Structural setting of the Southern Apennine fold-and-thrust belt (Italy) at hypocentral depth: The Calore Valley case history

The reconstruction of the main structural features of the Southern Apennines (Italy), in correspondence with the focal volume of some strong earthquakes that have affected this chain, can be attempted by analysing reflection seismic lines and deep well logs in comparison with surface geology.For instance, the Calore Valley and its surroundings have been the object of intense hydrocarbon exploration, and a wealth of subsurface data is available. Moreover, this area was affected by the 1688 Sannio earthquake (macroseismic magnitude 7.1), and a new location has recently been proposed for the related causative fault system. The present work defines the structural setting of the Southern Apennine chain in correspondence with this new location, and compares it with similar cases along the Italian peninsula.The analysis was focussed on the reconstruction of deep tectonic units (formed by the buried Apulia carbonate platform succession), which generally correspond to the hypocentral depths of strong earthquakes along the axis of the Southern Apennines. The results show that the Apulia platform succession is affected by three main thrusts, locally accompanied by backthrusts. The top of this succession is relatively shallow: the maximum depth does not exceed 1.8 s TWT (i.e. about 3500 m b.s.l.), while minimum depths occur in correspondence with the ramp anticlines culminations, at 0.5 s TWT (i.e. at about 500 m b.s.l.). Moreover, data suggest that the underlying Paleozoic basement is possibly involved in thrusting.In a regional perspective, extensional seismogenic structures along the axis of the Southern Apennines seem to share some common characteristics. Indeed, they develop (i) in correspondence with an uplifted Paleozoic basement; (ii) at the rear of a set of thrusts that account for the shallow Apulia units; (iii) at the surface, in proximity to the leading edge of a surficial tectonic unit formed by the Apennine carbonate platform succession. The 1688 seismogenic fault system fits in with these common traits. In the light of this, we finally speculate that these common characteristics in the architecture of the chain could provide a key to the location of the major seismicity along the axis of the Southern Apennines and an interpretative model for the identification of possible areas of seismic gap in this part of the Italian peninsula.     

Seismotectonic of Southern Apennines from recent passive seismic experiments

We used data of local earthquakes collected during two recent passive seismic experiments carried out in southern Italy in order to study the seismotectonic setting of the Lucanian Apennine and the surrounding areas. Based on continuous recordings of the temporary stations we extracted over 15,600 waveforms, which were hand-picked along with those recorded by the permanent stations of the Italian national seismic network obtaining a dense, high-quality dataset of P- and S-arrival times. We examined the seismicity occurring in the period 2001–2008 by relocating 566 out of 1047 recorded events with magnitudes M_L ≥ 1.5 and computing 162 fault-plane solutions. Earthquakes were relocated using a minimum one-dimensional velocity model previously obtained for the region and a V_p/V_s ratio of 1.83. Background seismicity is concentrated within the upper crust (between 5 and 20 km of depth) and it is mostly clustered along the Lucanian Apennine chain axis. A significant feature extracted from this study relates to the two E–W trending clusters located in the Potentino and in the Abriola–Pietrapertosa sector (central Lucania region). Hypocentral depths in both clusters are slightly deeper than those observed beneath the Lucanian Apennine. We suggest that these two seismic features are representative of the transition from the inner portion of the chain to the external margin characterized by dextral strike-slip kinematics. In the easternmost part of the study area, below the Bradano foredeep and the Apulia foreland, seismicity is generally deeper and more scattered. The sparse seismicity localized in the Sibari Plain, in the offshore area along the northeastern Calabrian coast and in the Taranto Gulf is also investigated thanks to the new recordings. This seismicity shows hypocenters between 12 and 20 km of depth below the Sibari Plain and is deeper (foci between 10 and 35 km of depth) in the offshore area of the Taranto Gulf. 102 well-constrained fault-plane solutions, showing predominantly normal and strike-slip character with tensional axes (T-axes) generally NE oriented, were selected for the stress tensor analysis. We investigated stress field orientation inverting focal mechanism belonging to the Lucanian Apennine and the Pollino Range, both areas characterized by a more concentrated background seismicity.     

Active tectonics of the outer northern Apennines: Adriatic vs. Po Plain seismicity and stress fields

The recent earthquake sequences of 2012 (northern Italy) and 2013 (Marche offshore) provided new, fundamental constraints to the active tectonic setting of the outer northern Apennines. In contrast to the Po Plain area, where the 2012 northern Italy earthquakes confirmed active frontal thrusting, the new focal mechanisms obtained in this study for the 2013 Marche offshore earthquakes indicate that only minor thrust fault reactivation occurs in the Adriatic domain, even for a theoretically favourably oriented maximum horizontal compression. Recent seismicity in this domain appears to be mainly controlled by transcurrent crustal faults dissecting the Apennine thrust belt. The along-strike stress field variation from the Po Plain to the Adriatic area has been quantitatively investigated by applying the multiple inverse method (MIM) to the analysis of the entire seismicity recorded from January 1976 to August 2014, from the top 12 km of the crust (fault plane solutions from 127 earthquakes with M_W ≥ 4), allowing us to obtain a comprehensive picture of the state of stress over the outer zone of the fold and thrust belt. The present-day stress field has been defined for 39 cells of 1.5° × 1.5° surface area and 12 km depth. The obtained stress field maps point out that, although the entire outer northern Apennines belt is characterized by a sub-horizontal maximum compressive axis (σ₁), the minimum compression (σ₃) is sub-vertical only in the Po Plain area, becoming sub-horizontal in the Adriatic sector, thus confirming that the latter region is dominated by an active tectonic regime of strike-slip type.     

Late Miocene shortening of the Northern Apennines back-arc

The inner Northern Apennines (western Tuscany and Tyrrhenian basin) is characterized by a relatively thin continental crust (∼20–25 km), high heat flow (>100 mW m⁻²), and the presence of relevant tectonic elision of stratigraphic sequences, a setting known as Serie Ridotta. These features are normally ascribed to an extensional deformation that affected the back-arc area above the subducting Adria plate since the Early-Middle Miocene (∼16 Ma). However, various geophysical studies image the continental crust to be currently affected by W-dipping thrust faults (and associated basement uplifts) that have not been obliterated by this claimed long-lasting extensional process. These observations raise the question whether the thrusts are older or younger than the continental extension. To address this question we have reprocessed and interpreted the deep seismic reflection profile CROP03/c that crosses the onshore hinterland sector, and investigated the structural setting of some of the Late Miocene-Pliocene hinterland basins (Cinigiano-Baccinello, Siena-Radicofani, Tafone, Albegna and Radicondoli basins) that are situated at the front or in-between the basement uplifts. The analysis of field structures and commercial seismic profiles has allowed the recognition that both substratum and basins’ infill have been intensely shortened. These findings and the architecture of the basins suggest that the latter developed under a contractional regime, which would have started around 8.5 Ma with the onset of the continental sedimentation. This compressive stress state followed an earlier phase of continental extension that presumably started at ∼16 Ma (with the blocking of the Corsica-Sardinia rotation), and thinned both the continental crust and sedimentary cover producing most of the Serie Ridotta. The main phases of basin shortening are bracketed between 7.5 and 3.5 Ma, and thus overlap with the increase in the exhumation rate of the metamorphic cores at ∼6–4 Ma determined through thermochronological data. We therefore propose a correlation between the basin deformation and the activity of the nearby basement thrusts, which would have thus shortened a previously thinned continental crust. This chronology of deformation may suggest a geodynamic model in which the back-arc and hinterland sector of the Northern Apennines was recompressed during Late Miocene-Early Pliocene times. This evolution may be explained through different speculative scenarios involving a blockage of the subduction process, which may vary between end members of complete slab detachment and stalled subduction.     

On the flux of water and sediment into the Northern Adriatic Sea

This paper focuses on the delivery of water and sediment to the northern Adriatic to better understand the short-term evolution of continental margin sedimentation under natural and human impact. For that reason, the Po and six Apennine rivers (Metauro, Musone, Potenza, Tronto, Chienti and Pescara) are investigated. The climate-driven hydrological model HydroTrend is used to simulate discharge and sediment loads where observational data are limited. The northern Apennine hinterland has a significant impact on the sediment flux leaving the Po River, contributing 56% of the sediment it delivers to the Adriatic Sea. The Po River experienced a strong decrease in its sediment load (17.2–6.4 Mt/yr) across 1933–1987, in contrast to a small increase in its water discharge. The rivers draining the southern Apennine hinterland contribute more than 50% of the sediment load entering the Adriatic Sea, and this is in spite of human modification of their discharge through numerous small reservoirs that invariably reduce a river's sediment load. As a result, hyperpycnal flows, which historically carried 20–40% of the sediment flux from these Apennine rivers, become rare. Sediment load reduction is also reflected by retreat of the Apennine coastline. Based on the ART model (used in HydroTrend), the total sediment load to the northern Adriatic is 43 MT/yr where the northern Alpine rivers contribute 8 MT/yr, the Po River 13 MT/yr and the Apennine rivers contribute 22 MT/yr.     

Post-seismic relaxation: An example of earthquake triggering in the Apennine belt (1915–1920)

The strong earthquake (M = 7) that occurred in the Fucino basin (central Italy) on January 13, 1915 was followed by six earthquakes of M > 5.5 and several other shocks of M > 5 in the major seismic zones of the northern Apennines from 1916 to 1920. This seismicity pattern is consistent with the implications of the present tectonic setting in the study area, which suggests that strong decoupling earthquakes in the central Apennines cause a significant increase of tectonic load, and possibly of seismicity, in the northern Apennines. A numerical simulation, carried out by an elastic-viscous model, of the stress diffusion induced by the Fucino and successive largest earthquakes, shows that each of the above shocks occurred when the respective zone was reached by the highest values of the strain and strain rate perturbation triggered by the previous events. Furthermore, the computed strain regime at each earthquake site is consistent with the known faulting pattern. The results provide important insights into the physical mechanism that controls the interaction of seismic sources in the central and northern Apennines.     

Tectono-metamorphic evolution of the Briançonnais zone (Modane-Aussois and Southern Vanoise units,Lyon Turin transect,Western Alps)

In the central Western Alps, a combined structural, petrological and ⁴⁰Ar–³⁹Ar geochronological study of the Modane-Aussois and Southern Vanoise units yields important constraints on the timing of deformation and exhumation of the Briançonnais zone. These data help to decipher the respective roles of oceanic subduction, continental subduction and collision in the burial and exhumation of the main units through time. In the Modane-Aussois unit top to the NW thrusting (D1) was followed by top to the east shearing (D2) interpreted by some as normal faulting and by others as backthrusting. Pseudosection calculations imply that D1 deformation occurred at 1.0 ± 0.1 GPa and 350 ± 30 °C. Analysis of chlorite–phengite pairs yield P–T estimates between 0.15 and 0.65 GPa and between 220 and 350 °C for the D2 event. Phengites along the D1 schistosity (sample M80) yields an ⁴⁰Ar–³⁹Ar age of 37.12 ± 0.39 Ma, while D2 phengites yield ages of 35.42 ± 0.38 (sample M173) and 31.60 ± 0.33 Ma (sample M196). It was not possible to test whether these ages are altered by excess argon or not. Our interpretation is that the D1/D2 transition occurred at ∼37 Ma at the beginning of decompression, and that D2 lasted until at least ∼32 Ma. Pseudosection calculation suggests that the Southern Vanoise unit was buried at 1.6 ± 0.2 GPa and 500–540 °C. D1 deformation occurred during exhumation until 0.7–10.5 GPa and 370 ± 30 °C. Published ages suggest that D1 deformation possibly started at ∼50 Ma and lasted until ∼37 Ma. D2 deformations started at P–T conditions close to that recorded in Modane-Aussois unit and lasted until 0.2 ± 0.1 GPa and 280 ± 30 °C at ∼28 Ma. The gap of 0.6 ± 0.3 GPa and 150 ± 130 °C between peak metamorphic conditions in the two units was concealed by thrusting of the South Vanoise unit on top of the Modane-Aussois unit during D1 Deformation. Top to the east deformation (D2) affects both units and is interpreted as backthrusting.Based on these data, we propose a geodynamic reconstruction where the oceanic subduction of the Piedmont unit until ∼50 Ma, is followed by its exhumation at the time of continental subduction of the continental Southern Vanoise unit until ∼45 Ma. The Southern Vanoise is in turn underthrusted by the Modane-Aussois unit until ∼37 Ma (D1). Between 37 and 31 Ma the Modane-Aussois and Southern Vanoise units exhume together during backthrusting to the east (D2). This corresponds to the collision stage and to the activation of the Penninic Thrust. In the ∼50 Ma to ∼31 Ma time period the main thrusts propagated westward as the tectonic context switched from oceanic to continental subduction and finally to collision. During each stage, external units are buried while internal ones are exhumed.     

The active fault system of SW Alps

Historical and active seismicity in the south-western Alps (France and Italy) shows the recurrence of relatively high-magnitude earthquakes (M ≥ 5.8), like the one that recently affected the Italian Apennine range (M = 6.3 on the 30th March 2009). However, up-to-date detailed mapping of the active fault network has been poorly established. The evaluation of seismological hazard in particular in the highly populated French and Italian coastal region cannot be done without this. Here, we present a detailed study of the main active fault system, based on geological observations along the south-western flank of the Alpine arc. This N140° right-lateral strike-slip active fault system runs along the edge of the Argentera-Mercantour range and can be followed down to the Mediterranean Sea. It is evidenced by (1) Holocene offsets of glacial geomorphology witnessing ongoing fault activity since 10 ka, (2) widespread recent (10–20 Ma) pseudotachylytes featuring long term activity of the faults, (3) active landslides along the main fault zone, (4) geothermal anomalies (hot springs) emerging in the active faults, (5) ongoing low-magnitude seismic activity and (6) localization of the main historical events. In the light of our investigations, we propose a new tectonic pattern for the active fault system in the south-western Alps.     

Raised Holocene paleo-shorelines along the Capo Vaticano coast (western Calabria,Italy): Evidence of co-seismic and steady-state deformation

Detailed mapping of geomorphological and biological sea-level markers around the Capo Vaticano promontory (western Calabria, Italy), has documented the occurrence of four Holocene paleo-shorelines raised at different altitudes. The uppermost shoreline (PS1) is represented by a deeply eroded fossiliferous beach deposit, reaching an elevation of ∼2.2 m above the present sea-level, and by a notch whose roof is at ∼2.3 m. The subjacent shoreline PS2 is found at an elevation of ∼1.8 m and is represented by a Dendropoma rim, a barnacle band and by a wave-cut platform. Shoreline PS3 includes remnants of vermetid concretions, a barnacle band, a notch and a marine deposit, and reaches an elevation of ∼1.4 m. The lowermost paleo-shoreline (PS4) includes a wave-cut platform and a notch and reaches an elevation of ∼0.8 m. Radiocarbon dating of material from individual paleo-shorelines points to an average uplift rate of 1.2–1.4 mm/yr in the last ∼6 ka at Capo Vaticano. Our data suggest that Holocene uplift was asymmetric, with a greater magnitude in the south-west sector of the promontory, in a manner similar to the long-term deformation attested by Pleistocene terraces. The larger uplift in the south-western sector is possibly related to the additional contribution, onto a large-wavelength regional signal, of co-seismic deformation events, which are not registered to the north-east. We have recognized four co-seismic uplift events at 5.7–5.4 ka, 3.9–3.5 ka, ∼1.9 ka and <1.8 ka ago, superposed on a regional uplift that in the area, is occurring at a rate of ∼1 mm/yr. Our findings places new constrains on the recent activity of border faults south of the peninsula and on the location of the seismogenic source the 1905 destructive earthquake.     

Low-temperature magnetic properties of rhodochrosite (MnCO3)

We have measured magnetic hysteresis loops, zero-field-cooled (ZFC) and field-cooled (FC) remanence, and low-field AC susceptibility as a function of temperature between 2 and 40 K for a single crystal several mm in size and for two powders of manganese carbonate (mineral rhodochrosite, MnCO₃), one ground from a natural precipitate (grainsize ∼100 μm) and another synthesized in the laboratory (grainsize ∼10 μm). For the single crystal, measurements carried out both in the basal (easy magnetization) plane and along the trigonal (hard magnetization) axis yielded, expectedly, grossly different magnetic properties. In the basal plane, hysteresis appears to be mostly controlled by domain wall movement at the two lowest temperatures studied, 5 and 15 K, as indicated by a fairly broad switching field distribution. At 25 K and above, however, magnetization reversal occurs at a single, well defined magnetic field, which we interpret as a characteristic field of the in-plane magnetic anisotropy. Hysteresis in the basal plane is observed up to 36 K which is above the nominal Néel temperature of rhodochrosite (34.3 K). In addition, a sharp coercivity peak occurs at 34.5 K. Rather unexpectedly, hysteresis is also observed for the magnetic field applied along the trigonal axis. It is very small at 5 K but develops gradually with increasing temperature, coercivity reaching maximum of 100 mT at 28 K and remanence peaking at slightly higher temperature (30–31 K). Hysteresis along the trigonal axis is observed up to 37 K. Hysteresis temperature dependence conforms with the AC susceptibility versus temperature curve which shows a maximum at 36.5 K. ZFC/FC remanence curves also closely match the temperature dependence of remanence extracted from hysteresis loops. We suggest that this behavior could be due to the presence of a minor, about 1 at.% amount of Fe²⁺ substituting for Mn in the crystalline lattice of rhodochrosite. Hysteresis measurements on powders have revealed a significant enhance in coercivity, up to 50 mT for the 100-μm powder and up to 150 mT for the 10-μm one. FC/ZFC ratio amounts to about 2 for the natural powder, while for the synthetic one, which is essentially pure material, it barely exceeds unity. FC/ZFC ratio can thus be viewed as a sensitive indicator of iron incorporation into rhodochrosite.     

Quantitative analysis of extensional joints in the southern Adriatic foreland (Italy), and the active tectonics of the Apulia region

The Adriatic foreland of the Apennines comes ashore only in Apulia (easternmost Italy). Its southern part, our study area, lacks any structural analysis devoted to define its recent-to-active tectonics. Throughout the Quaternary, this region was affected by mild brittle deformation with rare faults, characterized by small displacement, and widespread extension joints, frequently organized in sets. Therefore, we conducted a quantitative and systematic analysis of the joint sets affecting Quaternary deposits, by applying an inversion technique ad hoc to infer the orientation and ratio of the principal stress axes, R = (σ₂ ? σ₃)/(σ₁ ? σ₃). Within a general extensional regime, we recognized three deformational events of regional significance. The oldest event, constrained to the early and middle part of the Middle Pleistocene, is characterized by variable direction of extension and R between 0.64 and 0.99. The penultimate event, dated late Middle Pleistocene, is characterized by an almost uniaxial tension, with a horizontal σ₃ striking ～N43°E; R is high, between 0.85 and 0.99. The most recent event is characterized by the lowermost R values, that never exceed 0.47 and are frequently <0.30, indicating a sort of horizontal ‘radial’ extension. This event is not older than the Late Pleistocene and possibly reflects the active stress field still dominating the entire study area.     

Systematic deflection and offset of the Yangtze River drainage system along the strike-slip Ganzi-Yushu-Xianshuihe Fault Zone,Tibetan Plateau

The Ganzi-Yushu-Xianshuihe Fault Zone (GYXFZ) is a typical active strike-slip fault that has triggered many large historic earthquakes, including the 2010 M_w 6.9 Yushu earthquake in the central Tibetan Plateau. This fault zone extends for ca. 800 km from the central Tibetan Plateau to its southeastern margin and varies in trend from WNW-ESE in the northwestern segment of the fault zone to NNW-SSE in the southeastern segment, having the geometry of an arc projecting northeastwards. In this study, we present evidence for the systematical sinistral deflection and/or offset of the Yangtze River and its branch stream channels and valleys along the GYXFZ. Topographic analysis of three-dimensional (3D) perspective images constructed using digital elevation model (DEM) data, 0.5 m-resolution WorldView and GeoEye images, and 15 m-resolution Landsat-Enhanced Thematic Mapper (ETM+) images, together with analysis of geological structures, reveals the following: (i) the main river channels and valleys of the Yangtze River drainage system show systematic sinistral deflections and/or offsets along the GYXFZ; (ii) various amounts of sinistral offset have accumulated on the tributary stream channels, valleys, and gullies of the Yangtze River along the fault, with a linear relation, D = aL, between the upstream length L from the deflected point and the offset amount D with a certain coefficient a; (iii) the maximum amount of sinistral offset is up to ca. 60 km, which was accumulated in the past 13–5 Ma; and (iv) the long-term average strike-slip rate is ca. 4.6–12 mm/year. Geological and geomorphic evidence, combined with geophysical data, demonstrates that the GYXFZ is currently active as one of the major seismogenic faults in the Tibetan Plateau, dominated by left-lateral strike-slip motion. Our findings supply important evidence for the tectonic evolution of strike-slip faults in the Tibetan Plateau since the Eurasia-India continental collision.     

Ca. 13 Ma strike-slip deformation in coastal Sonora from a large-scale,en-echelon,brittle-ductile,dextral shear indicator: implications for the evolution of the California rift

The Rancho Nuevo semi-circular structure is a geomorphological structure defined by drainage patterns in coastal Sonora, about 160 km NW of Hermosillo. The structure is about 15 by 30 km, and it is cored by felsic to intermediate plutons (granodiorite, monzogranite, quartz-porphyry) covered by Miocene volcanic rocks. This work is focused on the deformation of the intrusives which cover most of study area.The plutons are a co-magmatic suite dated between 71 ± 1.1 and 67.9 ± 1.0 Ma (U-Pb zircon, LA-ICPMS). The most voluminous unit is a granodiorite characterized by conspicuous sigmoidal fractures at the scale of high resolution satellite images, along which rhyolite dikes were emplaced about 13.2 Ma. Magnetic fabric (AMS) and paleomagnetic data were collected from 27 sites in the granodiorite. Magnetic fabrics are weak but well developed, and are characterized by steep foliation planes with strikes that follow the sigmoidal fracture pattern and suggest NE-SW to NW-SE flattening after emplacement. The characteristic magnetization is of dual polarity, but it is dominantly reverse consistent with emplacement during chron C31r. The prevalent magnetization is southwest and moderately steep negative (ten sites), a discordant direction rotated clockwise about 41 ° ± 11 with respect to the expected Late Cretaceous reference direction, also indicating gentle southward tilt. There is, however, paleomagnetic evidence suggesting that the structure did not rotate as a rigid body, but it deformed internally instead. These data are interpreted to indicate that the Rancho Nuevo semicircular structure is a large-scale, dextral, brittle-ductile shear indicator. The age of the dikes and the fact that they are covered discordantly by rocks assigned to the tuff of San Felipe indicate that northwest, srike-slip, motion of Baja California peninsula (and thus the Pacific plate relative to North America) was accommodated by faults in coastal Sonora about 13 Ma ago.     

Composite mesoscopic and magnetic fabrics of the Paleo-Proterozoic Wangtu Gneissic Complex,Himachal Himalaya,India: Implications for ductile deformation and superposed folding of the Himalayan basement rocks

The present study demonstrates how the Paleo-Proterozoic Wangtu Gneissic Complex (WGC) of the Lesser Himalayan Crystalline sequence experienced superposed folding and doming prior to its exhumation, with the help of integrated field, microstructural, magnetic fabric anisotropy and geochronological studies. The WGC forms the basement of the Lesser Himalaya and is bounded by Vaikrita Thrust (VT) to the northeast and Munsiari Thrust (MT) to the southwest. The regional structure consists of upright large scale early folds (D1) trending NW–SE. The mesoscopic fabric is related to axial plane foliation of the D1 folds and, to a lesser extent, late D2 folds. The axis of maximum compression for D1 and D2 folds are mutually orthogonal. The D1 folds have formed simultaneously with the major Himalayan thrusts whereas the D2 folds have developed during a later deformation event. The magnetic lineation at the hangingwall of the VT is sub-horizontal indicating stretching along the strike of the thrust. In the interior parts of the WGC, the magnetic fabric is of two types: (i) magnetic lineation demarks the intersection of mesoscopic and magnetic foliation indicating superposed deformation and (ii) scattered distribution of magnetic lineations due to D2 folding on initially curved and non-cylindrical D1 surface. ⁴⁰Ar–³⁹Ar dating of biotite from one site from the core of WGC gives an age of 9.3 ± 0.3 (2σ) Ma. It is inferred that the doming of the WGC took place at ∼9 Ma and, instead of large scale thrusting, it is characterized by superposed folding and strike-parallel stretching along the VT zone. It is suggested that the effect of superposed folding and ductile deformation of the Himalayan basement rocks has to be taken into account before cross-section balancing or any estimation of crustal shortening is attempted.     

Drainage migration and out of sequence thrusting in Bhalukpong,Western Arunachal Himalaya,India

In the foreland regions of the Western Arunachal Himalaya (WAH), geological studies along the Kameng river (between Tipi village and the Himalayan Frontal Thrust (HFT)) reveal four levels of unpaired terraces and a paired terrace. In WAH, wrench deformation of HFT zone resulted in a SE propagation of the Balipara anticline and it is suggested that the Mikir high basement controls its orientation. Ages of terrace surfaces from Siwaliks suggest that since the Late Pleistocene, Kameng River migrated at a rate varying between ∼7.5 cm/yr in upper reaches and ∼13.5 cm/yr towards northeast due to HFT related uplift. In the Brahmaputra plains, luminescence ages of abandoned paleochannel deposits suggest eastward shifting of the Kameng river at an average rate of ∼1 m/yr. Field evidences between Bhalukpong and Tipi villages show Pliocene strath and Quaternary terrace surfaces, displaced by faults that do not correspond to the mapped faults in the foreland region. We interpret them as out-of-sequence thrusts (OOSTs). This is the first such report of OOST in the NE Himalaya. Presence of active OOST is inferred by similar age (∼1 ka) and differing incision rates of the surface of same terrace (T_2b) in adjacent locations. This suggests that OOSTs in the western Arunachal Siwalik are <1 ka. Average slip rate and horizontal shortening rate on OOST during the Holocene, are calculated as ∼12 mm/yr and 7 mm/yr respectively. Thus any estimation of Holocene shortening in the Siwalik therefore, needs to incorporate slip along the OOSTs given that it accommodates a significant amount of N-S compression of the Himalayan fold-and-thrust belt. The reason for OOST in the WAH Siwalik foreland is discussed in terms of the critical wedge dynamics arising from erosion via tectonics-climate interaction. We estimate a minimum slip rate of Siwalik as ∼27 mm/yr during the Holocene and suggest acceleration in shortening rates east of Bhutan.     

Structure and development of the Andean system between 36° and 39°S

One of the main morphological changes along the Southern Central Andes occurs from 36° to 39°S. The northern portion is characterized by prominent basement structures and a thick-skinned orogenic front with relief of over 2000 m with a deep level of exhumation where more than 4 km of section has been eroded. Contrastingly, the southern part is formed by mildly inverted basement structures restricted mainly to the hinterland zone, which reaches only 1500–1700 m relief. We quantify the variable contributions of two main contractional stages through the construction of three regionally balanced sections across the Andes, constrained by field and geophysical data. Extensional re-activation described for this segment in late Oligocene-early Miocene and Pliocene to Quaternary times, after the two main contractional episodes, suggests only 3 km of stretching that represents 30–10% of the original longitude. We, therefore, conclude that while initial Late Cretaceous to Eocene compression was similar along strike (∼10–7 km), it is the contrasting degrees of Neogene shortening (∼16–6 km) that have played the largest role in the along strike differences in structure and morphology along this portion of the southern Andes. Variable Neogene arc expansion could be responsible for the contrasting contractional deformation: In the north, late Miocene arc-related rocks cover most of the retroarc zone (>200 km with respect to the late Miocene arc front in the south), presumably driven by a shallow subduction episode in the area, whereas to the south they remain restricted to the continental drainage divide. Other factors involving architecture of previous rift structures, are proposed as additional mechanisms that accommodated variable shortening magnitudes through inversion.     

Modern sediment accumulation on the Po shelf,Adriatic Sea

Sediment accumulation over the past century on the continental shelf near the Po delta varies with distance from the most active distributary channels. Near the Pila and Goro distributaries, sediment accumulation is rapid (1–4 cm yr⁻¹) and occurs in pulses. In these areas, the seabed is dominated by physical sedimentary structures that can be related to flood sedimentation. Between the two distributaries and in the southern portion of the dispersal system, sediment accumulation is slower (rates reach a minimum of 0.23 cm yr⁻¹ at ∼50 km from the Pila mouth) and steady-state, reflecting more continuous dispersal of sediment during non-flood periods. Sedimentary strata in these locations are composed of finer (clayey silt), mottled sediment. The similarity in the spatial distribution of long-term (100-yr) sediment accumulation to deposition resulting from the 2000 flood event suggests that the Po shelf is flood-dominated.About half of the sediment delivered by the Po River on a 100-yr time scale can be accounted for in the seabed deposit within ∼50 km of the Pila mouth. The remaining sediment is likely transported southward by the prevailing circulation, and this sediment coalesces with inputs from the Apennine Rivers.     

Resistivity imaging of Pleistocene alluvial aquifers in a contractional tectonic setting: A case history from the Po plain (Northern Italy)

In this work we present the hydrogeophysical imaging of a key sector of the Quaternary Po foreland basin (northern Italy), focussing on the reconstruction of clastic aquifers and aquitards in a complex tectono-sedimentary subsurface architecture. The study area includes the relic reliefs of Casalpusterlengo and Zorlesco, two smooth morphological features involving uplifted and gently folded Pleistocene marine to alluvial sediments, plausibly linked to the buried Northern Apennines thrust and fold belt. The geophysical data include 35 Direct Current Vertical Electrical Soundings collected over a 37 km² wide area, acquired with Schlumberger array and maximum half-spacing of 500 m. 1-D resistivity-depth profiles were computed for each VES. An integrated hydrostratigraphic approach was applied, to constrain the interpretation of the geophysical data along several cross-sections, including the comparison of resistivity soundings to stratigraphic logs, borehole electric logs and the pore-water properties.The resistivity interfaces, traceable with the same laterally continuous vertical polarity, were used to develop an electrostratigraphic model in order to portray the stacking of electrostratigraphic units down to 200 m below ground surface. Their vertical associations show a general upward increase of electrical resistivity. This assemblage mimics the regional coarsening upwards depositional trend, from the conductive units of the Plio-Pleistocene marine-to-transitional depositional systems to the resistive units of the Middle–Late Pleistocene fluvial and alluvial plain depositional systems. Middle Pleistocene depositional systems host an alternation of North-dipping, high-to-intermediate permeability aquifer systems (70–180 Ωm, thickness of 5–70 m) separated by low permeability aquitards (20–50 Ωm, thickness up to 40 m). These units pinch out against the Casalpusterlengo and Zorlesco relic reliefs, where they cover the uplifted and folded regional aquitard (20–50 Ωm) formed by Pliocene-Lower Pleistocene clays to sandy silts with gravel lenses in agreement with borehole data. In the deepest part of the local stratigraphy, a broad low-resistivity anomaly (< 10 Ωm) was clearly mapped through the study area. By comparison with electrical borehole logs in deep oil-wells, it could be interpreted as the fresh–saltwater interface due to the presence of connate waters and brines hosted by the marine-to-transitional shales.     

Crustal deformation before the 2008 Wenchuan MS8.0 earthquake studied using GPS data

We used GPS velocities from approximately 700 stations in western China to study the crustal deformation before the Wenchuan M_S8.0 earthquake. The processing methods included analyses of the strain rate field, inversion of fault locking and the GPS velocity profiles. The GPS strain rate in the E-W direction in the Qinghai-Tibet block shows that extensional deformation was dominant in the western region of the block (west of 92.5° E), while compressive deformation predominated in the eastern region of the block (from 92.5° E to 100° E). On a regional scale, the hypocentral region of the Wenchuan earthquake was located at the edge of an intense compression deformation zone of about 1.9 × 10⁻⁸/a in an east-west direction. The characteristic deformation in the seismogenic fault was compressive with a dextral component. The compression deformation rate was greater in the fault's western region than in its eastern region, and the strain accumulation was very slow on the fault scale. The results of a fault locking inversion show that the locking fraction and slip deficit was greater in the middle-northern section of the seismogenic fault than in the southern section. The GPS velocity profile before the Wenchuan earthquake shows that the compression deformation was smaller than the dextral deformation, which is asymmetrical with respect to the distribution of co-seismic displacement. These deformation characteristics should provide some clues to the Wenchuan earthquake which occurred in the later period of the earthquake cycle.     

Lattice-preferred orientation and microstructure of peridotites from ODP Hole 1274A (15°39′N), Mid-Atlantic Ridge: Testing models of mantle upwelling and tectonic exhumation

Eleven harzburgites and one dunite from Ocean Drilling Program Leg 209 Hole 1274A preserve high-temperature mantle textures. Electron backscatter diffraction (EBSD) analysis shows moderately developed crystal lattice preferred orientations (LPOs) in olivine and orthopyroxene (M-indices ≈ 0.1) indicative of crystal-plastic deformation at ~ 1250 °C. These rocks preserve a protogranular texture with a weak olivine foliation, a very weak or absent orthopyroxene foliation that may be decoupled from the orthopyroxene LPO, and minor interstitial clinopyroxene and spinel. Olivine grain size distributions, along with melt-related microstructures in orthopyroxene, clinopyroxene and spinel suggest that high-temperature deformation textures have been overprinted by pervasive post-deformation melt-rock interaction. Paleomagnetic data constrain the olivine [100] axes to be subhorizontal and oriented at low angle (≤ 28.6° ± 10.6°) to the ridge axis at the onset of serpentinization. This orientation is consistent with either complex 3-D mantle upwelling or 2-D mantle upwelling coupled with complex 3-D tectonic emplacement to the seafloor.