Abstract The effects of Arabia-Eurasia collision are recorded in faults, basins, and exhumed metamorphic massifs across eastern and central Anatolia. These faults and basins also preserve evidence of major changes in deformation and associated sedimentary processes along major suture zones including the Inner Tauride suture where it lies along the southern (Ecemiş) segment of the Central Anatolian fault zone. Stratigraphic and structural data from the Ecemiş fault zone, adjacent NE Ulukışla basin, and metamorphic dome (Niğde Massif) record two fundamentally different stages in the Cenozoic tectonic evolution of this part of central Anatolia. The Paleogene sedimentary and volcanic strata of the NE Ulukışla basin (Ecemiş corridor) were deposited in marginal marine to marine environments on the exhuming Niğde Massif and east of it. A late Eocene–Oligocene transpressional stage of deformation involved oblique northward thrusting of older Paleogene strata onto the eastern Niğde Massif and of the eastern massif onto the rest of the massif, reburying the entire massif to >10 km depth and accompanied by left-lateral motion on the Ecemiş fault zone. A profound change in the tectonic setting at the end of the Oligocene produced widespread transtensional deformation across the area west of the Ecemiş fault zone in the Miocene. In this stage, the Ecemiş fault zone had at least 25 km of left-lateral offset. Before and during this faulting episode, the central Tauride Mountains to the east became a source of sediments that were deposited in small Miocene transtensional basins formed on the Eocene–Oligocene thrust belt between the Ecemiş fault zone and the Niğde Massif. Normal faults compatible with SW-directed extension cut across the Niğde Massif and are associated with a second (Miocene) re-exhumation of the Massif. Geochronology and thermochronology indicate that the transtensional stage started at ca. 23–22 Ma, coeval with large and diverse geological and tectonic changes across Anatolia.

Abstract The plate interface undergoes two transitions between seismogenic depths and subarc depths. A brittle-ductile transition at 20–50 km depth is followed by a transition to full viscous coupling to the overlying mantle wedge at ∼80 km depth. We review evidence for both transitions, focusing on heat-flow and seismic-attenuation constraints on the deeper transition. The intervening ductile shear zone likely weakens considerably as temperature increases, such that its rheology exerts a stronger control on subduction-zone thermal structure than does frictional shear heating. We evaluate its role through analytic approximations and two-dimensional finite-element models for both idealized subduction geometries and those resembling real subduction zones. We show that a temperature-buffering process exists in the shear zone that results in temperatures being tightly controlled by the rheological strength of that shear zone’s material for a wide range of shear-heating behaviors of the shallower brittle region. Higher temperatures result in weaker shear zones and hence less heat generation, so temperatures stop increasing and shear zones stop weakening. The net result for many rheologies are temperatures limited to ≤350–420 °C along the plate interface below the cold forearc of most subduction zones until the hot coupled mantle is approached. Very young incoming plates are the exception. This rheological buffering desensitizes subduction-zone thermal structure to many parameters and may help explain the global constancy of the 80 km coupling limit. We recalculate water fluxes to the forearc wedge and deep mantle and find that shear heating has little effect on global water circulation.

Abstract The Lake Malawi (Nyasa) Rift, in the East African Rift System (EARS), is an ideal modern analogue for the study of extensional tectonic systems in low strain rate settings. The seismically active rift contains the 700-m-deep Lake Malawi, one of the world’s oldest and largest freshwater lakes with one of the most diverse endemic faunal assemblages on Earth. Modern and reprocessed legacy multichannel seismic-reflection data are constrained by velocity information from a wide-angle seismic experiment to evaluate variability in extension, segmentation, and timing of fault development along the 550-km-long rift zone. Fault geometries and patterns of synrift sediment fills show that the Lake Malawi Rift is composed of three asymmetric rift segments, with intervening accommodation zone morphologies controlled by the degree of overlap between segment border faults. Most extension occurs on the basin border faults, and broadly distributed extension is only observed at one accommodation zone, where no border fault overlap is observed. Structural restorations indicate a weakly extended rift system (∼7 km), with diminishing values of extension and thinner rift fill from north to south, suggesting a progressively younger rift to the south. There is no evidence of diking, sill injection, or extrusives within the synrift fill of the Lake Malawi Rift, although the volcanic load of the Rungwe magmatic system north of the lake and related subsidence may explain the presence of anomalously thick synrift fill in the northernmost part of the lake. The thickest synrift depocenters (∼5.5 km) are confined to narrow 10- to 20-km-wide zones adjacent to each rift segment border fault, indicating concentration of strain on border faults rather than intrarift faults. Intrarift structures control axial sediment delivery in the North and Central rift segments, focusing sediment into confined areas resulting in localized overpressure and shale diapirs. The asymmetric, basement-controlled relief was established early in rift development. When overprinted with frequent high-amplitude hydroclimate fluctuations, which are well documented for this basin, the resulting highly variable landscape and lake morphometry through time likely impacted the diverse endemic faunas that evolved within the basin. New seismic-reflection data, augmented by wide-angle seismic data and age constraints from drill core, offer the most highly resolved 3D view to date of latest Cenozoic extensional deformation in East Africa and provide a foundation for hazards analysis, resource assessments, and constraining deformation in a low strain rate, magma-poor active rift.

Abstract Sandy trench-fill sediments at accretionary margins are commonly scraped off at the frontal wedge and rarely subducted to the depth of high-pressure (HP) metamorphism. However, some ancient exhumed accretionary complexes are associated with high-pressure–low-temperature (HP-LT) metamorphic rocks, such as psammitic schists, which are derived from sandy trench-fill sediments. This study used sandbox analogue experiments to investigate the role of seafloor topography in the transport of trench-fill sediments to depth during subduction. We conducted two different types of experiments, with or without a rigid topographic high (representing a seamount). We used an undeformable backstop that was unfixed to the side wall of the apparatus to allow a seamount to be subducted beneath the overriding plate. In experiments without a seamount, progressive thickening of the accretionary wedge pushed the backstop down, leading to a stepping down of the décollement, narrowing of the subduction channel, and underplating of the wedge with subducting sediment. In contrast, in experiments with a topographic high, the subduction of the topographic high raised the backstop, leading to a stepping up of the décollement and widening of the subduction channel. These results suggest that the subduction of stiff topographic relief beneath an inflexible overriding plate might enable trench-fill sediments to be deeply subducted and to become the protoliths of HP-LT metamorphic rocks.

Abstract The Miocene Columbia River Basalt Group (CRBG) is the youngest and best studied continental flood basalt province on Earth. The 210,000 km3 of basaltic lava flows in this province were fed by a series of dike swarms, the largest of which is the Chief Joseph dike swarm (CJDS) exposed in northeastern Oregon and southwestern Washington. We present and augment an extensive data set of field observations, collected by Dr. William H. Taubeneck (1923–2016; Oregon State University, 1955–1983); this data set elucidates the structure of the CJDS in new detail.The large-scale structure of the CJDS, represented by 4279 mapped segments mostly cropping out over an area of 100 × 350 km2, is defined by regions of high dike density, up to ∼5 segments/km−2 with an average width of 8 m and lengths of ∼100–1000 m. The dikes in the CJDS are exposed across a range of paleodepths, from visibly feeding surface flows to ∼2 km in depth at the time of intrusion. Based on extrapolation of outcrops, we estimate the volume of the CJDS dikes to be 2.5 × 102–6 × 104 km3, or between 0.1% and 34% of the known volume of the magma represented by the surface flows fed by these dikes. A dominant NNW dike segment orientation characterizes the swarm. However, prominent sub-trends often crosscut NNW-oriented dikes, suggesting a change in dike orientations that may correspond to magmatically driven stress changes over the duration of swarm emplacement. Near-surface crustal dilation across the swarm is ∼0.5–2.7 km to the E-W and ∼0.2–1.3 km to the N-S across the 100 × 350 km region, resulting in strain across this region of 0.4%–13.0% E-W and 0.04%–0.3% N-S. Host-rock partial melt is rare in the CJDS, suggesting that only a small fraction of dikes were long-lived.

Abstract Four areas (Loriu, Lojamei, Muranachok-Muruangapoi, Kamutile Hills) of well-developed Miocene-age dikes in the northern Kenya Rift (Turkana, Kenya) have been identified from fieldwork and satellite images; in total, >3500 dikes were mapped. Three areas display NNW-SSE– to N-S–oriented dike swarms, with straight, radial, and concentric patterns in zones <15 km long, and indicate NNW-SSE to N-S regional maximum horizontal principal stress (SHmax) directions in the early to middle Miocene. Individual dikes are typically <2 m wide and tens to hundreds of meters long and have accommodated <2% extension. In places (Loriu, Lojamei, Lokhone high), dikes trend at a high angle to the rift trend, suggesting some local influence (e.g., overpressured magma chamber, cracked lid–style dike intrusions over a sill or laccolith, preexisting fabric in basement) on orientation, in addition to the influence from regional stresses. Only a minor influence by basement fabrics is seen on dike orientation. The early- to middle-Miocene dikes and extrusive activity ended a long phase (up to 25 m.y.) of amagmatic half-graben development in central Kenya and southern Turkana, which lay on the southern edge of the early (Eocene–Oligocene) plume activity. The Miocene dike sets and extension on major border faults in Turkana contrast with larger, more extensive arrays of dikes in evolved systems in the Main Ethiopian Rift that are critical for accommodating crustal extension. By the Pliocene–Holocene, magmatism and intrusion along dikes had become more important for accommodating extension, and the tectonic characteristics began to resemble those of rift basins elsewhere in the eastern branch of the East African Rift.

Abstract The nexus of plate-boundary deformation at the northern end of the Coachella Valley in southern California (USA) is complex on multiple levels, including rupture dynamics, slip transfer, and three-dimensional strain partitioning on nonvertical faults (including the San Andreas fault). We quantify uplift of mountain blocks in this region using geomorphology and low-temperature thermochronometry to constrain the role of long-term vertical deformation in this tectonic system. New apatite (U-Th)/He (AHe) ages confirm that the rugged San Jacinto Mountains (SJM) do not exhibit a record of rapid Neogene exhumation. In contrast, in the Little San Bernardino Mountains (LSBM), rapid exhumation over the past 5 m.y. is apparent beneath a tilted AHe partial retention zone, based on new and previously published data. Both ranges tilt away from the Coachella Valley and have experienced minimal denudation from their upper surface, based on preservation of weathered granitic erosion surfaces. We interpret rapid exhumation at 5 Ma and the gentle tilt of the erosion surface and AHe isochrons in the LSBM to have resulted from rift shoulder uplift associated with extension prior to onset of transpression in the Coachella Valley. We hypothesize that the SJM have experienced similar rift shoulder uplift, but an additional mechanism must be called upon to explain the pinnacle-like form, rugged escarpment, and topographic disequilibrium of the northernmost SJM massif. We propose that this form stems from erosional resistance of the Peninsular Ranges batholith relative to more-erodible foliated metamorphic rocks that wrap around it. Our interpretations suggest that neither the LSBM nor SJM have been significantly uplifted under the present transpressive configuration of the San Andreas fault system, but instead represent relict highs due to previous tectonic and erosional forcing.

Abstract The assembly of Rodinia involved the collision of eastern Laurentia with southwestern Amazonia at ca. 1 Ga. The tectonostratigraphic record of the central Andes records a gap of ∼300 m.y. between 1000 Ma and 700 Ma, i.e., from the beginning of the Neoproterozoic Era to the youngest part of the Cryogenian Period. This gap encompasses the time of final assembly and breakup of the Rodinia supercontinent in this region.We present new petrographic and whole-rock geochemical data and U-Pb ages combined with Hf isotope data of detrital zircons from the volcanosedimentary Chilla beds exposed on the Altiplano southwest of La Paz, Bolivia. The presence of basalt to andesite lavas and tuffs of continental tholeiitic affinity provides evidence of a rift setting for the volcanics and, by implication, the associated sedimentary rocks. U-Pb ages of detrital zircons (n = 124) from immature, quartz-intermediate sandstones have a limited range between 1737 and 925 Ma. A youngest age cluster (n = 3) defines the maximum depositional age of 925 ± 12 Ma. This is considered to coincide with the age of deposition because Cryogenian and younger ages so typical of Phanerozoic units of this region are absent from the data.The zircon age distribution shows maxima between 1300 and 1200 Ma (37% of all ages), the time of the Rondônia–San Ignacio and early Sunsás (Grenville) orogenies in southwestern Amazonia. A provenance mixing model considering the Chilla beds, Paleozoic Andean units, and data from eastern Laurentia Grenville sources shows that >90% of the clastic input was likely derived from Amazonia. This is also borne out by multidimensional scaling (MDS) analysis of the data.We also applied MDS analysis to combinations of U-Pb age and Hf isotope data, namely εHf(t) and 176Hf/177Hf values, and demonstrate again a very close affinity of the Chilla beds detritus to Amazonian sources. We conclude that the Chilla beds represent the first and hitherto only evidence of Rodinia breakup in Tonian time in Andean South America.

Abstract The Northern Sierra terrane is one of a series of Paleozoic terranes outboard of the western Laurentian margin that contain lithotectonic elements generally considered to have originated in settings far removed from their present relative locations. The Lower to Middle Paleozoic Shoo Fly Complex makes up the oldest rocks in the terrane and consists partly of thrust-imbricated deep-marine sedimentary strata having detrital zircon age signatures consistent with derivation from the northwestern Laurentian margin. The thrust package is structurally overlain by the Sierra City mélange, which formed within a mid-Paleozoic subduction zone and contains tectonic blocks of Ediacaran tonalite and sandstone with Proterozoic to early Paleozoic detrital zircon populations having age spectra pointing to a non–western Laurentian source. Island-arc volcanic rocks of the Upper Devonian Sierra Buttes Formation unconformably overlie the Shoo Fly Complex and are spatially associated with the Bowman Lake batholith, Wolf Creek granite stock, and smaller hypabyssal felsic bodies that intrude the Shoo Fly Complex. Here, we report new results from U-Pb sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) dating of 15 samples of the volcanic and intrusive rocks, along with geochemical studies of the dated units. In addition, we report U-Pb laser ablation–inductively coupled plasma–mass spectrometry ages for 50 detrital zircons from a feldspathic sandstone block in the Sierra City mélange, which yielded abundant Ordovician to Early Devonian (ca. 480–390 Ma) ages. Ten samples from the composite Bowman Lake batholith, which cuts some of the main thrusts in the Shoo Fly Complex, yielded an age range of 371 ± 9 Ma to 353 ± 3 Ma; felsic tuff in the Sierra Buttes Formation yielded an age of 363 ± 7 Ma; and three felsic hypabyssal bodies intruded into the Sierra City mélange yielded ages of 369 ± 4 Ma to 358 ± 3 Ma. These data provide a younger age limit for assembly of the Shoo Fly Complex and indicate that arc magmatism in the Northern Sierra terrane began with a major pulse of Late Devonian (Famennian) igneous activity. The Wolf Creek stock yielded an age of 352 ± 3 Ma, showing that the felsic magmatism extended into the early Mississippian. All of these rocks have similar geochemical features with arc-type trace-element signatures, consistent with the interpretation that they constitute a petrogenetically linked volcano-plutonic system. Field evidence shows that the felsic hypabyssal intrusions in the Sierra City mélange were intruded while parts of it were still unlithified, indicating that a relatively narrow time span separated subduction-related deformation in the Shoo Fly Complex and onset of Late Devonian arc magmatism. Following recent models for Paleozoic terrane assembly in the western Cordillera, we infer that the Shoo Fly Complex together with strata in the Roberts Mountains allochthon in Nevada migrated south along a sinistral transform boundary prior to the onset of arc magmatism in the Northern Sierra terrane. We suggest that the Shoo Fly Complex arrived close to the western Laurentian margin at the same time as the Roberts Mountains allochthon was thrust over the passive margin during the Late Devonian–early Mississippian Antler orogeny. This led to a change in plate kinematics that caused development of a west-facing Late Devonian island arc on the Shoo Fly Complex. Due to slab rollback, the arc front migrated onto parts of the Sierra City mélange that had only recently been incorporated into the accretionary complex. In the mélange, blocks of Ediacaran tonalite, as well as sandstones having detrital zircon populations with non–western Laurentian sources, may have been derived from the Yreka and Trinity terranes in the eastern Klamath Mountains, where similar rock types occur. If so, this suggests that these Klamath terranes were in close proximity to the developing accretionary complex in the Northern Sierra terrane in the Late Devonian.