Entire Fault Zone Research Articles

Seismic Reflection and Borehole Constraints on the Tectonic Activity of the North Branch of the Lintong–Chang’an Fault: Implications for the Seismic Hazards near the Megacity of Xi’an, Central China

Abstract Active faults that develop in urban regions pose significant seismic hazards to cities with densely concentrated populations and wealth, as demonstrated by several destructive earthquake events in the recent decades. Lintong–Chang’an fault is a known active fault, which comprises multiple branches and traverses the urban area of Xi’an in Weihe Graben—a prominent Chinese megacity with a rich 3000-year-old civilization and a population of 13 million. High-resolution seismic reflection profiles and borehole transects, combined with Quaternary strata dating, reveal that: (1) to the south of Shenhe Loess Tableland, two northern branches of the fault zone follow the trend of the middle part and extend to the front of the Qinling mountains in the SW240° direction; (2) the strata since the late Middle Pleistocene on the borehole transect have been offset, with the vertical displacement of the ∼216 ka layer measured at 5.9 ± 0.3 m, the ∼118 ka layer at 3.8 ± 0.3 m, and 41 ka layer at 1.0 ± 0.1 m, indicating an average vertical slip rate of 0.02–0.04 mm/yr for the individual branch at the study site. Notably, the slip rate of the entire fault zone could be two to three times that of a single branch. Despite the relatively low-slip rate, the fault traverses the megacity of Xi’an, is situated in the relay zone of two large, strongly active basin boundary normal faults (Huashan and Qinling Piedmont faults) and is responsible for the formation of Xi’an ground fissures. Hence, it is necessary to pay special attention to this fault.

Fault Source Models Show Slip Rates Measured across the Width of the Entire Fault Zone Best Represent the Observed Seismicity of the Pallatanga–Puna Fault, Ecuador

AbstractWe explore how variation of slip rates in fault source models affect computed earthquake rates of the Pallatanga–Puna fault system in Ecuador. Determining which slip rates best represent fault-zone seismicity is vital for use in probabilistic seismic hazard assessment (PSHA). However, given the variable spatial and temporal scales slip rates are measured over, significantly different rates can be observed along the same fault. The Pallatanga–Puna fault in southern Ecuador exemplifies a fault where different slip rates have been measured using methods spanning different spatial and temporal scales, and in which historical data and paleoseismic studies provide a record of large earthquakes over a relatively long time span. We use fault source models to calculate earthquake rates using different slip rates and geometries for the Pallatanga–Puna fault, and compare the computed magnitude–frequency distributions (MFDs) to earthquake catalog MFDs from the fault zone. We show that slip rates measured across the entire width of the fault zone, either based on geodesy or long-term geomorphic offsets, produce computed MFDs that compare more favorably with the catalog data. Moreover, we show that the computed MFDs fit the earthquake catalog data best when they follow a hybrid-characteristic MFD shape. These results support hypotheses that slip rates derived from a single fault strand of a fault system do not represent seismicity produced by the entire fault zone.

Along-strike architectural variability of an exhumed crustal-scale seismogenic fault (Bolfin Fault Zone, Atacama Fault System, Chile)

Fault zone architecture and its internal structural variability play a pivotal role in earthquake mechanics, by controlling, for instance, the nucleation, propagation and arrest of individual seismic ruptures and the evolution in space and time of foreshock and aftershock seismic sequences. Nevertheless, the along-strike architectural variability of crustal-scale seismogenic sources over regional distances is still poorly investigated. Here, we describe the architectural variability of the >40-km-long exhumed, seismogenic Bolfin Fault Zone (BFZ) of the intra-arc Atacama Fault System (Northern Chile). The BFZ cuts through plutonic rocks of the Mesozoic Coastal Cordillera and was seismically active at 5–7 km depth and ≤ 300 °C in a fluid-rich environment. The BFZ includes multiple altered fault core strands, consisting of chlorite-rich cataclasites-ultracataclasites and pseudotachylytes, surrounded by chlorite-rich protobreccias to protocataclasites over a zone up to 60-m-thick. These fault rocks are embedded within a low-strain damage zone, up to 150-m-thick, which includes strongly altered volumes of dilatational hydrothermal breccias and clusters of epidote-rich fault-vein networks at the linkage of the BFZ with subsidiary faults. The strong hydrothermal alteration of rocks along both the fault core and the damage zone attests to an extensive percolation of fluids across all the elements of the structural network during the activity of the entire fault zone. In particular, we interpret the epidote-rich fault-vein networks and associated breccias as an exhumed example of upper-crustal fluid-driven earthquake swarms, similar to the presently active intra-arc Liquiñe-Ofqui Fault System (Southern Andean Volcanic Zone, Chile).

Stress drops of hydraulic fracturing induced microseismicity in the Horn River basin: challenges at high frequencies recorded by borehole geophones

SUMMARYThe ground motions caused by seismicity associated with fluid injection can pose a significant hazard. Borehole geophone arrays can provide access to tiny seismic events, which can extend the investigated magnitude range. However, the high frequency phase arrivals (i.e., >100 Hz) also present challenges associated with high frequency cut-offs (fmax), stronger attenuation and resonances within geophones. These effects limit our ability to accurately constrain attenuation models and high frequency source parameters. We investigate 112 −0.6 $\le \, M_{\rm w}\, \le$ 0.7 seismic events and calculate corner frequencies and stress drops from 90 of these events recorded during hydraulic fracturing treatment in the Horn River basin, British Columbia. High frequency resonances (>250 Hz) caused by spurious frequency excitation and/or coupling issues can significantly distort the shape of phase arrival spectra and affect source parameter estimates. Critically, resonances vary in strength between (nearly) colocated events, which may compromise the validity of a spectral ratio approach. For stations showing the cleanest spectra, the Brune model provides a decent fit to the displacement spectra. However, bandwidth limitations, low signal-to-noise ratios, high frequency cut-offs and significant attenuation still hinder our ability to retrieve high frequency source parameters. We find that a frequency independent Qp = 180 ± 40 provides a reasonable model for crustal attenuation but the large uncertainty caused by resonances prevents a robust constraint. From those events that show the best fits, we find a mean Madariaga corner frequency of 210 Hz ± 30 from P-phase arrivals, which is in the range of expected values if self-similarity extends into negative magnitudes. We also calculate a mean stress drop of 1.6 MPa ± 1.2, which is within the tectonic range but slightly lower than other deeper regional studies, which can be explained by lower effective stresses and/or a lower crustal shear strength. We find no evidence for a change in stress drop with depth or distance from the point of injection. A plausible explanation is that effective stresses are lowered relatively quickly over the entire fault zone via direct hydraulic connections. However, the large uncertainties make it difficult to interpret source parameter variability in detail. For high resolution monitoring and source properties of microseismicity, there is an urgent need for high quality high frequency recordings unaffected by spurious frequencies.

Multi-fault segments in the Pernambuco Lineament, Brazil: Implications for fault architecture in intraplate areas

Here we report the intraplate seismicity in the Pernambuco Lineaments (PeL), a Neoproterozoic continuous and vertical ductile shear zone, 700 km long and 5 km wide ~600 Ma in intraplate Brazil. We observed one fault segment in the PeL, here defined as the São Caetano segment, to assess (1) the seismogenic reactivation of the ductile Precambrian basement fabric and (2) the correlation between seismological and structural data derived from satellite imagery to understand fault reactivation, and (3) the identification of a complex multi-fault zone architecture revealed by our seismological observations. In the São Caetano Fault segment, the overall spatial distribution of the seismic clusters follows approximately an E-W direction with a 64° dip towards the N. However, the observed multi-fault architecture revealed by the hypocentral locations exhibits different clusters that do not form a single fault structure. Hence, composite solutions are not necessarily the only representative focal mechanism of an entire fault zone. The inversion of the stress tensor using the focal mechanism from the present study is consistent with S1 nearly vertical and S3 in the E-W direction. This indicates that the interplay between the stress field, inherited structures, and seismicity can be more complex and vary at the local scale of 10s of kilometers. In this intraplate region, we show that the stress field is variable, with long-wavelength variations overprinted by more local and subtle changes, which has profound implications for intraplate seismic hazard mapping since models to estimate hazard rely on simplified assumptions of the fault geometry.

Creep on the Sargent Fault over the Past 50 Yr from Alignment Arrays with Implications for Slip Transfer between the Calaveras and San Andreas Faults, California

ABSTRACT The 55-km-long Sargent fault connects the creeping Calaveras fault with the locked San Andreas fault through the Santa Cruz Mountains west of Gilroy, California. The position of the Sargent fault between these two faults may have implications for slip transfer and strain accumulation between a creeping and locked fault. The detection and measurement of creep on the Sargent fault would indicate where interseismic strain is accumulating adjacent to these neighboring faults. In 1969, two alignment arrays separated by 3.7 km were installed across the central section of the Sargent fault to investigate potential creep. These arrays were measured in 1970 and 1975, and comparison of these measurements yielded a creep rate of 3.4 ± 0.6 mm/yr across two fault strands in the northern array; results from the southern array were never published. In 2019 and 2020, we resurveyed both arrays using a total station and analyzed the results to determine accumulated fault creep. Our results show that between 1970 and 2020, a period of 49.3 yr, the northern array was dextrally offset 164 ± 25 mm across the same two fault strands that were active in the 1970s, yielding an average creep rate of 3.3 ± 1.3 mm/yr. Thus, it appears that the 5 and 50 yr creep rates at this site are similar. The southern array, which may not span the entire fault zone, was dextrally offset 84 ± 13 mm across two fault strands between 1970 and 2019, yielding an average creep rate of 1.7 ± 0.8 mm/yr over 48.9 yr. These recent surveys document continued creep on the Sargent fault, which may reduce seismic strain accumulation and therefore seismic hazard. However, continued aseismic slip on this fault may result in the redistribution of stress and strain to adjacent faults and should be an area of continued study.

Beyond Byerlee friction, weak faults and implications for slip behavior

Some faults are considered strong because their strength is consistent with the Coulomb criterion under Byerlee's friction, 0.6<μ<0.85. In marked contrast, numerous studies have documented significant fault weakening induced by fluid-assisted reaction softening that generally takes place during the long-term evolution of the fault. Reaction softening promotes the replacement of strong minerals with phyllosilicates. Phyllosilicate development within foliated and interconnected fault networks has been documented at different crustal depths, in different tectonic regimes and from a great variety of rock types, nominating fluid-assisted reaction softening as a general weakening mechanism within the seismogenic crust. This weakening originates at the grain-scale and is transmitted to the entire fault zone via the interconnectivity of the phyllosilicate-rich zones resulting in a friction as low as 0.1<μ<0.3.Collectively, geological data and results from laboratory experiments provide strong supporting evidence for structural and frictional heterogeneities within crustal faults. In these structures, creep along weak and rate-strengthening fault patches can promote earthquake nucleation within adjacent strong and locked, rate-weakening portions. Some new frontiers on this research topic regard: 1) when and how a seismic rupture nucleating within a strong patch might propagate within a weak velocity strengthening fault portion, and 2) if creep and slow slip can be accurately detected within the earthquake preparatory phase and therefore represent a reliable earthquake precursor.

Spatiotemporal Variations of Stress and Strain Parameters in the San Jacinto Fault Zone

We analyze the background stress field around the San Jacinto fault zone (SJFZ) in Southern California with a refined inversion methodology using declustered focal mechanisms of background seismicity. Stress inversions applied to the entire fault zone, and three focus areas with high level of seismicity, provide three-dimensional distributions of the maximum horizontal compression direction (SHmax), principal stress plunges, and stress ratio \(R = (\sigma_{1} - \sigma_{2} )/(\sigma_{1} - \sigma_{3} )\). The results are compared with coseismic strain parameters derived from direct summation of earthquake potencies and b values of frequency–size event statistics. The main stress regime of the SJFZ is strike-slip, although the northwest portion near Crafton Hills displays significant transtension. The SHmax orientation rotates clockwise with increasing depth, with the largest rotation (23°) observed near Crafton Hills. The principal stress plunges have large rotations below ~ 9 km, near the depth section with highest seismicity rates and inferred brittle–ductile transition zone. The rotations produce significant deviations from Andersonian strike-slip faulting, likely generating the observed increased dip-slip faulting of relatively deep small events. The stress ratio parameters and b value results are consistent with increasing number of dip-slip faulting below ~ 9 km. The derived coseismic strain parameters are in good agreement with the stress inversion results. No large-scale stress rotations are observed across the time of the 2010 Mw 7.2 El Mayor–Cucapah earthquake. The stress ratio near the Trifurcation area of the SJFZ changes after the El Mayor–Cucapah earthquake toward transpression, but this may have been produced by local M > 5.4 event or aseismic slip.

The Impact of the Low Throw Fault on the Stability of Roadways in a Hard Coal Mine

AbstractEnsuring roadways stability in hard coal mines is one of the main challenges faced by engineers. A changeable geological structure have caused the roadway’s conditions to vary, thus influencing its stability. One of the causes of those changes is the presence of a previously undiscovered fault zone (small faults crossed the roadway) within which a significant convergence or support deformation may occur.The paper presents the impact of low throw faults on the degree of convergence of roadways. Convergence is determined for two roadways in the hard coal mine. A special measuring stations have been installed in one of the roadways, and they have carried out constant measurements for 15 months. In the other roadway, the degree of convergence has been determined on the basis of an on-site verification and comparison of the measurements obtained and the initial values, based on the roadway’s records.On the basis of the obtained convergence results, the impact of a single fault and the entire fault zone on the roadway stability has been determined. The impact of a single, low throw fault results in a 30% higher vertical convergence than in the case of roadways free of geological disturbance. In the roadway section located in the fault zone, vertical convergence is 4 times higher than in the case of sections free of disturbance impact. The floor heaving constitutes ca. 90% of vertical convergence both for roadway sections situated within the faulted zones and for sections free of the influence of any additional factors.

Seismic constraints on the architecture of the Newport‐Inglewood/Rose Canyon fault: Implications for the length and magnitude of future earthquake ruptures

AbstractThe Newport‐Inglewood/Rose Canyon (NIRC) fault zone is an active strike‐slip fault system within the Pacific‐North American plate boundary in Southern California, located in close proximity to populated regions of San Diego, Orange, and Los Angeles counties. Prior to this study, the NIRC fault zone's continuity and geometry were not well constrained. Nested marine seismic reflection data with different vertical resolutions are employed to characterize the offshore fault architecture. Four main fault strands are identified offshore, separated by three main stepovers along strike, all of which are 2 km or less in width. Empirical studies of historical ruptures worldwide show that earthquakes have ruptured through stepovers with this offset. Models of Coulomb stress change along the fault zone are presented to examine the potential extent of future earthquake ruptures on the fault zone, which appear to be dependent on the location of rupture initiation and fault geometry at the stepovers. These modeling results show that the southernmost stepover between the La Jolla and Torrey Pines fault strands may act as an inhibitor to throughgoing rupture due to the stepover width and change in fault geometry across the stepover; however, these results still suggest that rupture along the entire fault zone is possible.

Shape of the Magnitude–Frequency Distribution for the Canterbury Earthquake Sequence from Integration of Seismological and Geological Data

Abstract We combine geological and seismological data to describe the shape of the magnitude–frequency distribution for the Canterbury earthquake sequence. In doing so, we take the opportunity of the new datasets to address a long‐standing debate in the literature as to whether the seismicity of individual faults or fault zones is best described by the Gutenberg–Richter (GR) relationship or the characteristic earthquake (CE) model. We find that the magnitude–frequency distribution for the entire composite fault zone of the Canterbury earthquake sequence is adequately described by the GR relationship when uncertainties in the GR curve (fitted to the instrumental catalog data) and the range of paleoseismically derived recurrence intervals for large (mainshock) earthquakes on the Greendale fault are collectively considered. In contrast, the magnitude–frequency distribution for the smaller area of the Greendale fault is better described by the CE model. The difference is one of scale, in that the composite fault zone represented by the Canterbury earthquake sequence shows a GR distribution, whereas individual faults within the zone show CE distributions. Definition of magnitude–frequency distributions for seismic‐hazard modeling must therefore take scaling considerations into account, rather than simply assuming that a single magnitude–frequency distribution shape is applicable to all fault definitions.

Seismic characterisation of fault damage in 3D using mechanical and seismic modelling

Although typically interpreted as 2D surfaces, faults are 3D narrow zones of highly and heterogeneously deformed rocks with petrophysical properties differing from the host rock. Fault zones have been extensively studied in outcrop, but in the subsurface they are barely explored, mainly because they are at the limit of seismic resolution and are rarely drilled and cored. We present a 3D synthetic workflow to assess the potential of seismic data for imaging and characterising fault damage and properties. The workflow is based on forward modelling techniques. First, we run a 3D discrete element model to simulate faulting and associated deformation. Then, we use simple relationships to modify the initial elastic properties of the model based on its volumetric strain. From this reflectivity cube, we apply a ray-based, pre-stack depth migration simulator. Finally, from the resultant seismic image, we use seismic attributes to characterise the fault volume. We illustrate the workflow for a large displacement normal fault in a sandstone-shale sequence for two cases, one with constant fault displacement and another with linearly variable displacement along strike. Seismic cubes of these models for a homogeneous overburden and several wave frequencies are generated. High frequencies show the impact of the fault on the offset and folding of the reflectors. In the variable fault slip model, the fault has less impact as the displacement decreases, and the fault tipline can be interpreted. We extract the fault geobody using three combined seismic attributes: dip, semblance and tensor. The geobody for the constant fault displacement model corresponds to an inner high-deformation area within the fault zone, while in the variable fault slip model the geobody captures better the entire fault zone. Cross plotting of amplitudes and strains shows that the geobody contains all range of strains, but almost all high strain values are within the geobody. This allows a direct comparison between the fault zone identified on the seismic image and that in the mechanical model.

Transtensional Tectonics of the Minto Flats Fault Zone and Nenana Basin, Central Alaska

Abstract Central Alaska is a broad zone of crustal deformation that is produced by collision and flat slab subduction. Within central Alaska, there are large‐scale right‐lateral strike‐slip faults, such as the Denali fault (2002 M w 7.9), as well as smaller‐scale fold‐and‐thrust belts and a set of left‐lateral strike‐slip fault zones, one of which is the Minto Flats fault zone (MFFZ). We use seismological evidence to document a pair of overlapping left‐lateral faults that define the MFFZ. Microseismicity delineates this 180‐km‐long fault zone. Using body waves and surface waves, we perform moment tensor inversions for the 11 best‐recorded earthquakes in the fault zone. Moment tensors reveal consistent left‐lateral faulting throughout the fault zone. A finite‐source model for the 1995 M w 6.0 earthquake is consistent with left‐lateral faulting and provides rupture details for the largest known event in the fault zone. The two main faults are separated by 10 km and overlap by 67 km, forming a releasing stepover geometry within a local transtensional setting. Between the faults is a 90‐km‐long, 12‐km‐wide, and 8‐km‐deep sedimentary basin (the Nenana basin). We interpret the transtensional faulting to be responsible for the development of the basin over the past 6 Ma. The distances of fault overlap and fault separation are key parameters for determining (1) the 3D morphology of the sedimentary basin and (2) the likelihood of earthquake ruptures jumping from one fault to the next. The structure of the Nenana basin is consistent with shear motion accommodated by the identified faults. The 10 km fault separation suggests that ruptures are not likely to span the entire fault zone. Earthquakes as large as M w 7.0–7.5 could occur on the faults. The transtensional fault zone provides an important constraint for understanding the larger‐scale intraplate tectonic setting of central Alaska. Online Material: Figures of waveform fits, depth estimation for moment tensors, variation in seismicity, and fault‐plane identification.

Experimental study and active tectonics on the Zhangjiakou-Penglai fault zone across North China

The Zhangjiakou-Penglai Fault Zone (ZPFZ), as a large-scale WNW-trending active fault zone with frequent seismic activity, is an area prone to moderate to intense earthquakes across North China. A detailed analysis of the ZPFZ has been carried out based on earthquake activity, deep structural response, dynamic mechanism and physical simulation by conducting an en échelon fault model experiment. The results show that the ZPFZ is a lithospheric active fault zone that has controlled some small en échelon Quaternary pull-apart basins and has triggered many earthquakes as seismogenic structures. The earthquake location and intensity distribution by the experimental simulation fit well with those of actual earthquakes. The earthquake distribution is controlled obviously by the en échelon structural properties. The intensity of seismic activity is strong in the middle and weak at the western and eastern ends of the fault zone, migrating from the southeast to the northwest along the entire fault zone. This WNW-trending fault zone in North China, is closely related to the surrounding plate activities. The ZPFZ is not only dominated by the collision of the Indian Plate to the Eurasian Plate, but is also influenced by the subduction of the Pacific Plate. It depends on the comprehensive reflection of two geodynamic settings from the western collision of the Indian to the Eurasian plates and the eastern subduction of the Pacific Ocean Plate to the Eurasian Plate.

Inelastic response of compliant fault zones to nearby earthquakes in three dimensions

Abstract Using dynamic modeling of earthquake rupture on a strike-slip fault and seismic wave propagation in a three dimensional inhomogeneous elastoplastic medium, we investigate the inelastic response of compliant fault zones to nearby earthquakes. We primarily examine the plastic strain distribution within the fault zone and the displacement field that characterizes the effects of the presence of the fault zone. We find that when the fault zone rocks are close to failure in the prestress field, plastic strain occurs along the entire fault zone near the Earth's surface and some portions of the fault zone in the extensional quadrant at depth, while the remaining portion deforms elastically. Plastic strain enhances the surface displacement of the fault zone, and the enhancement in the extensional quadrant is stronger than that in the compressive quadrant. Inelastic response may be distinguished from elastic response by sympathetic motion (or reduced retrograde motion) exhibited in the fault-parallel horizontal surface displacement in conjunction with enhanced vertical surface displacement in a strike-slip faulting environment. These findings suggest that taking into account both elastic and inelastic deformation of fault zones to nearby earthquakes may improve our estimations of fault zone structure and properties from small-scale surface deformation signals. Furthermore, identifying the inelastic response of nearby fault zones to large earthquakes may allow us to place some constraints on the absolute stress level in the crust.

Deformation style in the Munsiari Thrust Zone: a study in the Madlakia–Munsiari–Dhapa section in north-eastern Kumaun Himalaya

The Main Central Thrust demarcates the boundary between the Lesser Himalaya and the Higher Himalaya in the Himalayan orogen. Several definitions of the Main Central Thrust have been proposed since it was originally described as the southern boundary of the crystalline rocks (the Main Central Thrust mass) in the Kumaun-Garhwal Himalaya. The long-held contention that the Munsiari Thrust represents the Main Central Thrust has been negated by recent isotopic studies. One way to define the Main Central Thrust is that it is a ductile shear zone that is delimited by the Munsiari Thrust (MCT-I) in south and the Vaikrita Thrust (MCT-II) in north. The alternative proposition that the Vaikrita Thrust represents the Main Central Thrust is fraught with practical limitations in many parts of the Himalaya, including the study area. In the metamorphic rocks bounded between the Vaikrita Thrust and the Munsiari Thrust, the isoclinal folds of the earliest phase are routinely ascribed to the pre-Himalayan orogeny, whereas all subsequent folding phases are attributed to the Himalayan orogeny. This article elucidates the structural characteristics of the kilometre-thick Munsiari Thrust Zone and revisits the issue of pre-Himalayan orogenic signatures in the thrust zone. With the help of high-resolution field mapping and the analyses of mesoscopic scale structures, we demonstrate that the Munsiari Thrust is a typical fault zone that is made up of a fault core and two damage zones. The fault core traces the boundary between the quartzite and the biotite-gneiss. The damage zones consist of the low-grade metasedimentary rocks in the footwall and the gneiss-migmatite in the hanging wall. The entire fault zone shares an essentially common history of progressive ductile shearing. Successively developed mesoscopic folds trace various stages of progressive ductile shearing in the damage zones. Two recognizable stages of the shearing are represented by the early isoclinal folds and the late kink folds. As the strain during progressive deformation achieved the levels that were too high for accommodation by ductile flow, it was released by development of a tectonic dislocation along a mechanically weak boundary, the Munsiari Thrust. The isoclinal folds and the Munsiari Thrust were developed at different stages of a common progressive deformation during the Himalayan orogeny. Contrary to the popular notion of consistency with respect to orientation, the stretching lineations show large directional variability due to distortion during the late folding.

Crustal Anisotropy in the Eastern Sea of Marmara Region in Northwestern Turkey

Abstract The North Anatolia Fault Zone (NAFZ) is a transform zone 1600 km in length representing the plate boundary between the westward moving Anatolian Plate and stable Eurasia. Almost the entire fault zone has failed during the last century except for the Sea of Marmara section, which is located in direct vicinity to the city of Istanbul. In this study, we investigate the crustal anisotropy along the eastern Marmara section of the NAFZ based on shear‐wave splitting. We measure seismic anisotropy parameters, namely, the fast polarization direction (PD) and time delay (TD), by analyzing local seismicity recorded at selected seismographs operated throughout the eastern Sea of Marmara region. Our shear‐wave splitting (SWS) observations indicate a predominant northwest–southeast‐oriented PD, which is sub‐parallel to both the orientation of the regional S Hmax in northwest Turkey and the local NAFZ strike along the Princes’ Islands segment. Toward the south, at the Armutlu Peninsula, we find a different PD pattern reflecting local fault strikes, S Hmax as well as strain asymmetry between different crustal blocks across the fault zone. Applying strict quality criteria enables us to identify robust, preferred fast PDs, which suggests that initially observed PD heterogeneities are sometimes caused by second‐order effects in the data rather than by varying PDs. Comparing TD and hypocentral depth distribution, we find the depth extent of the anisotropy is confined to the uppermost 10‐km depth of crust. We combine our SWS results with those from previous studies conducted along the San Andreas fault (SAF) and NAFZ in order to investigate the relation of angular deviations of the PDs from regional S Hmax and local fault strikes with fault‐zone distance. We find that fast PDs are mainly controlled by the local fault structure in close proximity to a fault zone (5 and 10 km) while they are controlled by crustal stress at off‐fault locations (5 and 10 km).

Surprisingly young Rb/Sr ages from the Simav extensional detachment fault zone, northern Menderes Massif, Turkey

The present paper demonstrates that exposed semi-brittle–brittle detachment fault zones, in addition to footwall mylonites and syn-extensional granitoids, can be used to date the timing of faulting and constrain the history and evolution of metamorphic core complexes. We employed Rb–Sr geochronology on micas, sampled directly from a part of the Simav detachment fault (SDF) zone in the northern Menderes Massif (western Turkey). The exposed part of the fault zone is marked by ∼3-m thick zone of low-grade mylonites/foliated cataclasites, in which mylonitic fabrics in orthogneisses are overprinted by fabrics of semi-brittle deformation. The low-grade mylonites/foliated cataclasites are characterized by coexistence of brown and green biotites. Rb–Sr ages on muscovite and brown and green biotite from the low-grade mylonites/foliated cataclasites are ca. 30 Ma, 17–13 Ma and 12–10 Ma, respectively; green biotite ages are interpreted as dating fluid-assisted deformation-induced dynamic recrystallization and suggest that a part of the SDF was active during a 12–10 Ma interval. The ca. 30 Ma muscovite ages date dynamic crystallization and probably beginning of extensional exhumation of the northern Menderes Massif. The coexistence of brown and green biotites in the same sample indicates retrogressive processes associated within a detachment faulting during which green biotites have recrystallized from primary brown biotites with an age of 19 ± 2 Ma in this area. This further means that the isotopic system became opened during faulting and that the green biotite ages therefore record the activity of the SDF. We have also dated an orthogneiss sample exposed well away from the detachment fault zone (devoid of any retrogressive processes); muscovites and biotites from this sample yield Rb–Sr ages of 45.7 ± 0.6 and 18.17 ± 0.18 Ma, respectively. The biotite age is in accord with regional biotite ages (19 ± 2 Ma) and record cooling of the footwall rocks of the detachment fault. We argue that the ca. 46 Ma muscovite age record synkinematic recrystallization during, and lend credibility to interpretation of, the prograde Barrovian-type regional Main Menderes Metamorphism as an Eocene event and that, prior to the onset of extensional detachment formation, the northern Menderes Massif (or at least the rocks we measured) experienced Eocene temperatures in excess of 500 °C to get this age. New brown biotite and secondary green biotite ages together with previously published age data argues that the footwall rocks reached to biotite cooling temperature conditions by ca. 18 Ma and stayed at similar conditions for a longer period until ca. 12 Ma. The ca. 18–12 Ma time interval corresponds to a period of tectonic quiescence during the Neogene and is attributed to the locking of SDF activity by the intrusion and rapid cooling of I-type syn-extensional granites. Secondary green biotite Rb–Sr ages indicate that the fault then become active during 12–10 Ma. Published low-temperature apatite–zircon fission track and apatite (U–Th)/He ages further suggest that displacement along the entire fault zone has continued until 8.0 ± 0.5 Ma. Combined with previous radiometric and low-temperature thermochronologic ages, the new ages argue for episodic core-complex formation and the Simav detachment fault activity between ca. 30 and 8 Ma. The data therefore argues for a complex history of core-complex formation in the northern Menderes Massif; it occurs in pulses and involves periods of alternating rapid and slow cooling/denudation rates.

Along-strike growth of the Ostler fault, New Zealand: Consequences for drainage deflection above active thrusts

[1] Rarely are geologic records available to constrain the spatial and temporal evolution of thrust-fault growth as slip accumulates during repeated earthquake events. Here, we utilize multiple generations of dated and deformed fluvial terraces to explore two key aspects of the along-strike kinematic development of the Ostler fault zone in southern New Zealand over the past ∼100 k.y.: accumulation of fault slip through space and time and fixed-length thrust growth that results in patterns of drainage diversion suggestive of laterally propagating faults. Along the Ostler fault, surface deformation patterns revealed by topographic surveying of terrace profiles in nine transverse drainages define systematic variations in fault geometry and suggest deformation over both listric and planar thrust ramps. Kinematic modeling of folded terrace profiles and >100 fault-scarp surveys along major fault sections reveals remarkably similar slip distributions for multiple successions of geomorphic surfaces spanning ∼100 k.y. Spatially abrupt and temporally sustained displacement gradients across zones of fault section overlap suggest that either persistent barriers to fault propagation or interference between overlapping faults dominate the interactions of fault tips from the scale of individual scarps to the entire fault zone. Deformed terrace surfaces dated using optically stimulated luminescence and cosmogenic radionuclides indicate steady, maximum rates of fault slip of ∼1.9 mm/yr during the Late Quaternary. Slip data synthesized along the central Ostler fault zone imply that displacement accumulated at approximately constant fault lengths over the past ∼100 k.y. A northward temporal progression of abandoned wind gaps along this section thus reflects lateral tilting in response to amplification of displacement, rather than simple fault lengthening or lateral propagation. Oscillations of climate at ∼104-yr time scales modulate the formation and incision of geomorphic surfaces during successive glacial stages. Superimposed on apparently steadier rates of fault slip, such climate-dependent surfaces contribute to a pattern of progressive drainage deflection along the central Ostler fault zone that is largely independent of fault propagation.

Comparison of structural features of the fault zone developed at different protoliths: crystalline rocks and mudrocks

We investigated the mesoscopic and microscopic structural features of the Keumwang fault zone, Korea, to determine the effect of different protoliths (crystalline granitic gneiss and mudrock) on fault zone structure including discrete shear surfaces, shear bands, foliation, porphyroclasts and calcite veins. Overall, in crystalline rock, the shear deformation tends to be localized to a fault core or several fault gouge-dominated layers within the fault zone. The cataclastic flow appears to become stronger toward the fault core. Most of the shear deformation appears to be partitioned between slip on discrete shear surfaces and shear bands, and cataclastic flow, including syntectonic alteration in the fine-grained matrix of fault rocks. In contrast, within mudrocks, shear deformation tends to be relatively uniformly distributed within the entire fault zone. Shearing generates continuous S-foliations throughout the whole fault zone. Most of the shearing is accommodated by slip along these foliations and on discrete shear surfaces distributed within the entire fault zone.