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Most ophiolitic mélanges and chaotic rock units in exhumed subduction zone complexes and orogenic belts are commonly interpreted as the products of tectonic processes (e.g., underplating and return flow) acting at intermediate to great depths (depth > 10–15 km, T > 250 °C) at convergent margins. Conversely, observations from modern and ancient, non- to poorly metamorphosed subduction–accretion complexes (recognized as mélanges and chaotic rock units) around the world show that these rock associations: (1) likely formed at shallow structural levels first, and (2) were later subducted and became tectonically reworked. As such, they mainly consist of broken formations (> 21.5%), and sedimentary (c. 20%), polygenetic (> 13.7%) and/or diapiric (c. 6.7%) mélanges. Tectonic mélanges are limited to

Differences in REE patterns of calcite from extensional and shear veins of the Sestola Vidiciatico Tectonic Unit in Northern Apennines suggest variations in fluid source during the seismic cycle in an ancient analogue of a shallow megathrust (Tmax 100°-150°C). In shear veins, a positive Eu anomaly suggests an exotic fluid source, likely hotter than the fault environment. Small-scale extensional veins were derived instead from a local fluid in equilibrium with the fault rocks. Mutually crosscutting relations between two extensional veins sets, parallel and perpendicular to the megathrust, suggest repeated shifting of the σ 1 and σ 3 stresses during the seismic cycle. This is consistent with: (i) a seismic phase, with brittle failure along the thrust, crystallization of shear veins from an exotic fluid, stress drop and stress rotation; (ii) a post-seismic phase, with fault-normal compaction and formation of fault-normal extensional veins fed by local fluids; (iii) a reloading phase, where shear stress and pore pressure are gradually restored and fault-parallel extensional veins form, until the thrust fails again. The combination of geochemical and structural analyses in veins from exhumed megathrust analogues represents a promising tool to better understand the interplay between stress state and fluids in modern subduction zones. Supplementary material: Cathodoluminescence microphotographs, methodological details on the microstructural analysis, microphotographs of the location of analysed spots and the geochemical data table and are available at https://doi.org/10.6084/m9.figshare.c.4842165 Thematic collection: This article is part of the Polygenetic melanges collection available at: https://www.lyellcollection.org/cc/polygenetic-melanges

Deciphering the stress and strain distribution across plate boundary shear zones is critical to understanding the physical processes involved in the nucleation of megathrust faults and its behaviour. Plate boundaries at shallow depth represent complex and highly deformed zones showing structures from both distributed and localized deformation.As magnetic minerals are sensitive to stress regime, the investigation of the magnetic fabric has proven to be an effective tool in studying faulting processes at intraplate shear zones.Anisotropy of magnetic susceptibility (AMS) provides insights into the preferred orientation of mineral grains and the qualitative relationships between petrofabrics and deformation intensity.We present an approach of combined Contoured Diagram and Cluster Analysis to isolate the contribution of coexisting petrofabrics to the total AMS and evaluating the significance of magnetic fabric clusters.Our results reveal distinct subfabrics with reasonably straightforward correlations with structural data. Specific AMS pattern may be associated to the intensity of the reworking related to tectonic shearing and the structural position within the shear zone (i.e., the proximity to the main thrust faults).Close to the main thrust the magnetic fabric is dominantly oblate with magnetic foliation consistent to the S-C fabric and/or mélange foliation and the magnetic lineation parallel to the shear sense.Away from the thrust faults the degree of anisotropy as well as the ellipsoids oblateness gradually diminishes. Thus, the presence of subfabrics related to previous tectonic events or less intense deformation (i.e. intersection lineation fabric) became dominant. The discrimination of subfabrics also allowed to unravel the presence of minor thrust plane and qualitatively evaluate the heterogeneous registration of strain (i.e. distributed versus localized deformation).An abrupt change in magnetic ellipsoid shape and parameters is also observed below the basal décollements showing purely sedimentary magnetic fabric or previous deformation history with minor to absent evidences of shearing in the hanging wall.Then, the integration with anisotropy of magnetic remanence experiments in different coercivity windows (ApARM) allow to separate the contribution of different ferromagnetic subpopulation of grains, constraining the significance of the different magnetic pattern/clusters detected through the AMS analysis.In conclusion, our results show the potential of a combination of density diagrams and cluster analysis validated by ApARM experiments in distinguishing the superposition of deformation events, unravelling strain partitioning/concentration and thus to better understand the geodynamic evolution of subduction-accretion complexes.

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.

Smectite-rich fault gouges recovered during Integrated Ocean Drilling Program Expedition 343 (Japan Trench Fast Drilling Project (J-FAST)) from the plate boundary slip zone of the 2011 Mw 9.0 Tohoku-Oki earthquake were deformed at slip velocities of 10 µm s−1 to 3.5 m s−1 and normal stresses up to 12 MPa. Water-dampened gouges (1) are weaker (apparent friction coefficient, μ* 0.1 m s−1). A significant amount of amorphous material formed in room-humidity experiments at low- and high-slip velocities, likely by comminution and disordering of smectite. Our results indicate that the frictional properties of water-dampened gouges could have facilitated propagation of the Tohoku-oki rupture to the trench and large coseismic slip at shallow depths.

We present a critical appraisal of the role of subducted, medium (10-1000 km(2)) to giant (>= 1000 km(2)) and heterogeneous, mud-rich mass transport deposits (MTDs) in seismic behavior and mechanisms of shallow earthquakes along subduction plate interfaces (or subduction channels) at convergent margins. Our observations from exhumed ancient subduction complexes around the world show that incorporation of mud-rich MTDs with a "chaotic" internal fabric (i.e., sedimentary m langes or olistostromes) into subduction zones strongly modifies the structural architecture of a subduction plate interface and the physical properties of subducted material. The size and distribution of subducted MTDs with respect to the thickness of a subduction plate interface are critical factors influencing seismic behavior at convergent margins. Heterogeneous fabric and compositions of subducted MTDs may diminish the effectiveness of seismic ruptures considerably through the redistribution of overpressured fluids and accumulated strain. This phenomenon possibly favors the slow end-member of the spectrum of fault slip behavior (e.g., Slow Slip Events, Very Low Frequency Earthquakes, Non-Volcanic Tremors, creeping) compared to regular earthquakes, particularly in the shallow parts (T

The Sestola Vidiciatico tectonic Unit (SVU) accommodated the early Miocene convergence between the subducting Adriatic plate and the overriding Ligurian prism, and has been interpreted as a field analogue for the shallow portion of subduction megathrusts. The SVU incorporated sediments shortly after their deposition and was active down to burial depth corresponding to temperatures around 150 °C. Here, we describe the internal architecture of the basal thrust fault of the SVU through a multi-scale structural analysis and investigate the evolution of the deformation mechanisms with increasing burial depth. At shallow depth, the thrust developed in poorly lithified sediments which deformed by particulate flow. With increasing depth and lithification of sediments, deformation was accommodated in a meter scale, heterogeneous fault zone, including multiple strands of crack-and-seal shear veins, associated with minor distributed shearing in clay-rich domains and pressure solution. In the last stage, slip localized along a sharp, 20 cm thick shear vein, deactivating the fault zone towards the footwall. The widespread formation of crack-and-seal shear veins since the first stages of lithification indicates that failure along the thrust occurred at high fluid pressure and low differential stress already at shallow depth. Progressive shear localization occurs in the last phases of deformation, at temperatures typical of the transition to the seismogenic zone in active megathrusts.

Apatite fission-track (AFT) analyses were performed on 16 sandstone samples from a tectonic mélange unit exposed in three tectonic windows located near the inferred front of the early Miocene subduction system of the Northern Apennines of Italy. The tectonic windows display a block-on-block tectonic mélange present under the Ligurian Units. The mélange is formed by portions of the upper plate incorporated in the plate boundary shear zone as a consequence of a mechanism of frontal tectonic erosion during the Aquitanian (early Miocene). AFT and structural data, together with stratigraphic constraints, allow the reconstruction of a complete deformation cycle with a phase of underthrusting followed by underplating and early exhumation of the tectonic mélange. The exhumation, in particular, took place at ~10-20 km from the original subduction front. Moreover, the analysis suggests that multiple faults were active at the same time in the frontal part of the subduction zone.

The early to middle Miocene erosive plate boundary preserved in the Northern Apennines NW of the Sillaro Line is formed by two distinct units - the Sestola-Vidiciatico Tectonic Unit and the Subligurian Units. These two units occupy, respectively, the SE and the NW portions of the studied area. Upon closer examination, the features that distinguish these mapped units do not reflect differing plate boundary processes, but rather the incorporation or non-incorporation of forearc-toe mass-wasting deposits into the active subduction channel. In other respects, these two units document similar subduction channel processes, including the contemporaneous activity of multiple sub-parallel slip surfaces. This mode of subduction channel deformation leads to the ‘laminar' incorporation of distinct stacked slices within the channel.

Some faults seem to slip at unusually high angles (>45°) relative to the orientation of the greatest principal compressive stress1, 2, 3, 4, 5. This implies that these faults are extremely weak compared with the surrounding rock6. Laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre-existing frictionally weak surface7, 8, 9, weakening from chemical reactions10, elevated fluid pressure11, 12, 13 or dissolution–precipitation creep14, 15. Here we describe shear veins within the Chrystalls Beach accretionary melange, New Zealand. The melange is a highly sheared assemblage of relatively competent rock within a cleaved, anisotropic mudstone matrix. The orientation of the shear veins—compared with the direction of hydrothermal extension veins that formed contemporaneously—indicates that they were active at an angle of 80°±5° to the greatest principal compressive stress. We show that the shear veins developed incrementally along the cleavage planes of the matrix. Thus, we suggest that episodic slip was facilitated by the anisotropic internal fabric, in a fluid-overpressured, heterogeneous shear zone. A similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings. We therefore conclude that incremental slip along a pre-existing planar fabric, coupled to high fluid pressure and dissolution–precipitation creep, may explain active slip on severely misoriented faults.

In the Northern Apennines of Italy the Ligurian and Subligurian Units (LSU) represent far-travelled/allochthonous units which, since early Miocene to Recent, migrated towards NE, above the Tuscan-Umbrian foredeep deposits, reaching the present-day frontal ranges of the chain. In the NE-facing side of the western Northern Apennines the present-day geometry of the LSU does not reflect the shape they acquired during the late Oligocene, when the early orogenic collisional phases produced a wedge whose geometry tapered out towards the foreland area, i.e. to the E-NE. In fact, at present, geological evidences indicate that the LSU are characterized by a doubly-tapering geometry, whose thickness ranges from less than 1 km at the main ridge zone (SW) up to more than 4 km along the NE slope of the chain and then tips out again few km N of the Northern Apennines topographic front, underneath the late Messinian to Recent marine and continental deposits of the Po Plain foredeep. These data imply that since late Oligocene the LSU geometry has been deeply reshaped inside a context of overall growth of the orogenic wedge. Therefore the reshaping of the LSU is related to their post-early Miocene progressive emplacement over the foredeep units and to the late orogenic evolution of the Northern Apennines, characterized by the coeval activity of extensional and compressional tectonics. In order to put new constraints on the relationships existing between timing and modes of reshaping of the LSU and the Miocene to Recent evolution of the western Northern Apennines, we adopted a multidisciplinary approach which took into account field evidences, low temperature thermal and thermochronological data (vitrinite reflectance, clay mineral analyses, apatite fission track dating), numerical modelling of the AFT cooling ages through the use of Pecube finite element code, recently published works on the evolution of the external slope of the chain, and a new interpretation of seismic lines and boreholes data. This multidisciplinary approach allowed us to: 1) build a 3D representation of the LSU present-day geometry; 2) highlight two main stages of reshaping of the LSU, first during late Miocene (10 to 6 Ma), and subsequently during Pliocene to Recent (?) time (6 to 0 (?) Ma); 3) relate, during the first stage, the LSU thinning processes, achieved by means of low-angle extensional faults, to the tectonic denudation of the foredeep units; 4) envisage the importance of coupled tectonic (high-angle extensional tectonics) and progressively increased surface erosion processes as responsible for the denudation of the foredeep units during the second stage; 5) define estimates of the exhumation/erosion rates characterizing the investigated area during the last 10 My. These results allowed us to give new insights, temporal and spatial constraints on the morphostructural evolution of the western Northern Apennines, which has been mainly affected initially (10-6 Ma) by the interplay between deep compressional and coeval shallow extensional tectonics (< 20 and < 4 km, respectively), and subsequently by the progressively more relevant onset of surface erosion processes.

This guide provides background information and an itinerary for a two-day fi eld trip in the Northern Apennines leaving from Modena and ending at Riolunato (Modena). The proposed fi eld trip route leads through the Po Valley side of the Northern Apennines, in the Emilia region. The fi eld trip provides opportunities to examine the exposed geological features related to a phase of early-middle Miocene convergence between the European and Adria plates. In particular, outcrops have been selected that exhibit features characterizing deformation in an exhumed plate boundary shear zone, interpreted as an erosive plate boundary shear zone. The sedimentary evolution and deformation of the upper and lower plates are also highlighted.

A large and morphologically spectacular rock slope instability is detected in the northern Apennines of Italy. Ten of metres of blocks and columns, in calcareous rock formation, slide and spread on clay shale, dipping down slope 30°. The geological structures and the morphological features of the slope deformation have been identified with use of high-resolution digital elevation models obtained from ground based laser scanner and airborne LiDAR. The actual morphological setting of the slope is characterized, also far away form the main scarp, by some metres high columns in vertical position. This suggests that the main deformations are parallel to the slope. Inside the landslide, rock falls are also frequent, due to the failure of large blocks and columns through set of joints. The origin of the instability is probably related with the erosion of the clay formation, at the foot of the slope, by the close torrent. From the analysis of aero-photographs of the past fifty years is possible identified an enlargement of the mass instability in the up slope direction. A destructive event was recognized in the Seventies, when the road and the bridge below the landslide were destroyed. During the event was documented the passage between slow to very fast moving. In the summer 2009, after the increase of the slope deformations of the last years, a continuous monitor system was installed. The system is a robotic station that every two hours measures 20 benchmarks fixed in the rock blocks and columns in the landslide area. Today, tens centimetre of displacements are recorded. The data show an increase of the deformations rate with the distance from the main scarp. At the moment, the evolution of the slope is controlled by continuous creeping processes and by period with accelerates deformations. These last correlate with wet and snow melt periods, which are responsible of the variation in the groundwater regime and of the changes in the material behaviour within the sliding zone

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.

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.

The thickness of an active plate boundary fault is an important parameter for understanding the strength and spatial heterogeneity of fault behavior. We have compiled direct measurements of the thickness of subduction thrust faults from active and ancient examples observed by ocean drilling and fi eld studies in accretionary wedges. We describe a general geometric model for subduction thrust decollements, which includes multiple simultaneously active, anastomosing fault strands tens of meters thick. The total thickness encompassing all simultaneously active strands increases to ~100–350 m at ~1–2 km below seafl oor, and this thickness is maintained down to a depth of ~15 km. Thin sharp faults representing earthquake slip surfaces or other discrete slip events are found within and along the edges of the tens-ofmeters- thick fault strands. Although fl attening, primary inherited chaotic fabrics, and fault migration through subducting sediments or the frontal prism may build melange sections that are much thicker (to several kilometers), this thickness does not describe the active fault at any depth. These observations suggest that models should treat the subduction thrust plate boundary fault as

Stressed Out Large seismic events such as the 2011 magnitude 9.0 Tohoku-Oki earthquake can have profound effects not just on the severity of ground motion and tsunami generation, but also on the overall state of the crust in the surrounding regions. Lin et al. (p. 687 ) analyzed the stress 1 year after the Tohoku-Oki earthquake and compared it with the estimated stress state before the earthquake. In situ resistivity images were analyzed from three boreholes drilled into the crust across the plate interface where the earthquake occurred. Stress values indicate a nearly complete drop in stress following the earthquake such that the type of faulting above the plate boundary has changed substantially. These findings are consistent with observations that the sea floor moved nearly 50 meters during the earthquake.

The main science goal of the Japan Trench Fast Drilling Project (JFAST) is to understand the physical mechanisms and dynamics of large slip earthquakes, which is fundamental to understanding the huge tsunami that caused extensive damage during the 2011 Tohoku earthquake. Specifically, the level of frictional stress during the earthquake rupture and the physical characteristics of the fault zone are investigated through drilling. The objectives of JFAST include locating the fault that ruptured during the Tohoku event using logging while drilling (LWD); characterizing the composition, architecture, and fundamental mechanisms of dynamic frictional slip and healing processes along the fault by taking core samples; and estimating the frictional heat and stress within and around the fault zone by placing a temperature measurement observatory across the fault. During the main JFAST expedition (Integrated Ocean Drilling Program [IODP] Expedition 343 in April and May 2012), LWD was completed in a borehole drilled to 850.5 meters below seafloor (mbsf) (total depth [TD] = 7740 meters below sea level [mbsl]), and a coring hole was drilled to 844.5 mbsf (TD = 7734 mbsl) to acquire 21 cores that spanned the two main fault targets. Because of delays associated with severe weather and technical challenges of operating in great water depths, installation of the observatories for temperature and pressure measurements was not completed during the main expedition; however, temperature sensors were successfully deployed during a short technical extension of JFAST, IODP Expedition 343T (JFAST II), in July 2012. Principal results of Expedition 343/343T include the following: 1. The overall structure at the drill site consists of a prism of faulted and folded clayey to silty mudstones above, and in fault contact (at ~820 mbsf) with, a largely undeformed, relatively thin sequence of hemipelagic and pelagic sediments that were deposited on top of the incoming Pacific plate. The primary constituents of the mudstones that make up the prism are terrigenous silt and clay, vitric ash, and biogenic silica. The fault contact, interpreted as the plate boundary decollement, is defined by a subhorizontal

[1] Convergent plate boundaries accommodate intraplate displacement within a ∼100–1000 m thick shear zone. Marine geophysicists typically define this zone, the subduction channel (SC), as the sedimentary layer between the downgoing oceanic crust and the base of the upper plate. Geologists and modelers, instead, perceive the SC as a specific type of shear zone. The original theory of SCs was developed when the net accretion of marine sediments to the forearc was thought to typify a convergent margin. While erosive margins were briefly mentioned, their mechanics were not discussed in any detail. We now realize that subduction erosion is taking place at roughly half of the modern subduction margins. Here we review and revise the theory of erosive SCs (1) to unify this concept across disciplines, focusing on the meaning of the channel's boundaries; (2) to redefine the portions of the forearc included in the SC concept; and (3) to better idealize this dynamic system where material supply to the channel, fluid content, and the heterogeneity of deformation all influence the SC's upper and lower boundaries. Migration of the channel boundaries controls the downdip variation of tectonic mechanisms that shape the margin. Within the shallow

Subduction can be associated either with accretion or with removal of material from the overriding plate. These two processes can either coexist or alternate in time along the same margin. Their inception has the potential to change the dynamic equilibrium of a marginal wedge resulting in the development of out-of-sequence thrusts, normal and strike-slip faults, or large submarine landslides in the frontal part of the subduction zone. In this work we investigate the effects of the transition from frontal accretion to frontal erosion on the stability of a subduction complex through the study of an ancient example from the northern Apennines (Ligurian subduction complex). New structural data suggest that in the early Neogene (Aquitanian), the removal and underthrusting of the toe of the wedge, formed by both the accreted oceanic sediments and the overlying wedge-top basin fill (i.e., the Subligurian units), implied a process of frontal tectonic erosion. The presence, on top of the subduction complex, of a complete succession of mid-late Eocene to late Miocene slope-apron sediments—i.e., the Epiligurian succession—facilitates a reconstruction of the sedimentary response to this event. In the Aquitanian, large areas of the wedge were denuded from the lower-slope sedimentary cover through extensive gravitational mass movements. The subsequent deposition of a thick body of submarine debris flows is documented. The mass-wasting deposits are interpreted as the sedimentary response to the underthrusting of the frontal part of the Ligurian subduction complex formed by the Subligurian units.

An accretionary melange of Triassic age ocean floor sediments exposed in the Chrystalls Beach Complex, South Island, New Zealand, comprises competent sandstone and chert phacoids set in a cleaved mudstone matrix, deformed in a continuous–discontinuous style at subgreenschist conditions. Deformation structures include a pervasive anastomosing fault–fracture mesh of multiple shearing surfaces, subparallel to cleavage, coated with incrementally developed quartz–calcite slickenfibers. Microstructural observations reveal slickenfiber growth by ‘crack-seal’ shear slip increments of 10–100 μm, with incremental slip transfer of the same order accommodated by opening of extension fractures that link en echelon slip surfaces. Individual slip surfaces can be traced for meters to tens of meters so that the ratio of average slip, u, to potential rupture length, L, predominantly lies within the range, 10− 6 < u/L < 10− 5, characteristic of microearthquakes obeying ‘constant stress-drop’ scaling with a low stress-drop Δτ ~ 30 kPa, typical of low frequency earthquakes. The host-rock assemblage, metamorphic environment, inference of near-lithostatic fluid overpressures, low stress-drop and mixed continuous–discontinuous shearing, resemble conditions and characteristics of low frequency earthquakes as identified within the seismic signals recorded during episodic tremor and slow slip events, at the downdip end of the seismogenic subduction thrust interface and within accretionary prisms.

The late Eocene‐middle Miocene erosive plate boundary between the European and the Adriatic plates is exhumed in the Northern Apennines of Italy. The fossil fault zone is 500 m thick and the outcropping portion exposes the first 5 km of its depth. At this plate boundary basal and frontal tectonic erosion incorporated unlithified, fluid‐rich sediments into the fault zone. The deformation and nature of the material along the plate boundary define a fossil subduction channel. Here we couple a detailed structural analysis of the Apennine subduction channel, focusing, in particular, on calcite veins, with a stable isotope analysis to characterize the fluid regime along an active subduction channel. The 13C and 18O composition of calcite vein and host rock samples within the fault zone indicates that there is a deep metamorphic source of fluids migrating upward along the subduction channel, in addition to locally derived fluid components. Dewatering of subducting turbidites contributes significantly only in the shallowest part of the channel. Structural observations indicate fluid flow along and across the subduction channel. At deep levels fluid flow is associated with discrete deformation events on shear faults offset by dilational jogs filled with implosion breccias. At intermediate levels deformation is still cyclic and associated with repeated crack‐and‐seal events. At the shallowest levels deformation occurred, while portions of the subducting material were still unlithified. Here the deformation was quasicontinuous, without associated vein development. Both isotope and structural analyses indicate that this erosive subduction channel behaved as a weak fault with a vertical maximum principal stress.

This chapter reviews observations and theories for the aseismic-seismic transition in the megathrust between the incoming and overriding plates at a subduction zone. The temperature of the aseismic-seismic transition appears to be quite similar at erosive and accretionary margins, despite large differences between them in the lithology of the seismogenic subduction channel that composes the “megathrust” plate interface. This fact, and the recent laboratory demonstration that both smectite and illite are velocity-strengthening in creep, suggests that the oft-postulated change in mechanical behavior of the megathrust due to a smectite-illite clay mineral transformation at ∼150°C is not the cause of the onset in seismogenesis at these temperature conditions within the subduction channel. Field observations from fossil megathrust zones suggest that a temperature-dependent change in the availability of in situ fluid is likely to play a key role in the onset of seismogenesis. Perhaps the causal link is to the smectite-illite transformation and other metamorphic dewatering reactions that liberate water at ∼150°C, under conditions where these reactions are an important local source of hydrous fluids. Field studies of fossil megathrusts support the hypothesis that fluids “control” seismogenesis, and indicate that there are large fluid pressure variations during the seismic cycle. In the fossil erosive megathrust system preserved in the Apennines, two decollements are simultaneously active at the roof and base of the subduction channel. The uppermost (nonseismogenic) portion of the megathrust even appears to alternate between tensional and compressional modes of failure during the seismic cycle along the deeper portions of the megathrust.

A fossil erosive subduction channel (the shear zone marking the plate boundary) preserved in the Northern Apennines of Italy is identified. The fossil zone records the presence of two decollements which seem to have been simultaneously active at the top and base of the subduction channel. In modern subduction zones the onset of seismic activity is believed to occur at ∼150° C, however in the fossil channel the onset seems to have occurred at colder temperatures. Tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the Earth’s subduction zones1,2. In situ investigation of the geological processes at active erosive margins is extremely difficult owing to the deep marine environment and the net loss of forearc crust to deeper levels in the subduction zone. Until now, a fossil erosive subduction channel—the shear zone marking the plate boundary3—has not been recognized in the field, so that seismic observations have provided the only information on plate boundary processes at erosive margins. Here we show that a fossil erosive margin is preserved in the Northern Apennines of Italy. It formed during the Tertiary transition from oceanic subduction to continental collision, and was preserved by the late deactivation and fossilization of the plate boundary. The outcropping erosive subduction channel is ∼500 m thick. It is representative of the first 5 km of depth, with its deeper portions reaching ∼150 °C. The fossil zone records several surprises. Two decollements were simultaneously active at the top and base of the subduction channel. Both deeper basal erosion and near-surface frontal erosion occurred. At shallow depths extension was a key deformation component within this erosive convergent plate boundary, and slip occurred without an observable fluid pressure cycle. At depths greater than about 3 km a fluid cycle is clearly shown by the development of veins and the alternation of fast (co-seismic) and slow (inter-seismic) slip. In the deepest portions of the outcropping subduction channel, extension is finally overprinted by compressional structures. In modern subduction zones the onset of seismic activity is believed to occur at ∼150 °C, but in the fossil channel the onset occurred at cooler palaeo-temperatures.

The Sestola-Vidiciatico Tectonic Unit (SVTU) in the Northern Apennines is an underthrust tectonic melange presently sandwiched between the Tuscan-Umbrian foredeep units and the overlying Ligurian/Subligurian thrust-nappe. The SVTU has been generated during the collision between the European and the Adria plates and now it separates the former oceanic accretionary wedge Ligurian/Subligurian thrust nappefrom the underlying fold–and-thrust belt formed by Adria sedimentary units. The collision caused an eastward migrating foredeep basin and the overthrusting of the frontal part of the Ligurian/Subligurian thrust-nappe on the subducting Adria margin. Part of the inner lower-slope sediments of the migrating foredeep basin have been unconformably deposited on a frontal prism formed by material already accreted in the Ligurian/Subligurian prism gravitationally and tectonically reworked. The frontal prism and its sedimentary cover have been progressively dragged down along the plate boundary zone generating the SV...

In the Northern Apennines of Italy the Ligurian and Subligurian Units (LSU) were involved, since the Late Cretaceous, in the accretionary and orogenic processes related to the convergence between Europe and Adria. Since early Miocene the Ligurian Units completely overthrusted the Subligurian Units (REMITTI et alii, 2011) and both units stacked together (i.e. the LSU) translated towards NE, above the Tuscan-Umbrian foredeep deposits, until they reached the present-day topographic front in the NW portion of the Northern Apennines. In the NE-facing side of the western Northern Apennines the present-day geometry of the LSU does not reflect neither the shape of the late Cretaceous-middle Eocene oceanic accretionary wedge (MOLLI, 2008 and references therein) nor the shape acquired during the late Oligocene, when the early orogenic collisional phases produced a wedge whose geometry tapered out towards the foreland area, i.e. to the E-NE. In fact, at present, geologic evidences indicate that the LSU have a shape whose thickness ranges from less than 1 km at the main ridge zone (SW) up to more than 3,5 km along the NE slope of the chain and then tips out again few km N of the Northern Apennines topographic front, underneath the Pleistocene-to Recent sediments of the Po Plain foredeep. Therefore, the LSU thickness increases towards its NE external tip and tapers out again in the subsurface of the Po Plain. These data imply the activation of thinning processes in the inner portion of LSU and thickening processes in the central portion of LSU; these thinning and thickening processes have to be related to the post-early Miocene progressive emplacement of the LSU over the foredeep units and to the late orogenic extensional and compressional tectonics affecting the Northern Apennines (ARGNANI et alii, 2003, BOCCALETTI et alii, 2011). In particular, the thinning processes have been identified as being of tectonic nature in the innermost portion (mainly low- and high-angle normal faults: ARTONI et alii, 2006, BETTELLI et alii, 2002, VANNUCCHI et alii - 2008) and of sedimentary nature (mass-wasting deposits) (ARTONI et alii, 2010, PAPANI et alii, 1987, REMITTI et alii, 2011) in the outermost portion of the study area. The observed thickening in the central portion of the LSU body has been interpreted as a tectonic doubling caused by the late Miocene-Recent (?) thrusting which affected the whole Apennine orogenic wedge (CAMURRI et alii, 2001; CARLINI et alii, submitted) or, alternatively, as an accumulation zone of large scale and deep-seated gravitational processes (ARTONI et alii, 2006). In order to put new constraints on the timing and modes of thinning and thickening of the LSU we focussed on the western portion of the Northern Apennines, comprised between the main ridge, the Ceno and the Secchia rivers and the Apenninic topographic front. Here we adopted a multidisciplinary approach taking into account field evidences, low temperature thermal and thermochronological data (i.e. vitrinite reflectance, clay mineral analyses, apatite fission-tracks), numerical modelling of the cooling ages through the use of Pecube finite element code (BRAUN, 2003), results from recently published works on the evolution of the external slope of the chain, and a new interpretation of seismic lines and boreholes data. This multidisciplinary approach allowed us to: 1) build a 3D representation of the LSU present-day geometry; 2) constrain to the late Miocene-early Pliocene (in the innermost portion) and to the late Miocene-Pleistocene (?) (at the Apenninic front) the thinning of the LSU; 3) identify the tectonic exhumation and consequent denudation of the foredeep units as one of the main causes which triggered the thinning processes since late Miocene; 4) define first estimates of exhumation rates affecting the LSU and the underlying foredeep units. These results allow us to give new insights, temporal and spatial constraints on the interplay between shallow (< 4 km) and deep (< 20 km), compressional and extensional tectonics which appear to be acting at the same time and at different depths within the study area since late Miocene to Recent.