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Basalt is one of the main components of megathrust faults in subduction zones where the world's largest earthquakes are generated. Paradoxically, pseudotachylytes have never been reported from basaltic rocks deformed at conditions compatible with the thermally-defined seismogenic zone. We report the first discovery of pseudotachylyte from oceanic crustal basalt, identified by microstructural criteria. The fault is located within a Late Cretaceous subduction complex, the Mugi Melange, and either formed along the plate interface during subduction or within the upper plate during underplating. After solidifying, the pseudotachylytes were fragmented by cataclasis and subsequently partially hydrated to produce phyllosilicates. Previous experiments using dry, felsic lithologies as starting material have shown that pseudotachylytes can be as strong as their host rock, deterring re-activation and favoring their preservation over geologic time. However, we show that where fluid is present, pseudotachylyte can be replaced by phyllosilicate-rich layers with a low frictional strength. The rate of dissolution is dependent on the bulk composition of the pseudotachylyte, with dissolution of mafic glasses occurring at least three orders of magnitude faster than felsic glasses. Under hydrothermal conditions, the replacement of pseudotachylytes by layers of frictionally weak phyllosilicates is predicted to occur over time intervals shorter than the megathrust earthquake cycle. This process likely reduces fault strength and promotes reactivation.

Recent advances in the use of thermal proxies to identify past earthquake slip in exhumed faults have created an extraordinary opportunity to map the geometry of past ruptures in unprecedented detail. This approach can reveal along-strike differences in the structure and speed of earthquake slip. Here, we present organic thermal maturity data collected along the Muddy Mountain thrust in Nevada to investigate where it was within the fault that earthquake slip occurred and whether this is consistent along strike. We observe large changes in thermal maturity, which represent temperature-rise variations along the fault. Modeling of thermal maturity measurements yield peak temperatures of 760–1090 °C where the principal slip zone (PSZ) is narrowest (0.8 mm). Nearby where the PSZ is thicker (1.5 mm) these temperatures did not exceed 500 °C. Based upon estimates of temperature rise within a submillimeter PSZ, mean frictional work during earthquake slip was 6.3–7.1 MJm−2. These results show that thickness of the active slipping zone can vary at small spatial scales and development of a continuously narrow (

Earthquakes occur by overcoming fault friction; therefore, quantifying fault resistance is central to earthquake physics. Values for both static and dynamic friction are required, and the latter is especially difficult to determine on natural faults. However, large earthquakes provide signals that can determine friction in situ. The Japan Trench Fast Drilling Project (JFAST), an Integrated Ocean Discovery Program expedition, determined stresses by collecting data directly from the fault 1–2 years after the 2011 Mw 9.1 Tohoku earthquake. Geological, rheological, and geophysical data record stress before, during, and after the earthquake. Together, the observations imply that the shear strength during the earthquake was substantially below that predicted by the traditional Byerlee's law. Locally the stress drop appears near total, and stress reversal is plausible. Most solutions to the energy balance require off-fault deformation to account for dissipation during rupture. These observations make extreme coseismic weakening the preferred model for fault behavior. ▪ Determining the friction during an earthquake is required to understand when and where earthquakes occur. ▪ Drilling into the Tohoku fault showed that friction during the earthquake was low. ▪ Dynamic friction during the earthquake was lower than static friction. ▪ Complete stress drop is possible, and stress reversal is plausible.

EON-ROSE (Earth-System Observing Network - Réseau d’Observation du Système terrestrE) is a new initiative for a pan-Canadian research collaboration to holistically examine Earth systems from the ionosphere into the core. The Canadian Cordillera Array (CC Array) is the pilot phase, and will extend across the Cordillera from the Beaufort Sea to the U.S. border. The vision for EON-ROSE is to install a network of telemetered observatories to monitor solid Earth, environmental and atmospheric processes. EON-ROSE is an inclusive, combined effort of Canadian universities, federal, provincial and territorial government agencies, industry, and international collaborators. Brainstorming sessions and several workshops have been held since May 2016. The first station will be installed at Kluane Lake Research Station in southwestern Yukon during the summer of 2018. The purpose of this report is to provide a framework for continued discussion and development.RÉSUMÉEON-ROSE (Earth-System Observing Network - Réseau d’Observation du Système terrestrE) est une nouvelle initiative de collaboration de recherche pancanadienne visant à étudier de manière holistique les systèmes terrestres, depuis l’ionosphère jusqu’au noyau. Le Réseau canadien de la cordillère (CC Array) en est la phase pilote, laquelle couvrira toute la Cordillère, de la mer de Beaufort jusqu’à la frontière étasunienne. L’objectif d’EON-ROSE est d’installer un réseau d’observatoires télémétriques pour suivre en continu les processusterrestres, environnementaux et atmosphériques. EON-ROSE est un effort combiné et inclusif des universités canadiennes, des organismes gouvernementaux fédéraux, provinciaux et territoriaux, de l’industrie et de collaborateurs internationaux. Des séances de remue-méninges et plusieurs ateliers ont été tenus depuis mai 2016. La première station sera installée à la station de recherche du lac Kluane, dans le sud-ouest du Yukon, au cours de l’été 2018. Le but du présent rapport est de fournir un cadre de discussion et de développement continu.

Mid-crustal fault rheology is controlled by alternating brittle and plastic deformation mechanisms, which cause feedback cycles that influence earthquake behavior. Detailed mapping and microstructural observations in the Pofadder Shear Zone (Namibia and South Africa) reveal a lithologically heterogeneous shear zone core with quartz-rich mylonites and ultramylonites, plastically overprinted pseudotachylyte and active shear folds. We present evidence for a positive feedback cycle in which coseismic grain size reduction facilitates active shear folding by enhancing competency contrasts and promoting crystal plastic flow. Shear folding strengthens a portion of a shear zone by limb rotation, focusing deformation and promoting plastic flow or brittle slip in resulting areas of localized high stress. Using quartz paleopiezometry, we estimate strain and slip rates consistent with other studies of exhumed shear zones and modern plate boundary faults, helping establish the Pofadder Shear Zone as an ancient analogue to modern, continental-scale, strike-slip faults. This feedback cycle influences seismicity patterns at the scale of study (10s of meters) and possibly larger scales as well, and contributes to bulk strengthening of the brittle-plastic transition on modern plate boundary faults.

Declining gold discoveries underscore the importance of informed geological models for future exploration. The genesis of many gold deposits remains ambiguous, but integrated structural and chronological studies shed light on key questions of source, timing and geodynamic setting. The Hammond Reef gold deposit is unusual in that it is hosted in Mesoarchean tonalite of the Wabigoon superterrane, rather than in Neoarchean greenstone belt terranes, as is common elsewhere in the Superior Province. Gold deposition at Hammond Reef occurred in two stages: The main stage of disseminated low-grade mineralization was followed by discrete, channelized veins, both tied to regional tectonic events. Disseminated mineralization is associated with localized sulfidation and chlorite breakdown within a sericitized deformation zone adjacent to the locked greenstone-gneiss boundary. Chlorite-leached halos around small fractures at the deposit margins indicate that fluid penetration into the tonalite was guided by fractures concentrated in the broad deformation corridor. Pervasive alteration was enhanced by grain-scale flow. Hammond Reef may have been particularly susceptible to sulfidation and associated gold precipitation due to the greater local abundance of mafic dykes and chlorite-rich enclaves in the tonalite. The secondary gold mineralization phase is associated with younger and discrete auriferous quartz-carbonate veins that were episodically emplaced along pre-existing structures. Cross-cutting relationships show that both mineralization phases post-date the emplacement of the 2.889 Ga Diversion Stock. The kinematics of mineralized structures relate Hammond Reef to the 2.7 Ga northward subduction of the Wawa terrane beneath the Wabigoon superterrane. This places Hammond Reef in a supra-subduction setting, where an older dormant fault zone with enhanced permeability was infiltrated by auriferous fluids during the an orogeny up to 200 million years later. The two phases of gold enrichment present disseminated and channelized fluid flow through the available permeable pathways within the same regional tectonic setting.

The project benefited from funding by the Canadian Foundation for Innovation and McGill University.

Deformation adjacent to faults and shear zones is traditionally thought to correlate with slip. Inherited structures may control damage geometry, localizing fluid flow and deformation in a damage aureole around structures, even after displacement has ceased. In this paper we document a post-shearing anastomosing foliation and fracture network that developed to one side of the Mesoarchean Marmion Shear Zone. This fracture network hosts the low-grade, disseminated Hammond Reef gold deposit. The shear zone juxtaposed a greenstone belt against tonalite gneiss and was locked by an intrusion that was emplaced during the final stages of suturing. After cessation of activity, fluids channeled along fault- and intrusion-related fractures led to the pervasive sericitization of feldspars. Foliated zones resulted from flattening in the weaker sericite-rich tonalite during progressive alteration without any change in the regional NW-SE shortening direction. The anastomosing pattern may have been inherited from an earlier ductile fabric, but sericite alteration and flattening fabrics all formed post-shearing. Thus, the apparent foliated fracture network adjacent to the Marmion Shear Zone is a second-order effect of shear-related damage, distinct in time from shear activity, adjacent to an effectively dormant shear zone. This phenomenon has implications for understanding the relative timing of fault zone activity, alteration and (in this case) gold mineralization related to long-term fault zone permeability.

The Makgadikgadi basin and wider Middle Kalahari region of Botswana and beyond host landforms which have been attributed to Quaternary environmental change, including palaeolake level fluctuations and aeolian activity. Tectonic processes and landforms on the other hand, have mostly been linked to the Okavango graben and associated rift zone (ORZ) to the west of the Makgadikgadi. In this paper we establish the extent of tectonic surface expression associated with the Makgadikgadi Rift Zone (MRZ). We identify a series of parallel, NNE-SSW, normal faults and scarps, expressing horst and graben structures linked to seven major blocks in the northern Makgadikgadi basin, using both Shuttle Radar Topographic Mission (SRTM) and geomagnetic data. Subtle expression of rifting has controlled endorheic drainage topology, replicated regional dune-field patterns and displaced the 945 m palaoelake contour since lake desiccation. These observations underscore the role of neotectonic “piano key” block movement in shaping surface landforms across a large expanse of the Kalahari region. This paper provides the first detailed map and introduction to the Makgadikgadi Rift Zone (MRZ) and its geomorphology.

Pore fluids migrating through the deep section of continental strike-slip fault zones have been invoked to explain such phenomena as tectonic tremor, stress transfer across the brittle-ductile transition, and short timescales of co-seismic healing. In this contribution, we describe a coseismic mechanism for forming transient vertical fluid conduits within dilational jogs in strike-slip faults. We present field observations of breccias that formed coseismically at dilational stepovers in the dextral Pofadder Shear Zone, a ∼ 1 G a exhumed continental strike-slip fault in South Africa and Namibia. These breccias are interpreted to have formed when tensile fractures emanating from rupture tips intersected mylonitic foliation parallel to the rupture surface, which then failed, disaggregating the rock. We used quartz textures in the mylonites determined by electron backscatter diffraction to uniquely compare the orientation of each clast to the neighboring wall rock and constrain finite clast rotation within breccia bodies. Comparison of two- and three-dimensional rotation patterns show that clast trajectories are highly scattered when decoupled from wall rock, suggesting that Pofadder breccias were not formed by gradual plucking of clasts during slip. The dilational breccia bodies have sub-vertical geometries and high porosities relative to the host mylonites. We infer that the opening of these breccias may have created instantaneous, temporary vertical pathways for fluid draining through the brittle-plastic transition. These pathways healed post-seismically by cementation or ductile creep along the fault. The connection of many adjacent and overprinting breccia bodies through time provides a mechanism for fluid transport on a 10 s of km scale though the middle crust.

During earthquake propagation, geologic faults lose their strength, then strengthen as slip slows and stops. Many slip-weakening mechanisms are active in the upper-mid crust, but healing is not always well-explained. Here we show that the distinct structure and rate-dependent properties of amorphous nanopowder (not silica gel) formed by grinding of quartz can cause extreme strength loss at high slip rates. We propose a weakening and related strengthening mechanism that may act throughout the quartz-bearing continental crust. The action of two slip rate-dependent mechanisms offers a plausible explanation for the observed weakening: thermally-enhanced plasticity, and particulate flow aided by hydrodynamic lubrication. Rapid cooling of the particles causes rapid strengthening, and inter-particle bonds form at longer timescales. The timescales of these two processes correspond to the timescales of post-seismic healing observed in earthquakes. In natural faults, this nanopowder crystallizes to quartz over 10s–100s years, leaving veins which may be indistinguishable from common quartz veins., Tectonic faults weaken during slip in order to accelerate and produce earthquakes. Here the authors show a mechanism for weakening faults through the transformation of quartz to amorphous nanoparticulate wear powders that lubricate friction experiments, and transform back to quartz under geologic conditions.

This is a pre-copy-editing, author-produced PDF of an article accepted for publication in The Geological Society of America Special Papers following peer review. The definitive publisher-authenticated version: "Regalla, C., Rowe, C., Harrichhausen, N., Tarling, M. and Singh, J., 2018. Styles of underplating in the Marin Headlands Terrane, Franciscan Complex, California. GSA Special Publications no. 534" is available online at: http://rock.geosociety.org/Store/detail.aspx?id=spe534.

Exhumed fault zones offer insights into deformation processes associated with earthquakes in unparalleled spatial resolution; however it can be difficult to differentiate seismic slip from slow or aseismic slip based on evidence in the rock record. Fifteen years ago, Cowan (1999) defined the attributes of earthquake slip that might be preserved in the rock record, and he identified pseudotachylyte as the only reliable indicator of past earthquakes found in ancient faults. This assertion was based on models of frictional heat production (Sibson, 1975, 1986) providing evidence for fast slip. Significant progress in fault rock studies has revealed a range of reaction products which can be used to detect frictional heating at peak temperatures less than the melt temperature of the rock. In addition, features formed under extreme transient stress conditions associated with the propagating tip of an earthquake rupture can now be recognized in the rock record, and are also uniquely seismic. Thus, pseudotachylyte is no longer the only indicator of fossilized earthquake ruptures. We review the criteria for seismic slip defined by Cowan (1999), and we determine that they are too narrow. Fault slip at rates in the range 10−4−101 m/s is almost certainly dynamic. This implies that features reproduced in experiments at rates as low as 10−4 m/s may be indicators of seismic slip. We conclude with a summary of the rock record of seismic slip, and lay out the current challenges in the field of earthquake geology.

We determine the trace element stratigraphy of Site C0019, drilled during the Japan Fast Trench Drilling Project (JFAST) International Ocean Discovery Program (IODP) Expedition 343, to illuminate the structure of the plate boundary following the Tohoku-Oki earthquake of 2011. The stratigraphic units at the JFAST site are compared to undeformed Western Pacific sediments from two reference sites (Ocean Drilling Program (ODP) Site 1149 and Deep Sea Drilling Project (DSDP) Site 436). The trace element fingerprints in these reference sedimentary units can be correlated to individual JFAST samples. At the JFAST site, we find that the accretionary wedge and downgoing plate sediments in the core are composed primarily of Holocene to Eocene sediments. There are several age reversals and gaps within the sequence, consistent with multiple faults in the bottom 15 m of the JFAST core. Our results point to several candidate faults that could have slipped during the 2011 Tohoku-Oki earthquake, in addition to the pelagic clay layer that has been proposed as the main decollement fault.

Long-lived subduction complexes, such as the Franciscan Complex of California, include tectonic contacts that represent exhumed megathrust horizons that collectively accommodated thousands of kilometres of slip. The chaotic nature of melanges in subduction complexes has spawned proposals that these melanges form as a result of megathrust displacement. Detailed field and petrographic relationships, however, show that most Franciscan melanges with exotic blocks formed by submarine landsliding. Field relationships at El Cerrito Quarry in the eastern San Francisco Bay area suggest that subduction slip may have been accommodated between the blueschist facies metagreywacke of the Angel Island nappe above and the prehnite-pumpellyite facies metagreywacke of the Alcatraz nappe below. Although a 100–200 m-thick melange zone separates the nappes, this melange is a variably deformed, prehnite-pumpellyite facies sedimentary breccia and conglomerate deposited on the underlying coherent sandstone, so the melange is par...

High-velocity friction experiments have shown an almost complete loss of strength associated with silica gel formation on slip surfaces. The identification of frictional silica gel products in palaeo-seismic faults is, however, problematic, because there are multiple natural sources of silica gel and recrystallization of gel to quartz complicates preservation. The importance of gel formation on natural faults is therefore unknown. Here, we report a structurally distinct and semi-continuous, 0.5 – 10 mm thick layer of microcrystalline quartz along a major carbonate-hosted fault, the Olive fault, in the Naukluft Nappe Complex, Namibia. The quartz layer is distinguished by flow banding-like textures and unusual cathodoluminescence characteristics. The layer consists of ∼2-20 μm hexagonal quartz crystals, which include distinct, crystalline, pore-bearing micro-to nano-spheres, separated by pore geometries indicative of volumetric contraction, and with grain boundaries enriched in aluminium. We interpret these features to indicate that the quartz crystals formed from recrystallization and dehydration of a silica gel. Because it is found in a carbonate-hosted fault and crosscuts lithological layering, the silica source is not from comminution of local wall rocks. Rather, the gel likely formed from reshear of a quartz-coated fault surface, or incremental shear slip associated with precipitation of silica driven by co-seismic pressure drops. This example of fault-related silica gel may have formed by a different mechanism than the gels produced in high-velocity friction experiments, but once formed, may have comparable rheological effects.

Off-fault dynamic tensile cracks form behind an earthquake rupture front with distinct orientation and spacing. These cracks explode the wall rock and create breccias, which we hypothesize will preserve a unique fingerprint of dynamic rupture. Identification of these characteristic breccias may enable a new tool for identifying paleoseismic slip surfaces in the rock record. Using previous experimental and theoretical predictions, we develop a field-based model of dynamic dilational breccia formation. Experimental studies find that secondary tensile fracture networks comprise closely spaced fractures at angles of 70–90° from a slip surface, as well as fractures that branch at angles of ∼ 30 ° from a primary mode I fracture. The Pofadder Shear Zone, in Namibia and South Africa, preserves breccias formed in the brittle–ductile transition zone displaying fracture patterns consistent with those described above. Fracture spacing is approximately two orders of magnitude less than predicted by quasi-static models. Breccias are clast-supported, monomict and can display an abrupt transition from fracture network crackle breccia to mosaic breccia textures. Brecciation occurs by the intersection of off-fault dynamic fractures and wall rock fabric; this is in contrast to previous models of fluid pressure gradient-driven failure “implosion breccias”. This mechanism tends to form many similar sized clasts with particle size distributions that may not display self-similarity; where self-similarity is observed the distributions have relatively low D-values of 1.47 ± 0.37 , similar to other studies of dynamic processes. We measure slip distances at dilational breccia stepovers, estimating earthquake magnitudes between M w 2.8–5.8 and associated rupture lengths of 0.023–3.3 km. The small calculated rupture dimensions, in combination with our geologic observations, suggest that some earthquakes nucleated within the quartz-plastic transitional zone and potentially record deep seismic slip.

The unique structural architecture of Archaean terranes has generated competing models for early earth tectonics. Understanding the structural and metamorphic history of an individual terrane allows us to compare the deformation path to that predicted by tectonic models, and determine the best model for matching field observations. The Finlayson Lake greenstone belt is a Mesoarchaean terrane lying between three different gneiss terranes in the south-central Wabigoon subprovince in Canada. The belt has been interpreted as either a synformal keel sagducted between rising gneiss diapirs or as three fault-bounded allochtonous sub-belts of different ages. We present a detailed structural field study to define the deformation history of the Finlayson Lake greenstone belt and show that it is not consistent with either previous hypothesis. The Finlayson Lake greenstone belt is a single volcanic package that incorporates detritus from exposed felsic terranes similar in age and composition to the adjacent 3.0 Ga Marmion tonalites. The geometry of the greenstone belt is defined by two outward-facing palaeo way-up orientations (anticlinal) and geothermobarometry records a clockwise metamorphic path with a deep prograde event at 820 ± 40 MPa (27–30 km) and 600 ± 45 °C, followed by peak metamorphism at 635 ± 165 MPa (21–23 km) and 625 ± 25 °C. The deformation history records sinistral transpression during peak metamorphism and continued flattening during retrogression and exhumation from ductile to brittle regimes. The structural and metamorphic results are comparable with modern subduction-accretion style settings. The intensity of both retrogressive and brittle deformation fabrics during exhumation decrease from east to west away from the eastern boundary shear zone, the Marmion Shear Zone. Stronger deformation along the eastern margin of the Finlayson Lake greenstone belt, adjacent to the Marmion Shear Zone, suggest reactivation during exhumation and is likely related to the 2.7 Ga amalgamation of the Superior Province.

We report observations from a kilometer-scale thrust ramp on the Naukluft thrust, Namibia. The Naukluft thrust is a low angle thrust that was active at subgreenschist facies conditions and accommodated several tens of kilometers of displacement at the base of the Naukluft Nappe Complex in the Pan-African Damara Orogeny. The fault zone is generally planar and a few meters thick, comprising predominantly a dolomite-rich cataclasite. At the ramp, the fault-rock assemblage increases in thickness, and the hanging-wall, which elsewhere is relatively intact, contains a high density network of inclined quartz veins, subvertical dolomite and calcite veins, breccia zones, as well as injectites of cataclastic fault rock emanating from the fault surface. The geometry of the hanging-wall structures indicates local subhorizontal extension. Local tensile stress can be explained by bending in the hanging-wall as it deformed to slide above the ramp structure. High fluid pressures created dynamically during fast slip by decarbonation of carbonate fault rock, and by dewatering of the footwall under an impermeable fault during interseismic periods, led to additional reduction in local effective compressive stresses. In this location, the ramp is more optimally oriented for slip in the inferred regional stress field, and therefore likely to fail before the contiguous thrust flats that are subparallel to the maximum principal stress. As such, the ramp represents the likely location for nucleation of fault slip, which could both trigger dynamic failure of the adjacent thrust faults, and produce hanging-wall extensional structures.

We present a detailed kinematic study of the boundary between the Monapo Klippe and the underlying Nampula Block in northern Mozambique. The Monapo Klippe is an allochthonous klippe of granulite-facies metamorphic rocks with subsidiary granitic and mafic intrusive rocks. The klippe overlies the Nampula Block which is made up of ortho- and paragneisses that are lower metamorphic grade and considerably older than the rocks of the Monapo Klippe. The boundary between the klippe and the underlying Nampula Block is a distinct mylonitic shear zone of variable width. The mylonite composition varies with the local footwall lithology but the dominant composition is quartz-rich with minor feldspar augen and biotite. The mylonite is interpreted to have formed as an early broad ductile shear zone that evolved during multiple phases of shearing to a narrow high-strain, lower temperature mylonite horizon. Kinematic indicators observed in outcrop, thin section, and image slices from X-ray computed tomography of three-dimensional mylonitic fabrics record top-to-east motion. The granulite-facies Cabo Delgado Nappe Complex, which lies to the north of the Monapo Klippe, has been hypothesized as the root terrane for the klippe. The Cabo Delgado Nappe Complex records top to the north-west motion at around 630–610 Ma. However, granitic and pegmatitic rocks that intrude the Monapo Klippe, and which also cross-cut the mylonite fabric as well as being deformed by it, are considered to be part of the Cambrian Murrupula Suite dated at between 530 and 470 Ma. This suggests that the emplacement of the Monapo Klippe over the Nampula Block is part of a later extensional phase post 530 Ma associated with east-directed post-orogenic collapse following Gondwana assembly.

The Monapo Klippe in north-east Mozambique is an ovoid-shaped outcrop measuring approximately 35 × 40 km and is clearly visible on satellite and geophysical images. Based on recent field mapping, geochemical studies and new geochronological data, we present a revision of the lithostratigraphy of the klippe and offer a model for its origin and emplacement in the framework of regional tectonics. There are three main groups of rocks within the klippe: (1) the Metacheria Metamorphic Complex; (2) the Mazerapane Intrusive Suite; and (3) the Ramiane Intrusive Suite. The Metacheria Metamorphic Complex consists of a melange of granulite gneiss, including mafic, felsic, pelitic and carbonate rocks, characterised by a strong penetrative shear fabric. The largely undeformed Mazerapane and Ramiane Suites have intruded into the Metacheria Metamorphic Complex. The Mazerapane Suite consists of foid-bearing ultramafic and mafic gneisses and intrudes into the western half of the complex, whereas the Ramiane Suite is dominated by alkaline granitic rocks, contains no foid-bearing units and intrudes into the eastern half of the complex. In addition to these three main units, there are a number of minor but structurally important units, the main ones of which include amphibolite-facies tonalitic gneisses and the Evate calcite carbonatite. Underlying all of these units is a narrow, high strain mylonite zone. Undeformed pegmatite bodies and dykes cross-cut all rock types of the Monapo Klippe including the marginal mylonite. Near identical dates for the intrusion of the Ramiane Suite at 637 ± 5 Ma and metamorphism of the Metacheria Complex at 634 ± 8 Ma indicates a major episode of granulite-facies metamorphism and crust generation at this time. The ∼635 Ma age for the granulite-facies metamorphism is comparable to granulite-facies events identified in other parts of the East African Orogen in Tanzania, Madagascar and other parts of northern Mozambique to the north of the Lurio Belt. The absence of granulite-facies rocks in the underlying Nampula Block is consistent with structural arguments that the Monapo Klippe is the remnant of an allochthonous thrust sheet. In this context, the Monapo Complex is very similar to other granulite-facies “klippe” in East Africa, Antarctica and Sri Lanka, lending support to the idea of a Pan-African mega-nappe formerly existing across greater East Gondwanaland.

Melt-origin pseudotachylytes are the most widely accepted feature recording earthquake slip in the fault rock record. However, reports of pseudotachylytes are rare compared to the frequency and distribution of earthquakes in active faults, suggesting melting occurs only under exceptional circumstances and therefore that pseudotachylytes are rarely formed. In this paper, we document the processes whereby pseudotachylytes are overprinted, destroyed and otherwise removed from the rock record. We present examples of recrystallized, altered, and cataclastically and crystal plastically deformed pseudotachylytes from a variety of ancient faults. Based on these observations, we identify characteristics of pseudotachylytes that are resistant to change over geologic time and develop criteria to allow recognition of relict pseudotachylytes. Our results imply that pseudotachylytes are vastly under-reported due to their vulnerability to destruction and the resulting difficulty in identification. As a consequence, the significance of frictional melting is underestimated. The criteria we propose to distinguish relict pseudotachylytes can help to reconcile the observed frequency of earthquakes with the difficulty of demonstrating ancient seismic slip in the rock record.

Fault rocks such as pseudotachylyte melt or granular gouge are sometimes injected into cracks off a fault surface. The width to height ratio of the injection veins is a direct measure of the shear strain in the wall rock required to accommodate the injection, so the aspect ratios preserve a record of the overpressure opening the cracks and injecting melt or fluidized granular material from the fault zone. We measured the aspect ratios for 201 pseudotachylyte injections and 29 granular injections. They have aspect ratios of � 0:2 (0.177 0.025 and 0.227 0.077, respectively, within 99% confidence). These are significantly proportionally wider than the aspect ratios of ordinary dikes (from 10 � 4 to 10 � 3 ). The injection aspect ratios exceed the usual limit of elastic strain in the wallrock ( � 1%) so we infer that the injection veins were accommodated by permanent deformation by microcracking and slip on foliation surfaces in the host rock, which is difficult to detect observationally. The fully elastic model therefore offers an upper bound on the stress required to open these injection veins ( � 10 8 210 10 Pa, with higher pressures in stiffer host rock lithologies.) These overpressures cannot be achieved by expansion of the fault rock fluids during melting or gouge fluidization mechanisms, as the pressure along the fault is limited to � 10 7

Unequivocal geological signatures of seismic slip are rare, exceptionally so in carbonate-hosted faults where carbonate minerals dissociate at temperatures lower than those required for producing a friction melt. This thermal dissociation leads to significant fault weakening by increased fluid pressure and/or nanoparticle lubrication, preventing further heating of the fault surface. Pseudotachylyte is therefore unlikely to form in carbonate-hosted faults, and other evidence for seismic slip must be identified. We studied the lower Cambrian Naukluft Thrust which crops out in central Namibia. It contains a cataclastic dolomite fault rock, referred to as ‘‘gritty dolomite’’, which we interpret as a signature of coseismic carbonate dissociation and subsequent fluid–rock interactions. The fault was active at ambient temperatures below 2001C. ‘‘Gritty dolomite’’ contains: rounded, low aspect ratio dolomite clasts with a uniform Fe-rich dolomite coating, euhedral to subhedral magnetite, quartz, and K-feldspar in a fine-grained, massive to laminated carbonate matrix of particulate dolomite and crystalline calcite cement. The fault rock texture, combined with evidence of injectites of gritty dolomite into the wallrock, indicates the cataclasite deformed as a fluidized granular flow. At seismic slip velocities, frictional heating caused dissociation of dolomite to CO 2 and Ca-, Fe- and Mg-oxides. This release of CO2 decreased the pH of the pore fluid in the fault, causing dissolution and rounding of dolomite clasts within an inertial grain flow, and precipitation of carbonate coatings and euhedral silicates and oxides during subsequent cooling and CO2 escape. Examples of similar rocks having some, if not all of these characteristics have been described from other carbonate-hosted faults. The geological setting of the Naukluft Thrust is unique in spatial extent and quality of exposure, allowing us to eliminate alternative hypotheses for sources of CO2 to drive fluidization.

We are eager to understand more about the setting of deposition and deformation of the Pakhuis Formation and “Fold Zone” and welcome the contribution of Blignault and Theron (2010). The presentation of this data in peer-reviewed form is a major contribution to the regional understanding of the Ordovician southern-hemisphere glaciation. However, we wish to comment on a few aspects of the sedimentology and deformation model presented by Blignault and Theron (2010).

Active faults slip at different rates over the course of the seismic cycle: earthquake slip ( c . 1 m s −1 ), interseismic creep ( c . 10–100 mm year −1 ) and intermediate rate transients (e.g. afterslip and slow slip events). Studies of exhumed faults are sometimes able to identify seismic slip surfaces by the presence of frictional melts, and slow creep by textures diagnostic of rate-limited plastic processes. The Pasagshak Point Thrust preserves three distinct fault rock textures, which are mutually cross-cutting, and can be correlated to different strain rates. Ultrafine-grained black fault rocks, including pseudotachylyte, were formed during seismic slip on layers up to 30 cm thick. Well-organized S – C cataclasites 7–31 m thick were formed by slow creep, with pressure solution as a dominant, rate-limiting mechanism. These must have formed at strain rates consistent with long-term plate-boundary motion, but solution-creep healing acted to reduce porosity of the cataclasites and eventually restricted fluid connectivity such that creep by this mechanism could not continue. Disorganized, non-foliated, rounded clast cataclasites were formed at shear rates faster than solution creep and are interpreted as representing shear at intermediate strain rates. These could have formed during afterslip or delocalization of slip associated with an earthquake rupture.

On Kodiak Island, Alaska, decimeterthick black fault rocks are at the core of foliated cataclasites that are tens of meters thick. The cataclasites belong to melange zones that are regarded as paleodecollements active at 12–14 km depth and 230–260 °C. Each black layer is mappable for tens of meters along strike. The black fault rocks feature a complex layering made at microscale by alternation of granular and crystalline micro textures, both composed of micronscale subrounded quartz and plagioclase in an ultrafi ne, phyllosilicate-rich matrix. In the crystalline microlayers, tabular zoned micro lites of plagioclase make up much of the matrix. No such feldspars have been found in the cataclasite. We interpret these crystalline microlayers as pseudotachylytes. The granular microlayers show higher grainsize variability, crushed microlites, and textures typical of fl uidization and granular fl ow deformation. Crosscutting relationships between granular and crystalline microlayers include fl ow and intrusion structures and mutual brittle truncation. This suggests that each decimeters-thick composite black fault rock layer records multiple pulses of seismic slip. In each pulse, ultracomminuted fl uidized material and friction melt formed and deformed together in a ductile fashion. Brittle truncation by another pulse occurred after solidifi cation of the friction melt and the fl uidized rock. X-ray powder diffraction (XRPD) and X-ray fl uorescence (XRF) analyses show that black fault rocks have similar mineral composition and chemical content as the cataclasites. The observed systematic chemical differences cannot be explained by bulk or preferential melting of any of the cataclasite components. The presence of an open, fl uidinfi ltrated system with later alteration of black fault rocks is suggested. The geochemical results indicate that these subductionrelated pseudotachylytes differ from those typically described in crystalline rocks and other tectonic settings.

We present a detailed structural and lithologic map of Robben Island, offshore Cape Town, South Africa. Robben Island is underlain by the Tygerberg Formation, part of the Neoproterozoic to early Cambrian Malmesbury Group of the Saldania Belt. The depositional setting and structural history of the Tygerberg Formation are poorly constrained due to limited outcrop and lack of previous structural studies. Sedimentary structures are indicative of deposition at relatively high rates in a high energy environment and we concur with previous workers that deposition occurred on turbidite fan systems in a tectonically deepening basin. By comparison with active and ancient examples, we suggest that a forearc or trench slope, supra-subduction zone basin is a possible match to the setting of the Tygerberg Formation. However, limits on preservation and insufficient age data prevent comparisons in basin geometry and deposition rates which could be used to test depositional setting with more certainty. Northwest-southeast striking subvertical pressure solution cleavage is pervasive throughout the exposures. Upright folds, with axial planes parallel to the cleavage, plunge 10 to 15° to the northwest or southeast with approximately 20° variation in trend azimuth. The folds are limited in along-axis extent and often occur in asymmetric pairs. Subtle bedding-parallel shear zones divide folds of different plunge directions. This pattern of folds is consistent with experiments and observations of en echelon folding during distributed strain associated with oblique transpression. This finding is consistent with previous studies of parallel, slightly earlier orogenic belts to the north (Gariep and Kaoko Belts) although our observations do not allow us to distinguish whether transpressional strain was sinistral or dextral. Sinistral transpression is considered more likely given the dominantly sinistral strike-slip history on the nearby Colenso Fault and the southward migration of collision along the western margin of Africa during the late Neoproterozoic to early Cambrian.

During the Late-Ordovician glacial period, the upper ~50 m of the quartz arenite Peninsula Formation (Table Mountain Group) was subject to a large-scale, soft-sediment deformation event, forming ellipsoidal synforms with wavelengths of up to 150 m. The de Balie locality in the Western Province, South Africa, provides clear 3D exposures into the morphology of these folds. Comparisons with experimental soft-sediment deformation and modern observations of periglacial environments suggest a single deformation event during which large-scale fluidization and gravitational instabilities (load casts) were triggered by thermal destabilisation. The fold morphology is inconsistent with previous interpretations that the folds were formed by ice contact shearing.

International audience; We have used sandbox experiments to explore the mechanics of the frontal prism structures documented by seismic reflection data and new borehole from IODP Expedition 343 (JFAST). This study investigated the effects of down-dip (normal to trench axis) variations in frictional resistance along a decollement on the structural development of the frontal wedges near subduction zones. Interpretation of seismic reflection images indicates that the wedge has been effected by trench-parallel horst-and-graben structures in the subducting plate. We performed sandbox experiments with down-dip patches of relatively high and low friction on the basal decollement to simulate the effect of variable coupling over subducting oceanic plate topography. Our experiments verify that high frictional resistance on the basal fault can produce the internal deformation and fault-and-fold structures observed in the frontal wedge by the JFAST expedition. Subduction of patches of varying friction caused a temporal change in the style of internal deformation within the wedge and gave rise to two distinctive structural domains, separated by a break in the surface slope of the wedge: (i) complexly deformed inner wedge with steep surface slope and (ii) shallow taper outer wedge, with a sequence of imbricate thrusts. Our experiments further demonstrate that the topographic slope-break in the wedge develops when the hinterland part of the wedge essentially stops deforming internally, leading to in-sequence thrusting with the formation of an outer wedge with low taper angle. For a series of alternate high and low frictional conditions on the basal fault the slope of the wedge varies temporally between a topographic slope-break and uniformly sloping wedge.

The 2011 Mw9.0 Tohoku-oki earthquake ruptured to the trench with maximum coseismic slip located on the shallow portion of the plate boundary fault. To investigate the conditions and physical processes that promoted slip to the trench, Integrated Ocean Drilling Program Expedition 343/343T sailed 1 year after the earthquake and drilled into the plate boundary ∼7 km landward of the trench, in the region of maximum slip. Core analyses show that the plate boundary decollement is localized onto an interval of smectite-rich, pelagic clay. Subsidiary structures are present in both the upper and lower plates, which define a fault zone ∼5–15m thick. Fault rocks recovered from within the clay-rich interval contain a pervasive scaly fabric defined by anastomosing, polished, and lineated surfaces with two predominant orientations. The scaly fabric is crosscut in several places by discrete contacts across which the scaly fabric is truncated and rotated, or different rocks are juxtaposed. These contacts are inferred to be faults. The plate boundary decollement therefore contains structures resulting from both distributed and localized deformation. We infer that the formation of both of these types of structures is controlled by the frictional properties of the clay: the distributed scaly fabric formed at low strain rates associated with velocity-strengthening frictional behavior, and the localized faults formed at high strain rates characterized by velocity-weakening behavior. The presence of multiple discrete faults resulting from seismic slip within the decollement suggests that rupture to the trench may be characteristic of this margin.

Ultrafine-grained black fault rocks (BFRs) in the Pasagshak Point Thrust of the Kodiak accretionary complex are examples of fault rocks that have recorded seismicity along an ancient subduction plate boundary. Trace element concentrations and 87Sr/86Sr ratios of BFRs and surrounding foliated/non-foliated cataclasites were measured to explore the nature of fluid-rock interactions along a subduction thrust. Foliated and non-foliated cataclasites do not show significant geochemical anomalies, suggesting that they were formed by slowly distributed shear. BFRs are characterized by Li and Sr enrichment, Rb and Cs depletion, and a low 87Sr/86Sr ratio. These geochemical signatures can be explained by fluid-rock interactions at >350°C, which result in preferential removal of Rb and Cs and formation of plagioclase under the presence of fluids with high Li and Sr concentrations and low 87Sr/86Sr ratios. Geochemical anomalies recorded by the BFRs indicate both frictional heating and external fluid influx into the subduction thrust.

During earthquakes, faults heat up due to frictional work. However, evidence of heating from paleoearthquakes along exhumed faults remains scarce. Here we describe a method using thermal maturation of organic molecules in sedimentary rock to determine whether a fault has experienced differential heating compared to surrounding rocks. We demonstrate the utility of this method on an ancient, pseudotachylyte-hosting megathrust at Pasagshak Point, Alaska. Measurements of the ratio of thermally stable to thermally unstable compounds (diamondoids/n-alkanes) show that the melt-bearing rocks have higher thermal maturity than surrounding rocks. Furthermore, the mineralogy of the survivor grains and the presence of any organic molecules allow us to constrain the temperature rise during the ancient earthquakes to 840–1170 °C above ambient temperatures of ~260 °C. From this temperature rise, we estimate that the frictional work of the earthquake was ~105–228 MJ/m 2 . Using experimental friction measurements as a constraint, we estimate that the minimum slip necessary for heating was ~1–8 m. This paper demonstrates that biomarkers will be a useful tool to identify seismic slip along faults without frictional melt.

Deep Drilling for Earthquake Clues The 2011 M w 9.0 Tohoku-Oki earthquake and tsunami were remarkable in many regards, including the rupturing of shallow trench sediments with huge associated slip (see the Perspective by Wang and Kinoshita ). The Japan Trench Fast Drilling Project rapid response drilling expedition sought to sample and monitor the fault zone directly through a series of boreholes. Chester et al. (p. 1208 ) describe the structure and composition of the thin fault zone, which is predominately comprised of weak clay-rich sediments. Using these same fault-zone materials, Ujiie et al. (p. 1211 ) performed high-velocity frictional experiments to determine the physical controls on the large slip that occurred during the earthquake. Finally, Fulton et al. (p. 1214 ) measured in situ temperature anomalies across the fault zone for 9 months, establishing a baseline for frictional resistance and stress during and following the earthquake.