Formation Of New Faults Research Articles

How Hydrothermal Cooling and Magmatic Sill Intrusions Control Flip‐Flop Faulting at Ultraslow‐Spreading Mid‐Ocean Ridges

Abstract“Flip‐flop” detachment mode represents an endmember type of lithosphere‐scale faulting observed at almost amagmatic sections of ultraslow‐spreading mid‐ocean ridges. Recent numerical experiments using an imposed steady temperature structure show that an axial temperature maximum is essential to trigger flip‐flop faults by focusing flexural strain in the footwall of the active fault. However, ridge segments without significant melt budget are more likely to be in a transient thermal state controlled, at least partly, by the faulting dynamics themselves. Therefore, we investigate which processes control the thermal structure of the lithosphere and how feedbacks with the deformation mechanisms can explain observed faulting patterns. We present results of 2‐D thermo‐mechanical numerical modeling including serpentinization reactions and dynamic grain size evolution. The model features a novel form of parametrized hydrothermal cooling along fault zones as well as the thermal and rheological effects of periodic sill intrusions. We find that the interplay of hydrothermal fault zone cooling and periodic sill intrusions in the footwall facilitates the flip‐flop detachment mode. Hydrothermal cooling of the fault zone pushes the temperature maximum into the footwall, while intrusions near the temperature maximum further weaken the rock and promote the formation of new faults with opposite polarity. Our model allows us to put constraints on the magnitude of two processes, and we obtain most reasonable melt budgets and hydrothermal heat fluxes if both are considered. Furthermore, we frequently observe two other faulting modes in our experiments complementing flip‐flop faulting to yield a potentially more robust alternative interpretation for existing observations.

Emergence and growth of faults during earthquakes: Insights from a dynamic elasto-plastic continuum model

I use a dynamic two-dimensional plane strain model of an elasto-plastic (Mohr-Coulomb) material with velocity-weakening friction to numerically simulate the emergence and propagation of shear fractures that accumulate slip intermittently during earthquakes. The model considers a 10 × 10 km block of homogeneous crust with a small weak inclusion, subjected to slow simple shear. Results show that deformation is distinctly bimodal, consisting of long periods (lasting ca. 50–200 years) where deformation is dominantly elastic separated by rapid sliding events lasting a few seconds during which time deformation is strongly localised on narrow plastic shear bands (faults). In terms of energy expenditure, the interseismic period is overwhelming dominated by accumulation of elastic strain energy whereas during earthquakes, most of the drop in elastic strain energy is consumed (dissipated) by plastic work. Model earthquakes have maximum slip rates of a few meters per second, involve a few meters of slip and result in drop of mean differential stress of several megapascals. Most earthquakes are complex events involving near-simultaneous rupture of multiple fault segments that propagate at supershear velocities (close to the P-wave speed) while they also induce a significant amount of more diffuse off-fault inelastic deformation. Earthquakes may involve formation of new faults and/or reshear on previously formed faults, which are favourably reactivated owing to their low pressure. Most individual faults establish much of their length early, whereas during subsequent events they continue to accumulate displacement at virtually constant length. As the confining pressure is increased, ruptures are more dynamic, which leads to increasingly widespread inelastic deformation. Overall, the models highlight the complex temporal and spatial interactions that exist between different faults within an array that communicate especially during earthquakes through static and dynamic changes in the stress field.

Analysis of Vertical Deformation and Potential Formation of New Faults Cause by Earthquake Using the DInSAR Method (Case Study: 2022 Cianjur Earthquake)

In this study, we focus on the repetitive nature of earthquakes along known fault lines and their potential to create new fault segments. Our primary aim is to enhance our understanding of fault patterns and distribution, facilitating more efficient and accurate earthquake disaster mitigation efforts. Our research specifically targets the vertical deformation resulting from the 2022 Cianjur Earthquake. To achieve this, we employed a sophisticated technique involving data processing from the Sentinel 1A satellites using the Differential Interferometric Synthetic Aperture Radar (DInSAR) method. This method allowed us to derive earthquake-induced displacement values in meters with an impressive level of accuracy, down to the centimeter scale. Our analysis revealed a conspicuous fringe pattern in the interferogram phase data, which did not align with the nearby Cimandiri Fault. This discrepancy led us to suspect the emergence of a new fault or fault segment. By comparing Sentinel 1A satellite images before and after the earthquake, we observed phase differences ranging from -3.112 to 3.117 phases. These recurring phase differences clustered at coordinates 6?48'48" South Latitude and 107?06'05" East Longitude, pinpointing the epicenter of the Cianjur earthquake on November 21, 2022. Furthermore, we calculated line-of-sight (LoS) displacement values ranging from 0 to minus 102 millimeters, indicating vertical subsidence in the vicinity of the 2022 Cianjur earthquake. In conclusion, our research underscores the utility of the DInSAR method for assessing Earth's surface deformation patterns, making it a valuable tool for various earthquake-related analyses.

Determination of seismic generation zones of Fennoscandia according to data of analysis of seismicity and computer geodynamic modelling

The work is devoted to the problem of identifying zones of seismic generstion zones of Fennoscandia, for the solution of which structural-geomorphological interpretation was carried out, seismicity analysis and computer modeling of the latest geodynamics were performed. According to the results of structural-geomorphological interpretation, the territory under consideration was divided into 6 large blocks. Computer modeling showed that each of them is characterized by a certain type of stress state in recent times, and also allowed us to calculate the probability of the formation of new short-distance faults. The reliability of the constructed models is evidenced by the established numerical correlation between this parameter and the density of earthquake epicenters. Based on the data on the density of earthquake epicenters and areas of the possible formation of new faults, a map of seismic generation zones of Fennoscandia was developed.

Precambrian fault reactivation revealed by structural and K-Ar geochronological data from the spent nuclear fuel repository in Olkiluoto, southwestern Finland

Integrated structural/geochronological studies help unraveling complex brittle deformation histories. We have analysed the structural geological database of brittle faults from the ONKALO™ underground facility for spent nuclear fuel in Olkiluoto in southwestern Finland. Based on the structural geological data from eleven representative fault zones, we classify the Olkiluoto brittle structural features into four fault systems, referred to as Fault system I to IV. The classification is based on their structural properties and tectonic history, crosscutting relationships, fault rock mineralogical characterization and 3D modelling. Some constraints on the timing of faulting are provided by K-Ar dates on synkinematic illite from fault gouge samples. Our results show that the bedrock in southwestern Finland experienced numerous brittle deformation phases between ca. 1.75 and 0.9 Ga. N-S strike-slip faults (Fault systems I and II) formed at mid-crustal levels ca. 1.79–1.75 Ga ago in response to NW-SE/NNW-SSE compression soon after the Svecofennian orogeny. Later E-W striking oblique dextral/normal faults (Fault system III) are tentatively associated with the Gothian orogeny 1.6 Ga ago. These three fault systems were reactivated during NE-SW compression ca. 1.3–1.2 Ga ago, coeval with intrusion of a regional swarm of olivine diabase sills. E-W compression at the onset of the Sveconorwegian orogeny ca. 1.1–1.0 Ga ago resulted in the formation of SE dipping low-angle thrust faults (Fault system IV) and the selective reactivation of fault system II and III. Overall E-W extension during the collapse of the Sveconorwegian orogen ca. 0.97–0.87 Ga ago caused the localised reactivation of fault systems III and IV. Our research approach, which is integral to the siting process of repositories for spent nuclear fuel, demonstrates that the basement in southwestern Finland experienced repeated reactivation since the Mesoproterozoic, suggesting that future deformation localization is likely to be also accommodated by reactivation of existing brittle structures rather than formation of new faults.

Influence of two-phase extension on the fault network and its impact on hydrocarbon migration in the Linnan sag, Bohai Bay Basin, East China

Multiphase rifts produce fault networks formed by the non-coaxial faults, these networks evolve through the formation of new faults and pre-existing faults. Pre-existing faults have a great influence on the formation of new faults under later regional stress, resulting in a complex fault network. Here, abundant 3D seismic and log data are used to reveal the evolution of fault networks in the Linnan sag in southwestern East China, a complex basin that experienced multiple phases of extension during the Cenozoic and developed NE-SW-, ENE-WSW- and E-W-striking faults. (1) During deposition of the Kongdian formation (65-50 Ma), NE-SW-striking faults formed under regional NW extension. In contrast, the ENE-WSW-striking and E-W-striking faults are younger, as they show no impact on the Ek formation. (2) During deposition of the Shahejie 4 formation (50-42 Ma), faults of all orientations (NE-SW, ENE-WSW and E-W) were active. However, the pre-existing NE-SW-striking faults show dextral strike-slip characteristics. The ENE-WSW-striking faults in the central parts show shear properties. The minor faults controlled by the regional extension and local strike-slip faulting near the pre-existing NE-striking faults strike NE, and the E-W-striking faults are distributed far from the pre-existing fault. These phenomena all indicate that the stress field changed from NW-SE to N-S extension in this stage. (3) During deposition of the Shahejie 3 formation (42-38 Ma), all fault activity was the strongest, and NE-striking faults began to connect and control basin deposition. The ENE-WSW-striking faults became longer. The density of E-W-striking faults increased. (4) During deposition of the Shahejie 2-Dongying formations (38–23.5 Ma), all fault activity weakened, and the Linnan sag received sediment. These observations demonstrate that later extension with a different direction can form local stress near pre-existing faults and that faults with new strikes can enhance the geometric and kinematic complexity in the fault network in the late stage. This study provides a reference for the interpretation of other multiphase rift, where two-phase extension fault networks were controlled by regional and local stresses, the reactivated pre-exist faults and newly-formed faults coexist in non-coaxial extension. Additionally, such fault networks can have important controlling effects on the distribution of hydrocarbon accumulation.

New Macroseismic and Morphotectonic Constraints to Infer a Fault Model for the 9 (Mw6.1) and 11 January (Mw7.3) 1693 Earthquakes (Southeastern Sicily)

This study deals with the earthquakes which occurred in southeastern Sicily in 1693 (January 9 and 11, Mw ≈ 6.1 and 7.3, respectively). Although they have been largely studied, robust and commonly accepted seismic sources are still missing. We performed a revision of the 1693 macroseismic data and, for the fore and main-shocks, modeled new NNE-SSW trending seismic sources. In the Hyblean Plateau area, we carried out an analysis of DEM and aerial photos to map tectonic features. Then, we performed field surveys on the main faults, and a morphotectonic study with the aim of characterizing the activity of mapped faults. The study revealed the presence of three main fault systems. The first is the Palazzolo-Villasmundo Fault System, composed of NNE-SSW and NE-SW trending north-west-dipping normal faults. Some of these faults could be reactivated as reverse faults. The second is the Augusta-Floridia Fault System, made of NNW-SSE and NW-SE normal faults. The third is composed of faults which have never been mapped before: the Canicattini-Villasmundo Fault System that shows both a segmented and stepping pattern, almost N-S trending and west-dipping normal faults; some of these faults show a left-lateral movement. The morphotectonic study demonstrated that the fault systems are active. Furthermore, both strike and kinematics of the studied faults well match with the regional stress field characterized by a NW-SE σ1, which in the study area is probably both affecting some pre-existing faults, the Palazzolo-Villasmundo and the Augusta-Floridia Fault Systems, and causing the formation of new faults, the Canicattini-Villasmundo Fault System. The latter system lies across the Hyblean Plateau with a total length of 35 km and, due to its aligned segmented pattern, it can be the surface expression of a master fault that seems dividing the Hyblean Plateau in two blocks. Moreover, the Canicattini-Villasmundo Fault System well fits the southern part of the 1693 revaluated seismic sources and matches with a current alignment of shocks mainly characterized by left-lateral focal mechanisms on almost N-S fault planes. Considering the possible rupture length in depth, it could manage to release Mw ≈ 7.1 earthquakes, representing a valuable candidate source for the 1693 earthquakes.

Cross-scale analysis of nickel Superalloy fatigue using Markov state model-molecular dynamics method

Molecular dynamics (MD) simulation is an advanced method in microscale modelling of material but it depends on the complexity of the model. The performance of MD simulation is poor once the model size is huge. To accelerate the computing of MD simulation, the Markov state model (MSM) can be applied because of the ability to predict a future state of a stochastic system. With the advantage of MSM and MD applied in material modelling, a good result could be expected where the time scale limitation of MD simulation is bridged by MSM method. In this research, an MSM method based on the MD microstates in which a nickel superalloy's atomic model arrangements and their microstructure evolution have been treated with the Markov properties is presented. This MSM is based and classified by a dislocation model which is a fundamental of the microstructural tessellation evolution. The results indicate that the microstructure evolution in a situation of energy minimisation favours the formation of new faults alongside existing ones. And dislocation accumulation on the grain boundary was observed during fatigue resolving. Some dislocations formatted and grown in the middle of coarse grain and penetrated through the grain.

Competition between slow slip and damage on and off faults revealed in 4D synchrotron imaging experiments

In the continental crust, faults may accommodate deformation through aseismic creep, slow slip events, or seismic slips that produce dynamic damage, or a combination of these endmembers. A variety of parameters controls the occurrence of these mechanical behaviors. In a series of laboratory experiments, we image centimeter-scale faults during sliding under in situ conditions. We perform four experiments of slip on centimeter-scale crystalline rock samples prepared with a saw-cut interface at 45° from the direction to the maximum compressive stress and at stress conditions of 2–3 km depth. We image fault slip and off-fault fracture development using 4D synchrotron X-ray microtomography. Three faults have an initial rough interface, and deformation occurs with increasing differential stress by a combination of slow slip events and off-fault damage, until catastrophic failure and the formation of new faults. Conversely, the pre-cut fault with a smooth initial surface deforms mainly by slow slip, develops numerous striations along its slip plane, and no microfractures are detected in the wall rock. Our experiments reproduce aseismic and seismic faulting behavior, and demonstrate that the roughness of the fault plane is one of the parameters that control the transition between these two behaviors. A fault with a rougher interface may tend to develop more off-fault damage and seismic behavior. For the rough fault experiments, the secondary faulting occurs along a network of faults oriented at high angles from the pre-existing saw-cut plane, a behavior similar to several earthquake sequences that occurred along orthogonal faults.

Новые данные о новейшем напряженном состоянии земной коры острова Сахалин (по структурно-геоморфологическим индикаторам тектонических напряжений)

To verify the ideas about neotectonic and modern stresses of Sakhalin, we analyze structural and geomorphological signs of the stress state of this region, discovered during field work in 2019–2020. Along with updated field measurements using the structural-geomorphological method, data on crustal deformation based on GPS/GLONASS measurements are presented. Data from geophysical studies (seismological and borehole methods) are given. The identification of three types of areas with different geodynamic regime: transtension, transpression and strike – slip (simple shift) is confirmed. Variations of the current stress field at the boundaries of regions with different geodynamic regime for the formation of new faults are noted. Northern Sakhalin has specific directions of compression axes of neotectonic stresses, expressed in North-Eastern orientations, in contrast to the prevailing sublatitudinal orientations on the entire island. Studies have shown that in the south of Sakhalin, the border between the Amur and Okhotsk microplates runs along the West Sakhalin fault rather than the Central Sakhalin fault.

Impact of range‐parallel sediment transport on 2D thermo‐mechanical models of mountain belts: Application to the Kyrgyz Tien Shan

AbstractEvolving mountain belts dynamics is very sensitive to surface processes. The surface processes affect tectonics by enhancing crust exhumation and thermal weakening, and depositing soft yet cold sediments in surrounding basins. While 2D plane strain models approximate cylindrical tectonic structures well, simple 1D mass transfer cannot capture erosion–sedimentation complexity. The Eastern Kyrgyz Tien Shan, where structures, basins and exhumation rates are well constrained, is used here to illustrate this issue. Thermo‐mechanical models demonstrate that 1D transport cannot adjust both basin geometry and Apatite Fission Track exhumation ages. When out‐of‐plane sediment transfer is considered, the amount of evacuated sediment delays or accelerates the formation of new faults, affecting the relative timing of exhumation. For our case study, lateral drainage must evacuate 80% of the sediments to match the geological constraints, which is consistent with other source to sink analyses. This indicates that lateral drainage should not be neglected in regional 2D models.

From Extension to Shortening: Tectonic Inversion Distributed in Time and Space in the Alboran Sea, Western Mediterranean

Abstract2D seismic reflection data tied to biostratigraphical and log information from wells in the central and southeastern Alboran Sea have allowed us to constrain the spatial and temporal distribution of rifting and inversion. Normal faults, tilted basement blocks, and growth wedges reveal a thinned continental crust that formed in response to NW‐SE extension. To the east, a secondary SW‐NE trend of extension affects the transitional crust adjacent to the oceanic Algerian Basin. The maximum thickness of syn‐rift sediments is ~3.5 km, and the oldest recorded deposits are Serravallian. The WNW‐ESE Yusuf fault formed a buttress separating and accommodating variable extension between two different tectonic domains: the thinned continental crust of Alboran and the oceanic spreading of the Algerian Basin. Late Tortonian to present‐day NW‐SE Africa/Eurasia plate convergence drove shortening and reactivation of some of the earlier extensional structures as reverse and strike‐slip faults, forming complex, compartmentalised subbasins. Tectonic inversion coexisted with the formation of new faults and folds. Inversion was partial along the Habibas Basin and Al‐Idrisi fault, but complete along the Alboran Ridge, where some SW‐NE trending faults were perpendicular to the recent NW‐SE plate convergence and were reactivated as thrusts. The WNW‐ESE Yusuf fault is oblique to the convergence vector, and therefore, reactivation is mainly expressed as transpressional deformation. Volcanic rocks intruded along the Alboran Ridge and Yusuf faults during the latest stages of extension formed rheological anisotropies that localised the later inversion.

The strength of earthquake-generating faults

This paper summarizes the observations and methods that have been used to study the strength of active earthquake-generating (seismogenic) faults. Indirect inferences based upon a range of geophysical and geological observations suggest that faults fail in earthquakes at shear stresses of less than c. 50 MPa, equivalent to effective coefficients of friction of less than 0.3, and possibly as low as 0.05. These low levels of effective friction are likely to be the result of a combination of high pore fluid pressures, which could be local or transient, and the frictional properties of phyllosilicate-rich fault rocks. The dip angles of new faults forming in oceanic outer rises imply that intrinsically low-friction fault rocks must control the fault strength in at least that setting. When combined with the much higher fault strengths inferred from borehole studies and some laboratory measurements, the observations are most consistent with weak faults embedded in strong surroundings, providing a clear reason for the prevalence of fault reactivation. However, the conditions required for the formation of new faults, and the reasons for an apparent wide variability in the degree of fault healing through time, remain unknown.

Influence of fault reactivation during multiphase rifting: The Oseberg area, northern North Sea rift

Multiphase rifts tend to produce fault populations that evolve by the formation of new faults and reactivation of earlier faults. The resulting fault patterns tend to be complex and difficult to decipher. In this work we use seismic reflection data to examine the evolution of a normal fault network in the Oseberg Fault Block in the northern North Sea Rift System – a rift system that experienced Permian – Early Triassic and Middle Jurassic – Early Cretaceous rifting and exhibits N-S, NW-SE and NE-SW oriented faults.Both N-S- and NW-SE-striking faults were established during the Permian – Early Triassic rifting, as indicated by Triassic growth packages in their hanging walls. In contrast, the NE-SW-striking faults are younger, as they show no evidence of Permian – Early Triassic growth, and offset several N-S- and NW-SE-striking faults. Structural analysis show that a new population of NW-SE-striking faults formed in the Lower – Middle Jurassic (inter-rift period) together with reactivation of N-S-striking Permian – Early Triassic faults, indicating a NE-SW inter-rift extension direction.During the Middle Jurassic – Early Cretaceous rifting, faults of all orientations (N-S, NW-SE and NE-SW) were active. However, faults initiated during the Middle Jurassic – Early Cretaceous rifting show mainly N-S orientation, indicating E-W extension during this phase. These observations suggest a reorientation of the stress field from E-W during the Permian – Early Triassic rift phase to NE-SW during inter-rift fault growth and back to E-W during the Middle Jurassic – Early Cretaceous rift phase in the Oseberg area. Hence, the current study demonstrates that rift activity between established rift phases can locally develop faults with new orientations that add to the geometric and kinematic complexity of the final fault population.

Improved concept of lithospheric strength and earthquake activity at shallow depths based upon the fan-head dynamic shear rupture mechanism

The typical depth–frequency distribution of earthquake hypocentres (DFDE) demonstrates that, below an upper cutoff, the earthquake frequency increases with depth up to a maximum value and then decreases and ceases at a lower cutoff. Such regular behaviour of earthquakes implies the existence of some fundamental mechanisms responsible for the distribution. Conventional models of lithospheric strength based upon the assumption that the frictional strength along pre-existing faults represents a lower limit on the rock shear strength do not provide any intrinsic logic for the observed DFDE. The paper shows that these models ignore the specific properties of intact hard rocks which can exhibit extremely low transient strength (significantly lower than the frictional strength) during failure under the high confining stresses corresponding to seismogenic depths. The low transient strength is provided by a recently identified fan-head shear rupture mechanism which can be initiated in intact rocks in the proximity of pre-existing faults. The low transient shear strength of intact rock determines the correspondingly low transient strength of the lithosphere, which favours generation of new earthquake faults in the intact rock mass adjoining pre-existing faults in preference to frictional stick-slip instability along these faults. The efficiency of the fan-mechanism within the seismogenic layer is variable, with maximum efficiency at the middle range between the upper and lower cutoffs, thus providing minimum transient strength of the lithosphere and maximum earthquake frequency at that depth. We believe that this intrinsic property of hard rocks is responsible for the observed DFDE. Importantly, the formation of new faults in intact rock generated by the fan-mechanism can be accompanied by very small stress-drops (similar to, or lower than, stress-drops for frictional stick-slip instability) combined with abnormally high energy release. The paper proposes an improved concept of lithospheric strength and earthquake activity at seismogenic depths.

Constraining fault friction in oceanic lithosphere using the dip angles of newly-formed faults at outer rises

We investigate the mechanical properties of the oceanic lithosphere using earthquake focal mechanisms from subduction zone outer rises. We study regions where faulting oblique to the pre-existing mid-ocean ridge fabric implies the formation of new faults. The nodal-plane dips of dip-slip earthquakes on these faults are dominantly in the range 30–60°, with a strong peak concentrated around 45°. This distribution is inconsistent with the pattern that would result from high coefficients of friction (e.g. 0.6, equivalent to normal faults forming at 60° and thrust faults at 30°). We instead suggest that the observed distribution of dips implies that faults in the oceanic lithosphere have a low coefficient of friction, due to either low-friction clay minerals formed by hydrothermal alteration at the ridge, or due to an intrinsic level of friction that is lower than that suggested by laboratory studies.

The growth of northeastern Tibet and its relevance to large‐scale continental geodynamics: A review of recent studies

AbstractRecent studies of the northeastern part of the Tibetan Plateau have called attention to two emerging views of how the Tibetan Plateau has grown. First, deformation in northern Tibet began essentially at the time of collision with India, not 10–20 Myr later as might be expected if the locus of activity migrated northward as India penetrated the rest of Eurasia. Thus, the north‐south dimensions of the Tibetan Plateau were set mainly by differences in lithospheric strength, with strong lithosphere beneath India and the Tarim and Qaidam basins steadily encroaching on one another as the region between them, the present‐day Tibetan Plateau, deformed, and its north‐south dimension became narrower. Second, abundant evidence calls for acceleration of deformation, including the formation of new faults, in northeastern Tibet since ~15 Ma and a less precisely dated change in orientation of crustal shortening since ~20 Ma. This reorientation of crustal shortening and roughly concurrent outward growth of high terrain, which swings from NNE‐SSW in northern Tibet to more NE‐SW and even ENE‐WSW in the easternmost part of northeastern Tibet, are likely to be, in part, a consequence of crustal thickening within the high Tibetan Plateau reaching a limit, and the locus of continued shortening then migrating to the northeastern and eastern flanks. These changes in rates and orientation also could result from removal of some or all mantle lithosphere and increased gravitational potential energy per unit area and from a weakening of crustal material so that it could flow in response to pressure gradients set by evolving differences in elevation.

The role of frictional plasticity in the evolution of normal fault systems

Tectonic extension applied to the upper crust commonly results in the development of sub-parallel arrays of faults and fault belts. A computational model of an extending elastic-viscous-plastic crust is presented, which describes the evolution of stress, strain and displacement in the vicinity of an active normal fault. Within the upper crust discrete episodes of fault slip accommodate up to half the applied extension rate, with the balance accommodated by bands of frictional-plastic shearing representing young proto-faults. Repeated coseismic shear stress reductions in the near-fault vicinity impose a stress-failure shadow where the formation of new faults is prohibited. A near-surface compressional stress regime in the footwall causes the failure shadow to extend disproportionally into this fault block, thereby favoring the formation of new faults in the hanging wall. A continuous shallowing of fault dip is also documented and supports a scenario of migration of faulting into the hanging wall of existing faults, consistent with descriptions of the structural evolution in the western Gulf of Corinth, Greece.

Seismic evidence for large‐scale compositional heterogeneity of oceanic core complexes

Long‐lived detachment faults at mid‐ocean ridges exhume deep‐seated rocks to form oceanic core complexes (OCCs). Using large‐offset (6 km) multichannel seismic data, we have derived two‐dimensional seismic tomography models for three of the best developed OCCs on the Mid‐Atlantic Ridge. Our results show that large lateral variations in P wave velocity occur within the upper ∼0.5–1.7 km of the lithosphere. We observe good correlations between velocity structure and lithology as documented by in situ geological samples and seafloor morphology, and we use these correlations to show that gabbros are heterogeneously distributed as large (tens to >100 km2) bodies within serpentinized peridotites. Neither the gabbros nor the serpentinites show any systematic distribution with respect to along‐isochron position within the enclosing spreading segment, indicating that melt extraction from the mantle is not necessarily focused at segment centers, as has been commonly inferred. In the spreading direction, gabbros are consistently present toward the terminations of the detachment faults. This suggests enhanced magmatism during the late stage of OCC formation due either to natural variability in the magmatic cycle or to decompression melting during footwall exhumation. Heat introduced into the rift valley by flow and crystallization of this melt could weaken the axial lithosphere and result in formation of new faults, and it therefore may explain eventual abandonment of detachments that form OCCs. Detailed seismic studies of the kind described here, when constrained by seafloor morphology and geological samples, can distinguish between major lithological units such as volcanics, gabbros, and serpentinized peridotites at lateral scales of a few kilometers. Thus such studies have tremendous potential to elucidate the internal structure of the shallow lithosphere and to help us understand the tectonic and magmatic processes by which they were emplaced.

Fault plane orientations of intermediate‐depth earthquakes in the Middle America Trench

Intermediate‐depth earthquakes are often attributed to dehydration embrittlement reactivating preexisting weak zones. The orientation of presubduction faults is particularly well known offshore of Middle America, where seismic reflection profiles show outer rise faults dipping toward the trench and extending >20 km into the lithosphere. If water is transported along these faults and incorporated into hydrous minerals, the faults may be reactivated later when the minerals dehydrate. In this case, the fault plane orientations should be the same in the outer rise and at depth, after accounting for the angle of subduction. To test this hypothesis, we analyze the directivity of 54 large (MW ≥ 5.7) earthquakes between 35 and 220 km depth in the Middle America Trench. For 12 of these earthquakes, the directivity vector allows us to identify the fault plane of the focal mechanism. Between 35 and 85 km depth, we observe both subhorizontal and subvertical fault planes. The subvertical fault planes are consistent with the reactivation of outer rise faults, whereas the subhorizontal fault planes suggest the formation of new faults. Deeper than 85 km, we only observe subhorizontal faults, indicating that the outer rise faults are no longer being reactivated. The similarity with previous results from the colder Tonga‐Kermadec subduction zone suggests that the mechanism generating these earthquakes, and controlling fault plane orientations, depends on pressure rather than temperature or other tectonic parameters and that the observed rupture characteristics constitute a basic feature of intermediate‐depth seismicity. Exclusively subhorizontal faults may result from isobaric rupture propagation or the hindrance of seismic slip on preexisting weak subvertical planes.