Extensional Fault Systems Research Articles

Fault Systems and their Control of Deep Gas Accumulations in Xujiaweizi Area

Abstract:A study of faults and their control of deep gas accumulations has been made on the basis of dividing fault systems in the Xujiaweizi area. The study indicates two sets of fault systems are developed vertically in the Xujiaweizi area, including a lower fault system and an upper fault system. Formed in the period of the Huoshiling Formation to Yingcheng Formation, the lower fault system consists of five fault systems including Xuxi strike‐slip extensional fault system, NE‐trending extensional fault system, near‐EW‐trending regulating fault system, Xuzhong strike‐slip fault system and Xudong strike‐slip fault system. Formed in the period of Qingshankou Formation to Yaojia Formation, the upper fault system was affected mainly by the boundary conditions of the lower fault system, and thus plenty of multi‐directionally distributed dense fault zones were formed in the T2 reflection horizon. The Xuxi fault controlled the formation and distribution of Shahezi coal‐measure source rocks, and Xuzhong and Xudong faults controlled the formation and distribution of volcanic reservoirs of Y1 Member and Y3 Member, respectively. In the forming period of the upper fault system, the Xuzhong fault was of successive strong activities and directly connected gas source rock reservoirs and volcanic reservoirs, so it is a strongly‐charged direct gas source fault. The volcanic reservoir development zones of good physical properties that may be found near the Xuzhong fault are the favorable target zones for the next exploration of deep gas accumulations in Xujiaweizi area.

Deposition and deformation in the deepwater sediment of the offshore Barreirinhas Basin, Brazil

The Barreirinhas Basin is an ideal location to study shale-dominated gravity-driven thrusting systems because of the limited areal extent of the deformed areas compared to other areas in the world. Regional seismic reflection profiles across the Barreirinhas Basin on the Brazilian Equatorial margin show two major deepwater fold and thrust belts linked landward to extensional fault systems. Thrust faults are interpreted to be products of shortening caused by gravity driven extension on the continental margin that involve rocks of both the shelf and the slope. Results show two main deformation events during the Cretaceous (between ca. 89 and 84 Ma) and several episodes during the Later Cenozoic (ca. 55–0 Ma). All events were characterized by displacement along a detachment fault linking a landward system of normal faults to a basinward system of folds and thrust faults. The Cretaceous deformation involved a

From surface geology to aftershock analysis: Constraints on the geometry of the L’Aquila 2009 seismogenic fault system

The aim of this study is the definition of the geometry and structural style of the extensional fault system involved during the L'Aquila 2009 seismic sequence. The surface tectonic setting of the late Quaternary faults in the epicentral areas (i.e., the Paganica, Mt. Gorzano and Montereale faults in northwestern Abruzzo) was defined by integrating data from the literature with original field work. The geometry of the faults at depth was interpreted using high-resolution aftershock distributions (based on a dataset covering 6 April to 27 December 2009, Ml ≥?1.9). The 3D fault model was reconstructed through 39 sections variously oriented across the epicentral area and represented in a map view as depth contour lines.Consistent geometric and kinematic correlations between the geological and seismological data were observed. The seismic sequence reactivated four well-distinguished fault sources differing in geometry, size and the degree of involvement. In order of importance, these sources may be ranked as follows: 1) the SW-dipping Paganica fault system activated during the 6 April mainshock (Mw 6.1/6.3) from the surface to a depth of ~10.5 km (along-strike length, L, 21 km; surface rupture length 13 km); 2) the NE-dipping hidden Ocre source activated during the strongest aftershock (9 April, Mw 5.4/5.5) at depths between 11 km and 16 km; 3) the WSW-dipping Mt. Gorzano fault activated by four relevant aftershocks (6 to 13 April, Mw ~5.0 to 5.5) at depths between 6 km and 12 km; 4) the WSW-dipping Montereale fault system and its continuation into the S-dipping Mt. San Franco fault activated at depths between 6 km and 10 km in a large number of minor events with Mw up to 3.6.The hypocentre of the 6 April main event is sited at a depth of ~9.5 km close to the intersection line between the SW-dipping Paganica fault and the S-dipping Mt. Stabiata fault, suggesting that the fault plane complexities (in this case, the sharp bend of the fault strike) may represent a favoured site for earthquake nucleation. The L'Aquila 2009 seismic sequence did not reactivate the entire surface of the involved faults. Nevertheless, on the basis of other geological and seismological constraints, the overall fault extent is schematically reconstructed and parameterised. Some considerations related to the structural style of the intra-Apennine Quaternary extensional structures and their patterns at depth are advanced.

Large normal-sense displacement on the South Tibetan fault system in the eastern Himalaya

The extensional South Tibetan fault system (STFS) crops out for more than 2000 km along the length of the Himalayan range crest. Interpretations of the significance of this structure in Himalayan tectonics vary. While many regard it as rooted northward beneath Tibet, with offset estimates ranging as high as >200 km, others regard it as a relatively minor, rootless structure. Such inconsistent interpretations are possible because, except for the Everest region, where glacial erosion has exposed a cross section of the STFS, there are few direct constraints on the amount of slip on this feature. Arguments for very large displacements have been based on indirect lines of evidence (e.g., geothermobarometric data) that are multiply interpretable. However, new field work and remote sensing data reported here directly support the petrologic arguments for large displacements, at least in the eastern Himalaya. Near the western boundary of Bhutan, the trace of the STFS is apparently offset ∼70 km across a NNE-SSW–trending graben, the Yadong cross structure. This offset has been attributed by previous authors to left-lateral slip along the Jomolhari fault system, which bounds the eastern edge of the graben. In fact, the basal structure of the STFS is not offset, but forms a regionally continuous low-angle detachment surface. The outcrop pattern of the structure, kinematic indicators in fault-related tectonites, and the lack of matchable correlative hanging wall and footwall units require a minimum displacement of ∼65 km for the detachment. Such displacements are comparable with estimates for major contractional structures in the eastern Himalaya, such as the Main Central thrust system, and confirm the importance of STFS extension in the evolution of the orogenic system.

Fault-related fluid flow history in shallow marine sediments from carbonate concretions, Crotone basin, south Italy

We present the results of a multidisciplinary study on the relationships between the structural architecture of an extensional fault system in poorly lithified shallow marine sands and the associated pattern of diagenetic carbonate concretions. Based on their shape, spatial distribution, cement texture and chemistry, and isotopic signature, carbonate concretions are grouped into (1) tabular (encompassing also nodular and lens-shaped) concretions developed within the fault zones, and (2) elongate and coalescent strata-bound concretions formed adjacent and mostly parallel to fault zones. Cement chemistry and morphology, as well as stable carbon–oxygen isotopic signatures, indicate that tabular concretions formed during early diagenesis in the vadose mixing marine–meteoric zone, possibly during coseismic rupture propagation and, in the interseismic periods, as a result of slow capillary suction along the low-permeability fault zones. On the other hand, elongate concretions formed after regional uplift and during telodiagenesis by precipitation from meteoric, phreatic water flowing parallel to the fault zones, which acted as a hydraulic barrier. The concretion patterns record the evolution of fluid flow orientation and the changing chemistry of fault-zone fluids, which were fundamentally driven by fault system propagation and resulting fault architecture during two major diagenetic stages.

Application of spectral decomposition and seismic attributes to understand the structure and distribution of sand reservoirs within Tertiary rift basins of the Gulf of Thailand

The Tertiary rift basins in the Gulf of Thailand are the major hydrocarbon producing areas of Thailand. The reservoirs in these basins are predominantly Lower to Middle Miocene fluvial channels and overbank sands and these reservoirs are segmented due to post-depositional faulting (Racey, 2011). There are small closely spaced sealing faults, which compartmentalize the sand reservoirs and are not easy to map on conventional full spectrum seismic data. This fault system is oblique to the regional north-south extensional fault system and is related to graben shifts/transfer zones associated with changes in dip of graben provinces and zig-zag map pattern of the regional north-south faults (Kornsawan and Morley, 2002). Most wells until now have been drilled on structures along the faults and it is common to encounter different sand facies with different fluid contents in closely spaced wells at the same depth intervals. Because of these structural and stratigraphic complexities, it is required to develop geophysical workflows to better image faults related with graben shifts and map the reservoir sands in the area. In this study, we analyzed a 3D seismic data set over a gas field in the Pattani Basin (Figure 1) where we applied a spectral decomposition technique to resolve these exploration issues. Moreover, semblance and sweetness attributes were also calculated to assist in the fault and sand detection, respectively.

A step metallogenetic model for gold deposits in the northwestern Shandong Peninsula, China

Based on our study of the nature and variation of mineralization-controlling faults in their deep extension and the spatial distribution of gold orebodies in the northwestern Shandong Peninsula (NSP), here we propose that the gold deposits in the Jiaojia gold belt are controlled by listric faults and the gold deposits in the NSP are controlled by extensional fault system. We also suggest that there is a multiple mineralization space in the deep part of the NSP gold belt. As steps or benches occur along the deep extention of the listric fault where its dipping angle changes from steep to gentle, a number of gold deposits are correspondingly distributed as a steplike pattern, which is named as a step metallogenetic model for gold deposits in the NSP. The gold deposits in Shandong Peninsula were formed in the Cretaceous when the crust in the region experienced extention and thinning following the strong collision between North China Plate and Yangtze Plate during the Triassic. Crust anataxis, fluid remobilization, and extention-detachement of the crust are the leading factors for the gold mineralization.

Constraints on exhumation and extensional faulting in southwestern Nevada and eastern California, U.S.A., from zircon and apatite thermochronology

Eastern California and southwestern Nevada represent an area of Tertiary and Quaternary extensional and dextral transtensional deformation. We used zircon and apatite fission-track thermochronology to study the distribution and timing of tectonic exhumation resulting from extensional and transtensional detachment faulting in this area. Sampling efforts were focused on Paleozoic and Precambrian clastic sedimentary and metasedimentary rocks. Sixty-nine new apatite and zircon fission-track cooling ages from 50 samples, analyzed in conjunction with published fission-track data from the region, indicate a distinct population of young (Miocene) fission-track ages and a population of irregularly distributed older (pre-Miocene) fission-track ages. Miocene (young population) fission-track ages become younger toward the west—indicating westward migration of the cooling front, consistent with well-documented Miocene extension of the Basin and Range Province. The younging pattern is also consistent with west-northwest displacement of the hanging wall of a crustal-scale extensional fault system and consequent progressive footwall exhumation. The active trailing edge of the hanging wall of this system generally coincides with Death Valley. Migration rates of the cooling front in the footwall of this system are on the order of 10–11 mm/yr. Based on the distribution of the Miocene fission-track ages, we interpret that the crustal faults that defined the eastern edge of the detachment system originated as separate normal faults that were linked by the formation of a transfer fault. Extrapolation of apatite fission-track closure ages from two transects across the eastern margin of the Death Valley region suggests that exhumation along the eastern margin of the system continues beneath Death Valley today.

Syn-thrust deformation across a transverse zone: the Grem–Vedra fault system (central Southern Alps, N Italy)

The lateral continuity of the E–W trending thrust sheets developed within the Lower to Middle Triassic cover of the central Southern Alps (Orobic belt) is disturbed by the occurrence of several N–S trending transverse zones, such as the poorly known Grem–Vedra Transverse Zone (GVTZ). The GVTZ developed during the emplacement of the up to six S-verging thrust sheets consisting of Lower to Middle Triassic units, occurring immediately south of the Orobic Anticlines. The transverse zone, active during thrust emplacement related to the early Alpine compressions which pre-date the Adamello intrusion, includes three major vertical shear zones, the Grem, Pezzel and Zuccone faults. The major structure of the transverse zone is the dextral Grem fault, forming a deep lateral ramp between thrust sheets 3 and 5. A similar evolution also occurred along the Zuccone and Pezzel faults, which show a left-lateral displacement of syn-thrust folds. The Grem fault was later reactivated as an oblique tear fault during the emplacement of the Orobic Anticlines, due to back-thrusting along out-of-sequence thrust surfaces (Clusone fault). Transpressional deformations along the fault zone are recorded by the rotation of major syn-thrust folds, which also suggest a horizontal offset close to 0.5 km. Records of the first stage of evolution of the Grem fault are better preserved along its northern segment, and structural relationships suggest that it propagated southward and downward in the growing thrust stack. The study of the meso and megascopic structures developed along the GVTZ constrains the evolution of the transverse zone, illustrating the complex deformational phenomena occurring in a transpressional regime. The GVTZ probably reflects the existence of pre-existing tectonic lineaments with a similar orientation. Evidence of pre-existing structures are not preserved in the exposed units, nevertheless the N–S extensional fault systems that characterize the Norian to Jurassic rifting history of the Lombardian basin are valid candidates.

Structure of the Southern Patagonian Andes at 49ºS, Argentina

This paper describes Late Paleozoic Gondwanan and Late Cretaceous to Early Cenozoic Andean structures in the Southern Patagonian Andes and an associated Extra-Andean region between lakes San Martin and Viedma. The study area encompasses a 200-km-long W-E section between the Patagonian icefield and the 72oW longitude meridian, in Argentine Patagonia. The oldest structures are of Late Paleozoic age and developed through at least two deformation phases during the Gondwanan Orogeny. The first deformation phase (Dg1) includes isoclinal and N-overturned WNW trending folds and associated thrusts, including duplexes. The second deformation phase includes NNE trending open folds (Dg2). Deformation occurred in non-metamorphic to very low-grade metamorphic conditions. A spaced rough cleavage is found near the first phase fold hinges. The Eocene and Miocene Andean structural compression resulted in a N-S oriented fold and thrust belt. This belt is comprised of three morphostructural zones from W to E, with distinctive topographic altitudes and structural styles: Andean; Sub-Andean; and Extra-Andean zones. The first corresponds to the inner fold and thrust belt, while the last two are part of the outer fold and thrust belt. The Andean zone (3400–2000m above sea level) is characterized by N-S to NNE trending, E-vergent, Cenozoic reverse faults and associated minor thrusts. The northern part of the Sub- Andean zone (2000–1500m above sea level) consists of W-vergent reverse faults and some NNE open folds. The southern part of the Andean zone includes tight folds with box and kink geometries, related to thrusts at deeper levels. In the Extra-Andean zone, with maximum heights of 1500m, the deformation is less intense, and gentle folds deform the Upper Cretaceous sediments. An inherited Jurassic N-S extensional fault system imposed a strong control on this morphostructural zonation. Also the variation of the Austral Basin sedimentary thickness in the N-S direction seems to have influenced the structural styles of the outer fold and thrust belt. Those differences in sedimentary thickness may be related to S-dipping transfer zones associated to W-E Jurassic extension. In turn, the transfer zones may have been controlled by the N-vergent WNW, Dg1, Gondwanan structural fabric.

Tectonic evolution of a low‐angle extensional fault system from restored cross‐sections in the Northern Apennines (Italy)

Broad geological and geophysical documentation is available on regional extensional systems driven by low‐angle normal faults. However, little information exists about the three‐dimensional geometry and the offset distribution of such extensional structures. We present a new set of balanced geological sections across the extensional fault system driven by the Altotiberina low‐angle normal fault in the Northern Apennines of Italy. We document this extensional system throughout a large set of surface (field surveys and geological maps) and subsurface data (seismic reflection profiles and boreholes). The subsurface data allowed us to define the fault deep geometry and to obtain its structural contours. The fault geometry is characterized by both along‐dip and along‐strike irregularities. In cross‐section, the fault displays a staircase trajectory with the shallowest part being dome‐shaped and flattened to horizontal. This bending could be due to the footwall uplift triggered by a footwall uploading greater than about 115 MPa. The sequential restoration of five geological cross‐sections yields a maximum extension of about 10 km accumulated over approximately 3 Ma. The resulting long‐term slip‐rate is about 3 mm/yr, which is of the same order as the present‐day extensional rate measured by GPS (2.5–3.0 mm/yr), suggesting an almost steady state extension over the last 3 Ma. The distribution of the extension values along the fault strike is bell‐shaped, as expected for a continuous surface.

The Campania‐Lucania Extensional Fault System, southern Italy: A suggestion for a uniform model of active extension in the Italian Apennines

By integrating new field data, seismic lines interpretation and a critical review of the literature, this work highlights a regional array of normal faults in the southern Apennines of Italy, which have been active during the Quaternary and are referred to as the “Campania‐Lucania Extensional Fault System” (CLEFS). The CLEFS consists of three main NW‐SE striking alignments of normal faults, which for the first time are considered genetically linked and methodically described in their geometry, kinematics and displacement. The CROP04 seismic profile, crossing the central part of the CLEFS, was strictly constrained by detailed geological surveys and reinterpreted to define the downdip trajectory of the major normal faults. These latter have been observed to splay from an east dipping low‐angle detachment surface penetrating the upper crust to depths of 12–13 km. The time‐space evolution of the faults and the associated basins was defined through a review of the stratigraphic data on the syntectonic deposits. As regards the overall geometry and the associate sense of shear (top‐to‐east), the CLEFS sensibly differs from the extensional features described previously in this region but shows surprising affinities with the “Etrurian Fault System,” an extensional megastructure of the northern Apennines. Remarkable similarities concern the extent, the surface and subsurface geometry, the timing of activity and the amount of the associated deformation. These common characters recognized over a belt nearly 600 km long lead to formulation of a low‐angle normal fault–driven extension geometrical‐kinematic model, which is sound for the whole active extensional belt of Italy.

The fracture pattern of the Sant Corneli Bóixols oblique inversion anticline (Spanish Pyrenees)

In this work we present and discuss the macro and mesostructural deformation patterns of the Late Cretaceous Sant Corneli Bóixols oblique inversion anticline (Spanish Pyrenees). This E–W striking and south-verging structure developed by inversion of an Early Cretaceous extensional fault system, formed by WNW–ESE striking faults (oriented perpendicular to the Early Cretaceous stretching direction) and transfer faults. During inversion, the shortening direction was oriented NNW–SSE, i.e. oblique to both basin margin and inherited extensional deformation structures. A pre to early-folding layer-parallel shortening (LPS) stage caused the strike-slip reworking of inherited extensional structures and the development of extensional and contractional structures oriented parallel and perpendicular to the shortening direction, respectively. Few faults clearly developed during the late stages of fold growth, in response to a stress field having principal axes parallel to those of the layer-parallel shortening event, i.e. oblique to the fold axis. WSW–ENE striking bedding-perpendicular joints and veins postdate the LPS pattern and can be associated with a syn-folding layer-parallel stretching. Analogously, few reverse faults having a top-to-the crest shear sense are associated with flexural-slip folding. Both layer-parallel stretching and shear sense provided by flexural-slip related elements are not perpendicular to the fold axial trend. All these assemblages were overprinted by a set of post-folding extensional structures oriented oblique to the strike of the anticline.Presented data show that syn-inversion fractures in the Sant Corneli Bóixols Anticline are neither parallel nor perpendicular to the fold axial trend, and their development has been controlled by inherited extensional structures and the regional shortening direction.

Late Quaternary tectonics of the Vallo di Diano basin (southern Apennines, Italy)

We discuss new data concerning the late Quaternary tectonics of the Vallo di Diano basin, the largest intramontane depression in the southern Apennines chain (Italy). The southern Apennines axis is currently subject to NE–SW extension. Here, active NW-trending normal faults are associated with strong earthquakes ( M > 6), with average recurrence times >1 ka. The Vallo di Diano basin is a half-graben bounded by an extensional fault system VDFS (Vallo di Diano Fault System), featuring three major NW-trending, SW-dipping, ∼15–17 km long fault segments. Holocene faulting and possible historical seismicity have been documented only for the northern fault segment. No major historical earthquakes can be associated with the other fault segments, and the recent sedimentary and structural evolution of the basin is still poorly constrained. As a consequence, its seismogenetic potential has probably been underestimated. We found evidence of late Pleistocene–Holocene faulting in the relay zone of the central and southern fault segments bordering the basin. Here, late Pleistocene volcaniclastic layers at the apex of an alluvial fan are affected by normal displacements on the order of ∼1 m. Additionally, in the same area we recognized a previously unreported, 9 km long and 0.5–1.4 km wide set of scarplets (ranging in height between ∼1 m and 2.5 m) affecting late Pleistocene–late Holocene alluvial fans. The last deformation is younger than 3 ka BP. Due to the geometric arrangement of these scarplets and the long-term displacement along the Vallo di Diano Fault System, we hypothesize that the central and southern fault segments belong to a ∼32 km long hard-linked system, capable of generating Mw > 6.5 earthquakes. The Vallo di Diano basin is thus bounded to the east for its entire length (∼45 km) by an active segmented extensional fault system. Therefore it has to be considered one of the major seismogenic sources in southern Italy.

Geometry and evolution of low‐angle normal faults (LANF) within a Cenozoic high‐angle rift system, Thailand: Implications for sedimentology and the mechanisms of LANF development

At least eight examples of large (5–35 km heave), low‐angle normal faults (LANFs, 20°–30° dip) occur in the Cenozoic rift basins of Thailand and laterally pass into high‐angle extensional fault systems. Three large‐displacement LANFs are found in late Oligocene–Miocene onshore rift basins (Suphan Buri, Phitsanulok, and Chiang Mai basins), they have (1) developed contemporaneous with, or after the onset of, high‐angle extension, (2) acted as paths for magma and associated fluids, and (3) impacted sedimentation patterns. Displacement on low‐angle faults appears to be episodic, marked by onset of lacustrine conditions followed by axial progradation of deltaic systems that infilled the lakes during periods of low or no displacement. The Chiang Mai LANF is a low‐angle (15°–25°), high‐displacement (15–35 km heave), ESE dipping LANF immediately east of the late early Miocene Doi Inthanon and Doi Suthep metamorphic core complexes. Early Cenozoic transpressional crustal thickening followed by the northward motion of India coupled with Burma relative to east Burma and Thailand (∼40–30 Ma) caused migmatization and gneiss dome uplift in the late Oligocene of the core complex region, followed by LANF activity. LANF displacement lasted 4–6 Ma during the early Miocene and possibly transported a late Oligocene–early Miocene high‐angle rift system 35 km east. Other LANFs in Thailand have lower displacements and no associated metamorphic core complexes. The three LANFs were initiated as low‐angle faults, not by isostatic rotation of high‐angle faults. The low‐angle dips appear to follow preexisting low‐angle fabrics (thrusts, shear zones, and other low‐angle ductile foliations) predominantly developed during Late Paleozoic and early Paleogene episodes of thrusting and folding.

Internal geometry and growth history of a thrust-related anticline in a deep water fold belt

Although 3D seismic imaging has become a fundamental tool for the analysis of extensional fault geometries and their development, it has not been utilised to the same degree to investigate the structure and growth history of thrust faults. Here, the structural evolution of a fold and thrust belt is investigated through the analysis of an isolated fold imaged in the deep water Niger Delta. A quantitative description of thrust fault displacement through transfer zones is presented to investigate the internal structure of a complex fault-related fold, while syn-kinematic growth packages are used to chart fold growth through time, so as to also understand the early growth history. Results reveal that a number of initially separate folds coalesced along the axial trends into a single structural culmination. Numerous thrust faults with similar and opposing dip, link and transfer displacement within the structure as it changes vergence along strike. The distribution of bulk deformation along the fault-related fold is simple, systematic and similar to that observed for extensional fault systems. Deficits in fault heave, both in the form of perturbations on non-linking or aggregated profiles, and displacement minima within transfer zones, are compensated to some degree by an increase in fold-related strain. Despite this accommodation, fold amplitude or crest elevation is largely unaffected by variations in fault heave. The observations for this isolated fold should be widely applicable to fold and fault development in more complex areas.

Timing and nature of formation of the Ios metamorphic core complex, southern Cyclades, Greece

Abstract We apply low-temperature thermochronology, Rb/Sr geochronology, petrological data, and structural mapping to constrain the timing and kinematics of the Ios metamorphic core complex. Top-to-north extension in the lower plate Headland Shear Zone was active at 18–19 Ma under metamorphic conditions of 475–610 °C and 0.65–1.1 GPa. The South Cyclades Shear Zone/Ios Detachment Fault (SCSZ/IDF) system shows top-to-south extensional shear active at c. 19 Ma at 380–550 °C, with local top-to-north bands. Extensional shear above the SCSZ/IDF is dominantly top-to-south to top-to-SW. PT estimates from an eclogite boudin constrain Eocene high-pressure metamorphism to 430–560 °C and 1.21±0.42 GPa to 0.66±0.37 GPa. Similar low-temperature thermochronometric ages across Ios demonstrate that ductile extensional movement ceased by c. 15 Ma. Exhumation to shallow crustal levels took place between c. 15 and 9 Ma at cooling rates of up to 120 °C Ma −1 with a slow down to <20 °C Ma −1 between 12 and 9 Ma, most likely accommodated by extensional slip at rates of c. 3 km Ma −1 along the top-to-SW Coastal Fault System. We propose a model of bivergent extension for exhumation of the Ios core complex between 19 and 9 Ma, with Ios forming a secondary antithetic top-to-south to top-to-SW extensional fault system to a more dominant top-to-north Naxos/Paros detachment system.

Geochemical and temporal evolution of Cenozoic magmatism in western Turkey: mantle response to collision, slab break-off, and lithospheric tearing in an orogenic belt

Abstract Post-collisional magmatism in western Anatolia began in the Eocene, and has occurred in discrete pulses throughout the Cenozoic as it propagated from north to south, producing volcano-plutonic associations with varying chemical compositions. This apparent SW migration of magmatism and accompanying extension through time was a result of the thermally induced collapse of the western Anatolian orogenic belt, which formed during the collision of the Sakarya and Tauride–Anatolide continental blocks in the late Paleocene. The thermal input and melt sources for this prolonged magmatism were provided first by slab break-off-generated aesthenospheric flow, then by lithospheric delamination-related aesthenospheric flow, followed by tectonic extension-driven upward aesthenospheric flow. The first magmatic episode is represented by Eocene granitoid plutons and their extrusive carapace that are linearly distributed along the Izmir–Ankara suture zone south of the Marmara Sea. These suites show moderately evolved compositions enriched in incompatible elements similar to subduction zone-influenced subalkaline magmas. Widespread Oligo-Miocene volcanic and plutonic rocks with medium- to high-K calc-alkaline compositions represent the next magmatic episode. Partial melting and assimilation-fractional crystallization of enriched subcontinental lithospheric mantle-derived magmas were important processes in the genesis and evolution of the parental magmas, which experienced decreasing subduction influence and increasing crustal contamination during the evolution of the Eocene and Oligo-Miocene volcano-plutonic rocks. Collision-induced lithospheric slab break-off provided an influx of aesthenospheric heat and melts that resulted in partial melting of the previously subduction-metasomatized mantle lithosphere beneath the suture zone, producing the Eocene and Oligo-Miocene igneous suites. The following magmatic phase during the middle Miocene (16–14 Ma) developed mildly alkaline bimodal volcanic rocks that show a decreasing amount of crustal contamination and subduction influence in time. Both melting of a subduction-modified lithospheric mantle and aesthenospheric mantle-derived melt contribution played a significant role in the generation of the magmas of these rocks. This magmatic episode was attended by region-wide extension that led to the formation of metamorphic core complexes and graben systems. Aesthenospheric upwelling caused by partial delamination of the lithospheric root beneath the western Anatolian orogenic belt was likely responsible for the melt evolution of these mildly alkaline volcanics. Lithospheric delamination may have been caused by ‘peeling off’ during slab rollback. The last major phase of magmatism in the region, starting c .12 Ma, is represented by late Miocene to Quaternary alkaline to super-alkaline volcanic rocks that show OIB-like geochemical features with progressively more potassic compositions increasing toward south in time. These rocks are spatially associated with major extensional fault systems that acted as natural conduits for the transport of uncontaminated alkaline magmas to the surface. The melt source for this magmatic phase carried little or no subduction component and was produced by the decompressional melting of aesthenospheric mantle, which flowed in beneath the attenuated continental lithosphere in the Aegean extensional province. This time-progressive evolution of Cenozoic magmatism and extension in western Anatolia has been strongly controlled by the interplay between regional plate-tectonic events and the mantle dynamics, and provides a realistic template for post-collisional magmatism and crustal extension in many orogenic belts.

The Vallo di Diano and Auletta extensional basins in the southern Apennines (Italy): a simple model for a complex setting

AbstractThe aim of this paper was to provide a significant case‐history concerning the evolution of a segmented system of extensional faults and related basins, investigated by a set of seismic reflection profiles. We investigated two kinematically linked semi‐grabens, developed at the hangingwall of two opposite‐dipping normal faults: the Vallo di Diano and Auletta basins, located in the southern Apennines, one of the most seismically active regions of the Italian peninsula. Our interpretation suggests that the Pliocene–Quaternary tectonic history consists of a single extensional phase, where the major NW–SE trending normal faults, generating the basins, and the strike‐slip and transtensional faults connecting adjacent normal fault segments, simultaneously act, being part of the same extensional system. We also conclude that major normal faults, bordering the basins, should be considered as potential seismogenic sources in the seismic hazard evaluation.

Major structural elements of the Miocene section, Burgos Basin, northeastern Mexico

Major Miocene structural elements of the Burgos Basin include a regionwide detachment system that connects extensional fault systems throughout the basin with an active diapir belt downdip, a regionwide pattern of downthrown extensional rollover folds, pervasive secondary faults, and salt and shale diapiric masses in the eastern part of the basin. An interpretation of two-dimensional seismic data suggests that the Burgos Basin Miocene section can be divided into four structural domains: expanded zone, Lamprea trend, Corsair-Wanda trend, and diapir belt. The westernmost unexpanded zone is the footwall of the expanded system part of the basin, which overlies a domain of Oligocene extension. Remaining trends represent an extensional accommodation related to the basinward migration of mobile salt and shale, which has produced a relatively uniform structural style in the Miocene section. The structural style observed in the Burgos Basin appears to define a transitional zone between gravitational collapse in the offshore Laguna Madre-Tuxpan shelf to the south and salt-related raft tectonics of the south Texas Gulf Coast.