Episodes Of Tectonic Activity Research Articles

Tectonic Evolution of the SE West Siberian Basin (Russia): Evidence from Apatite Fission Track Thermochronology of Its Exposed Crystalline Basement

The West Siberian Basin (WSB) is one of the largest intracratonic Meso-Cenozoic basins in the world. Its evolution has been studied over the recent decades; however, some fundamental questions regarding the tectonic evolution of the WSB remain unresolved or unconfirmed by analytical data. A complete understanding of the evolution of the WSB during the Mesozoic and Cenozoic eras requires insights into the cooling history of the basement rocks as determined by low-temperature thermochronometry. We presented an apatite fission track (AFT) thermochronology study on the exposed parts of the WSB basement in order to distinguish tectonic activation episodes in an absolute timeframe. AFT dating of thirteen basement samples mainly yielded Cretaceous cooling ages and mean track lengths varied between 12.8 and 14.5 μm. Thermal history modeling based on the AFT data demonstrates several Mesozoic and Cenozoic intracontinental tectonic reactivation episodes affected the WSB basement. We interpreted the episodes of tectonic activity accompanied by the WSB basement exhumation as a far-field effect from tectonic processes acting on the southern and eastern boundaries of Eurasia during the Mesozoic–Cenozoic eras.

Magnetostratigraphy of deep drilling core 15YZK01 in the northwestern Qaidam Basin (NE Tibetan Plateau): Tectonic movement, salt deposits and their link to Quaternary glaciation

The thick Cenozoic sediments in the Qaidam Basin contain considerable information about the uplift of Tibetan Plateau, climate change and salt deposits. In this paper, a palaeomagnetic study was performed on the 1400-m-long drilling core 15YZK01 from the Yiliping playa in the northwestern Qaidam Basin to obtain a timescale documenting this information. The polarity sequence consisted of 17 pairs of normal and reversed zones and can be correlated well with subchrons C2Ar-C1n of the geomagnetic polarity timescale GPTS 2012 (from ca. 4.1 to 0.002 Ma). Three periods of high sedimentation rates occurred at ca. 4.1–3.207 Ma, 2.581–1.945 Ma, and 0.781–0.002 Ma. These 3 periods correspond to 4 episodes of tectonic activity on the Tibetan Plateau: the Qingzang Movement (3.6 Ma), the B phase of the Qingzang Movement (2.6 Ma), the Kunlun-Huanghe Movement (1.1–0.7 Ma) and the Gonghe Movement (0.15 Ma). The dips of the formations in the drilling core indicate that the Yiliping area was uplifted at ca. 3.236 Ma. After this uplift, the sedimentation rate sharply decreased from 39.1 cm/ka to 19.2 cm/ka. In the Pliocene, the Yiliping area mainly featured a freshwater environment, and the earliest gypsum sediment formed at ca. 3.530 Ma after the Qingzang Movement. After the B phase of the Qingzang Movement, the Yiliping area alternated between freshwater and semi-saline lake environments, and 20 layers of gypsum-bearing strata began to appear since 2.103 Ma. After the Kunlun-Huanghe Movement, the Yiliping area alternated between semi-saline and saline lake environments, and salt deposits began to form at 0.733 Ma or 0.746 Ma in the middle Pleistocene. Four salt-forming periods in the Yiliping area can be correlated with Quaternary glaciations on the Tibetan Plateau: the S1 deposit corresponds to the Wangkun Glaciation; the S3 and S4 deposits and the mirabilite-bearing stratum between them correspond to the Penultimate Glaciation; and S5 began to form in the Last Glaciation. The calculated formation age of S2 has to be considered with caution, and it is very close to the timing of the Wangkun Glaciation. A reduction in the water supply of the salt lakes during the Quaternary glaciations was an important driving factor in the formation of salt deposits in the Qaidam Basin, and the Quaternary glaciations were closely related to the tectonic activity of the Tibetan Plateau.

Tectonothermal history of the NE Jiangshan–Shaoxing suture zone: Evidence from 40Ar/39Ar and fission-track thermochronology in the Chencai region

The Jiangshan–Shaoxing suture zone, a component of the Neoprotozoic orogenic belt between the Yangtze and Cathaysia blocks in South China, has experienced a complex history of tectonic reactivation. Previous investigations of this significant tectonic boundary have focused on earlier stages of its tectonic history pre-dating the Early Paleozoic Wuyi–Yunkai orogeny, with more recent episodes rarely addressed. We here improve understanding of the later reactivation history of this suture zone through new 40Ar/39Ar and fission track thermochronological constraint derived for the ductilely strained Chencai Complex and the Lipu–Huangshan quartz diorite, together with sedimentological investigations of the proximal Wuzao Formation.This combined constraint demonstrates the occurrence of as many as five discrete episodes of tectonic reactivation and deformation of the Jiangshan–Shaoxing suture zone. The 431±4Ma 40Ar/39Ar biotite cooling age of the Lipu–Huangshan quartz diorite records retrograde cooling of the Chencai region following high-grade metamorphism at c. 460–450Ma during the Early Paleozoic Wuyi–Yunkai orogeny. Consistent younger 40Ar/39Ar ages of the gneissic Chencai Complex suggest that this retrograde cooling episode may extend to c. 390–400Ma, with later ductile strain and/or thermal overprinting of biotite at c. 386Ma suggesting that the terminal stage of the Early Paleozoic Wuyi–Yunkai orogeny may have continued significantly later than previously assumed. Although not reflected directly in the thermotectonic records compiled for the Chencai Complex, lithoclasts of these distinctive ductilely deformed gneisses represent a significant component of the conglomeratic gravels appearing in the late Triassic Wuzao Formation, demonstrating substantial uplift and erosion of the suture zone at this time, coincident with the Indosinian orogeny. Zircon fission track thermochronology of the samples from both Lipu–Huangshan quartz diorite and Chencai Complex yield much younger Cretaceous cooling ages of 144±8 to 103±6Ma, with apatite fission track ages ranging from 66±4 to 41±3Ma and mean apatite fission track lengths between 12.1 and 13.1μm. Thermal history modeling of these low-temperature controls demonstrates significant cooling during the Cenozoic. These later activities reflect brittle deformation in the upper crust and erosional exhumation of these rocks coincident with the Yanshanian extension event and Himalayan tectonism.These results together imply that the Jiangshan–Shaoxing suture zone represents a significant and long-lived lithospheric weakness that was preferentially reactivated during at least two episodes of tectonic activity in regional far-field domains. Examining the complex reactivation history of the fossil suture zone thus provides a useful window into the timing of episodes of broader tectonic activity across the wider region.

The Ridgeway Conglomerate Formation of SW Wales, and its implications. The end of the Lower Old Red Sandstone?

AbstractThe Devonian Old Red Sandstone Ridgeway Conglomerate Formation crops out in Pembrokeshire, SW Wales. It was deposited as part of a dryland alluvial fan, axial fluvial valley deposystem. It conformably overlies the mid Lochkovian Freshwater West Formation and probably predates deposition of the Lower Cosheston Group Mill Bay Formation indicating an Early Devonian (latest Lochkovian to earliest Pragian) age, rather than a Middle Devonian age as suggested by previous workers. It therefore represents the youngest preserved formation of the Milford Haven Group south of the Ritec Fault. The Formation thickens drastically into the Ritec Fault, indicating its control on sedimentation. The half‐graben topography initiated deposition of a hangingwall alluvial fan that was sourced from a southerly Lower Palaeozoic/Precambrian provenance within the present‐day Bristol Channel. The Formation is heterolithic in nature, with deposits on the fan reflecting a mixture of processes. Conglomerates were deposited primarily by laterally extensive sheetfloods, and as bars in low‐relief, laterally accreted channels. Sandstones were also predominantly deposited by sheetfloods. Gritty mudrocks in comparison demonstrate deposition by cohesive debris flows.The fan prograded northward and interfingered with a low‐gradient, high‐sinuosity fluvial channel system dominated by inclined and non‐inclined heterolithic stratification. Thinly laminated mudstone and sandstone interbeds were deposited in ephemeral fan‐toe and axial valley lakes that may have developed during sub‐humid climatic episodes. The lacustrine heterolithic association has associated matgrounds and possible ‘algal roll‐up’ structures. Calcretized peetee structures and root traces comprise a lake margin calcrete association.Fan gravels prograded into the axial fluvial valley during periods of increased sediment flux that may represent semi‐arid conditions and/or episodes of tectonic activity. Calcretes of varying development were established in both the fan and axial valley zones. Calcretes with lower stages of development are more proximal to the Ritec Fault reflecting decreased soil residence times and high deposition rates within the axial valley. More strongly developed soil profiles on the fan may indicate sequence boundaries associated with low sediment flux, or increased soil residence time due to active fan‐channel migration (the pedofacies concept). Groundwater calcretes have sharp‐based and layer‐bound calcrete profiles. Gully‐bed cements are locally developed within the fan gravels. Copyright © 2007 John Wiley & Sons, Ltd.

Anisotropy across the Sorgenfrei–Tornquist Zone from shear wave splitting

During the TOR-1 passive seismic experiment in 1996/97, a maximum of 139 temporary seismograph stations were operating over the Sorgenfrei–Tornquist Zone (STZ) in an area extending from northern Germany through Denmark to central Sweden. One of the objectives was to study horizontal anisotropy directions in the subcrustal lithosphere and asthenosphere across the Trans-European Suture Zone. To achieve this goal, broad-band and intermediate-period (5s) data of the TOR-1 stations and additional stations of permanent networks (GRSN, GEOFON) were analysed for splitting of SKS and SKKS phases. As a result of the relatively dense station spacing, the method offers good lateral resolution of anisotropy. Preliminary results suggest that the directions of the fast horizontal S wave velocity are affected by the STZ. In central Europe and southern Sweden, far away from the STZ, fast S wave directions are approximately E–W while they turn more northerly closer to the STZ where they are approximately parallel to the trend of the STZ. No significant shear wave splitting was observed north of 57°N and east of 14°E. Small delay times between 0.2 and 0.5s observed at the northernmost TOR-1 station T40S and T60S may be controlled by anisotropy in a thickened crust. The mantle contribution of horizontal anisotropy within the STZ is probably constrained to an approximately 60-km-thick zone in the depth range between 70 and 300km. The observations are consistent with a model where azimuthal anisotropy is not governed by present-day mantle flow in the asthenosphere, but rather is frozen into the subcrustal lithosphere during the last episode of tectonic activity.

Geochemistry of the Namurian Lismore Formation, northern mainland Nova Scotia: sedimentation and tectonic activity along the southern flank of the Maritimes Basin

Geochemical and isotopic data from the clastic rocks of the Namurian Lismore Formation in mainland Nova Scotia identify key episodes of tectonic activity during the development of the Maritimes Basin in Atlantic Canada. The Lismore Formation forms part of the Mabou Group and is an upward-coarsening 2500 m thick fluvial sequence deposited in the Merigomish sub-basin along the southern flank of the Maritimes Basin. Based on stratigraphic evidence, the Lismore Formation can be divided into upper and lower members which reflect variations in depositional environment and paleoclimate. The geochemical and isotopic data may also be subdivided into two groupings that primarily reflect varying contributions from accessory phases, clay minerals, or rock fragments. This subdivision occurs 115 m above the base of the upper member. The data from the lower grouping (group A) show an important contribution from underlying Silurian rocks, with a relatively minor contribution from Late Devonian granitoid rocks from the adjacent Cobequid Highlands and possibly metasedimentary rocks from the Meguma Terrane to the south. The data from the upper grouping (group B) reveal a more important contribution from the Cobequid Highlands granitoid rocks. This variation in geochemistry is thought to constrain the age of renewed motion and uplift along the faults along the southern flank of the Maritimes Basin and, more generally, suggests that geochemical and isotopic data of continental clastic rocks may help constrain the age of tectonic events that influence deposition of basin-fill rocks.

Structural Development of Billings and McKenzie Counties, Southwestern North Dakota

Billings and southern McKenzie counties contain about 40 active oil and gas fields. In the same area, two major structural highs and several minor ones can be recognized. The Billings anticline extends approximately 60 km (18 mi) along a north-south line in central Billings County and is composed of two parallel folds. A second major structure, the Rough Rider anticline, lies west of the Billings anticline and also extends about 60 km (18 mi) along a north-south line from north-central Billings County into southern McKenzie County. Both of these structures are fault bounded on their eastern sides and are sites of significant hydrocarbon accumulation. Episodes of tectonic activity within the area were identified by study of isopach patterns derived from wireline log picks and by subsidence analysis. Although the subsidence of the Williston basin was the principal regional tectonic activity, the structural features in Billings and McKenzie counties were intermittently decoupled from the larger scale basin movement, producing local highs. The two major positive features, as well as several minor ones, have behaved independently of each other through time. Most of the development of these structures took place during the Devonian and Mississippian.