Detachment Surface Research Articles

Progradational slope architecture and sediment distribution in outcrops of the mixed carbonate-siliciclastic Bone Spring Formation, Permian Basin, west Texas

AbstractSediment transport and distribution are the keys to understanding slope-building processes in mixed carbonate-siliciclastic sediment routing systems. The Permian Bone Spring Formation, Delaware Basin, west Texas, is such a mixed system and has been extensively studied in its distal (basinal) extent but is poorly constrained in its proximal upper-slope segment. Here, we define the stratigraphic architecture of proximal outcrops in Guadalupe Mountains National Park in order to delineate the shelf-slope dynamics of carbonate and siliciclastic sediment distribution and delivery to the basin. Upper-slope deposits are predominantly fine-grained carbonate lithologies, interbedded at various scales with terrigenous (i.e., siliciclastic and clay) hemipelagic and gravity-flow deposits. We identify ten slope-building clinothems varying from terrigenous-rich to carbonate-rich and truncated by slope detachment surfaces that record large-scale mass wasting of the shelf margin. X-ray fluorescence (XRF) data indicate that slope detachment surfaces contain elevated proportions of terrigenous sediment, suggesting that failure is triggered by changes in accommodation or sediment supply at the shelf margin. A well-exposed terrigenous-rich clinothem, identified here as the 1st Bone Spring Sand, provides evidence that carbonate and terrigenous sediments were deposited contemporaneously, suggesting that both autogenic and allogenic processes influenced sediment accumulation. The mixing of lithologies at multiple scales and the prevalence of mass wasting acted as primary controls on the stacking patterns of terrigenous and carbonate lithologies of the Bone Spring Formation, not only on the shelf margin and upper slope, but also in the distal, basinal deposits of the Delaware Basin.

Mesozoic compressional to extensional tectonics in the Central East Iranian Microcontinent: evidence from the Boneh Shurow metamorphic core complex

The Boneh Shurow metamorphic core complex (BSMCC) in the Central East Iranian Microcontinent (CEIM) provides a good example of the Mesozoic succession of nonsynchronous compressional and extensional deformation events attributed to the transitional Cimmerian events. The D1 compression developed subvertical dextral ductile shear zones and corresponds to continental accretion and crustal thickening, producing kyanite- and sillimanite-grade rocks and migmatites in the Early Cimmerian Orogeny in the CEIM. The D2 deformation event is marked by extension during the mid-Cimmerian Orogeny. It is characterized by top-to-the-NE normal sense of shear along a low-angle detachment surface. Field evidence for cross-cutting relationships of D1- by D2-related structures reveals that the occurrence of Barrovian facies metamorphism and associated partial melting in the core of the BSMCC formed during compressional tectonic events. These structures formed before the initiation of extension and the formation of the low-angle detachment shear zone. Finally, during the Late Cimmerian D3 event, the east and west Boneh Shurow reverse faults ruptured on both sides of the MCC. Recognition of the complicated origin and exhumation mechanisms of the BSMCC provides crucial constraints on the prolonged evolution of Palaeo- and Neotethys ocean basins and collisional and post-collisional events in this region.

Pattern of active crustal deformation in a part of a Lesser Himalayan tectonic window, Himachal Pradesh, India

Overriding the underthrusting Indian plate along the low angle detachment surface of the Main Himalayan Thrust (MHT), the entire Himalayan wedge, consisting of several allochthonous and autochthonous sheets is actively deforming due to ongoing India-Asia convergence. However, the information on active tectonics of autochthonous sheets exposed in tectonic windows has often been eluded due to lack of directed investigations. The present study pertains to the active tectonics of the eastern part of an autochthonous sheet exposed in Kullu-Larji-Rampur window of the Lesser Himalaya. Geomorphic features and statistically analyzed geomorphic indices of Drainage Basin Shape (Bs), Hypsometric Integral (HI), Stream Length-Gradient Index (SL), Transverse Topography Symmetry Factor (T), and Basin Asymmetry Factor (AF) reveal active crustal deformation in the window caused by tectonic activities of spatially varying magnitudes. While the spatial distribution of Bs, HI, and SL suggest differential uplift of the anticlinally folded autochthonous sheet along the MHT, the spatial distributions of AF and T suggest an overall southeastward preferential shifting of the trunk streams of 3rd-order drainage basins of the terrane; the hinge zone and northwestern part of the sheet are experiencing relatively faster uplift. The faster uplift in the northwest has caused an overall southeastward down-tiling of the terrane, and thus preferential stream migration in that direction. The faster uplift of the hinge zone is considered to indicate active tightening of the fold.

Tectonostratigraphic evolution of the Orphan Basin and Flemish Pass region – Part 1: Results from coupled kinematic restoration and crustal area balancing

The Orphan Basin and Flemish Pass region on the Newfoundland continental margin is a frontier area in terms of oil and gas exploration and remains poorly understood in terms of structural evolution and rift development. The area has few exploration wells and, until recently, sparse seismic data coverage. Existing gravity inversion and seismic refraction data from the area suggest that stretched continental crust in the Orphan Basin is highly attenuated (locally to < 5 km) but previously published structural restorations have been unable to fully restore the continental crust below the Orphan Basin to pre-deformed thicknesses (30–32 km). Here we perform a structural restoration of a regional, WNW-ESE oriented cross-section to investigate crustal structure, rifting style, and structural evolution of the Orphan Basin and Flemish Pass region. Interpretation is constrained by modern regional depth converted broadband 2D and 3D seismic reflection data, well data, and published gravity inversion results. Present-day crustal thicknesses are used as constraints for crustal area balancing and estimation of crustal thinning across the basin. We find that low-angle extensional detachments are widespread in the area, with offsets on individual structures of up to 42 km. Zones of attenuated continental crust are coincident with the presence of low-angle detachment surfaces displaying isostatic uplift of detachment footwalls. Comparison of total extension derived from crustal area balancing vs. structural restoration suggests approximately 39% of extension across the cross-section is unresolvable in seismic data. We attribute this to (i) ductile deformation prior to brittle faulting, (ii) subseismic-scale faulting, (iii) uncertainties in detachment breakaway positions, and (iv) unresolvable later offset of detachment surfaces. Widespread and relatively evenly distributed extension across the Orphan Basin and Flemish Pass region from Middle-Late Jurassic to Early Cretaceous influences sediment distribution across the region. Calibrated regional seismic mapping of the Orphan Basin suggests that significant thicknesses of Jurassic strata exist throughout the area, including in the northwestern part of the study area, where Kimmeridgian and Tithonian strata reach combined thicknesses of up to 4.9 km.

A Paleoseismic Record Spanning 2‐Myr Reveals Episodic Late Pliocene Deformation in the Western Qaidam Basin, NE Tibet

AbstractThe western Qaidam Basin, NE Tibet contains numerous NW‐SE‐trending thrusts that extend over a distance of ∼300 km along the Altyn Tagh Fault and north of the Kunlun Range. However, little is known about the long‐term seismo‐tectonic evolution of this active thrust zone due to the absence of an extended paleoseismic record. We present a 2‐Myr‐long disturbance record established from a core drilled on the crest of a thrust‐cored anticline. Based on detailed sedimentological analysis, the disturbances (micro‐faults, soft‐sediment deformation, slumps, and detachment surfaces) are interpreted as paleoearthquake/tectonic indicators. The core records five seismite clusters which occurred at 3.6‐3.5, 3.4‐3.2, 3.15‐3.1, 3.0‐2.9, and 2.8‐2.75 Ma. This suggests the rate of tectonic strain accommodated by the folds and thrusts in the region varies and thus reveals episodic local deformation. During the clusters, regional deformation is concentrated more in the fold‐and‐thrust system than along regional major strike‐slip faults.

3D geometry and kinematics of the Niudong Fault, Baxian Sag, Bohai Bay Basin, Eastern China—Insights from high-resolution seismic data

Baxian sag is a typical half-graben-like depression whose controlling boundary is a normal fault, the Niudong Fault. We studied the three-dimensional (3D) geometry and kinematics of this fault based on high-resolution 3D pre-stack depth migration seismic data and drilling data from the Baxian Sag. By analysing the geometries and structural deformation of the hanging wall units of the Niudong Fault, we defined the spatial variation and kinematical characteristics of the fault. The Niudong Fault is marked by a major listric normal fault that soles into a detachment surface at depth. Based on fault strikes and dips, we divided the Niudong Fault into three sections and seventeen zones. Geometrically, the Niudong Fault seems to be a single fault. While kinematically, it is not a single fault in the strict sense as various blocks of the fault have their own growth activities dominated by unidirectional and lateral growth. Map-view restoration of the Niudong Fault shows that the hanging wall was dominated by anticlockwise rotation; while cross-section restoration demonstrates that, the hanging wall was deformed by inclined shear.

The 9 April 2013 Kaki earthquake (Mw 6.3) in SW Iran occurred along a blind backthrust in the Fars geological province of the Zagros Fold and Thrust Belt

Abstract The Zagros Fold and Thrust Belt (ZFTB) in southern Iran is a seismically active tectonic zone, where SW-vergent thrust faults and NW–SE- and NE–SW-oriented strike-slip fault systems accommodate crustal shortening, resulting from the active Arabia–Eurasia collision. The majority of earthquakes in Iran occur within the ZFTB, posing a major hazard for society. The 9 April 2013 Kaki Earthquake (Mw 6.3) in the southern part of the ZFTB took place along a fault that was previously unknown regarding its surface expression, geometry and kinematics. We have used surface–subsurface distributions and focal mechanism solutions of the Kaki Earthquake aftershocks to characterize the fault system responsible for the quake. Our results indicate that it was a NE-vergent thrust fault with a minor dextral component that slipped c. 7–17 km at depth, causing the Kaki Earthquake. There were no surface ruptures, although some surface fissures developed in fluvial terraces during the main shock. We interpret this fault as a blind backthrust, which probably represents a reactivated Mesozoic basement fault emanating from the Zagros detachment surface. An upper shallow décollement zone within the Miocene Gachsaran Salt facilitated its upward propagation on the back-limb of an overturned syncline.

The effect of pre-existing structures on the Moosfluh landslide and its lateral propagation (Great Aletsch Glacier, Switzerland)

The retreat of the Great Aletsch Glacier is accompanied by a series of slope failures in solid bedrock, which are heavily influenced by the presence of pre-existing deformation structures. Since the 1880's, the Great Aletsch Glacier has shortened by more than 3 km and decreased about 400 m in thickness. As a reaction to the loss of the stabilizing effect of the ice, one of the largest active deep-seated landslides in the European Alps with an affected surface area of about 1.5 km2, called Moosfluh landslide, is evolving. In this study, a multimethod approach combining field work, remote sensing techniques and microseismic monitoring is used to assess the effect of pre-existing structures on the landslide deformation processes.The landslide evolution from 2008 to 2018 could be reconstructed with high spatial resolution. Surface deformation analysis reveals the concentration of high deformation in narrow zones, allowing to directly link pre-existing tectonic and exhumation structures with landslide deformation processes. Toppling as the main gravity-driven process is enabled by reactivation of NE-SW striking, steeply SE dipping Alpine Handegg phase shear zones. Differences in the lateral detachment processes are attributed to shear zone bridges in the NE as well as fractures and shear zones similarly oriented to Alpine Oberaarb phase shear zones in the SW. At the landslide toe, a transition from toppling to sliding mechanism due to the formation of a continuous basal detachment surface can be observed, which is favored by the presence of exfoliation joints. The dramatic acceleration of the Moosfluh landslide in autumn 2016 is directly related to an increase in glacier height loss rate, which implies that glacier retreat is the main trigger of the landslide. A temporal stabilization of the landslide is recorded after 2017, most probably caused by the self-stabilizing properties of flexural toppling. However, microseismic data records a lateral propagation of the landslide, following the retreating Great Aletsch Glacier.

Seismicity trends and detachment fault structure at 13°N, Mid-Atlantic Ridge

AbstractAt slow-spreading ridges, plate separation is commonly partly accommodated by slip on long-lived detachment faults, exposing upper mantle and lower crustal rocks on the seafloor. However, the mechanics of this process, the subsurface structure, and the interaction of these faults remain largely unknown. We report the results of a network of 56 ocean-bottom seismographs (OBSs), deployed in 2016 at the Mid-Atlantic Ridge near 13°N, that provided dense spatial coverage of two adjacent detachment faults and the intervening ridge axis. Although both detachments exhibited high levels of seismicity, they are separated by an ∼8-km-wide aseismic zone, indicating that they are mechanically decoupled. A linear band of seismic activity, possibly indicating magmatism, crosscuts the 13°30′N domed detachment surface, confirming previous evidence for fault abandonment. Farther south, where the 2016 OBS network spatially overlapped with a similar survey done in 2014, significant changes in the patterns of seismicity between these surveys are observed. These changes suggest that oceanic detachments undergo previously unobserved cycles of stress accumulation and release as plate spreading is accommodated.

From Caledonian Collapse to North Sea Rift: The Extended History of a Metamorphic Core Complex

AbstractExtensional systems evolve through different stages due to changes in the rheological state of the lithosphere. It is crucial to distinguish ductile structures formed before and during rifting, as both cases have important but contrasting bearings on the structural evolution. To address this issue, we present the illustrative ductile‐to‐brittle structural history of a metamorphic core complex (MCC) onshore and offshore western Norway. Combining geological field mapping with newly acquired 3‐D seismic reflection data, we correlate two distinct onshore basement units (BU1 and BU2) to corresponding offshore basement seismic facies (SF1 and SF2). Our interpretation reveals two 40 km wide domes (one onshore and one offshore), which both show characteristic kilometer‐scale, westward plunging upright folds. The gneiss domes fill antiformal culminations in the footwall of a &gt;100 km long, shallowly west dipping, extensional detachment. Overlying Caledonian nappes and Devonian supradetachment basins occupy saddles of the hyperbolic detachment surface. Devonian collapse of the Caledonian orogen formed dome and detachment geometries. During North Sea rifting, brittle reactivation of the MCC resulted in complex fault patterns deviating from N‐S strike dominant at the eastern margin of the rift. Around 61°N, only minor N‐S faults (&lt;100 m throw) cut through the core of the MCC. Major rift faults (≤5 km throw), on the other hand, reactivated the detachment and follow the steep flanks of the MCC. This highlights that inherited ductile structures can locally alter the orientation of brittle faults formed during rifting.

Geomorphological and geophysical surveys with InSAR analysis applied to the Picerno earth flow (southern Apennines, Italy)

Landslides represent a major geomorphological feature influencing the evolution of the southern Apennine slopes with earth flows being one of the most representative types of landslide. The development of earth flows in the southern Apennines is facilitated by the widespread occurrence of clayey lithologies. Earth flows can either produce marked scars along the slopes, when their activity is high, or give rise to bumpy areas that are frequently covered by vegetation, when their activity is low. In this latter case, although the risk is significantly reduced, the earth flow can still represent a problem for buildings and infrastructure. The Picerno earth flow in the Basilicata region is an example of a low-activity earth flow. This NW-SE–oriented landslide extends for a length of ~ 5.5 km, has an average width of ~ 680 m, and affects the village of Picerno. Our investigations mainly focused on the terminal portion of the landslide and revealed that some significant linear infrastructures (e.g., the Potenza–Naples railway) and important provincial and municipal roads are affected by the earth flow. In order to better define the amount of displacement characterizing the Picerno earth flow and gain insights into the lateral extension and the depth of the main detachment surface, we undertook a detailed geological and geomorphological survey. Interferometry synthetic aperture radar data provided by the processing of SAR images, with the aim of highlighting the regions of the landslide that are currently active, together with electrical resistivity tomography data which have been utilized to define the geometry of the landslide body. Our results provide useful suggestions for planning appropriate actions aimed at stabilizing the landslide body.

Discovery of Mega‐Sheath Folds Flooring the Liwan Subbasin (South China Sea): Implications for the Rheology of Hyperextended Crust

AbstractSheath folds recognized on kilometer scale are rare and only described in salt tectonics in seismic data. Here, we present mega‐sheath folds spectacularly imaged in a 3‐D seismic survey in the seismic basement flooring the hyperextended Liwan subbasin in the northern margin of the South China Sea. The sheath folds show eye structures delimited by anastomosing discontinuities. The axial planes of the sheath folds are subparallel to a rift‐related, extensional detachment surface separating hyperextended seismic basement from the syn‐rift sedimentary sequence. The sheath folds as well as the anastomosing discontinuities formed during hyperextension by ductile shearing of the forearc inherited and prestructured metasediments. The ductile shearing eventually resulted in the metasediments derived from midcrust unroofed at the footwall of an extension detachment fault. The discovery of mega‐sheath folds and ductile deformation within the basement conflicts with the general assumption that hyperextended domains are in the brittle field. The new seismic observations show the importance of forearc inherited, prestructured anisotropic basement controlling the crustal rheology during hyperextension.

The Cerro Guanaco mass movements: A geophysical and morphometric approach on a megalandslide in the Fuegian Andes (Southern Patagonia)

The Fuegian Andes, in the southernmost part of South America, are characterized by a relatively young deglacial landscape, where the glacial erosive relief is strongly influenced by regional-scale geological structures, resulting in deep longitudinal valleys with steeply dipping hillslopes. The rock-slope stability response to receding ice masses and the contribution of the gravitational processes in landscape evolution of the southernmost Andes, however, have not been assessed so far. The remoteness of the region, comprising a mountainous area hardly to access and piedmont and valley floors densely covered by forest, peatbogs or water bodies might partly explain the lack of such studies. Therefore, the objective of this contribution is to document for the first time the occurrence of massive slope failures in the Fuegian Andes using a multidisciplinary approach. This involves the use of indirect methods such as applied geophysics and landscape morphometrics. The southwest-facing slope of Cerro Guanaco, near Ushuaia, hosts several slope failures, but a massive failure stands out. The morphostructural approach allowed the geometrical description of the main dimensional characteristics of the first megalandslide recognized in the Fuegian Andes, which involved almost 1,2 km3 of material. On the other hand, the geophysical data has proved valuable in estimating the depth and characteristic of the buried detachment surface. The slope failure consisted of an initial translational movement followed by secondary slides and was developed shortly after the glacier recession. The gravitational processes represent an important element in the Fuegian deglacial landscape evolution.

Large-scale gravity-driven phenomena in the Crotone Basin, southern Italy

The aim of this study is to better define the scale, timing and driving mechanisms of a seaward-gliding landmass currently affecting the Crotone Basin, a forearc basin on the Ionian side of Calabria (southern Italy). An integrated approach of seismic, well and field data, together with literature information, have revealed that the slide includes an onshore updip extensional domain compatible with the presence of a curved seaward-dipping fault system to the NW, which is connected southward to an offshore downdip compressional domain through a buried detachment surface lying at a Messinian halite layer. The landmass, which involves Messinian-Pleistocene deposits, has started to slide since the Zanclean, and experienced a paroxysmal episode between late Zanclean and early Piacenzian linked to transpressional tectonics. After a phase of inactivity during the Calabrian, the gravitational collapse underwent a second reactivation since middle Pleistocene due to regional uplift, which seems to be restricted to the offshore sector, the Crotone peninsula and probably the eastern coastal area. • A better definition of the scale, timing and driving mechanisms of the Crotone Megalandslide is provided. • A first phase of activity started during the Zanclean and ended during the Gelasian, and involved the whole basin. • A second phase of activity persists since the middle Pleistocene and involved only the Crotone peninsula and the offshore area.

Numerical simulation of underwater air bubble formation under rolling conditions

Underwater gas bubbles are universal phenomenon in various industries. These bubbles appear under ocean conditions especially rolling conditions, that affect the mass transfer and produce different acoustic signals. The bubble formation and oscillation frequency characteristics under rolling conditions were investigated in this study by using a numerical method. Results showed that the bubbles followed two basic steps under rolling conditions, namely, expansion and detachment. The bubble detachment surface area and shape exhibited periodic changes because of the periodical motion of the surrounding water. A correlation was established to predict the bubble detachment surface area under rolling conditions. Similar to the bubble detachment surface area, the bubble oscillation frequency periodically changed. The oscillation mode of the bubbles under rolling conditions was the same as those under motionless condition, that is zeroth mode. A modified Minnaert formula was established to predict the bubble oscillation frequency under rolling conditions.

Two Earthquake Sequences Nearly a Century Apart Reveal a Conjugate Seismogenic System in Central Taiwan

Abstract Seismically active central Taiwan is considered part of an orogenic wedge with low-angle east-dipping active faults above a detachment surface and an active mountain-building process later. In 2013, two moderate reverse-faulting earthquakes of magnitudes ML 6.2 and 6.5 occurred in Nantou. They brought to mind the historically damaging sequence of four earthquakes in the same area that claimed a total of 71 lives in 1916. The 2013 earthquake sequence provides a good opportunity to study the 1916 sequence. We compared the historical Omori record of the main event in the 1916 sequence, discovered in the Seismogram Archives at the Earthquake Research Institute, University of Tokyo, and the corresponding simulated Omori records of the 2013 events. Our comparison shows significant similarity among the earthquakes, although they are separated by nearly 100 yr. To understand the seismogenic structure associated with these earthquake sequences, we further studied the source rupture properties of earthquakes in this region since 1999 using local broadband records to determine the rupture fault planes. Results show that all events have similar focal mechanisms with one low-angle east-dipping and another high-angle west-dipping nodal planes. Rupture plane determination indicates that whereas events at shallow depths (&amp;lt;20 km) ruptured on the low-angle east-dipping plane, events at greater depths (&amp;gt;20 km) slipped on the high-angle west-dipping plane in a conjugate fault system. The comparison also suggests that the 1916 sequence occurred on the low-angle east-dipping plane of this conjugate fault system in the orogenic wedge as part of a mountain-building process. Given the active mountain-building process in central Taiwan, occurrences of this type of earthquake must be addressed in seismic hazard mitigation efforts.

The central Fuegian fold and thrust belt in Tierra del Fuego: Strike‐slip tectonics superimposed onto compressional deformation

The boundary between South America and Scotia plates was established during Neogene times when a wrench tectonic regime was developed along the central area of Tierra del Fuego. This boundary includes the Magallanes–Fagnano fault system: a regional diffuse deformation zone, which involves the development of the satellite Yehuin–Chepelmut fault zone and a series of pull‐apart basins overprinting the Fuegian fold and thrust belt. The analysis of regional seismic lines allowed to reconstruct and recognize the structure of the external fold and thrust belt. The interpreted structures were integrated into a regional transect and a 3D block diagram that display the different structural domains and, mainly, the wrench deformation associated with the Magallanes–Fagnano fault system. Geophysical data from gravity and magnetic anomaly maps provided further support for the location and development of the pull‐apart basins along the plate boundary. The deformation of the external fold and thrust belt is characterized by an imbricated fan system with a Paleogene detachment surface, which was later cut across by the Yehuin–Chepelmut fault zone and the Magallanes–Fagnano fault system.

Reduction of peduncle detachment surface discoloration in Ananas comosus L. By pre-harvest spraying with potassium phosphite

Peduncle detachment surface discoloration (PDSD) in fresh pineapple fruit has rendered serious economic losses in pineapple exports from Taiwan. This experiment was conducted to investigate the post-harvest PDSD process and to evaluate the effect of potassium phosphite (KPhi) solution on pineapple PDSD and fruit quality. PDSD often start to appear in pineapple fruit, especially at room temperature (25 °C), quickly between 3 and 4 days after harvest. To evaluate its efficacy in controlling pineapple PDSD, KPhi was sprayed on pineapple leaves at the pre-harvest stage, at four concentrations: 0 (control), 2 g·L−1, 4 g·L−1, and 10 g·L−1. All treated fruits were stored at 18℃ and 80–90 % relative humidity for 1 week and then shift to room temperature (25℃) for 4 days to simulate the transportation and shelf-life conditions of exported pineapples. The results showed that KPhi treatment significantly reduced the severity of PDSD compared to that of control (water-only spray). All the KPhi treatments did not affect the total soluble solid content significantly. A pre-harvest spray of 10 g·L−1 resulted in better skin color but yielded the worst crown leaf condition and shell appearance of the just-harvested fruit, however, with no significant difference from the control in skin color or shell appearance after the export condition simulation. Most of the fruit characteristics for the other two KPhi treatments were better than those of the control and 10 g·L−1 treatments. The results indicate that pre-harvest KPhi spraying at 4 g·L−1 is well expected and suggested as a good alternative fungicide treatment to maintain and ensure good pineapple fruit quality for export.

Magmatism versus serpentinization—crustal structure along the 13°N segment at the Mid-Atlantic Ridge

SUMMARY A region of oceanic core complexes (OCCs) exists at 13°N on the Mid-Atlantic Ridge that is regarded as a type site. This site includes two OCCs at 13°20′N and 13°30′N, thought to be in the active and dying stages of evolution, and two together called the Ashadze Complex (centred at 13°05′N) that are considered to be relict. Here we describe the results of S-wave seismic modelling along an ∼200-km-long 2-D transect traversing, south-to-north, through both the Mercurius and Marathon fracture zones, the southern outside corner of the 13°N segment, the OCCs, the ridge axis deviation in trend centred at 13°35′N, and the youngest oceanic crust of the eastern ridge flank to the north. Our inversion model, and the corresponding Vp/Vs ratio, show that the majority of the crust beneath the 13°30′N OCC comprises metamorphosed lithologies that have been exhumed to the shallowest subseabed level, while basaltic lithologies underlie the 13°20′N OCC. The transition between these contrasting crustal structures occurs over a distance of &amp;lt;5 km, and extends to at least ∼2 km depth below seafloor. The northern and southern OCCs of the Ashadze Complex have contrasting structures at shallow depth, with the northern OCC having a faster S-wave velocity in the upper crust. A Vp/Vs ratio of &amp;gt;1.9 (and equivalent Poisson's ratio of &amp;gt;0.3) indicates exhumed and/or metamorphosed lithologies beneath the bathymetric depression between them and within the crust beneath the southern OCC. Between the northern and southern flanks of the Marathon fracture zone and northern flank of Mercurius fracture zone, the lower crust has a relatively low Vp/Vs ratio suggesting that the deformation associated with Marathon fracture zone, which facilitates fluid ingress, extends laterally within the lower crust. Marathon fracture zone itself is underlain by a broad zone of low S-wave velocity (∼2.0 km s−1) up to ∼20 km wide from the seabed to at least the mid-crust, that is mirrored in a high Vp/Vs ratio and lower density, particularly deeper than ∼1 km below seabed within its bathymetric footprint. Volcanic domains are highlighted by a low Vp/Vs ratio of &amp;lt;1.6 (and equivalent Poisson's ratio of &amp;lt;0.15). Our combined seismic and density models favour the localized model of OCC evolution. They also show a considerable ridge-parallel variability in the amount and distribution of magmatic versus metamorphosed crust. Our results suggest that the current focus of magmatism lies to the north of the 13°20′N OCC, where the magmatic accretion-type seabed morphology observed is mirrored in the pattern of microseismicity, suggesting that its inward-facing median-valley-wall fault may link to the 13°20′N OCC detachment surface. Magmatism and active faulting behind (to the west) the footwall breakaway of the 13°30′N OCC, and the microseismicity concentrated in a band along its southern flank, suggest a readjustment of ridge geometry along axis is underway. As part of this, a transform offset is forming that will ultimately accommodate the 13°30′N OCC in its inside corner on the eastern flank of the ridge axis to the north.

Structural style and kinematic analysis of folding in the southern Dezful Embayment oilfields, SW Iran

Compression of a region containing multiple detachment layers produces large-scale disharmonic folding, where individual competent units separated by detachment fold independently within the succession. Here, we present a case study of the kinematic evolution of three subsurface structures (the Karanj, Paranj and Parsi oilfields) bounded by multiple detachments, located in the southern Dezful Embayment, a major petroleum province of the Zagros Fold-Thrust Belt in SW Iran. Interpretation of seismic sections and the construction of corresponding balanced cross-sections reveals different geometries along the folds and the complete decoupling of these structures from the surface geology. This is controlled by the reactivation of basement faults, and flow of the Gachsaran incompetent units (Miocene) as the upper detachment, and the Kazhdumi (Upper Cretaceous),Pabdeh-Gurpi and Dashtak Formations as the middle detachments, respectively. Detachment folding with limb rotation started in the Middle Miocene and continued with the migration of the basal detachment horizon (equivalent Hormuz series) into the core of the anticlines. Flow of both the basal and upper detachments continued until no more material is available, whereupon the shortening is accommodated by fault development. The development of a decoupled, disharmonic folding style was controlled by the upper detachment surface through migration from the crests of the anticlines to the synclinal areas and by the reactivation of the basement faults. From NW to SE part of the area The total shortening amounts are changed between 21% and 12%.