Ralf Hetzel

An active bivergent rolling-hinge detachment system: Central Menderes metamorphic core complex in western Turkey

Two symmetrically arranged detachment systems delimit the central Menderes metamorphic core complex and define a bivergent continental breakaway zone in the Anatolide belt of western Turkey. Structural analysis and apatite fission-track thermochronology show that a large east-trending syncline within the Alpine nappe stack in the central part of the orogen is related to late Miocene‐early Pliocene to recent core-complex formation. The syncline formed as a result of two opposite-facing rolling hinges in the footwalls of each of the two detachments. Back-rotation of the syncline limbs suggests that the detachments rotated from an initial dip of 408‐608 to a currently shallow orientation of 08‐208.

Slip rate variations on normal faults during glacial-interglacial changes in surface loads.

Geologic and palaeoseismological data document a marked increase in the slip rates of the Wasatch fault and three adjacent normal faults in the Basin and Range Province during the Late Pleistocene/Early Holocene epochs. The cause of this synchronous acceleration of fault slip and the subsequent clustering of earthquakes during the Holocene has remained enigmatic, although it has been suggested that the coincidence between the acceleration of slip and the shrinkage of Lake Bonneville after the Last Glacial Maximum may indicate a causal relationship. Here we use finite-element models of a discrete normal fault within a rheologically layered lithosphere to evaluate the relative importance of two competing processes that affect fault slip: postglacial unloading (the removal of mass), which decreases the slip rate, and lithospheric rebound, which promotes faster slip. We show that lithospheric rebound caused by regression of Lake Bonneville and deglaciation of adjacent mountain ranges provides a feasible mechanism for the high Holocene rates of faulting in the Wasatch region. Our analysis implies that climate-controlled changes in loads applied to Earths surface may exert a fundamental control on the slip history of individual normal faults.

Low slip rates and long-term preservation of geomorphic features in Central Asia.

In order to understand the dynamics of the India–Asia collision zone, it is important to know the strain distribution in Central Asia, whose determination relies on the slip rates for active faults. Many previous slip-rate estimates of faults in Central Asia were based on the assumption that offset landforms are younger than the Last Glacial Maximum (∼20 kyr ago). In contrast, here we present surface exposure ages of 40 to 170 kyr, obtained using cosmogenic nuclide dating, for a series of terraces near a thrust at the northern margin of the Tibetan Plateau. Combined with the tectonic offset, the ages imply a long-term slip rate of only about 0.35 mm yr-1 for the active thrust, an order of magnitude lower than rates obtained from the assumption that the terraces formed after the Last Glacial Maximum. Our data demonstrate that the preservation potential of geomorphic features in Central Asia is higher than commonly assumed.

Late Pleistocene/Holocene slip rate of the Zhangye thrust (Qilian Shan, China) and implications for the active growth of the northeastern Tibetan Plateau

luminescence dating, and 10 Be exposure dating. The seismically active Zhangye thrust transects late Pleistocene alluvial fan deposits and forms a prominent north facing scarp. The fault consists of two segments that differ in orientation, scarp height, and age. A series of loess-covered terraces records the uplift history of the western thrust segment. Loess accumulation on all terraces started at 8.5 ± 1.5 kyr and postdates terrace formation. Gravels from the

Precise U–Pb ages of syn-extensional Miocene intrusions in the central Menderes Massif, western Turkey

Western Turkey is an area which has experienced large-scale extension of continental crust. Here we report precise crystallization ages of two intrusions in the central Menderes Massif, the Turgutlu and Salihli granodiorites, using U–Pb dating. Both intrusions occur in the southern footwall of the seismically active Alasehir graben and were emplaced syntectonically in an extensional top-to-the-NNE shear zone which was active at retrograde greenschist-facies conditions. The U–Pb ages of 16.1 ± 0.2 Ma (monazite, Turgutlu granodiorite) and 15.0 ± 0.3 Ma (allanite, Salihli granodiorite) document that tectonic exhumation of middle-crustal rocks in the central Menderes Massif was already underway at the Early to Middle Miocene transition. Combined with published geochronological, structural and sedimentological data, the new U–Pb ages point to a continued extension since at least 16 Ma. There is no convincing evidence for a late Miocene/Pliocene phase of tectonic shortening.

The tectono-metamorphic evolution of gneiss complexes in the Middle Urals, Russia: a reappraisal

Abstract The Middle Urals are characterized by a major virgation in the linear trend of the Urals orogen, and represent the most highly contracted part of the late Palaeozoic collisional belt. This part of the orogen is dominated by metamorphic complexes and major fault and shear zones. The Main Uralian Fault zone (MUF), the east-dipping suture of the orogen containing low-grade metamorphic rocks, separates the Sysert Complex in the east from the Ufaley Complex in the west. The Sysert Complex in the hanging wall of the MUF consists of intensely deformed gneisses, granitic intrusions and a metamorphosed melange zone. Tectonic and isotopic investigations suggest the following stages for the evolution of the Sysert Complex: (a) pre-orogenic rifting and magmatism during Ordovician and Silurian times; (b) oceanic closure, island arc formation related to convergence and subduction during Devonian times; (c) major ductile deformation under amphibolite facies conditions related to NW-directed thrusting associated with crustal stacking during collision in Carboniferous times; (d) exhumation and contractional intracontinental tectonics during Permian times; and (e) closing of isotope systems related to cooling and the end of orogenic shortening through Triassic times. The Ufaley Complex, in the footwall of the MUF, is interpreted as an east-dipping crustal stack that records an amphibolite facies Uralian metamorphism. Lithologically the complex can be divided into pre-orogenic European basement (West Ufaley) and intensely deformed Palaeozoic metasediments and amphibolites (East Ufaley). High-pressure relics in the East Ufaley Complex are interpreted to be the result of subduction, whereas intense ductile deformation is related to overthrusting onto West Ufaley. The West Ufaley Complex is composed of gneisses, amphibolites, migmatites and granitic intrusions and has been thrust onto Devonian limestones along a major shear zone. In both Sysert and Ufaley Complexes, NW-trending stretching lineations and top-to-the-NW kinematic indicators suggest an oblique plate convergence with a significant sinistral component. The MUF is interpreted as a major normal fault that developed congruent with continental subduction and that compensated lithospheric thickening and the rapid exhumation of subducted crust in the footwall.

Subduction- and exhumation-related fabrics in the Paleozoic high-pressure–low-temperature Maksyutov Complex, Antingan area, southern Urals, Russia

We use structural and petrologic data from a cross section through the high-pressure‐lowtemperature Maksyutov Complex to develop a new model for its tectonometamorphic evolution. The Maksyutov Complex is located within the southern Urals, the only Paleozoic orogen that apparently preserved its collisional architecture without overprinting by late orogenic extensional deformation. The high-pressure complex constitutes a large antiform in the footwall of the east-dipping Main Uralian fault and is composed of two tectonometamorphic units. The core of the antiform exposes wellpreserved eclogites and blueschists in the structurally lower unit 1 that underwent peak metamorphic conditions of ~17 kbar and ~570 °C. In contrast, the structurally overlying unit 2 contains lawsonite-bearing assemblages indicating both lower peak pressure (<8 kbar) and temperature (<450 °C). Both units exhibit a composite foliation S 1 affected by northwestvergent F 2 folds. F 2 fold axes and S 1 /S 2 intersection lineations trend northeast-southwest, oblique to the present north-south trend of the Maksyutov antiform. The D 1 /D 2 fabrics record a progressive northwest-directed shearing under prograde metamorphic conditions and are interpreted as the result of eastward subduction beneath the Irendyk island arc during oblique northwest-southeast‐directed plate convergence at 370‐380 Ma. After their subduction to different depths, the structurally lower unit 1 was tectonically juxtaposed against the upper unit 2 by a ductile, top-to-the-northeast extensional D 3 shear zone associated with the retrograde metamorphic evolution. The exhumation of unit 1 occurred in Late Devonian‐Early Carboniferous time, during continuous plate convergence that was accommodated by a thrust that imbricates the basement of the East European platform and is situated below the high-pressure rocks. Further exhumation of the Maksyutov Complex to a shallow crustal level was accomplished by ductile D 4 shear zones exhibiting east-west‐trending stretching lineations present at the margins of the complex. Large-scale folding of the Maksyutov antiform and minor top-to-the-east backthrusting on the western limb took place during a late stage of the Uralian orogeny, coeval with formation of the foreland thrust-and-fold belt in Permian time.

Postglacial slip-rate increase on the Teton normal fault, northern Basin and Range Province, caused by melting of the Yellowstone ice cap and deglaciation of the Teton Range?

Along the eastern front of the Teton Range, Wyoming, prominent fault scarps offset Pinedale deposits by up to 30 m and document that multiple earthquakes ruptured the range-bounding Teton normal fault after the last glacial period. Paleoseismological data suggest that ~70% of the postglacial slip on the southern Teton fault accumulated during or shortly after deglaciation, before 8 ka. Here, we use a three-dimensional fi nite-element model to show that melting of the Yellowstone ice cap and the valley glaciers in the Teton Range may have caused the postglacial slip-rate increase on the Teton fault. During deglaciation, slip on our model fault accelerates by a factor of ~6 with respect to the long-term rate. Our model further shows that the impact of the melting Yellowstone ice cap on fault slip increases along-strike of the fault from south to north and is everywhere larger than the effect of the former valley glaciers in the Teton Range. The results demonstrate that postglacial slip on faults in glaciated regions may not have been uniform through time. Rather, a signifi cant fraction of slip may have accumulated within a few thousand years after the last glaciation. We hypothesize that the rebound caused by the Yellowstone ice cap has also triggered clusters of earthquakes on other normal faults in the surrounding Basin and Range Province.

21Ne versus 10Be and 26Al exposure ages of fluvial terraces: the influence of crustal Ne in quartz

Abstract The accuracy of 21 Ne surface exposure ages depends critically on the correction for a trapped Ne component. Commonly, the amount of cosmogenic Ne used to calculate 21 Ne exposure ages is considered to be the Ne excess relative to a trapped component of atmospheric composition ( 21 Ne/ 20 Ne=0.00296). Here, we document a trapped Ne component in quartz samples from a series of river terraces at the northern margin of Tibet [Hetzel et al., Nature 417 (2002) 428–432], which has a non-atmospheric 21 Ne/ 20 Ne ratio varying from 0.00299 to 0.00398. Vacuum crushing of amalgamated samples, each derived from 30–80 quartz clasts, revealed that the non-atmospheric trapped component is present in fluid inclusions. It has probably been incorporated into the quartz crystals during their growth in veins at low-grade metamorphic conditions. Only if the amount of cosmogenic 21 Ne is determined relative to this trapped component are the calculated 21 Ne exposure ages consistent with the relative ages of the tectonically induced terraces and in agreement with independent 10 Be and 26 Al exposure ages of the fluvial terraces. A significant in situ production of nucleogenic 21 Ne by the reaction 18 O(α,n) 21 Ne in the Paleozoic quartz minerals is ruled out by extremely low U contents of the order of 1–5 ppb. All ages have been corrected for an inherited cosmogenic component that results from cosmic ray exposure during erosion of the host rock and transport of the clasts to the terraces. The origin of the trapped component from crustal fluids is supported by high 40 Ar/ 36 Ar ratios of 2000–6000 and slightly elevated 136 Xe/ 132 Xe ratios relative to air, which can be explained by radioactive decay of 40 K and spontaneous fission of U in the crust.

Late Mozambique Belt structures in western Kenya and their influence on the evolution of the Cenozoic Kenya Rift

Abstract The N-S-trending Late Proterozoic Mozambique Belt in western Kenya is characterized by steep Edipping foliation, generated during orogen-parallel shearing in sinistral ductile shear zones (Barsaloian event, ∼580 Ma). During the final stages of orogenic evolution, two NW-trending brittle sinistral fault zones, crossing the present Elgeyo Escarpment, were active (Loldaikan event, 580-530 Ma). One of these fault zones generated well-developed pseudotachylytes that are described in detail. A later dextral reactivation of the two fault zones is related to the latest event (Kipsingian event, 530-470 Ma) of the Mozambique orogeny. Petrological, geophysical and geological data show that the Kenya Rift follows the trace of an important crustal boundary between the Archean Tanzania Craton and the Proterozoic Mozambique Belt. Miocene extensional reactivation of the steep E-dipping ‘Barsaloian’ foliations at the Elgeyo and Nguruman Escarpments led to the formation of asymmetric rift basins bounded by E-dipping normal faults along the western rift margin. The brittle fault zones at the Elgeyo Escarpment, trending obliquely to the basement foliation are responsible for segmentation of the Elgeyo border fault and the abrupt change in the orientation of the northern Kenya Rift.