-frequency ratio and emission type. A simple model incorporating an abrupt transition from wave- to place-fixed behavior near the middle of the cochlea accounts for key features of distortion-OAE phase.

Intracratonic stability: A comparison of paleomagnetic data from the north and the south of Dharwar Craton, India

The prominent bend of Dharwar craton, India was recently suggested to post-date the ‘stabilization’ phase of the craton via a ~30°, anticlockwise, vertical-axis intracratonic rotation. The prima facie evidence for this rotation was the realignment of dyke trends following correction of the inferred deformation. The proposal also appeared to be consistent with the observed structural grain in the host Archean basement. We argue that using the trend of dyke swarms is not the ideal method to assess intracratonic rotation and instead we use variations in paleomagnetically determined declination data from these dykes. We find that multiple early Paleoproterozoic dyke swarms (the 2367, 2216, and 2082 Ma generations) preserve a systematic offset with a large uncertainty. The mean paleomagnetically permissible anticlockwise rotation is 19° ± 17°. Dykes emplaced during the ~1888 Ma LIP do not show the same offset, which suggests that rotation occurred prior to this time. Therefore, the 2082-1888 Ma interval brackets the window for any deformation and tectonism required to bend the Dharwar craton by about ~20°. No large-scale deformational events are documented in the Dharwar craton during this critical interval apart from the earliest stages of Cuddapah basin formation. We argue that the ‘observed’ rotation results from biases associated with how trends of dykes and their magnetic directions are compiled when age data are limited. The curvature of geological features in Dharwar craton is likely the tectonic inheritance of the Neoarchean collision between the Western Dharwar and Eastern Dharwar cratons.

Secondary Fault Activity of the North Anatolian Fault near Avcilar, Southwest of Istanbul: Evidence from SAR Interferometry Observations

Strike-slip faults may be traced along thousands of kilometers, e.g., the San Andreas Fault (USA) or the North Anatolian Fault (Turkey). A closer look at such continental-scale strike faults reveals localized complexities in fault geometry, associated with fault segmentation, secondary faults and a change of related hazards. The North Anatolian Fault displays such complexities nearby the mega city Istanbul, which is a place where earthquake risks are high, but secondary processes are not well understood. In this paper, long-term persistent scatterer interferometry (PSI) analysis of synthetic aperture radar (SAR) data time series was used to precisely identify the surface deformation pattern associated with the faulting complexity at the prominent bend of the North Anatolian Fault near Istanbul city. We elaborate the relevance of local faulting activity and estimate the fault status (slip rate and locking depth) for the first time using satellite SAR interferometry (InSAR) technology. The studied NW-SE-oriented fault on land is subject to strike-slip movement at a mean slip rate of ~5.0 mm/year and a shallow locking depth of <1.0 km and thought to be directly interacting with the main fault branch, with important implications for tectonic coupling. Our results provide the first geodetic evidence on the segmentation of a major crustal fault with a structural complexity and associated multi-hazards near the inhabited regions of Istanbul, with similarities also to other major strike-slip faults that display changes in fault traces and mechanisms.

Volar Marginal Rim Fracture Fixation With Volar Fragment-Specific Hook Plate Fixation

To review the outcomes of patients treated with a volar hook plate specifically designed to capture volar marginal rim fractures. A retrospective study was performed over 18 months of patients treated with a volar hook plate in the management of AO type B or C distal radius fractures with a volar marginal rim fragment. Clinical and radiographic outcomes were evaluated. The series included 26 wrists in 25 patients, average age 55 years. Average follow-up was 9 months (range, 3-30 mo). Twenty patients had AO type C fractures and 6 had AO type B fractures. All 6 AO type B were B3 fractures. Of the AO type C, 1 had C1, 7 had C2, and 12 had C3. No patients had loss of fixation of the critical volar ulnar corner and there was no evidence of carpal subluxation. Five patients required hardware removal. Four patients experienced hardware irritation requiring removal of all hardware including the volar hook plate. One patient required partial hardware removal that did not include the volar hook plate. All patients with volar hardware irritation had hook plates that were of second-generation design that had a prominent bend, which has since been modified. There were no cases of tendon rupture. Volar marginal rim fragments of intra-articular distal radius fractures are not amenable to standard volar plate fixation. Fragment-specific fixation using a volar hook plate designed specifically for these fragments allowed for stable fixation when combined with other fragment-specific fixation techniques. There was no loss of fixation of the critical corner in this series. Although hardware irritation can occur, fully seated hooks and subsequent modification of the design of the hook bend has diminished this complication. Therapeutic IV.

Slip distributions on active normal faults measured from LiDAR and field mapping of geomorphic offsets: an example from L'Aquila, Italy, and implications for modelling seismic moment release

Surface slip distributions for an active normal fault in central Italy have been measured using terrestrial laser scanning (TLS), in order to assess the impact of changes in fault orientation and kinematics when modelling subsurface slip distributions that control seismic moment release. The southeastern segment of the surface trace of the Campo Felice active normal fault near the city of L'Aquila was mapped and surveyed using techniques from structural geology and using TLS to define the vertical and horizontal offsets of geomorphic slopes since the last glacial maximum (15±3ka). The fault geometry and kinematics measured from 43 sites and throw/heave measurements from geomorphic offsets seen on 250 scarp profiles were analysed using a modification of the Kostrov equations to calculate the magnitudes and directions of horizontal principal strain-rates. The map trace of the studied fault is linear, except where a prominent bend has formed to link across a former left-stepping relay-zone. The dip of the fault and slip direction are constant across the bend. Throw-rates since 15±3ka decrease linearly from the fault centre to the tip, except in the location of the prominent bend where higher throw rates are recorded. Vertical coseismic offsets for two palaeo earthquake ruptures seen as fresh strips of rock at the base of the bedrock scarp also increase within the prominent bend. The principal strain-rate, calculated by combining strike, dip, slip-direction and post 15±3ka throw rate, decreases linearly from the fault centre towards the tip; the strain-rate does not increase across the prominent fault bend. The above shows that changes in fault strike, whilst having no effect on the principal horizontal strain-rate, can produce local maxima in throw-rates during single earthquakes that persist over the timescale of multiple earthquakes (15±3ka). Detailed geomorphological and structural characterisation of active faults is therefore a critical requirement in order to properly define fault activity for the purpose of accurate seismic hazard assessment. We discuss the implications of modelling subsurface slip distributions for earthquake ruptures through inversion of GPS, InSAR and strong motion data using planar fault approximations, referring to recent examples on the nearby Paganica fault that ruptured in the Mw 6.3 2009L'Aquila Earthquake.

Anatomy of an exposed vertical salt weld and flanking strata, La Popa Basin, Mexico

Abstract La Popa Weld in La Popa Basin, Mexico, is a 24 km long near-vertical structure with a prominent bend approximately halfway along its length. Halokinetic folding, local unconformities and diapir-derived detritus in flanking strata document a precursor salt wall. Shortening during the latest Cretaceous to Eocene Hidalgoan Orogeny squeezed the salt wall to form the weld. Deformation varies significantly along the weld. The northwestern third has remnant gypsum (including a diapir at the northwestern end), little large-scale folding of flanking strata and only background fracture intensity. Directly NW of the bend are pods of gypsum linked by complete welds, a large-scale cuspate anticlinal geometry and significant fracturing within 5–10 m of the weld. The southeastern half is completely welded with no remnant gypsum, a prominent cuspate anticlinal geometry and a 50 m wide damage zone. The variable deformation was controlled by the original width of the salt wall and the amount and direction of shortening. Where orthogonal to the wall, shortening locally closed the diapir but little further deformation took place. Where oblique, shortening caused post-weld dextral strike-slip movement and significant fracturing and shearing of the wall rock. The resulting deformation variability likely impacted the sealing capability of the weld.

Protein-Induced Membrane Curvature Investigated through Molecular Dynamics Flexible Fitting

In the photosynthetic purple bacterium Rhodobacter (Rba.) sphaeroides, light is absorbed by membrane-bound light-harvesting (LH) proteins LH1 and LH2. LH1 directly surrounds the reaction center (RC) and, together with PufX, forms a dimeric (RC-LH1-PufX)2 protein complex. In LH2-deficient Rba. sphaeroides mutants, RC-LH1-PufX dimers aggregate into tubular vesicles with a radius of ∼250–550 Å, making RC-LH1-PufX one of the few integral membrane proteins known to actively induce membrane curvature. Recently, a three-dimensional electron microscopy density map showed that the Rba. sphaeroides RC-LH1-PufX dimer exhibits a prominent bend at its dimerizing interface. To investigate the curvature properties of this highly bent protein, we employed molecular dynamics simulations to fit an all-atom structural model of the RC-LH1-PufX dimer within the electron microscopy density map. The simulations reveal how the dimer produces a membrane with high local curvature, even though the location of PufX cannot yet be determined uniquely. The resulting membrane curvature agrees well with the size of RC-LH1-PufX tubular vesicles, and demonstrates how the local curvature properties of the RC-LH1-PufX dimer propagate to form the observed long-range organization of the Rba. sphaeroides tubular vesicles.

Plate-scale stress fields driving the tectonic evolution of the central Ouachita salient, Oklahoma and Arkansas

Two stages of regional cross-fold jointing refl ect the rearrangement of a lithosphericplate-scale stress fi eld during the Ouachita (i.e., Alleghanian) orogeny. These joints overprint northward-verging thrust-related folds in the Ouachita foreland. Stage-one jointing during the Ouachita orogeny is associated with a stress fi eld recorded by an Atokan and earlier regional joint set, which has a NNE strike. Stage-one jointing is separated into two phases in the central Ouachita salient, where a secondorder recess is marked by a prominent bend in the Windingstair fault below the forelandmost sheet of the central thrust belt. The Big Cedar recess developed when the Windingstair sheet overrode the frontal imbricate zone, thereby causing the Big Cedar pinch zone, a structure refl ecting fold-parallel shortening proximal to localized dextral transpression in the Choctaw thrust sheet. A later regional joint set, representing the second stage of jointing during the Ouachita orogeny, affects rocks in the Arkoma foreland as well as in the fold belt. Stage-two jointing records a stress fi eld associated with NNW-directed closure during the fi nal stage of plate convergence; this stage persisted into the Desmoinesian.

Radiometric ages for basement rocks from the Emperor Seamounts, ODP Leg 197

The Hawaiian‐Emperor seamount chain is the “type” example of an age‐progressive, hot spot‐generated intraplate volcanic lineament. However, our current knowledge of the age distribution within this province is based largely on radiometric ages determined several decades ago. Improvements in instrumentation, sample preparation methods, and new material obtained by recent drilling warrant a reexamination of the age relations among the older Hawaiian volcanoes. We report new age determinations (40Ar‐39Ar incremental heating method) on whole rocks and feldspar separates from Detroit (Sites 1203 and 1204), Nintoku (Site 1205), and Koko (Site 1206) Seamounts (Ocean Drilling Program (ODP) Leg 197) and Meiji Seamount (Deep Sea Drilling Project (DSDP) Leg 19, Site 192). Plateaus in incremental heating age spectra for Site 1203 lava flows give a mean age of 75.8 ± 0.6 (2σ) Ma, which is consistent with the normal magnetic polarity directions observed and biostratigraphic age assignments. Site 1204 lavas produced discordant spectra, indicating Ar loss by reheating and K mobilization. Six plateau ages from lava flows at Site 1205 give a mean age of 55.6 ± 0.2 Ma, corresponding to Chron 24r. Drilling at Site 1206 intersected a N‐R‐N magnetic polarity sequence of lava flows, from which six plateau ages give a mean age of 49.1 ± 0.2 Ma, corresponding to the Chron 21n‐22r‐22n sequence. Plateau ages from two feldspar separates and one lava from DSDP Site 192 range from 34 to 41 Ma, significantly younger than the Cretaceous age of overlying sediments, which we relate to postcrystallization K mobilization. Combined with new dating results from Suiko Seamount (DSDP Site 433) and volcanoes near the prominent bend in the lineament [Sharp and Clague, 2002], the overall trend is increasing volcano age from south to north along the Emperor Seamounts, consistent with the hot spot model. However, there appear to be important departures from the earlier modeled simple linear age progression, which we relate to changes in Pacific plate motion and the rate of southward motion of the Hawaiian hot spot.

The Emperor Seamounts: southward motion of the Hawaiian hotspot plume in Earth's mantle.

The Hawaiian-Emperor hotspot track has a prominent bend, which has served as the basis for the theory that the Hawaiian hotspot, fixed in the deep mantle, traced a change in plate motion. However, paleomagnetic and radiometric age data from samples recovered by ocean drilling define an age-progressive paleolatitude history, indicating that the Emperor Seamount trend was principally formed by the rapid motion (over 40 millimeters per year) of the Hawaiian hotspot plume during Late Cretaceous to early-Tertiary times (81 to 47 million years ago). Evidence for motion of the Hawaiian plume affects models of mantle convection and plate tectonics, changing our understanding of terrestrial dynamics.

An overview of the U[sbnd]Pb geochronology of the Paleoproterozoic Torngat Orogen, Northeastern Canada

The Paleoproterozoic Torngat Orogen, northern Labrador and northeastern Quebec, Canada, is the result of oblique collision between the Nain (North Atlantic) and Rae cratons. A comprehensive set of UPb ages allows the chronology of events to be closely bracketed. Subduction occurred below the Nain craton from ca 1910-1870 Ma, and collision between the cratons may have occurred between 1869 and 1844 Ma. Sinistral deformation continued along the intervening Abloviak Shear zone until ca 1824 Ma, and convergence was ultimately impeded by the partial subduction of the southern Rae craton. Post-collisional isostatic uplift of the southern Rae craton initiated deformation in the Komaktorvik zone ca 1798 Ma. A prominent bend in the Abloviak Shear zone may be the result of differential uplift of the western side of the orogen in the south. Sinistral strike-slip and east- as well as west-side-up deformation continued episodically in the Komaktorvik zone until at least 1710 Ma. The tectonic development of the orogen can be explained in terms of modern plate tectonic processes.

Solution structure sf r(gaggacug):d(CAGTCCTC) hybrid: implications for the initiation of HIV-1(+)-strand synthesis

The three-dimensional solution structure of the hybrid duplex r(gaggacug):d(CAGTCCTC) has been determined by two-dimensional NMR, distance geometry (DG), restrained molecular dynamics (rMD) and NOE back-calculation methods. This hybrid, consisting of a purine-rich RNA strand and a pyrimidine-rich DNA strand, is related to the polypurine (+)-strand primer formed after (−)-strand DNA synthesis and RNase H degradation of the viral RNA strand and contains the site of a specific cleavage by reverse transcription (RT) RNase H at the end of the HIV-1 polypurine tract. This polypurine primer is an important intermediate in the formation of virally encoded double-stranded DNA prior to HIV-1 retrovirus integration. The correct processing of this primer is vital in the life cycle of the human immunodeficiency virus type (HIV-1) retrovirus. The structure of the r(gaggacug):d(CAGTCCTC) hybrid, as determined in solution by NMR, is intermediate between canonical A-type and B-type double helices, and has mixed structural characteristics. It is quantitatively different from the previously determined solution structures of other RNA-DNA hybrids, particularly in the width and shape of the major groove, which is wider than the major groove of other hybrids and is close to the dimension of the major groove of B-type DNA duplexes. The structure of this hybrid duplex contains a prominent bend in the double helix with a magnitude and direction similar to the bend in Okazaki fragments. The structural features of the present duplex may explain the unique interactions of this sequence with HIV-1 RT during both (−)-strand and (+)-strand DNA synthesis.

Solution structure of an RNA x DNA hybrid duplex containing a 3'-thioformacetal linker and an RNA A-tract.

Neutral and achiral backbone linkers are promising replacements for the phosphodiester linkages of antisense oligonucleotides that target mRNA sequences. Results are presented here for the solution structure elucidation by NMR of an RNA x DNA hybrid duplex, r(GCGCAAAACGCG) x d(CGCGTT-SCH2O-TTGCGC) (designated RIII), containing a 3'-thioformacetal (3'-TFMA) backbone substitution in the DNA strand. The 3'-thioformacetal linker can be accommodated in the hybrid duplex in a conformation that is drastically different from its form in a DNA x DNA duplex but close to that of the canonical A-form helix, reflecting the sequence requirement for hybridization. While the global features of RIII are similar to what are described in the literature, the 3'-TFMA modification drives sugar puckers of the adjacent residues to more C3'-endo-like conformations and causes distortions in related twist angles and helical rises. The helical conformation analyses of each of the two strands and the hybrid duplex enable a clear account of the conformational variability of both the DNA and RNA strands. The A-tract in the RNA strand features an overall straight helix and a more prominent bend at the 3'-end CG step. The structure of RIII provides a structural basis for the improved thermal stability of RIII compared to the corresponding DNA x DNA duplex and insights into the factors that are important concerns for the design of new, effective antisense oligonucleotides.

Construction of plastic contact deformation maps on ceramics: a case study on aluminum nitride

Modern polishing, precision machining and microindentation techniques allow the processing and mechanical characterization of ceramics at nanometric scales and within entirely plastic deformation regimes. The mechanical response of most ceramics to such highly constrained contact is not predictable from macroscopic properties and the microstructural deformation patterns have proven difficult to characterize by the application of any individual technique. In this study, TEM techniques of contrast analysis and CBED are combined with stereographic analysis to construct a three-dimensional microstructure deformation map of the surface of a perfectly plastic microindentation on macroscopically brittle aluminum nitride.The bright field image in Figure 1 shows a lg Vickers microindentation contained within a single AlN grain far from any boundaries. High densities of dislocations are evident, particularly near facet edges but are not individually resolvable. The prominent bend contours also indicate the severity of plastic deformation. Figure 2 is a selected area diffraction pattern covering the entire indentation area.

Focal mechanisms of large earthquakes in the South Island of New Zealand: implications for the accommodation of Pacific-Australia plate motion

The plate motion model NUVEL-1 predicts oblique convergence between the Pacific and Australian plates in the South Island of New Zealand. We used P and SH body waveform analysis to constrain the focal mechanisms of the 15 largest earthquakes (Ms > 5.8) that have occurred in this region since 1964, in order to see how the plate motion is accommodated. At the southern end of the Alpine Fault, convergence is achieved by oblique slip movement along a concentrated zone of deformation. In the southern offshore region one event may be related to thrusting of the Australian plate beneath the Pacific plate, and another strike-slip event probably demonstrates movement on an active strike-slip fault system parallel to, but offset from, the southern limit of the Alpine Fault. This geometry provides a possible mechanism for the rapid uplift of the Fiordland region. Deformation in the northern South Island is more distributed. In the south-west Marlborough region partitioning occurs between strike-slip faulting in the SE and reverse faulting farther NW in the Buller region. We suggest that the partitioning developed as a consequence of an increasing component of shortening that was accommodated by slip on reactivated pre-existing normal faults in the Buller region. Shortening in the Buller region may have deflected the NE end of the Alpine Fault towards the NW, forming the prominent bend. The Marlborough Fault System, with its youngest and most active faults to the SE, probably developed in an attempt to maintain a through-going strike-slip structure as each of the strike-slip faults was transported towards the north-west. Partitioning of the opposite polarity (with reverse faulting SE of the strike-slip faulting) occurs in north-east Marlborough. The boundary between the two different styles of partitioning in NE and SW Marlborough appears to coincide with a change in the nature of the downgoing slab and a change in strike of faults of the Marlborough Fault System. A normal faulting earthquake on the northern edge of the Chatham rise probably results from a complex interaction of the buoyant continental crust in that region with the subduction zone and the overlying Marlborough Fault System.

10. A positive cis-element of the human c-myc gene with homo-purine/homo-pyrimidine-like sequence binds proteins with unusual properties

The three-dimensional solution structure of the hybrid duplex r(gaggacug):d(CAGTCCTC) has been determined by two-dimensional NMR, distance geometry (DG), restrained molecular dynamics (rMD) and NOE back-calculation methods. This hybrid, consisting of a purine-rich RNA strand and a pyrimidine-rich DNA strand, is related to the polypurine (+)-strand primer formed after (−)-strand DNA synthesis and RNase H degradation of the viral RNA strand and contains the site of a specific cleavage by reverse transcription (RT) RNase H at the end of the HIV-1 polypurine tract. This polypurine primer is an important intermediate in the formation of virally encoded double-stranded DNA prior to HIV-1 retrovirus integration. The correct processing of this primer is vital in the life cycle of the human immunodeficiency virus type (HIV-1) retrovirus. The structure of the r(gaggacug):d(CAGTCCTC) hybrid, as determined in solution by NMR, is intermediate between canonical A-type and B-type double helices, and has mixed structural characteristics. It is quantitatively different from the previously determined solution structures of other RNA-DNA hybrids, particularly in the width and shape of the major groove, which is wider than the major groove of other hybrids and is close to the dimension of the major groove of B-type DNA duplexes. The structure of this hybrid duplex contains a prominent bend in the double helix with a magnitude and direction similar to the bend in Okazaki fragments. The structural features of the present duplex may explain the unique interactions of this sequence with HIV-1 RT during both (−)-strand and (+)-strand DNA synthesis.

Structural analysis of a ductile-brittle Precambrian shear zone in the Sierra Madre, Wyoming: western extension of the Cheyenne belt?

Abstract Existing plate tectonic models for the Cheyenne belt, a Proterozoic suture at the southern margin of the Archean Wyoming craton, are based largely on data from the Medicine Bow Mountains, southern Wyoming. This paper tests the hypothesis that a major structural and geochronological discontinuity in the Sierra Madre of southern Wyoming is the western extension of the Cheyenne belt. The Sierra Madre shear zone consists of a cataclastic western segment and a mylonitic eastern segment that comprises several mylonite zones. The eastern segment records early ductile thrusting (ca. 1750 Ma) and later dextral strike-slip faulting as revealed by macrostructural, microstructural, and quartz fabric analysis. Structures associated with these events are kinematically compatible and coeval with structures within the suture zone in the Medicine Bow Mountains. The correlation of shear zones in the two ranges is supported further by lithologic similarity of distinctive rocks north of and within the shear zones and the presence of unusual quartz fabrics documenting 〈 c 〉 slip in synkinematic granites in both ranges. Isolated exposures of mylonite along the dominantly cataclastic western segment suggest that the mylonitic shear zone was formerly more extensive. The western segment of the Sierra Madre shear zone is a single zone of intense cataclasis that truncated and dismembered the earlier ductile shear zone. Kinematic analysis within the cataclastic zone suggests that it formed during north-directed thrusting. East-striking, south-dipping thrust faults and associated overturned-to-the-north recumbent folds are developed in supracrustal rocks directly north of the cataclastic fault zone. This spatial association of north-vergent structures and their apparent kinematic compatibility suggests that they are genetically related. Two major northwest-striking, cataclastic, dextral strike-slip in the eastern Sierra Madre appear to merge with the cataclastic thrust fault. These cataclastic faults are interpreted as parts of a north-vergent, thrust-tear system that cut across the original suture zone as rocks in the upper plate of the thrust were transported to the north. It is not known whether this deformational event occurred in response to continued, post-suture convergence to the south or if it represents a separate and distinct Precambrian event. The prominent bend in the Sierra Madre shear zone may be an oroclinal flexure produced as a consequence of northward translation along the cataclastic thrust system.

Aftershocks of the Coyote Lake, California, earthquake of August 6, 1979: A detailed study

Aftershock hypocenters and focal mechanism solutions for the Coyote Lake, California, earthquake reveal a geometrically complex fault structure, consisting of multiple slip surfaces. The faulting surface principally consists of two right stepping en echelon, northwest trending, partially overlapping, nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 Parkfield, California, earthquake. The overlap occurs near a prominent bend in the surface trace of the Calaveras fault at San Felipe Lake. Slip during the main rupture, as inferred from the distribution of early aftershocks, appears to have been confined to a 14‐km portion of the northeastern sheet between 4‐ and 10‐km depth. Focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex, with left stepping imbricate structure. Seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock. Aftershocks in this zone define a single vertical plane 8 km long between 3‐ and 7‐km depth. Within the overlap zone between the two main slip surfaces, the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone, in close agreement with the stress perturbations predicted by crack interaction models. Aftershock activity in the overlap zone is not associated with a simple dislocation surface. Space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone. This alternation is consistent with local stress perturbations predicted by crack interaction models. We conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which, in turn, control the displacements.Table 5 is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J82‐006; $1.00. Payment must accompany order.