Along-strike Variations Research Articles

Along-Strike Rapid Structural and Geomorphic Transition From Transpression to Strike-Slip to Transtension Related to Active Microplate Rotation, Papua New Guinea

The area of southeastern Papua New Guinea includes three active microplates – the Trobriand, Woodlark, and Solomon Sea plates – that are being deformed by regional convergence between the much larger Pacific and Australian Plates. The landward extent of the plate boundary between the Trobriand and Australian Plates corresponds to the Owen-Stanley Fault Zone (OSFZ), an onland and continuous 510 km-long left-lateral strike-slip fault that forms a linear, intermontane valley within the elongate Owen-Stanley Range (OSR) and continues as a 250 km-long low-angle normal fault along the margins of Goodenough and Woodlark basins. GPS geodesy reveals that the Trobriand microplate has undergone rapid counter-clockwise rotation since the Late Miocene (8.4 Ma) and that this rotation about a nearby pole of rotation predicts transpressional deformation along the 250 km-long northwestern segment of the OSFZ, strike-slip motion along a 100 km-long central segment, and transtension along the 270 km-long ESE-trending southeastern segment of OSFZ. In order to illustrate the along-strike variations in neotectonic uplift resulting from the changing structure of the OSFZ, we delineated 3903 river segments in the northeastern side of the OSR drainage divide and derived river longitudinal profiles along each river segment. Normalized steepness indices (ksn) and knickpoint clusters are the highest and most concentrated, respectively, for the northwestern transpressional segment of the OSR, moderately high and concentrated along the southeastern segment of the OSR, and the lowest and least concentrated along the central strike-slip segment. These geomorphological indices indicate that most of the plate boundary uplift occurs along the transpressional and transtensional segments that are connected by the central strike-slip zone. Within this overall pattern of structural variation, abrupt changes in the azimuth of the OSFZ create more localized anomalies in the geomorphological indices.

Differential Erosion and Sedimentation Process at the Longmenshan Foreland Basin, Eastern Margin of the Tibetan Plateau: Evidence From Analog Experiments

We use analog experiments to investigate the influence of rapid filling of a foreland basin system during the development of a fold-and-thrust belt, in particular, the change of erosion–sedimentation along the strike in the Longmenshan foreland basin. A negative relationship between wedge geometries and the magnitude of erosion can be found; increased erosion results in out-of-sequence thrusting and fault reactivation in the wedge hinterland, to limit the forelandward propagation of the wedge. In contrast, increased sedimentation facilitates the forelandward propagation of the wedge. We focus on a natural example of the Longmenshan foreland basin, where a change in erosion–sedimentation along the strike during the Late Cretaceous to Cenozoic is well documented. The comparison between our model and seismic sections indicates that such along-strike variation results in a rejuvenated foreland basin restricted to the southwestern part of the western Sichuan Basin in the Cenozoic.

Evolution of a complex early Permian coarse-grained shoreline along a rift basin margin

ABSTRACT Tectonic activity in extensional basins has a profound control on accommodation and sediment supply through the interplay between footwall uplift and hanging-wall subsidence, and thus largely influences the three-dimensional architecture of syn-rift sequences. This is emphasized in areas close to major rift-border faults, where steep coastal reliefs and fluvial gradients produce compound facies zonation and stratigraphic styles with strong lateral variability. The lower Permian High Cliff Sandstone was deposited in an array of shallow marine environments along the margin of the northern Perth Basin during a protracted late Paleozoic rifting episode in Western Australian basins. The formation is composed of fluvio-deltaic and nearshore strata sharply overlying a thick succession of offshore mudstone that was deposited during a phase of tectonic quiescence. This basal contact likely reflects submarine erosion and is, therefore, interpreted as a regressive surface of marine erosion generated in response to forced regression. The facies arrangement consisting of interbedded sandstone, conglomerate, and heterolithic facies chiefly records the evolution of a low- to high-gradient paleoshoreline punctuated by coastal streams, steep sea cliffs, and back-barrier lagoons. Extraformational outsized clasts were probably emplaced by the erosion of exhumed basement and older sedimentary rocks through fluvial incision, wave sapping, or landsliding. The along-strike variability between low- and high-gradient shoreline deposits indicates a dynamic depositional setting with a complex tectonic influence. The basal regressive surface of marine erosion is attributed to footwall uplift during the early reactivation stage of basin-bounding normal faults and, therefore, records the initiation of a new syn-rift phase in the northern Perth Basin.

Seismicity and velocity structure in the vicinity of repeating slow slip earthquakes, northern Hikurangi subduction zone, New Zealand

Using an 11-month (April 2012 to March 2013) deployment of Ocean Bottom Seismographs along the northern Hikurangi subduction zone, New Zealand, we located 2005 earthquakes (M 0.7-4.5) and obtained detailed 3-D Vp and Vs models for a region where a spectrum of subduction fault slip behavior is observed. Our analysis over a 120 × 110 km2 onshore-offshore area reveals high Vp/Vs (>1.8) in the subducting 10 km thick Hikurangi Plateau oceanic crust below an area of repeating slow slip events (SSEs), that are assumed to occur on the plate megathrust. Low Vp/Vs is mapped in a 15 km wide domain along the coastline, offshore Gisborne, marked by low Qs/Qp. Seismicity is absent from the core of high Vp/Vs anomalies (∼2.0) and within the band with lower Vp/Vs, but earthquakes are distributed vertically and are widespread in adjacent high Vp/Vs (∼1.85-1.95) regions along the trend of the coastline. Down-dip and along-strike variation of the Vp/Vs structure of the subducting Hikurangi Plateau could be the result of dehydration process caused by reactivated bending-related faults. Zones of vertical extensional fault–fracture in the subducting crust may act as conduits for the discharge of overpressured fluid and episodic hydrofracturing. During our deployment a large SSE (Mw6.8) occurred in February 2013. In response, seismicity became active in central Hawke Bay and at the down-dip edge of a previously mapped high-amplitude reflectivity zone. Microseismicity appeared synchronous with the slip propagation of the SSEs and may have been initiated by the slip. Our results hold implications for understanding of the physical environments conducive to slow slip generation and represent an important baseline for future seismicity work in the region.

Structural Significance of the Mid-level Décollement Within the Western Sichuan Fold-And-Thrust Belt (WSFTB), Insights From Sandbox Modeling

The WSFTB is located outboard of the eastern Tibetan Plateau, western China. It has received great attention due to high earthquake risks and rich resources of oil and gas. For both issues, the detailed structural configuration and deformation mechanism behind it are of great importance, but remain unclear due to the complexity created by the presence of multiple décollements. The effect of regionally distributed shallow Triassic salt décollement (SD) and the basal one (BD) has been well understood. In this paper, we focus on the third décollement situated between them. We conducted three sandbox experiments by varying this mid-level décollement (MD) from absence to presence, and from frictional to viscous, to test the effect on diversity of regional structural configuration. Our experimental results illustrated that 1) Absence of MD facilitated decoupling on SD, forming the greatest contrast between subsurface deformation front and the blind one beneath SD; 2) Frictional MD itself showed little decoupling, while its weakness reduced the bulk strength of deep structural level, lowering decoupling effect on SD and leading to approximating deformation fronts in the shallow and deep; 3) The viscous MD, along with SD relieved the resistance on their interbed layer. Consequently, the fastest deformation propagation rate and farthest deformation front (in all the experiments) occurred in the middle structural level. The modeled fold and thrust structures are comparable with the southern, central and northern WSFTB respectively, suggesting that varied MD may control the along-strike structural variations presented. The results also indicate that MD can alter the deformation partition in depth of any other multiple décollement system.

Along-strike variations in intermediate-depth seismicity and arc magmatism along the Alaska Peninsula

The Alaska Peninsula section of the Aleutian-Alaska subduction zone exhibits significant along-strike variations in plate coupling, earthquakes, and arc magmatism. Here we use regional and teleseismic data from 1990 to 2018 to investigate intermediate-depth intra-plate seismicity beneath the Alaska Peninsula. The distribution of these events shows five distinct segments from southwest to northeast: Unimak, Pavlof, Shumagin, Semidi, and Kodiak. We identify two double seismic zones (DSZs) in the Pavlof and Semidi segments but not in others. The magnitude-frequency relationship of these earthquakes (b-value) also varies along strike, suggesting more fluids in the southwestern segments and closer to the slab top interface. We also evaluate trace elements of arc lavas in the Alaska Peninsula. Th/La ratios indicate a significant amount of sediments subducted in the Semidi segment that includes Aniakchak and Veniaminof, and the sediment signature peaks beneath the Black Peak volcano. In contrast, high Pb/Th but low Th/La ratios from southwestern volcanoes suggest high fluids released from the subducted slab. In sum, our observations suggest that the slab is more hydrated and thus undergoing more intensive dehydration in the southwest than in the northeast, whereas more sediments are subducted in the northeast than in the southwest. These along-arc changes are generally in agreement with previous studies of plate coupling and the crustal structure of the incoming plate. Moreover, the complexities beyond the monotonic changes imply that the intermediate-depth intra-plate seismicity and sub-arc melting are influenced by not only the pre-existing fabrics in the incoming plate, but also the subducted sediments and plate stress state.

Joint inversion of ground gravity data and satellite gravity gradients between Nepal and Bhutan: New insights on structural and seismic segmentation of the Himalayan arc

Along-strike variation in the geometry of lithospheric structures is a key control parameter for the occurrence and propagation of major interplate earthquakes in subduction and collision zones. The lateral segmentation of the Himalayan arc is now well-established from various observations, including topography, gravity anomalies, exhumation rates, and present-day seismic activity. Good knowledge of the main geometric features of these segments and their boundaries is thus the next step to improve seismic hazard assessment in this area. Following recent studies, we focus our approach on the transition zone between Nepal and Bhutan where both M > 8 earthquakes and changes in the geometry of the Indian plate have been documented. Ground gravity data sets are combined with satellite gravity gradients provided by the GOCE mission (Gravity Field and Steady-State Ocean Circulation Explorer) in a joint inversion to assess the location and the geometry of this transition. We obtain a ca. 10 km wide transition zone located at the western border of Bhutan that is aligned with the Madhupur fault in the foreland and coincides with the Dhubri–Chungthang fault zone and the Yadong-Gulu rift in Himalaya and southern Tibet, respectively. This sharp segment boundary at depth can act as a barrier to earthquake rupture propagation. It can possibly restrict the size of large earthquakes and thus reduce the occurrence probability of M > 9 earthquakes along the Main Himalayan Thrust.

Along-strike variations in protothrust zone characteristics at the Nankai Trough subduction margin

AbstractSignificant along-strike changes in the protothrust zone at the toe of the Nankai Trough accretionary prism were imaged in new high-resolution seismic reflection data. The width of the protothrust zone varies greatly along strike; two spatially discrete segments have a wide protothrust zone (∼3.3–7.8 km, ∼50–110 protothrusts), and two segments have almost no protothrust zone (∼0.5–2.8 km, <20 protothrusts). The widest protothrust zone occurs in the region with the widest and thickest sediment wedge and subducting turbidite package, both of which are influenced by basement topography. The trench wedge size and lithology, the lithology of the subducting section, and the basement topography all influence the rate of consolidation in the trench wedge, which we hypothesize is an important control over the presence and width of the protothrust zone. We conclude that protothrusts are fractures that form from shear surfaces in deformation band clusters as the trench fill sediment is consolidated. Strain localization occurs at sites with a high density of protothrusts, which become the probable locations of future frontal thrust propagation. The frontal thrust may propagate forward with a lower buildup of strain where it is adjacent to a wide protothrust zone than at areas with a narrow or no protothrust zone. This is reflected in the accretionary prism geometry, where wide protothrust zones occur adjacent to fault-propagation folds with shallow prism toe surface slopes.

Pleistocene palaeoenvironments in the Lower Volga region (Russia): Insights from a comprehensive biostratigraphical study of the Seroglazovka locality

There are many Middle-Upper Pleistocene outcrop sections in the Lower Volga region (Caspian Lowland, Russia) in between cities of Volgograd and Astrakhan. The present study focuses on the Seroglazovka locality, which is among the most famous key sites within the Lower Volga region. The Seroglazovka locality is of great regional interest, because of its clear exposure of genetically variable Middle and Upper Pleistocene deposits containing diverse complexes of fauna and flora. Even though the Seroglazovka locality was studied by many researchers, the diversity and along-strike variability of the depositional facies often makes the stratigraphic interpretation challenging. In this study, we performed an extensive and integrated study of the Seroglazovka locality including detailed outcrop mapping with lithological description, as well as diverse biostratigraphy analysis and OSL-dating. In addition to outcrop data, we used recently obtained borehole data near the Seroglazovka locality, which cover almost the entire Quaternary interval. This multidisciplinary approach permitted us to confidently stratify the Quaternary interval of the Seroglazovka locality into a series of regional horizons and subhorizons including the Akchagyl, Apsheron, Tyurkyan, Baku, Khazar and Khvalyn. Furthermore, we correlated the interpreted section to the adjacent parts of the Caspian Lowland and the Regional Stratigraphic Scale. Our results allowed us to determine the main Pleistocene palaeoenvironments and their evolution within the study area. Our findings are important to understand the processes, which led to the modern faunistic composition of the Lower Volga region, and to clarify the role of glacial refugia in the settlement of certain species and further colonization of water basins and land areas with habitable environments. Finally, we suggest the Seroglazovka locality as the key reference Pleistocene section for the Lower Volga region.

Asymmetrical wave-dominated siliciclastic shorelines with evidence of along-strike variability of sedimentary processes: A revised interpretation for the Toarcian Marrat red beds, central Arabia

Marine siliciclastic red beds are common in the geological record; however, distinguishing them from continental red beds can be challenging without detailed sedimentological and stratigraphic analyses. Owing to the common tendency in the geological literature to relate the reddish color to the oxidization processes within the vadose zone, siliciclastic red beds of marine genesis can be misinterpreted as continental deposits. Unlike the other Toarcian successions in the Tethyan basins surrounding Arabia, the sedimentological and stratigraphic characteristics of the Marrat Formation of Arabia in general and its middle unit's red siliciclastic beds, in particular, have not been well constrained. For more than 50 years, the red siliciclastic beds of the Toarcian Marrat Formation have been interpreted as continental to marginal marine deposits based on their reddish color and the absence of fossils. The lack of supporting sedimentological and stratigraphic evidence to this interpretation raises critical concerns regarding its validity.Along the Jurassic outcrop belt in central Arabia, three outcrop localities of the middle Marrat reddish siliciclastic deposits (with a total thickness of more than 120 m) demonstrate sedimentological and ichnological features that are contrary to the non-marine to marginal marine interpretation. The integration between sedimentology and ichnology reveals the existence of asymmetrical wave-dominated shorelines with evidence of river hyperpycnal flows and tidal processes during the middle Marrat time. A total of seventeen facies are identified and grouped into four facies associations: mixed wave and river-dominated delta front to prodelta mudstone (FA1), shoreface (strandplain) and underlying shelf mudstone (FA2), tidally-influenced distributary channel (FA3), and offshore carbonates (FA4). Stratigraphically, six major parasequences (each with a thickness of ~ 3–13 m) were mapped and correlated between the three measured sections with clear overall progradational to retrogradational stacking patterns.To the best of our knowledge, this study is the first to document the presence of Toarcian shallow-marine siliciclastic red beds on the Arabian Plate. It displays an example of a case study where ichnology and process-based sedimentology were integrated to constrain the depositional environment of these red beds. This study provides new insights into the depositional settings of the Toarcian red siliciclastic beds of the Marrat Formation, with implications to the stratigraphy of Arabia and the Early Jurassic paleogeography of the western Tethys and, in particular, the Arabian Plate.

Subcretionary tectonics: Linking variability in the expression of subduction along the Cascadia forearc

A number of seismic and other geophysical phenomena exhibit significant lateral heterogeneity along the strike of the Cascadia forearc. Both the overriding and downgoing plate have been invoked to play the dominant role in controlling along-strike correlations between seismogenic behavior, potential field measurements, morphological/tectonic characteristics, and seismic structure; however, significant feedbacks likely exist between the two. In this study, we apply a 3D velocity correction to receiver function data and interpret the resulting discontinuity model alongside a recently published shear-wave velocity model to understand the possible causative relationships between correlative along-strike variations. Our discontinuity model indicates that the forearc crust thickens as it approaches the mantle wedge corner before progressively thinning toward the magmatic arc, likely due to the basal accretion of material from the downgoing plate to the overriding plate. In the northern and southern portions of the forearc, this “subcreted” material is characterized by thick (∼10 km) anomalously low shear-wave velocity zones. The thickness, high internal reflectivity, and low Bouguer gravity signatures associated with the low velocity zones likely indicate that this subcreted material is composed of dominantly (meta)sedimentary material that has been emplaced through successive subcretion events over geologic timescales. Furthermore, the anomalously low velocities and spatial correlation with high non-volcanic tremor (NVT) density and short slow slip recurrence intervals indicate that these regions are fluid-rich. While first-order variations in the fluids that control NVT and slow slip likely result from differences in the permeability of the downgoing slab as inferred from its stress state and the distribution of intraslab seismicity, these subcreted packages likely represent thick, vertically-impermeable regions in the lower crust that further accentuate this correlation. Variability in the amount of subcretion explains patterns of exhumation and uplift along the Cascadia margin and the resulting forearc topography over geologic timescales, and is likely controlled by some combination plate interface geometry/rheology and overriding plate architecture.

Illuminating high-resolution crustal fault zones using multi-scale dense arrays and airgun source

High-resolution imaging of fault zone structure and its temporal changes can not only advance our understanding of earthquake physics, but is also critical for better seismic hazard preparation and mitigation. In the past a few years, we deployed multi-scale dense arrays across the Chenghai fault system in Binchuan, Yunnan, China. The first array consisted of 381 intermediate-period three-component seismometers with an average station spacing of ∼2 ​km. The array has been deployed in the field for ∼3 months in 2017 and recorded numerous local and teleseismic earthquakes. Travel time analyses based on teleseismic earthquakes and an airgun source in the region indicated clear signature of low-velocity fault zones in the southern branch of the Chenghai fault system. In 2018 we deployed two other linear arrays using the same instruments with much smaller inter-station spacing, e.g. 30–50 ​m, across the southern branch the Chenghai fault. The profile lengths were 8 and 5 ​km, respectively. Record sections of the airgun source on the two linear arrays clearly marked a low-velocity zone (LVZ) within the southern array but no such signature in the northern array, suggesting along-strike variation of the LVZ. Although the instruments within our dense arrays had an intermediate frequency band, we demonstrated that they were capable of characterizing crustal structure with techniques commonly applied to broadband signals such as receiver functions. To our best knowledge, this was the first time to have multi-scale across-fault dense arrays with three-component seismometers in such apertures. These results lay out the pavement to comprehensively investigate fault zone structures as well as to derive subsurface structural changes using dense arrays and the airgun source.

Anatomy and evolution of the Astoin diapiric complex, sub-Alpine fold-and-thrust belt (France)

The structure of the southwestern branch of the Alpine orogen is affected by the extensive Late Triassic evaporites. These evaporites have been involved in polyphased salt tectonics since the early Liassic, coeval with the Tethyan rifting, and are the décollement level for thrusts in the external parts during Alpine orogeny. The role of salt tectonics in this branch of the Alpine arc is re-evaluated in order to determine the relative importance of early deformation related to salt motion with respect to deformation related to main Alpine compressional events. This paper focuses on one structure identified as diapiric since the 1930’s: the Astoin diapir (Goguel, 1939). Analysis of geological maps together with new field work have allowed to better define diapirism in the Upper Triassic evaporites outcrops around Astoin. Study of the diapir and the surrounding depocenters reveals a major involvement of salt in the structuration of the area, since the Liassic. Several salt ridges are linked to a main diapiric structure, explaining why we call it the “diapiric complex” of Astoin. Salt tectonics was initiated during the Liassic rifting, and a few locations show evidence of reactive diapirism whereas in others evidence of passive diapirism as early as the Liassic is seen. Passive diapirism continued during the post-rift stage of Alpine margin history in the Late Jurassic and Cretaceous when an allochthonous salt sheet was emplaced. Diapirism also occurred during the Oligocene while the Alpine foreland basin was developing in this part of the European margin of the Alps. Serial interpretative cross-sections have been drawn in order to illustrate the lateral variations of diapirism and structural style. Sequential evolutions for each cross-section are proposed to reconstruct the diapiric complex evolution through time. The Astoin diapir shows a complex structural framework with an important along-strike variation of diapiric activity. Most of the geometries are inherited from salt tectonics that occurred during extension, and in some places these early structures are overprinted by Alpine compressional structures.

Three-dimensional shear velocity structure of the Mauléon and Arzacq Basins (Western Pyrenees)

We present a 3-D shear wave velocity model of the Mauléon and Arzacq Basins from the surface down to 10 km depth, inverted from phase velocity maps at periods between 2 and 9 s. These phase velocity maps were obtained by analyzing coherent surface wave fronts extracted from ambient seismic noise recorded by the large-N Maupasacq seismic array with a matched filtering approach. This new model is in good agreement with a local earthquake tomography study performed on the same acquisition dataset. Our passive imaging models reveal the upper crustal architecture of the Mauléon and Arzacq Basins, with new details on the basement and its relationship with the overlying sedimentary cover. Combining these new tomographic images with surface and subsurface geological information allows us to trace major orogenic structures from the surface down to the basement. In the basin, the models image the first-order basin architecture with a kilometric resolution. At depth, high velocity anomalies suggest the presence of dense deep crustal and mantle rocks in the hanging wall of north-vergent Pyrenean Thrusts. These high velocity anomalies spatially coincide with a positive gravity anomaly in the western Mauléon Basin. In addition, our models reveal major changes from the Chaînons Béarnais to the western Mauléon Basin across a set of orogen-perpendicular structures, the Saison and the Barlanès transfer zones. These changes reflect the along-strike variation of the orogenic evolution that led to the preservation of the former rifted domain and its underlying mantle in the orogenic wedge of the Western Pyrenees. We discuss the implications of these results for the 3-D architecture of the Mauléon Basin and its underlying basement.

Seismic facies, stratigraphy, geomorphology, and seismic modelling of a Lower Cretaceous carbonate platform

Abstract Globally, Lower Cretaceous carbonate platforms form important hydrocarbon reservoirs. An exceptional seismic dataset allowed us to examine controls on the stratigraphy, seismic geomorphology, facies architecture and along-strike variability of a Lower Cretaceous platform. Within the platform succession, a partitioning of facies is developed between Transgressive, Highstand and Forced Regressive seismic sequences. Spectacular seismic geomorphological images of the platform using spectral decomposition techniques reveal spatial organization within both depositional and diagenetic (i.e. palaeokarst) patterns. The study was undertaken in order to optimize a well location. Predrill seismic facies interpretations were interpreted with the aid of offset well data and drill cores. Core data encompass a range of slope, platform margin and platform interior facies with inferred palaeoenvironments that are matched to the seismic geomorphology of the platform. 2D seismic forward modelling was integrated in our workflow to investigate the extent of potential sealing lithologies and the reservoir potential of the individual seismic sequences. Nearby wells were used to calibrate a modified rock physics model for the different lithofacies. Multiple seismic model realizations were generated to assess reservoir quality in alternative well locations and the continuity and quality of sealing strata. Results of the predrill forward-modelled poro-perm prediction are presented, showing a good fit with the final well observations after drilling.

A Downgoing Indian Lithosphere Control on Along-Strike Variability of Porphyry Mineralization in the Gangdese Belt of Southern Tibet

Abstract The E-trending Gangdese porphyry copper belt in southern Tibet is a classic example of porphyry mineralization in a continental collision zone. New zircon U-Pb geochronological, zircon Hf-O, and bulk-rock Sr-Nd isotope data for the Miocene mineralizing intrusions from the Qulong, Zhunuo, Jiru, Chongjiang, and Lakange porphyry copper deposits and Eocene igneous rocks from the western Gangdese belt, together with literature data, show that both Paleocene-Eocene igneous rocks and Miocene granitoids exhibit coupled along-arc isotopic variations, characterized by bulk-rock ɛNd(t) and zircon ɛHf(t) values increasing from ~84° to ~92°E and then decreasing toward ~95°E. These are interpreted to reflect increasing contributions of subducted Indian continental materials from ~92° to ~84°E and from ~92° to ~95°E, respectively. The Miocene mineralizing intrusions were derived from subduction-modified Tibetan lower crust represented isotopically by the Paleocene-Eocene intrusions, with contributions from Indian plate-released fluids and mafic melts derived from mantle metasomatized by subducted Indian continental materials. Involvement of isotopically ancient Indian continental materials increased from east (Qulong) to west (Zhunuo), which is interpreted to reflect an increasingly shallower angle of the downgoing Indian slab from east to west, consistent with geophysical imaging. Exploration of Gangdese Miocene porphyry copper deposits should focus on the Paleocene-Eocene arc where the subarc mantle was mainly enriched by fluids from the subducted Neo-Tethyan oceanic slab. Neodymium-Hf isotope data for mineralizing igneous rocks from porphyry copper deposits globally show no obvious correlations with Cu endowment. Although Nd-Hf isotopes are useful for imaging lithospheric architecture through time, caution must be taken when using Nd-Hf isotopes to evaluate the potential endowment of porphyry copper deposits, because other factors such as tectonic setting, crustal thickening, magma differentiation, fluid exsolution, and ore-forming processes all play roles in determining Cu endowments and grades.

Systematic characterization of morphotectonic variability along the Cascadia convergent margin: Implications for shallow megathrust behavior and tsunami hazards

AbstractStudies of recent destructive megathrust earthquakes and tsunamis along subduction margins in Japan, Sumatra, and Chile have linked forearc morphology and structure to megathrust behavior. This connection is based on the idea that spatial variations in the frictional behavior of the megathrust influence the tectono-morphological evolution of the upper plate. Here we present a comprehensive examination of the tectonic geomorphology, outer wedge taper, and structural vergence along the marine forearc of the Cascadia subduction zone (offshore northwestern North America). The goal is to better understand geologic controls on outer wedge strength and segmentation at spatial scales equivalent to rupture lengths of large earthquakes (≥M 6.7), and to examine potential linkages with shallow megathrust behavior.We use cross-margin profiles, spaced 25 km apart, to characterize along-strike variation in outer wedge width, steepness, and structural vergence (measured between the toe and the outer arc high). The width of the outer wedge varies between 17 and 93 km, and the steepness ranges from 0.9° to 6.5°. Hierarchical cluster analysis of outer wedge width and steepness reveals four distinct regions that also display unique patterns of structural vergence and shape of the wedge: Vancouver Island, British Columbia, Canada (average width, linear wedge, seaward and mixed vergence); Washington, USA (higher width, concave wedge, landward and mixed vergence); northern and central Oregon, USA (average width, linear and convex wedge, mixed and seaward vergence); and southern Oregon and northern California, USA (lower width, convex wedge, seaward and mixed vergence). Variability in outer wedge morphology and structure is broadly associated with along-strike megathrust segmentation inferred from differences in oceanic asthenospheric velocities, patterns of episodic tremor and slow slip, GPS models of plate locking, and the distribution of seismicity near the plate interface. In more detail, our results appear to delineate the extent, geometry, and lithology of dynamic and static backstops along the margin. Varying backstop configurations along the Cascadia margin are interpreted to represent material-strength contrasts within the wedge that appear to regulate the along- and across-strike taper and structural vergence in the outer wedge. We argue that the morphotectonic variability in the outer wedge may reflect spatial variations in shallow megathrust behavior occurring over roughly the last few million years. Comparing outer wedge taper along the Cascadia margin to a global compilation suggests that observations in the global catalog are not accurately representing the range of heterogeneity within individual margins and highlights the need for detailed margin-wide morphotectonic analyses of subduction zones worldwide.

Along-strike permeability variation in carbonate-hosted fault zones

Variation in permeability along-strike of 10 faults, from differing lithofacies, host porosities and juxtaposition types, has been examined to improve our ability to understand controls on fault core development in carbonates, which in turn will help to predict the likely impact of faults on fluid flow. A large range in the permeability values has been recorded, up to 7 orders of magnitude variation in one example, created by significant fault core microstructural heterogeneity. A number of factors influence fault core development, and hence also control along-strike variation in microstructures and permeability. Juxtaposition of lithofacies is one key factor that appears to dictate the degree of heterogeneity along the fault core, where an increased microstructural heterogeneity is observed with juxtaposition of differing lithofacies, due to a variety of mechanisms active. This leads to a fault core with highly variable permeability values. Conversely, an increased microstructural homogeneity is observed in fault cores where the same or similar lithofacies are juxtaposed, with limited mechanisms active. However, a wide range in permeability has also been documented in these scenarios, but often far lower than scenarios of juxtaposition of different lithofacies: up to a maximum of 4 orders of magnitude has been recorded. However, the permeability range is usually much narrower, at around 2 orders of magnitude, with fractured outliers causing an increase to the spread of permeability values. Although the range of mechanisms active are not varied, the intensity of deformation and diagenetic processes does vary. Further, any later-stage processes such as fracturing and/or dissolution overprints the original textures. Both factors act to increase the permeability heterogeneity of the fault core.

Factors controlling margin instability during the Plio-Quaternary in the Gela Basin (Strait of Sicily, Mediterranean Sea)

This study presents novel findings on the Pliocene and Quaternary evolution of the Gela Basin (Strait of Sicily, Mediterranean Sea), an area recording the interaction between tectonics, climate change at Milankovitch and sub-Milankovitch timescales, and dynamic water masses exchange between the eastern and western Mediterranean Sea. The calibration of seismic profiles with exploration boreholes allowed for the refining of the chronostratigraphic framework of the Gela Basin and highlighted the main phases of margin growth. Since the Pliocene, the margin has recorded the deposition of 100 m high shelf-edge clinothems, accompanied by sediment drifts on the slope and mass-transport deposits (MTDs), possibly triggered by seismic activity. Through the Plio-Quaternary the locus of deposition of sediment drifts migrated upslope due to a progressive shift of bottom currents. After the Middle Pleistocene Transition (MPT) the margin experienced an accelerated outbuilding with the deposition of a 700 m thick succession in only 0.8 Myr. At this time, a marked change in sedimentary architecture reflects the growth of shelf-edge clinothems and associated MTDs on the slope, and the spreading of contourite deposits over a broader and generally shallower area. Sediment flux to the basin and the intensity of bottom currents appear both paced at 100 kyr eccentricity orbital cycles. The growth of bottom current deposits on a large portion of the upper slope and outer shelf likely reflects constraints to the bottom current flow by the margin morphology, inherited from Miocene and Pliocene tectonics. Overall, a combination of long-term tectonic activity, climate change and shifts in oceanographic regime resulted in a complex along-strike variability of the margin morphology and stratigraphic architecture and affected where and when MTDs were emplaced. These conclusions show how climate cyclicity influence sediment supply which combined with margin morphology can promote slope instability in continental margins.

Sequence of deformation at the front of an orogen: Lublin basin case study (Poland)

We provide a detailed description of a late Carboniferous fold-and-thrust belt in the Lublin Basin based on seismic lines, including new, high-resolution acquisitions, and legacy borehole and gravimetric data. A series of regional cross-sections integrate results of the joint seismic-borehole-gravimetric interpretation. Cross-section restoration provides estimates of the minimum incremental and total shortening and reveals its along-strike variations. The Lublin Basin straddles a leading edge of the Variscan orogen and its undeformed foreland; therefore, it experienced only a minor deformation. It preserves a fossilized record of early phases of shortening: seismic scale, divergent intra-formational thrusts and folds in horizontal strata that were later passively rotated by subsequent folds developed above a regional decollement. Lateral variations of deformation styles – from displacement of the entire basin with little internal deformation to dispersion of shortening by numerous contractional structures – is attributed to along-strike changes in lithology and thickness of the Silurian detachment horizon. Lateral change of detachment strength is quantified using the critical wedge theory. Our observations deliver a new perspective on factors controlling the localization of incipient structures, their differentiation as a function of detachment strength and interplay between folding and thrusting along a leading edge of an organic system.