Tectonics

We present estimates of slip rates for active faults in the External Dinarides. This thrust‐and‐fold belt formed in the Adria‐Eurasia collision zone by the progressive formation of NE‐dipping thrusts in the footwalls of older structures. We calculated the long‐term horizontal velocity field, slip rates and related uncertainties for active faults using a thin‐shell finite element method. We incorporated active faults with different effective fault frictions, rheological properties, appropriate geodynamic boundary conditions, laterally varying heat flow and topography. The results were obtained by comparing the modeled maximum compressive horizontal stress orientations with the World Stress Map database. The calculated horizontal velocities decrease from the southeastern External Dinarides to the northwestern parts of the thrust‐and‐fold belt. This spatial pattern is also evident in the long‐term slip rates of active faults. The highest slip rate was obtained for the Montenegro active fault, while the lowest rates were obtained for active faults in northwestern Slovenia. Low slip rates, influenced by local active diapirism, are also characteristic for active faults in the offshore central External Dinarides. These findings are contradictory to the concept of Adria as an internally rigid, aseismic lithospheric block because the faults located in its interior release a part of the regional compressive stress. We merged the modeling results and available slip rate estimates to obtain a composite solution for slip rates.

We present the first thermal model for the lithosphere in Turkey, which shows a highly heterogeneous pattern associated with mosaics of the Tethyan and modern subduction systems. We calculate a regionally average crustal density of 2.90 g/cm³ consistent with the presence of large volumes of mafic material. The Moho temperature with a regionally average value of 650–850 °C shows strong short‐wavelength variations. Lithosphere thinning to 50–75 km in most of western Anatolia may have developed in response to the Hellenic slab rollback, while the Neoproterozoic block in the Menderes Massif preserves a 150 km deep lithosphere root. In central Anatolia, the lithosphere thickness decreases southward from 100–150 to 50–60 km along a linear belt of young basaltic volcanism, followed by a belt of a 150 km thick lithosphere. We interpret this characteristic pattern by a SE dipping paleoslab beneath the western Taurides, which may cause the Cyprus subduction melting zone to deviate toward NW and NE. The Eastern Pontides‐Lesser Caucasus have 150–200 km thick lithosphere roots caused by collisional tectonics. The East Anatolian Plateau is underlain by a 80–140 km thick lithosphere, which suggests the presence of significant continental fragments; the patchy pattern of its thermal heterogeneity may be explained by teared and fragmented Tethyan slabs. A poor correlation between the lithosphere thermal structure, heat flux, the Neogene volcanic regions, and mantle seismic velocities implies that seismic anomalies are essentially controlled by heterogeneous mantle hydration by subduction systems of different ages and cannot be explained by temperature variations alone.

The widely cited Sibson‐Scholz conceptual fault zone model suggests that seismically active, upper crustal brittle faults pass downward across a predominantly thermally controlled transition at 10–15 km depth into ductile shear zones in which deformation occurs by aseimic viscous creep. The crustal‐scale Outer Hebrides Fault Zone (OHFZ) in NW Scotland has been described as the type example of such a continental fault zone. It cuts Precambrian basement gneisses and is deeply exhumed, allowing direct study of the deformation products and processes that occur across a wide range of crustal depths. A number of fault rock assemblages are recognized to have formed during a long‐lived displacement history lasting in excess of 1000 Myr. During Caledonian movements that are recognized along much of the 190 km onshore fault trace, brittle, cataclasite‐bearing faults in the west of the OHFZ are unequivocally overprinted to the east by a younger fabric related to a network of ductile shear zones. Field observations and regional geochronological data demonstrate that there is no evidence for reheating of the fault zone due to thrust‐related crustal thickening or shear heating. Microstructural observations show that the onset of viscous deformation was related to a major influx of hydrous fluids. This led to retrogression, with the widespread development of new finegrained phyllosilicate‐bearing fault rocks (“phyllonites”), and the onset of fluid‐assisted, grain size‐sensitive diffusional creep in the most highly deformed and altered parts of the fault zone. Phyllonitic fault rocks also occur in older, more deeply exhumed parts of the fault zone, implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature (depth) range within crustal‐scale faults. Our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal‐scale faults containing interconnected networks of phyllosilicate‐bearing fault rocks will be characterized by long‐term relative weakness and shallow (∼5 km) frictional‐viscous transition zones. Similar processes acting at depth may provide an explanation for the apparent weakness of presently active structures such as the San Andreas Fault.

The Arabian passive margin formed at the southern margin of the Neo‐Tethys ocean during the breakup of Pangea. In the Lurestan region of the Zagros mountain belt, the deformed Arabian continental paleo‐margin can be reconstructed as originally consisting of distinct crustal domains, including a proximal sector and a distal continental ribbon, separated by a deep‐water trough, known as the Radiolarite Basin. Such an architecture was shaped by the continental rifting process, thus reflecting timing and style of continent separation, which is generally assumed to have occurred during the Permo‐Triassic interval. This study reports evidence of syn‐sedimentary extensional faults, unconformities, and facies changes in the Mesozoic stratigraphic succession of the Lurestan region, which point to a major Jurassic extensional pulse. In detail, extension reached its climax at the end of the Early Jurassic, when tectonically driven drowning of the long‐lived Triassic to Early Jurassic carbonate platform led to the transition from shallow‐ to deep‐water environments in large areas of the inner margin, coevally with the development of the Radiolarite Basin. Our findings suggest a two‐step continental rifting in this area, with the first Permo‐Triassic phase predating an Early Jurassic one.

The cratonic Sichuan Basin is located east of the Tibetan Plateau, and is surrounded by mountains that have undergone complex deformation and uplift since the Cenozoic. Imaging mantle structure is important for understanding its formation, and to date most models suggest a deep cratonic root underlies the basin, blocking eastward extrusion of lithospheric material beneath Tibet. Here, we obtain detailed upper mantle structure from teleseismic tomography in the region utilizing travel time data from earthquakes recorded at 506 seismic stations, including 25 new stations in the poorly sampled Sichuan Basin. Contrasting to previous models, we show eastward and southeastward dipping high‐velocity anomalies extending eastward ∼150–400 km into the upper mantle from the Sichuan Basin. We suggest, the southeastward subduction of the Yangtze Block occurred in the Mesozoic and may be reactivated in the Cenozoic, with the relatively thin and weak lithosphere to the east of the Sichuan Basin prone to deformation in response to the eastward growth of the Tibetan Plateau. A west‐dipping high‐velocity anomaly beneath eastern Tibet is interpreted as delaminated lithosphere. This delamination may accelerate the development of the Xianshuihe fault zone and the horizontal extrusion of the Tibetan Plateau. Beneath the East Qinling orogen, the eastward extrusion of the plateau material is not obvious suggesting limited horizontal lithospheric extrusion is present north of the Sichuan Basin.

The Xianshuihe (XSH) fault in eastern Tibet is one of the most active faults in China, with the next large earthquake most likely to occur along its SE part, where the fault splits into three parallel branches: Yalahe, Selaha and Zheduotang (ZDT). Precisely quantifying their slip rates at various timescales is essential to evaluate regional earthquake hazard. Here, we expand our previous work on the Selaha fault to the nearby ZDT and Moxi (MX) faults, and add observations on the Yalahe fault and on the newly discovered Mugecuo South fault zone. Using tectonic‐geomorphology approaches with¹⁰Be dating, we had previously determined average late Quaternary slip rates of 9.75 ± 0.15 and 4.4 ± 0.5 mm/yr along the NW and SE Selaha fault, respectively. Using the same methods here, we determine a slip rate of 3.4–4.8 mm/yr on the ZDT fault and of 9.6–13.4 mm/yr on the MX fault. This is consistent with the southeastward slip rate increase we had proposed along the XSH fault system from 6‐8 mm/yr (Ganzi fault) to ∼10 mm/yr (Selaha fault), and >9.6 mm/yr (MX fault). We propose a new model for the SE XSH fault, where the large‐scale Mugecuo pull‐apart basin lies within an even larger scale compressive uplift zone in a restraining bend of the XSH fault, where the highest peak in eastern Tibet is located (Gongga Shan, 7,556 m). Our slip rate determination helps to constrain a relatively high regional M_w ∼ 7 earthquake hazard at present on the SE XSH fault.

Lithospheric extension is sited, preferentially, along orogenic belts because they have a thicker continental crust, contain structural inhomogeneities, and suffer extensional orogenic collapse caused by body forces resulting from isostatically compensated elevation and sharp elevation gradients. Collapse occurs especially where rapid advective thinning of the shortened thermal boundary conduction layer occurs beneath an orogen and causes rapid uplift. Where boundary forces are compressional, extension is balanced by radial thrusting to form oroclinal loops around collapsed extensional basins. Where, as in the disruption of Pangea, boundary forces change rapidly from compressional to tensional, body force collapse is continued by general extension which may lead to continental splitting. Even where overall convergence is continuing, orogenic collapse may be enhanced by subduction rollback into small remnant oceans. The extensional collapse of orogens offers a partial explanation for why oceans cyclically close and reopen in roughly the same places, preservation of very high pressure metamorphic rocks, for the return of orogenic large crustal thicknesses to normal without very much erosional denudation with the widespread preservation of supracrustal sequences, high temperature metamorphic assemblages and the minimum‐melting granite suite.

The External Betic‐Rif arc, which lies between the converging African and Iberian plates, is one of the tightest orogenic arcs on Earth. It is a thin‐skinned fold and thrust belt formed in Miocene time around the periphery of the Alborán Domain, an older contractional orogen that underwent extensional collapse coevally with the formation of the thrust belt. Restoration of four sections across the thrust belt, together with kinematic and paleomagnetic analysis, allows a reconstruction of the prethrusting geometry of the Alborán Domain, and the identification of the following processes that contributed to the formation of the arc: (1) The Alborán Domain moved some 250 km westward relative to Iberia and Africa during the Miocene. This initiated the two limbs of the arc on its NW and SW margins, closing to the WSW in the region of Cherafat in northern Morocco. The overall convergence direction on the Iberian side of the arc was between 310° and 295°, and on the African side it was between 235° and 215°. The difference in convergence direction between the two sectors was primarily a result of the relative motion between Africa and Iberia. (2) Extensional collapse of the Alborán Domain during the Miocene modified the geometry of the western end of the arc: the Internal Rif rotated anticlockwise to form the present north trending sector of the arc, and additional components of displacement produced by extension were transferred into the external thrust belt along a series of strike‐slip faults and shear zones. These allowed the limbs of the arc to rotate and extend, tightening the arc, and creating variations in the amounts and directions of shortening around the arc. The Betic sector of the arc rotated clockwise by 25° during this process, and the southern Rif rotated anticlockwise by ∼55°. (3) Oblique convergence on the two limbs of the arc, dextral in the Betics and sinistral in the southern Rif, resulted in strongly noncoaxial deformation. This had three related effects: (1) large rotations of individual thrust sheets resulted from the oblique propagation of thrusts away from the thrust front, followed by pinning and rotation as the thrust sheets peeled off, (2) continued oblique convergence resulted in distributed shear, particularly in the rear of the thrust wedge, causing rotation of stacks of thrust sheets on the scale of a few tens of kilometers, and (3) distributed shear in the orogen resulted in the rotation of folds as they amplified, the hinges migrating through the rock body, and rotating at a slower rate than the rock. These rotations were substantially larger than the bulk rotations of the limbs of the arc, and they strongly modified the orientations of folds, thrust traces, and the structural indicators of fault slip directions.

In recent years, the origin of the Betic‐Rif orocline has been the subject of considerable debate. Much of this debate has focused on mechanisms required to generate rapid late‐orogenic extension with coeval shortening. Here we summarize the principal geological and geophysical observations and propose a model for the Miocene evolution of the Betic‐Rif mountain belts, which is compatible with the evolution of the rest of the western Mediterranean. We regard palaeomagnetic data, which indicate that there have been large rotations about vertical axes, and earthquake data, which show that deep seismicity occurs beneath the Alboran Sea, to be the most significant data sets. Neither data set is satisfactorily accounted for by models which invoke convective removal or delamination of lithospheric mantle. Existing geological and geophysical observations are, however, entirely consistent with the existence of a subduction zone which rolled or peeled back until it collided with North Africa. We suggest that this ancient subducting slab consequently split into two fragments, one of which has continued to roll back, generating the Tyrrhenian Sea and forming the present‐day Calabrian Arc. The other slab fragment rolled back to the west, generating the Alboran Sea and the Betic‐Rif orocline during the early to middle Miocene.

The western Mediterranean subduction zone (WMSZ) extends from the northern Apennine to southern Spain and turns around forming the narrow and tight Calabrian and Gibraltar Arcs. The evolution of the WMSZ is characterized by a first phase of orogenic wedging followed, from 30 Ma on, by trench retreat and back‐arc extension. Combining new and previous geological data, new tomographic images of the western Mediterranean mantle, and plate kinematics, we describe the evolution of the WMSZ during the last 35 Myr. Our reconstruction shows that the two arcs form by fragmentation of the 1500 km long WMSZ in small, narrow slabs. Once formed, these two narrow slabs retreat outward, producing back‐arc extension and large scale rotation of the flanks, shaping the arcs. The Gibraltar Arc first formed during the middle Miocene, while the Calabrian Arc formed later, during the late Miocene‐Pliocene. Despite the different paleogeographic settings, the mechanism of rupture and backward migration of the narrow slabs presents similarities on both sides of the western Mediterranean, suggesting that the slab deformation is also driven by lateral mantle flow that is particularly efficient in a restricted (upper mantle) style of mantle convection.