Right-lateral Strike-slip Fault System Research Articles

Internal deformation of the North Andean Sliver in Ecuador-southern Colombia observed by InSAR

Summary In the Northern Andes, partitioning of oblique subduction of the Nazca plate beneath the South American continent induces a northeastward motion of the North Andean Sliver. The strain resulting from this motion is absorbed by crustal faults, which have produced magnitude 7 + earthquakes historically in the Andean Cordillera of Ecuador and southern Colombia. In order to quantify the strain in that area, we derive a high-resolution surface velocity map using InSAR time-series processing. We analyzed 6 to 8 years of Sentinel-1 data and combined different satellite line-of-sight directions to produce a reliable velocity map in the East direction. We use interpolated GNSS data to express the velocity map with respect to Stable South America and remove the long-wavelength pattern due to the post-seismic deformation following the 2016 Mw 7.8 Pedernales earthquake. The InSAR velocity map finds high E-W shortening strain rates along N-S trending structures within the Western Cordillera and the Interandean valley, with little deformation taking place east of them. This result strengthens the previous proposition of a ∼350 km long Quito-Latacunga tectonic block, forming a restraining bend in the overall right-lateral strike-slip fault system accommodating the northeastward escape motion of the North Andean Sliver. However, the high spatial resolution provided by InSAR indicates that previously proposed boundaries for this block need to be revised. In particular, InSAR results highlight high strain rate (>300 nstrain/yr) along undescribed active structures, south and west of the proposed limits for the Quito-Latacunga block, respectively in Peltetec and Ibarra regions. Interestingly, the two areas with the largest strain rates spatially correlate with the proposed areas of large historical earthquakes. Modeling of the InSAR and GNSS velocities in these areas suggests shallow coupling and high slip rates on structures which, previously, were not identified as active. We also demonstrate a slow-down of the shallow aseismic slip on the Quito fault after the Pedernales earthquake, suggesting that stress changes following large megathrust events might trigger transient slip behaviors on crustal faults. The high-resolution strain map provided by this work provides a new basis for future tectonic models in the Ecuadorian and southern Colombian Andes, and will contribute to the seismic hazard assessment in this highly populated area of the Andes.

Flexural-slip folding in buckling phases of orogenic belts: Insight into the tectonic evolution of fault splays in the East Iran orogen

Introduction: The East Iran orogen has experienced multiple buckling phases resulting in the formation of strike-slip fault splays. The geometric and kinematic characteristics of these splays are influenced by folding mechanisms. This study focuses on investigating the structural characteristics and tectonic evolution model of the Khousf splay, located in the northern terminus of the Nehbandan right-lateral strike-slip fault system.Methods: Field visits and geometrical properties from map views were used to analyze the structural features of the Khousf splay. The splay was found to consist of a multi-plunging anticline and syncline, referred to as the Khousf anticline and Khousf syncline, respectively. Flexural slip was identified as a significant mechanism for the formation of these structures. Structural evidence, including parasitic folds, active folds, and strike-slip duplexes, suggested that flexural slip occurred on discrete movement horizons among the rock units.Results: Analysis of the parasitic folds in the cores and limbs of the Khousf anticline and syncline revealed M, W, Z, and S shapes, with complex slicken-line patterns observed on faults parallel to the beds at the limbs. The analysis results indicated strain partitioning and inclined left- and right-lateral transpressional zones. Shortening estimates obtained from profiles in the Shekarab inclined transpressional zone were approximately 33%, 65%, and 68% for NE-SW, N-S, and NW-SE profiles, respectively. In the Arc area, which is the core of the anticline, shortening estimates from NE-SW and N-S profiles ranged from 14% to 10%. Structural analysis of the folds in this area revealed broad, close, semi-elliptical, and parabolic shapes, suggesting that secondary folds with NW-SE axis directions have been superimposed on the first-generation folds with E-W axis directions in the Khousf refolded splay.Discussion: The findings of this study highlight the structural characteristics and tectonic evolution model of the Khousf splay in the northern terminus of the Nehbandan right-lateral strike-slip fault system. The results suggest that flexural slip played a crucial role in the formation of the multi-plunging anticline and syncline in the Khousf splay. The presence of parasitic folds and complex slicken-line patterns on faults indicate the complexity of deformation processes. The observed strain partitioning and inclined transpressional zones suggest a complex tectonic history in the study area. The superimposition of secondary folds with different axis directions on first-generation folds adds further complexity to the structural evolution of the Khousf refolded splay. Overall, this study provides new insights into the structural characteristics and tectonic evolution of the Khousf splay in the East Iran orogen.

Active faulting and continental slope instability in the Gulf of Patti (Tyrrhenian side of NE Sicily, Italy): a field, marine and seismological joint analysis

The Gulf of Patti and its onshore sector represent one of the most seismically active regions of the Italian Peninsula. Over the period 1984–2014, about 1800 earthquakes with small-to-moderate magnitude and a maximum hypocentral depth of 40 km occurred in this area. Historical catalogues reveal that the same area was affected by several strong earthquakes such as the Mw = 6.1 event in April 1978 and the Mw = 6.2 one in March 1786 which have caused severe damages in the surrounding localities. The main seismotectonic feature affecting this area is represented by a NNW–SSE trending right-lateral strike-slip fault system called “Aeolian–Tindari–Letojanni” (ATLFS) which has been interpreted as a lithospheric transfer zone extending from the Aeolian Islands to the Ionian coast of Sicily. Although the large-scale role of the ATLFS is widely accepted, several issues about its structural architecture (i.e. distribution, attitude and slip of fault segments) and the active deformation pattern are poorly constrained, particularly in the offshore. An integrated analysis of field structural geology with marine geophysical and seismological data has allowed to better understand the structural fabric of the ATLFS which, in the study area, is expressed by two major NW–SE trending, en-echelon arranged fault segments. Minor NNE–SSW oriented extensional structures mainly occur in the overlap region between major faults, forming a dilatational stepover. Most faults display evidence of active deformation and appear to control the main morphobathymetric features. This aspect, together with diffused continental slope instability, must be considered for the revaluation of the seismic and geomorphological hazard of this sector of southern Tyrrhenian Sea.

Late Oligocene to present contractional structure in and around the Susitna basin, Alaska—Geophysical evidence and geological implications

The Cenozoic Susitna basin lies within an enigmatic lowland surrounded by the Central Alaska Range, Western Alaska Range (including the Tordrillo Mountains), and Talkeetna Mountains in south-central Alaska. Some previous interpretations show normal faults as the defining structures of the basin (e.g., Kirschner, 1994). However, analysis of new and existing geophysical data shows predominantly (Late Oligocene to present) thrust and reverse fault geometries in the region, as previously proposed by Hackett (1978). A key example is the Beluga Mountain fault where a 50-mGal gravity gradient, caused by the density transition from the igneous bedrock of Beluga Mountain to the >4-km-thick Cenozoic sedimentary section of Susitna basin, spans a horizontal distance of ∼40 km and straddles the topographic front. The location and shape of the gravity gradient preclude a normal fault geometry; instead, it is best explained by a southwest-dipping thrust fault, with its leading edge located several kilometers to the northeast of the mountain front, concealed beneath the shallow glacial and fluvial cover deposits. Similar contractional fault relationships are observed for other basin-bounding and regional faults as well. Contractional structures are consistent with a regional shortening strain field inferred from differential offsets on the Denali and Castle Mountain right-lateral strike-slip fault systems.

Structural evolution of the El Salvador Fault Zone: an evolving fault system within a volcanic arc.

The El Salvador Fault Zone, firstly identified after the 13th February 2001 Mw 6.6 El Salvador earthquake, is a 150 km long, 20 km wide right-lateral strike-slip fault system. Ruptures along the ESFZ are thought to be responsible for most of the historical destructive earthquakes along the El Salvador Volcanic Arc, as well as for most of the current seismicity of the area. In this work, we focus on the geological setting of the fault zone by describing its geomorphology and structure, using field-based observations, digital terrain modelling, and aerial photograph interpretation with the aim at contributing to the understanding of the ESFZ slip behaviour. In particular, we address the ESFZ structure, kinematics and evolution with time. The ESFZ is a complex set of traces divided in major rupture segments characterized by different geometry, kinematics and geomorphic expressions. Natural fault exposures and paleoseismic trenches excavated along the fault show that the strike-slip deformation is distributed in several planes. Both geometry and kinematics of the fault zone are consistent with a transtensional strain regime. The estimated geological slip rate for the main fault segments by paleoseismic trenches and displaced geomorphic features implies a deficit in velocity of the fault compared to the available GPS velocities data. The high vertical scarps of some fault segments would require Quaternary slip rates not coherent neither with measured GPS velocities nor with slip rates obtained from paleoseismic analysis. This mismatch suggests a pre-existing graben structure that would be inherited from the previous regional roll back related extensional stage. We consider that the ESFZ is using this relict structure to grow up along it. As a result, we ropose a model for ESFZ development consistent with all these observations.

New insights in the geodynamics of the Lipari–Vulcano area (Aeolian Archipelago, southern Italy) from geological, geodetic and seismological data

Geological, geodetic and seismological data have been analyzed in order to frame the Lipari–Vulcano complex (Aeolian archipelago, southern Italy) into the geodynamic context of the southeastern Tyrrhenian Sea. It is located at the northern end of a major NNW–SSE trending right-lateral strike-slip fault system named “Aeolian–Tindari–Letojanni” which has been interpreted as a lithospheric discontinuity extending from the Aeolian Islands to the Ionian coast of Sicily and separating two different tectonic domains: a contractional one to the west and an extensional one to the north-east. Structural field data consist of structural measurements performed on well-exposed fault planes and fractures. The mesostructures are mostly represented by NW–SE striking normal faults with a dextral-oblique component of motion. Minor structures are represented by N–S oriented joints and tension gashes widespread over the whole analyzed area and particularly along fumarolized sectors. The analyzed seismological dataset (from 1994 to 2013) is based on earthquakes with magnitude ranging between 1.0 and 4.8. The hypocenter distribution depicts two major alignments corresponding to the NNW–SSE trending Aeolian–Tindari–Letojanni fault system and to the WNW–ESE oriented Sisifo–Alicudi fault system. GPS data analysis displays ∼3.0mm/yr of active shortening between the two islands, with a maximum shortening rate of about 1.0×10−13s−1, between La Fossa Caldera and south of Vulcanello. This region is bounded to the north by an area where the maximum values of shear strain rates, of about 0.7×10−13s−1 are observed. This major change occurs in the area south of Vulcanello that is also characterized by a transition in the way of the vertical axis rotation. Moreover, both the islands show a clear subsidence process, as suggested by negative vertical velocities of all GPS stations which exhibit a decrease from about −15 to −7mm/yr from north to south. New data suggest that the current kinematics of the Lipari–Vulcano complex can be framed in the tectonic context of the eastward migrating Sisifo–Alicudi fault system. This is dominated by transpressive tectonics in which contractional and minor extensional structures can coexist with strike-slip motion.

Reconstruction of Late-Quaternary sea- and lake-level changes in a tectonically active marginal basin using seismic stratigraphy: The Gulf of Cariaco, NE Venezuela

The Gulf of Cariaco is a marginal basin located between the Cariaco Basin and the Paria Gulf, offshore NE Venezuela, along a system of active right-lateral strike-slip faults. It is connected to the Caribbean Sea via a shallow 58-m-deep sill implying that the gulf was disconnected from the global ocean during eustatic lowstands. A dense grid of high-resolution reflection seismic profiles has been used to determine the overall tectonic structure of the gulf and to establish the seismic stratigraphy of its sedimentary infill. Six unconformity-bounded seismic–stratigraphic units were identified in the upper ~200m of the sedimentary infill. Detailed seismic–stratigraphic and seismic-facies analysis allowed defining a series of sedimentary features that can be used as indicators of past sea or lake level in the Gulf of Cariaco: i) delta offlap breaks, ii) evaporites, and iii) erosional unconformities. Using accurate measurements of these various indicators at several locations in the gulf and a simple total subsidence model, a relative sea/lake-level history encompassing the last 130kyr could be reconstructed. In periods of connection with the open ocean, reconstructed relative sea level correlates well with eustatic sea level. In times of disconnection, distinct lake-level fluctuations occurred, which sometimes resulted in total dessication of the gulf. Lake-level fluctuations appear to correlate with major Heinrich Events, stadials and interstadials. MIS 4, the LGM and the Younger Dryas were thus identified in the Gulf of Cariaco sedimentary record. The last reconnection to the Caribbean Sea occurred during MWP1b (around 11.5kyr). The very good fit of the Cariaco sea/lake-level curve with the eustatic sea-level curves (both in terms of amplitude and of timing) underscores potential for future paleoclimate research of the sedimentary record contained in this marginal basin, despite its active tectonic setting.

The transition between Makran subduction and the Zagros collision: recent advances in its structure and active deformation

Abstract SE Iran is the site of a rare case of young transition between subduction and collision. We have synthesized recent results in geodesy, tectonics, seismology and magnetism to help understand the structure and kinematics of the Zagros–Makran transition. Surface observations (tectonics, magnetism and geodesy) indicate a transpressive discontinuity consisting of several faults striking obliquely to the convergent plate motion, whereas deeper observations (seismology) support a smooth transition across the fault system. No lithospheric transform fault has been created, although the transition already behaves like a major boundary in terms of tectonic style, seismic structure, lithology and magnetism. The Zendan–Minab–Palami fault system consists of several faults that accommodate a transpressive tectonic regime. It is the surface expression of a southward propagation of the north–south-trending right-lateral strike-slip fault system of Jiroft–Sabzevaran. Within each system the numerous faults will coalesce into a single, lithospheric, wrench fault.

Quaternary slip rates along the northeastern boundary of the Arabia-Eurasia collision zone (Kopeh Dagh Mountains, Northeast Iran)

The Kopeh Dagh is accommodating a large portion of the northward motion of central Iran with respect to Eurasia, involving a major right-lateral strike-slip fault system (Bakharden-Quchan). This fault system corresponds to the northeastern boundary of the Arabia-Eurasia collision, and can be considered as a lithospheric-scale tectonic feature. We present a well-constrained estimation of late Quaternary slip rates along two major strike-slip faults (the Baghan and Quchan faults) in this fault system, using in situ-produced 36Cl nuclide to date two offset alluvial fan surfaces. Combining detailed satellite image and digital topographic data analyses complemented with geomorphic fieldwork allows quantifying the cumulative offset values of 940 ± 100 and 360 ± 50 m of the fan surfaces along the Baghan and Quchan faults, respectively. A total of 12 carbonate boulders from the fan surfaces were collected and dated. This yields minimum age of two episodes of fan abandonment at 280 ± 16 (Baghan fault) and 83 ± 4 ka (Quchan fault). Age estimates and measured offsets of the fans are consistent with respective maximum long-term fault slip rates of 2.8 ± 1.0 and 4.3 ± 0.6 mm yr−1 for the Baghan and Quchan faults over the Middle-Late Pleistocene. Applying the slip rates to cumulative post-folding offsets along the Baghan and Quchan faults indicates that strike-slip motion within the Kopeh Dagh may have started ∼4 Ma. This constrains the timing of a major tectonic reorganization in the Kopeh Dagh, previously recorded through Arabia-Eurasia collision between 3 and 7 Ma. At the regional scale, the sum of total cumulative strike-slip offsets is about 35–40 km, which implies a total maximum slip rate of about 9 ± 2 mm yr−1 in the Central-Eastern Kopeh Dagh. This is resolved to average northward and westward slip rates of ∼8 and ∼4 mm yr−1, respectively, for the Western Kopeh Dagh with respect to Eurasia. Our results also suggests that the localized strike-slip faulting in the Central Kopeh Dagh can be considered as an intercontinental movement between northeast Iran and Eurasia, accommodating about 80 per cent of northward motion of central Iran with respect to Eurasia.

Structural characteristics of the Tan-Lu fault zone in Cenozoic basins offshore the Bohai Sea

The Tan-Lu fault zone across the eastern margin of the Cenozoic basins offshore the Bohai Sea is a NNE-trending right-lateral strike-slip fault system developed in the Cenozoic basin cover. It cuts through NE-to NNE-striking major extensional faults that controlled the formation of Paleogene basins. Recent petroleum exploration indicates that Cenozoic structural activities of the Tan-Lu fault system have directly or indirectly affected oil and gas distribution offshore the Bohai Sea. As part of a deep fault zone the Tan-Lu fault zone has been activated since the Oligocene, and obviously affected the tectonic evolution of offshore Bohai basins since then. The formation of Paleogene rift basins offshore the Bohai Sea has utilized the pre-existing structural elements of the Tan-Lu fault zone that developed in the late Mesozoic.

Role of transverse strike-slip faults in the structure of the Karpinsky Ridge and their kinematics

The segmented structure of the Karpinsky Ridge is determined by NE-trending transverse strikeslip faults with offsets of approximately 30–40 km. The newly recognized Pribrezhny Fault and the well-known Agrakhan Fault are the largest. A new correlation scheme for structural elements of the ridge’s eastern segment and its underwater continuation is proposed with account of offset along the Pribrezhny Fault. According to this scheme, the Semenovsky Trough rather than the Dzhanai Trough is an onshore continuation of the underwater Zyudevsky Trough. The uplift located south of the Zyudevsky Trough is correlated with the Promyslovy-Tsubuk Swell offset along the Pribrezhny Fault. In turn, this uplift is displaced along the right-lateral strike-slip fault that coincides with the Agrakhan Fault. The transverse faults were formed during the Early Permian collision related to the closure of the basin, which was presumably underlain by the oceanic crust. The faults were active during the Early Triassic rifting and Late Triassic inversion. Judging from the map of the surface of the Maikop sediments, the Agrakhan Fault does not cross the Terek-Caspian Trough. Bending arcwise, the fault joins a system of right-lateral strike-slip faults that border the Daghestan Wedge in the east. A system of rightlateral strike-slip faults may also be traced along the western coast of the Caspian Sea. The Agrakhan Fault as a northern element of this system functioned mostly in the Late Paleozoic-Early Mesozoic in connection with the formation of the fold-thrust structure of the Karpinsky Ridge. In the east the faults of the southern segment bound the Caucasus syntaxis of the Alpine Belt; they have retained their activity to the present day.

Intraplate termination of transform faulting within the Antarctic continent

The sector of Antarctica facing Australia east of 139 °E is characterized by the abundance of exceptionally long oceanic fracture zones that are collinear to post-rift right-lateral strike–slip fault systems developed at the northeastern edge of the Antarctic continent. High-resolution reflection seismic profiles indicate recent strike–slip activity at the southeastern edge of the Balleny Fracture Zone, similar to what is observed onshore in North Victoria Land. The architecture, kinematics, and timing of this intraplate deformation at the northeastern edge of Antarctica cannot be reconciled with typical plate tectonic kinematics, in particular, with a classical divergent plate boundary environment. Here we show that combined geological and geophysical data in northeastern Antarctica support the post-rift southeastward reactivation of the passive margin east of 139 °E along intraplate right-lateral strike–slip deformation belts. These deformation belts include oceanic transform faults and their collinear oceanic fracture zone and continental shear zone extensions. A striking consequence is that there is intraplate accommodation of transform fault slip in this region of Earth's surface along fracture zones and a long-active region of intracontinental deformation that is ‘reusing’ prior plate boundary fault zones. As the intraplate termination of plate boundary transform faulting is not predicted by classical plate tectonic theory; this region is one of the most clear examples of the transition from rigid to semi-rigid plate tectonic deformation during the formation and long-lived incubation of a potential new plate boundary.

Upheaval Down Under

GEOLOGY New Zealand sits astride a transition from a west-dipping subduction zone toward the north (responsible for the volcanism of the North Island) to an east-dipping subduction zone toward the south. The transition forms a system of right-lateral strike-slip faults that have produced the dramatic topography of the South Island, as well as several large earthquakes. One of these was the 1855 magnitude 8.2 temblor on the Wairarapa Fault just east of the city of Wellington. Rodgers and Little remeasured offsets produced by this earthquake and conclude that the ground slipped by as much as 18 m, an enormous amount for a strike-slip fault. For comparison, the devastating 1906 San Francisco earthquake produced a maximum of about 6 m of slip at the surface. Furthermore, the earthquake extended laterally only about 150 km (versus 480 km for the 1906 quake). An earlier quake may have produced surface slip of 14 m. The authors explain the paradox of a huge slip and short surface rupture by suggesting that the Wairarapa Fault extends deep into the crust, connecting with the northern-dipping subduction zone at depth. — BH J. Geophys. Res. 111 , B12408 (2006).

Consistent kinematic architecture in the damage zones of intraplate strike-slip fault systems in North Victoria Land, Antarctica and implications for fault zone evolution

Cumulative, polymodal normal (Gaussian distribution) statistics was applied to subsidiary fault data collected from damage zones associated with the Cenozoic Lanterman and Priestley intraplate right-lateral strike-slip fault systems in North Victoria Land, Antarctica. Results show that five Gaussian peaks out of seven in the Lanterman Fault and five out of nine in the Priestley Fault have almost coincident azimuthal values. We named these Gaussian peak pairs as consistent fault sets, arranged in a consistent kinematic architecture that is compatible with the Cenozoic regional strike-slip environment. Angular and kinematic relationships among subsidiary fault sets within the consistent kinematic architecture provide constraints for the inference of the state of stress along the Lanterman and Priestley fault systems. We interpret the fault pattern of the consistent kinematic architecture to be produced by early localisation of the principal displacement zone along pre-existing mechanical discontinuities inherited from the Early Paleozoic Ross Orogeny. Shear localisation was followed by subsidiary faulting at an angle to the principal displacement zone according to the Mohr–Coulomb–Byerlee failure criterion.

Late Pleistocene and Holocene paleoseismology of an intraplate seismic zone in a large alluvial valley, the New Madrid seismic zone, Central USA

Late Pleistocene and Holocene paleoseismology of an intraplate seismic zone in a large alluvial valley, the New Madrid seismic zone, Central USA

Paleoseismic evidence of legendary earthquakes: The apparition of Archangel Michael at Monte Sant'Angelo (Italy)

The Gargano promontory, a carbonate massif belonging to the Adria microplate, the foreland of both the Apenninic and Dinaric mountain chains, is a well-known seismogenic area, with historic earthquake up to Imax=X MCS (July 30,1627, M=6.7). Nevertheless, paleoseismic data on the region are still scarce, and no trench fault stratigraphy has been analysed along the major, E–W trending, right-lateral strike-slip fault system that cuts through the whole massif, the South Gargano Line (SGL). The over 60 km long on-shore trace of the SGL, the Mattinata Fault, is clearly segmented. This study focused on the eastern active fault segment, the Monte Sant'Angelo Fault (MSA), to verify the associated seismic potential.Analysis of contemporary descriptions of the August 10,1893 earthquake (M=5.2) coupled with field-work allowed identification of surface faulting along the MSA. Along the fault there is paleoseismic evidence of the occurrence of larger earthquakes. After collecting geomorphic evidence of paleo-earthquakes, we selected two trench sites at the base of two prominent fault escarpments, where the MSA is characterized by significant vertical components of fault displacement. The preliminary results demonstrate that: i) the recent vertical slip-rate of the MSA range between 0.2–0.3 and 0.7 mm/yr; ii) the MSA moves with incremental slip episodes, with vertical surface offset up to several decimeters; iii) based on the stratigraphic and geomorphic evidence of surface faulting observed and assuming a rupture length of around 20+ km, the seismic potential of the MSA segment would be similar to that displayed by the western SGL segment during the 1627 event.A large earthquake on the MSA is reported only in ancient legends, the most relevant of which being the one describing the apparition of Archangel Michael at Monte Sant'Angelo traditionally dated AD 493. Using a multidisciplinary approach, combining paleoseismic and historical data it has been possible to interpret the legend, and hereby identify an event of surface faulting before 8th century AD. A recent scarplet, up to 1 m high, on the MSA can be seen to be related to this legendary earthquake, which in turn seems to correspond to the maximum possible magnitude. Geologic phenomena observed during the earthquake at the site of the sanctuary appear to lie at the very origin of the veneration of the place.

40Ar– 39Ar dating of pseudotachylytes: the effect of clast-hosted extraneous argon in Cenozoic fault-generated friction melts from the West Antarctic Rift System

Fault-generated pseudotachylytes are the product of frictional melting during high-velocity slip associated with coseismic faulting. 40Ar– 39Ar dating of pseudotachylytes represents a powerful tool to directly determine the age of brittle faulting which is otherwise dated only indirectly. However, the pronounced spatial heterogeneity of most endogenically generated pseudotachylytes, due to the intimate coexistence of unmelted mineral clasts from the source rock, frictional glass and neogenic igneous minerals, often precludes a straightforward interpretation of argon data. This requires careful evaluation of the role of clasts in plaguing the age record of the pseudotachylyte matrix. This study exploits the full potential of the 40Ar– 39Ar method, using laser step-heating and laser in situ techniques in conjunction with textural and chemical characterisation at the microscale to disentangle the complexity of the first record of pseudotachylytes generated in the right-lateral fault systems which dissect the western shoulder of the West Antarctic Rift System. Pseudotachylytes occur as fault and injection veins. They exhibit glassy matrices with a potassium feldspar-like composition, and invariably contain, at a millimeter to microscopic scale, unmelted quartz and subordinate feldspars from the source rock. Injection veins characteristically contain much fewer mineral clasts. In situ ultraviolet (UV) laser analyses indicate that quartz from the host rock and from clasts within the fault veins contains significant amounts (∼10–50 ppb) of parentless 40Ar, most probably hosted in microscopic to submicroscopic fluid inclusions, associated with high Cl/K ratios. Infrared laser step-heating experiments on fault vein pseudotachylyte matrices yield strongly discordant age spectra with an overall saddle shape characterised by a minimum at ∼34 Ma, unrealistically old ages at high temperatures associated with high Cl/K ratios and total gas ages of 41.5–44.3 Ma. Petrographic data and the pronounced compositional similarity between in situ UV laser data on quartz and the high-temperature end member of step-heating analyses suggest that quartz is the main contaminant of the pseudotachylyte age record. In situ laserprobe analyses on pseudotachylyte matrices give concordant ages at ∼34 Ma for an injection vein and systematically older ages, clustering at ∼40–48 Ma, for fault veins. Although the age cluster fits into the Cenozoic tectonic framework of the region, it is an artefact due to the low spatial resolution of the argon laserprobe compared to the size and spatial distribution of quartz clasts within the pseudotachylyte. The isochron age of 34.11±0.96 Ma, derived from in situ data from the injection vein, overlaps with the emplacement age of syn-tectonic dykes from nearby areas and is interpreted to date a single episode of coseismic faulting. Regionally, the age of the studied pseudotachylytes represents the first direct onshore evidence of right-lateral strike–slip fault system activity in Victoria Land during the Cenozoic.

Structural architecture and displacement accommodation mechanisms at the termination of the Priestley Fault, northern Victoria Land, Antarctica

We present the results of field investigations carried out at the southern termination of the Priestley Fault, a major Cenozoic right-lateral strike-slip fault system in northern Victoria Land, Antarctica. The Priestley Fault was originally traced from offshore seismic profiles. Its onshore prosecution was inferred from the regional geological picture. Our work provides the first robust dataset on the kinematics and structural architecture of the Priestley Fault. The fault system includes a principal displacement zone (PDZ), where most horizontal displacement is accounted for, and a transtensional splay zone (TSZ) in the southern side consisting of a major basin at the fault tip, the Terror Rift, and minor ones paralleling it cratonward. Part of the strike-slip displacement is transferred by fault splaying from the principal displacement zone to the basin-boundary faults in the transtensional splay zone. The principal displacement zone is characterised by strongly transpressive deformation and this contrasts with the extensional component occurring in the transtensional splay zone. Such a contrast is only apparent because the two regions are kinematically linked. Transfer of the residual right-lateral strike-slip displacement from the tip region of the principal displacement zone to the basin-boundary fault systems of the Terror Rift represents the most effective displacement compensation mechanism at the termination of the Priestley Fault. It provides an additional solution for reducing the well known and still fully unsolved discrepancy between large translations of adjacent crustal blocks and the lack of comparable displacement values in their boundary strike-slip fault systems. The unquestionable evidence of intense Cenozoic brittle deformations in north Victoria Land imposes a re-examination of the regional tectonic framework, commonly interpreted as the mere result of the Early Paleozoic Ross Orogeny.

The San Andreas Fault in the San Francisco Bay Region, California: Structure and Kinematics of a Young Plate Boundary

Recently acquired high-resolution aeromagnetic data delineate offset and/or truncated magnetic rock bodies of the Franciscan Complex that define the location and structure of, and total offset across, the San Andreas fault in the San Francisco Bay region. Two distinctive magnetic anomalies caused by ultramafic rocks and metabasalts east of, and truncated at, the San Andreas fault have clear counterparts west of the fault that indicate a total right-lateral offset of only 22 km on the Peninsula segment, the active strand that ruptured in 1906. The location of the Peninsula segment is well defined magnetically on the northern peninsula where it goes offshore, and can be traced along strike an additional ˜6 km to the northwest. Just offshore from Lake Merced, the inferred fault trace steps right (northeast) 3 km onto a nearly parallel strand that can be traced magnetically northwest more than 20 km as the linear northeast edge of a magnetic block bounded by the San Andreas fault, the Pilarcitos fault, and the San Gregorio-Hosgri fault zone. This right-stepping strand, the Golden Gate segment, joins the eastern mapped trace of the San Andreas fault at Bolinas Lagoon and projects back onshore to the southeast near Lake Merced. Inversion of detailed gravity data on the San Francisco Peninsula reveals a 3 km wide basin situated between the two strands of the San Andreas fault, floored by Franciscan basement and filled with Plio-Quaternary sedimentary deposits of the Merced and Colma formations. The basin, ˜1 km deep at the coast, narrows and becomes thinner to the southeast along the fault over a distance of ˜12 km. The length, width, and location of the basin between the two strands are consistent with a pull-apart basin formed behind the right step in the right-lateral strike-slip San Andreas fault system and currently moving southeast with the North American plate. Slight nonparallelism of the two strands bounding the basin (implying a small component of convergence with continued strike-slip movement) may explain the progressive narrowing of the basin to the southeast and the puzzling recent uplift of the Merced Formation in a predominantly extensional (pull-apart basin) setting. The 1906 San Francisco earthquake may have nucleated within the step-over region, and the step-over places a strand of the San Andreas fault 3 km closer to downtown San Francisco than previously thought.

Source character of microseismicity in the San Francisco Bay block, California, and implications for seismic hazard

Abstract We examine relocated seismicity within a 30-km-wide crustal block containing San Francisco Bay and bounded by two major right-lateral strike-slip fault systems, the Hayward and San Andreas faults, to determine seismicity distribution, source character, and possible relationship to proposed faults. Well-located low-level seismicity (Md ≦ 3.0) has occurred persistently within this block throughout the recording interval (1969 to 1995), with the highest levels of activity occurring along or directly adjacent to (within ∼5 km) the bounding faults and falling off toward the long axis of the bay. The total seismic moment release within the interior of the Bay block since 1969 is equivalent to one ML 3.8 earthquake, one to two orders of magnitude lower than activity along and within 5 km of the bounding faults. Focal depths of reliably located events within the Bay block are generally less than 13 km with most seismicity in the depth range of 7 to 12 km, similar to focal depths along both the adjacent portions of the San Andreas and Hayward faults. Focal mechanisms for Md 2 to 3 events within the Bay block mimic focal mechanisms along the adjacent San Andreas fault zone and in the East Bay, suggesting that Bay block is responding to a similar regional stress field. Two potential seismic source zones have been suggested within the Bay block. Our hypocentral depths and focal mechanisms suggest that a proposed subhorizontal detachment fault 15 to 18 km beneath the Bay is not seismically active. Several large-scale linear NW-trending aeromagnetic anomalies within the Bay block were previously suggested to represent large through-going subvertical fault zones. The two largest earthquakes (both Md 3.0) in the Bay block since 1969 occur near two of these large-scale linear aeromagnetic anomalies; both have subvertical nodal planes with right-lateral slip subparallel to the magnetic anomalies, suggesting that structures related to the anomalies may be capable of brittle failure. Geodetic, focal mechanism and seismicity data all suggest the Bay block is responding elastically to the same regional stresses affecting the bounding faults; however, continuous Holocene reflectors across the proposed fault zones suggest that if the magnetic anomalies represent basement fault zones, then these faults must have recurrence times one to several orders of magnitude longer than on the bounding faults.