Extension Of Upper Crust Research Articles

The Early Mesozoic NE–SW Extensional Model and Exhumation Processes at the Southeastern Margin of the Central Asian Orogenic Belt: Insights from the Strain and Kinematic Vorticity Analysis of the Sonid Zuoqi Ductile Detachment Zone

AbstractThe Sonid Zuoqi ductile detachment zone is located at the southeastern margin of the Central Asian orogenic belt (CAOB), striking EW and dipping to the S. The major rock type of the Sonid Zuoqi ductile detachment zone is mylonite derived from granite. The sequence of mylonite features is: (1) S and C foliations of mylonite, and (2) extensional crenulation cleavage (ecc) or C' and the kinematic vorticity (Wk) value changed from 0.70 to 0.95 and from 0.37 to 0.69, respectively; the strain type of the mylonites within the Sonid Zuoqi ductile detachment zone is compressional to planar strain. The strong deformation mylonite and Halatu plutons yielded a zircon U‐Pb age of 244 Ma and a zircon (U‐Th)/He age of 214 Ma, respectively. Based on the strain and kinematic vorticity analysis, together with the zircon U‐Pb and zircon (U‐Th)/He ages and the regional tectonic background, the study area experienced three stage evolution: tangential simple‐shear (244 Ma), simple‐shear‐dominated general shear represented by upper crustal extension (224 Ma) and pure–shear–dominated general shear represented by the Halatu pluton doming (214 Ma), which constrained the early Mesozoic NE‐SW crustal extension at the southeastern margin of the CAOB. This NE‐SW extension probably originated from the post‐orogenic extensional collapse of the CAOB, subsequent exhumation being controlled by the far afield effects of the closure of the Mongol‐Okhotsk belt.

Syn‐ and post‐rift lower crustal flow under the Sunda Shelf, southern Vietnam: A role for climatically modulated erosion

AbstractTectonic subsidence on rifted, passive continental margins are largely controlled by patterns of extension and the nature of strain partitioning in the lithosphere. The Sunda Shelf, adjacent to the SW South China Sea, is characterized by deep basins linked to regional Cenozoic extension associated with propagating seafloor spreading caused by slab pull from the south. Analysis of seismic reflection profiles and drilled sections crossing the Nam Con Son and Cuu Long basins highlight Oligocene extension, with most of the thinning concentrated in the ductile mid‐lower crust. Upper crustal extension was modest and ductile flow is inferred to be directed northwestwards, towards the oceanic crust. Basin inversion occurred in the Mid Miocene, associated with the collision of the Dangerous Grounds Block and Borneo. Subsequent accelerated tectonic subsidence exceeded predictions from uniform extension models assumed to relate to extensional collapse after inversion. We correlate this to a period of faster erosion onshore driven by strong monsoon rains in Indochina and Peninsular Thailand at that time. Erosion of the onshore basement, inducing rock uplift and coupled with loading of the basins offshore, drives ductile mid‐lower crustal flow, likely to the northeast under Indochina, and/or to the west where Plio‐Pleistocene subsidence of the shelf is very slow. Significant sediment delivery from the Mekong River into the Cuu Long Basin began in the Late Miocene and migrated seawards as the basin filled. Mass balancing suggests that the basins of this part of the Sunda Shelf are filled through erosion of bedrock sources around the Gulf of Thailand. There is no need for sediment delivery from a major river draining the Tibetan Plateau to account for the deposited volumes.

Early Cretaceous extensional allochthons in the Taihang Shan associated with destruction of the North China Craton

Whilst geophysical exploration and petrological analyses imply that the Early Cretaceous North China Craton (NCC) destruction may have impacted the Taihang Shan in the central NCC, associated evidence indicative of contemporary upper-crustal extension is lacking. We herein investigate the formation mechanism and timing of the allochthons that consist of Mesoproterozoic to Mesozoic sedimentary rocks within the Paleoproterozoic Fuping complex in the middle Taihang Shan, based on field investigation and zircon U-Pb analysis. Our results indicate that the western boundary of the selected allochthon features wide collapsed breccias filled with synchronous Early Cretaceous (∼134 Ma) pyroclastics. These allochthons likely formed under extension along with the basement doming of the Fuping complex in response to crustal stretching in Early Cretaceous. Our results provide explicit evidence of upper-crustal extension in the Taihang Shan associated with the Early Cretaceous NCC destruction.

Whole-lithosphere shear during oblique rifting

Abstract Processes controlling the formation of continental whole-lithosphere shear zones are debated, but their existence requires that the lithosphere is mechanically coupled from base to top. We document the formation of a dextral, whole-lithosphere shear zone in the Death Valley region (DVR), southwest United States. Dextral deflections of depth gradients in the lithosphere-asthenosphere boundary and Moho are stacked vertically, defining a 20–50-km-wide, lower lithospheric shear zone with ~60 km of shear. These deflections underlie an upper-crustal fault zone that accrued ~60 km of dextral slip since ca. 8–7 Ma, when we infer that whole-lithosphere shear began. This dextral offset is less than net dextral offset on the upper-crustal fault zone (~90 km, ca. 13–0 Ma) and total upper-crustal extension (~250 km, ca. 16–0 Ma). We show that, before ca. 8–7 Ma, weak middle crust decoupled upper-crustal deformation from deformation in the lower crust and mantle lithosphere. Between 16 and 7 Ma, detachment slip thinned, uplifted, cooled, and thus strengthened the middle crust, which is exposed in metamorphic core complexes collocated with the whole-lithosphere shear zone. Midcrustal strengthening coupled the layered lithosphere vertically and therefore enabled whole-lithosphere dextral shear. Where thick crust exists (as in pre–16 Ma DVR), midcrustal strengthening is probably a necessary condition for whole-lithosphere shear.

High‐ and Low‐Angle Normal Fault Activity in a Collisional Orogen: The Northeastern Granada Basin (Betic Cordillera)

AbstractUnderstanding active tectonics and seismicity in extensional settings requires the analysis of high‐angle normal faults (HANFs) and the transfer of deformation at depth. The debate surrounds the role of low‐angle normal faults (LANFs) in triggering high magnitude earthquakes. The central Betic Cordillera is an active seismic zone affected by the NNW‐SSE Eurasia‐Nubia convergence and orthogonal extension. The seismicity and present‐day stress determined by earthquake focal mechanisms reveals the activity of a NE‐SW extensional system in the shallowest 12 km of the Granada Basin. The structure of the sedimentary infill, as derived by geological field and gravimetric techniques, suggests the formation of a half‐graben tilted to the N‐NE. Seismologic data suggest the activity of HANFs above 6–7 km depth and a LANF zone around 6–12 km depth, with related earthquakes of up to Mw 4.0 and 20° to 30° fault dips. High‐precision leveling lines highlight the importance of the Granada Fault in the system, with average vertical displacement rates of 0.35–1.1 mm/yr. These data suggest creep fault behavior at the surface and increased seismicity at depth. The upper crustal extension in the collisional Betic Cordillera is accommodated by a top‐to‐the‐WSW extensional detachment related to westward motion and rollback in the Gibraltar Arc and the gravitational collapse of the cordillera, in a framework of NNW‐SSE shortening. This comprehensive study draws a new scenario that advances understanding of relationships between HANFs and LANFs.

Thermo-kinematic modeling of detachment-dominated extension, northeastern Death Valley area, USA: Implications for mid-crustal thermal-rheological evolution

The deep structure of continental detachment faults remains debated. Thermo-mechanical models generate detachments that either transect the lithosphere or become distributed shear zones in the mid-lower crust, depending on prescribed thermo-rheological conditions. However, these geometries and prescribed conditions remain little constrained by geology-based reconstructions. We present stepwise, balanced reconstructions of a 160 km-long cross-section through two detachment faults in the southwest USA. Reconstructions form the basis of iteratively improved 2D forward thermo-kinematic numerical simulations of detachment fault slip, footwall exhumation, heat advection, and footwall zircon (U-Th)/He cooling ages. Thermo-kinematic model solutions are calibrated iteratively against surface heat flow, pre- and post-extensional geotherms, inferred Moho temperatures, and thermochronometric data from one detachment footwall. Best-fit models predict the thermal and geometric evolution of the crust and detachments, respectively, during extension. The detachment initially rooted into a mid-crustal shear zone (~7.5–12 km depth) and was probably delocalized in the deep middle crust (>12–15 km). The maximum principal stress was likely non-vertical in the middle crust at detachment initiation, possibly due to mantle upwelling. Our reconstructions suggest that the upper crust and lower crust-mantle lithosphere were decoupled by a weak, mid-crustal layer during early detachment faulting. The weak layer was thinned, cooled, partially embrittled, and therefore strengthened by continued detachment slip. This increased lithospheric mechanical coupling and caused the locus of upper-crustal extension to shift. Thinning of a weak mid-crustal layer, as is thought to precede coupled hyperextension and mantle exhumation during rifting, was mostly complete in our study area by ~7–6 Ma.

Localized foundering of Indian lower crust in the India–Tibet collision zone

The deep structure of the continental collision between India and Asia and whether India's lower crust is underplated beneath Tibet or subducted into the mantle remain controversial. It is also unknown whether the active normal faults that facilitate orogen-parallel extension of Tibetan upper crust continue into the lower crust and upper mantle. Our receiver-function images collected parallel to the India-Tibet collision zone show the 20-km-thick Indian lower crust that underplates Tibet at 88.5-92°E beneath the Yarlung-Zangbo suture is essentially absent in the vicinity of the Cona-Sangri and Pumqu-Xainza grabens, demonstrating a clear link between upper-crustal and lower-crustal thinning. Satellite gravity data that covary with the thickness of Indian lower crust are consistent with the lower crust being only ∼30% eclogitized so gravitationally stable. Deep earthquakes coincide with Moho offsets and with lateral thinning of the Indian lower crust near the bottom of the partially eclogitized Indian lower crust, suggesting the Indian lower crust is locally foundering or stoping into the mantle. Loss of Indian lower crust by these means implies gravitational instability that can result from localized rapid eclogitization enabled by dehydration reactions in weakly hydrous mafic granulites or by volatile-rich asthenospheric upwelling directly beneath the two grabens. We propose that two competing processes, plateau formation by underplating and continental loss by foundering or stoping, are simultaneously operating beneath the collision zone.

Petroleum geology controlled by extensive detachment thinning of continental margin crust: A case study of Baiyun sag in the deep-water area of northern South China Sea

Abstract The relationships between crustal stretching and thinning, basin structure and petroleum geology in Baiyun deep-water area were analyzed using large area 3D seismic, gravity, magnetic, ocean bottom seismic (OBS), deep-water exploration wells and integrated ocean drilling program (IODP). During the early syn-rifting period, deep-water area was a half-graben controlled by high angle faults influenced by the brittle extension of upper crust. In the mid syn-rifting period, this area was a broad-deep fault depression controlled by detachment faults undergone brittle-ductile deformation and differentiated extension in the crust. In the late syn-rifting period, this area experienced fault-sag transition due to saucer-shaped rheology change dominated by crustal ductile deformation. A broad-deep fault depression controlled by the large detachment faults penetrating through the crust is an important feature of deep-water basin. The study suggests that the broad-deep Baiyun sag provides great accommodation space for the development of massive deltaic-lacustrine deposition system and hydrocarbon source rocks. The differentiated lithospheric thinning also resulted in the different thermal subsidence during post-rifting period, and then controlled the development of continental shelf break and deep-water reservoir sedimentary environment. The high heat flow background caused by the strong thinning of lithosphere and the rise of mantle source resulted in particularities in the reservoir diagenesis, hydrocarbon generation process and accumulation of deep-water area in northern South China Sea.

Orogen‐parallel deformation of the Himalayan midcrust: Insights from structural and magnetic fabric analyses of the Greater Himalayan Sequence, Annapurna‐Dhaulagiri Himalaya, central Nepal

AbstractThe metamorphic core of the Himalaya (Greater Himalayan Sequence, GHS), in the Annapurna‐Dhaulagiri region, central Nepal, recorded orogen‐parallel stretching during midcrustal evolution. Anisotropy of magnetic susceptibility and field‐based structural analyses suggest that midcrustal deformation of the amphibolite facies core of the GHS occurred under an oblate/suboblate strain regime with associated formation of low‐angle northward dipping foliation. Magnetic and mineral stretching lineations lying within this foliation from the top of the GHS record right‐lateral orogen‐parallel stretching. We propose that oblate strain within a midcrustal flow accommodated oblique convergence between India and the arcuate orogenic front without the need for strain partitioning in the upper crust. Oblate flattening may have also promoted orogen‐parallel melt migration and development of melt‐depleted regions between km3 scale leucogranite culminations at ~50–100 km intervals along orogen strike. Following the cessation of flow, continued oblique convergence led to upper crustal strain partitioning between orogen‐perpendicular convergence on thrust faults and orogen‐parallel extension on normal and strike‐slip faults. In the Annapurna‐Dhaulagiri Himalaya, orogen‐parallel stretching lineations are interpreted as a record of transition from midcrustal orogen‐perpendicular extrusion to upper crustal orogen‐parallel stretching. Our findings suggest that midcrustal flow and upper crustal extension could not be maintained simultaneously and support other studies from across the Himalaya, which propose an orogen‐wide transition from midcrustal orogen‐perpendicular extrusion to upper crustal orogen‐parallel extension during the mid‐Miocene. The 3‐D nature of oblate strain and orogen‐parallel stretching cannot be replicated by 2‐D numerical simulations of the Himalayan orogen.

Lithostratigraphy and structure of the early Archaean Doolena Gap greenstone belt, East Pilbara Terrane, Western Australia

The early Archaean East Pilbara Terrane of the Pilbara Craton represents the archetypical granite dome – greenstone keel terrain and preserves much of the primary geological features of dome-and-keel formation. Here we present the first detailed lithostratigraphic and structural transect from marginal gneisses of the Muccan Granitic Complex through 6km of steeply dipping strata of the western Doolena Gap greenstone belt.Based on overprinting relationships, we identified five deformational events (D1–D5) that can be correlated between four structural domains: (i) marginal orthogneisses of the polyphase Muccan Granitic Complex, (ii) a dominantly mylonitic shear zone (South Muccan Shear Zone) representing a granite dome – greenstone keel detachment zone, (iii) a Central Fold Belt of dominantly mafic greenschists, and (iv) a lower greenschist- to sub-greenschist facies southern Low-Strain Belt.The D1 event in the Low-Strain Belt is recorded in syn-depositional normal faulting (f1) during eruption of deep water low vesicular pillow basalts of the ca. 3.47Ga Mount Ada Basalt, followed by a conformable transition towards shallow water stromatolites in the Duffer Formation. We interpret the D1 event as the result of upper-crustal extension and subsequent uplift above the Muccan Granitic Complex.The D2 event shows the most intense deformation with syn-anatectic tight folds within the Muccan Granitic Complex and tight to isoclinal transposed folds (F2) and fluid-induced normal faults (f2) within the Central Fold Belt. The D2 event resulted in a regional EW composite S1/2 foliation, and in the development of the mylonitic South Muccan Shear Zone along the dome-keel interface and within the Central Fold Belt, mineral lineations (L2) in conjunction with increasing southward D2l-tectonite. This D2 event is not observed in a ∼1km-thick sandstone package (ascribed to the <3.43Ga Strelley Pool Formation) in the Low-Strain Belt that was deposited over an angular unconformity indicating 45° SSW tilting of the underlying Mount Ada Basalt and Duffer Formation. This constrains the D2 event to between 3.47 and 3.43Ga.We propose that the D1 and D2 events record locally significant initiation of Muccan Granitic Complex doming and associated sagduction of greenstone keel rocks, suggesting that partial convective overturn initiated early in the evolution of the East Pilbara Terrane.

Seismic evidence for Mesozoic strata in the northern Nansha waters, South China Sea

According to previous studies, Mesozoic deposits have been unequivocally identified in the northeastern Nansha waters (southern margin of the South China Sea, SCS). Thick lower structural layers in the north Nansha waters have not clearly been identified as either Mesozoic or Cenozoic strata. These strata are characterized by strong top erosion, tilted layer or folded anticlines. New long-offset multi-channel seismic data show refracted phases from the top of the lower structural layer in the northern Nansha waters. A major velocity leap (approximately from 1.6km/s to 3.8km/s or 2.9km/s to 5.3km/s), calculated from the refraction wave of seismic data, is found across a prominent angular unconformity, indicating a major sedimentary hiatus. According to the stratigraphic characteristics and velocity range of the lower structural layer, velocity leap at the top of lower structural layer and ubiquitous absence of upper Cretaceous strata in the Nansha waters, the lower structural layer of the northern Nansha waters are interpreted as Mesozoic. Based on the similarities in stratigraphic characteristics of the lower structural layers between the northern and central Nansha waters, previous studies from gravity data and multi-channel seismic data, we propose that lower structural layers over central Nansha waters may also Mesozoic. This further suggests that the intensity of upper crustal extension was moderate in Nansha waters during the Cenozoic, which related to a combination of the Cenozoic slab pull of the proto-SCS and lithosphere delamination over an ancient orogenic belt between the northern and southern continental margins of the SCS, which may weaken extension of upper crust over the Nansha waters.

Continental subduction, intracrustal shortening, and coeval upper-crustal extension: P-T evolution of subducted south Iberian paleomargin metapelites (Betics, SE Spain)

The Nevado–Filabride complex represents the basement and Permo-Triassic cover of the south Iberian passive margin, metamorphosed during Miocene continental subduction under the Alboran crustal domain. It forms a nappe stack with the thick Calar-Alto unit (6km thick) sandwiched between the Ragua unit below and the Bédar-Macael unit above. The nappe stacking occurred by ductile flow along basal syn-metamorphic shear zones. Structural analysis of the Calar-Alto unit together with thermobarometric data of samples from both the bottom and top of the unit served us to understand the tectonic significance of this episode of intracrustal shortening in the context of a strongly extended orogen. Multiequilibrium thermobarometry shows that the Calar-Alto schist underwent a prograde P-T path at mantle depths from 1.6GPa at 450–500°C to 1.0GPa at 550–570°C during the growth of the schistosity (S1), parallel to the compositional layering. After detachment from the downgoing slab and underplating, the unit underwent nearly isobaric cooling by up to 250°C from peak metamorphic conditions. The main crenulation cleavage (S2) and the mylonitic foliation (Sm) developed within the crustal nappe stack during decompression and heating. Sm at the base of the unit grew between 0.8GPa at 400°C and 0.4GPa at 500°C, while S2 at the top of the unit developed between 0.6GPa at 300°C and 0.2GPa at 400°C. These shortening structures developed simultaneously to the activity of overlying middle Miocene extensional detachments. Intracrustal shortening and coeval upper-crustal extension resulted in a mode of exhumation where the material rise entailed an important horizontal component of displacement at rates of approximately 15mm/year, being exhumed along an S-shaped P-T path after peak metamorphic conditions and its detachment from the subducting slab.

Gravitational instability of mantle lithosphere and core complexes

For a wide range of viscosity structures, convergent and downward flow of the mantle lithosphere during the growth of gravitational instability induces not only thickening of overlying crust but also concurrent horizontal extension in the upper crust. Such extension, if it occurred in the Earth, would include normal faulting of the upper crust above a region of horizontal shortening in the lower crust and uppermost mantle. Convergent flow in the lower crust would also create shear stress on horizontal planes and localized upward flow of the lower crust. These features—extension of upper crust and exhumation of strained lower crust—characterize metamorphic core complexes exposed in regions of normal to thick continental crust. Thus, convergent flow and downwelling mantle lithosphere might contribute to the development of core complexes, at least in some settings. If horizontal shortening and crustal thickening at depth do occur simultaneously with normal faulting at the surface of the Earth today, evidence of this process does not seem obvious, but perhaps it has occurred concurrently with widespread regional crustal extension in places like the Basin and Range Province, Tibet, the Pamir, or the Aegean. If such mantle flow does participate in the development of core complexes, a weak lower crust might not be a prerequisite for their formation.

Kinematics of Neogene to Recent upper-crustal deformation in the southern Central Andes (23°–28°S) inferred from fault–slip analysis: Evidence for gravitational spreading of the Puna Plateau

Elucidating the tectono-morphologic evolution of the Central Andes is important for understanding the mechanical behaviour of non-collisional orogens at convergent plate boundaries. Fault-kinematic data from the southern Central Andes are generally interpreted in terms of changes in plate (boundary) kinematics or vertical-axis rotation associated with the formation of the Andean orocline. However, none of these hypotheses accounts adequately for Neogene to Recent horizontal extension and vertical shortening of upper crust documented from the Puna Plateau and the adjacent Eastern Cordillera. Based on new and compiled fault-kinematic data, in total 4746 brittle faults at 317 stations, we propose that strain axis configurations in the southern Central Andes are controlled by (1) local deformation kinematics, (2) the far-field deformation caused either by the orogen-normal shortening component of plate convergence or by removal of lithospheric mantle and (3) differences in lithospheric thickness and respective surface elevation, i.e., gravitational potential energy. Specifically, the Puna Plateau and Eastern Cordillera have been undergoing horizontal shortening and gravitational spreading of the upper crust, whereas the topographically lower Pampean Ranges continue to undergo horizontal shortening only. Although less apparent in the fault-kinematic data than the E–W and NW–SE extension, orogen-parallel extension is inherent to the Puna Plateau and Eastern Cordillera and compensates likely for overall WNW–ESE shortening and enhanced surface elevation. The importance of orogen-parallel extension in the deformation kinematics of topographically elevated parts of the southern Central Andes is underscored by the strike–slip components on prominent reverse and thrust faults. The data supports geodynamic interpretations in which the evolution of topography and lithospheric thickness influence significantly changes in upper-crustal kinematics of the southern Central Andes.

Normal upper plate fault reactivation in northern Chile and the subduction earthquake cycle: From geological observations and static Coulomb Failure Stress (CFS) change

The northern Chile subduction zone constitutes a seismic gap where no great interplate earthquakes (Mw>8.8) have nucleated since 1877. Active normal faulting occurs conspicuously in the Coastal Forearc at the southern end of this gap, between 22.7°S and 24°S. There, upper crustal extension is present right above a suggested subduction segment boundary of the Nazca-South America interplate contact. In this work, by using Coulomb Failure Stress Change (CFS) models, we explore the effects of coseismic and interseismic stages of the subduction earthquake cycle on the reactivation of pre-existing upper plate normal faults in this area. We attempt to unravel the first-order interplate conditions required to explain this extensional pattern, putting specific focus on the influence of the megathrust segmentation defined by the Mejillones Peninsula. It is suggested that the restricted occurrence of active normal faulting in the study area is tightly related to the structure of the interplate seismogenic zone. Positive CFS values are induced in the study area by the coseismic and interseismic stages of the subduction earthquake cycle, in a region characterized by lower coupling beneath the Mejillones Peninsula. We propose that fault reactivation in this zone is particularly favored during the coseismic stage by earthquakes occurring offshore of the Mejillones Peninsula and surrounding areas. In the adjacent subduction segments, coseismic and interseismic CFS values tend to counterbalance each other, resulting in no tensional conditions capable to cause normal fault reactivation. We suggest that the coupling variation in the seismogenic zone is long term controlled by the geological structure of the upper plate.

Himalayan gneiss dome formation in the middle crust and exhumation by normal faulting: New geochronology of Gianbul dome, northwestern India

A general lack of consensus about the origin of Himalayan gneiss domes hinders accurate thermomechanical modeling of the orogen. To test whether doming resulted from tectonic contraction (e.g., thrust duplex formation, antiformal bending above a thrust ramp, etc.), channel flow, or via the buoyant rise of anatectic melts, this study investigates the depth and timing of doming processes for Gianbul dome in the western Himalaya. The dome is composed of Greater Himalayan Sequence migmatite, Paleozoic orthogneiss, and metasedimentary rock cut by multiple generations of leucogranite dikes. These rocks record a major penetrative D2 deformational event characterized by a domed foliation and associated NE-SW–trending stretching lineation, and they are flanked by the top-down-to-the-SW (normal-sense) Khanjar shear zone and the top-down-to-the-NE (normal sense) Zanskar shear zone (the western equivalent of the South Tibetan detachment system). Monazite U/Th-Pb geochronology records (1) Paleozoic emplacement of the Kade orthogneiss and associated granite dikes; (2) prograde Barrovian metamorphism from 37 to 33 Ma; (3) doming driven by upper-crustal extension and positive buoyancy of decompression melts between 26 and 22 Ma; and (4) the injection of anatectic melts into the upper levels of the dome—neutralizing the effects of melt buoyancy and potentially adding strength to the host rock—by ca. 22.6 Ma on the southwestern flank and ca. 21 Ma on the northeastern flank. As shown by a northeastward decrease in 40 Ar/ 39 Ar muscovite dates from 22.4 to 20.2 Ma, ductile normal-sense displacement within the Zanskar shear zone ended by ca. 22 Ma, after which the Gianbul dome was exhumed as part of a rigid footwall block below the brittle Zanskar normal fault, tilting an estimated 5°–10°SW into its present orientation.

The relationship between extension of lower crust and displacement of the shelf break

With deep sea petroleum explorations become more and more popular, some geological phenomena have emerged: extension of lower crust and upper crust is inhomogeneous; shelf break has been moved rapidly after crustal stretching. These geological phenomena are important to the continental margin evolution. To investigate the thinning of the whole crust and the contribution of the upper crust versus the lower crust to the crustal stretching since the Cenozoic, we calculated the stretching factors of the upper and the lower crust based on the 13 seismic lines in the Baiyun Sag from CNOOC. The results indicated that the whole crustal thickness decreases seaward while the whole crustal stretching factor increases from shelf to slope. Our calculations showed that the lower crustal stretching factor is higher than that of the upper crust in the Baiyun Sag. In the Cenozoic, deformation of the Baiyun Sag is controlled mainly by ductile shearing rather than brittle shearing. Based on the numerical modeling, we can conclude the initial crust in the Baiyun Sag is thermally attenuated. The stretching factor (β) of the lower crust increases from the north to the south of the continental margin, indicating two stretching centers: the Baiyun Sag and the Liwan Sag. The geometry of the shelf break and the β isoline trap have the similar trend in 23.8 and 13.8 Ma, both located in the intense deforming zone of the lower crust, and therefore we conclude the stretching and flowing of the lower crust cause the displacement of the shelf break before and after 23.8 Ma.

Fault-controlled sedimentation in a progressively opening extensional basin: the Palaeoproterozoic Vargfors basin, Skellefte mining district, Sweden

The Vargfors basin in the central part of the Skellefte mining district is an inverted sedimentary basin within a Palaeoproterozoic (1.89 Ga) marine volcanic arc. The fault-segmented basin formed from upper-crustal extension and subsequent compression, following a period of intense sub-marine volcanism and VMS ore formation. New detailed mapping reveals variations in stratigraphy attributed to syn-extensional sedimentation, as well as provenance of conglomerate clasts associated with tectonic activity at the transition from extension to compression. The onset of fan delta to alluvial fan sedimentation associated with basin subsidence indicates that significant dip-slip displacement accommodating rapid uplift of the intrusive complex and/or subsidence of the adjacent volcano-sedimentary domain took place along a major fault zone at the southern margin of the intrusive complex. Subsidence of the Jorn intrusive complex and/or its burial by sedimentary units caused a break in erosion of the intrusion and favoured the deposition of a tonalite clast-barren conglomerate. Clast compositions of conglomerates show that the syn-extensional deposits become younger in the south-eastern parts of the basin, indicating that opening of the basin progressed from north-west to south-east. Subsequent basin inversion, associated with the accretion to the Karelian margin, involved reverse activation of the normal faults and development of related upright synclines. Progressive crustal shortening caused the formation of break-back faults accompanied by mafic volcanic activity that particularly affected the southern contact of the Jorn intrusive complex and the northern contact of the Vargfors basin.

Upper‐crustal extension during oblique collision: the Temsamane extensional detachment (eastern Rif, Morocco)

Terra Nova, 24, 505–512, 2012AbstractThe subducted North Maghrebian passive margin was exhumed in the Tortonian (11–7 Ma) by an upper‐crustal brittle‐ductile extensional detachment and brittle low‐angle normal faults in a continental subduction transform setting. The Temsamane detachment in the eastern Rif is defined by a ductile shear zone approximately 100‐m thick with a low‐angle ramp geometry that cuts down into the Temsamane fold‐nappe stack. The shear zone shows south‐westward kinematics and separates lower‐greenschist (≈400 °C) metapelites of the Temsamane units below from the anchizone and diagenetic rocks (&lt;300–200 °C) of the Ketama‐Aknoul units above. To the east, the detachment becomes brittle, branching into a listric‐fan that cuts through 10–6 Ma sediments and volcanoclastics in the Tres Forcas cape. Both the North Maghrebian and the South Iberian subducted passive margins were exhumed in the Betic and Rif branches of the Gibraltar arc by SW‐directed brittle‐ductile detachments during the late Miocene in an oblique collisional setting.

Middle Miocene to early Pliocene oblique extension in the southern Gulf of California

A multichannel seismic (MCS) experiment spanning 600 km across the Alarcon Rise and its conjugate rifted margins in the southern Gulf of California (western North America) provides insight into the spatial and temporal evolution of extension between Baja California and the mainland (Mexico). Stratigraphic analysis of multiple rift basins within the Alarcon spreading corridor indicates an initial stage of oblique extension starting ca. 14–12 Ma. This initial phase of extension was characterized by the formation of several large basins in the center of the gulf and on the southeast margin with negligible synrift sedimentation. A second phase of oblique extension, likely synchronous with large-scale basin opening in the central and northern Gulf of California, began ca. 8–5 Ma and was characterized by the formation of smaller half-grabens distributed across the conjugate margins that contain both synrift and postrift deposits. A key feature imaged within the MCS data is a highly reflective, ropey layer at the top of basement, interpreted to be either volcanic rocks from the 25–12 Ma Comondu Group, and/or early rifting volcanic rocks that are between 11 and 9 Ma, or younger. This volcanic layer is extensively faulted, suggesting that it predates the episode of early extension. Upper crustal extension appears to be equally distributed across conjugate margins, forming a symmetrical continental rift. Two styles of rifted basin are observed; older basins (estimated as 14–11 Ma using sedimentation rates) show distributed extension with extensive basement faulting. In contrast, the younger basins (likely post–6 Ma) are asymmetrical with synrift deposits thickening into the basin-bounding faults. The northeast-southwest geomorphic expression of the Tamayo bank and trough and other features provides additional evidence that northwest-southeast oblique extension began ca. 12 Ma. These new spatial and temporal constraints, when combined with a crustal thickness profile obtained across the entire Alarcon corridor, suggest that significant northwest-southeast oblique extension within the Gulf of California started well before 6 Ma, in contrast to earlier models.