Northward Convergence Research Articles

The Uttarakhand Himalaya: An image of the main Himalayan thrust, Moho, and lithosphere-asthenosphere boundary

We use seismic velocity tomography and Common Conversion Point (CCP) stacked radial PRFs to model the main Himalayan thrust (MHT), Moho, and Lithosphere-Asthenosphere boundary (LAB) beneath the Uttarakhand Himalaya using broadband data from 55 three-component seismographs. The Main Himalayan Thrust (MHT) between 8 and 20 km depth, which our modelling identifies as the source of the majority of the recent micro-earthquake activity, is characterised by larger negative Ps/P amplitudes, a 10-20% drop in Vp and Vs, and a 10-15% increase in Vp/Vs. We simulate a strongly undulating northeast-dipping crust-mantle interface between 30 and 55 km depth (with larger positive Ps/P amplitudes). In addition, we establish a lithosphere-asthenosphere boundary with a northeast-dipping depth between 130 and 180 km (with larger negative Ps/P amplitudes). Both the presence of metamorphic/aqueous fluids and the predicted double Moho structure are hypothesised to result from the continued northward convergence of the Indian plate beneath the most active rupture zone in the Garhwal - Kumaon Himalaya. One possible explanation for the double Moho structure is that the Indian crust and upper mantle became entangled during the collision of the Indian and Eurasian plates. Above the aforementioned rupture zone, where all Mw6.5 events have occurred so far, the upper crust is likely accumulating the strain energy needed to generate moderate to large earthquakes on the MHT.

Crustal velocity images of northwestern Türkiye along the North Anatolian Fault Zone from transdimensional Bayesian ambient seismic noise tomography

SUMMARY Türkiye poses a complex crustal structure and tectonic settings owing to the northward convergence of the Arabian and African plates with respect to the Anatolian and Eurasian plates. A reliable 3-D crustal structure of the unruptured segment of the North Anatolian Fault Zone (NAFZ) in the Sea of Marmara is thus of utmost importance for seismic hazard assessments considering that the megacity Istanbul—with more than 15 million habitants—is close to this seismic gap. This study provides high-resolution shear wave velocity images of northwestern Türkiye, including the NAFZ, revealed from ambient seismic noise tomography. We extract over 20 000 Green’s functions from seismic ambient noise cross-correlations and then construct group velocity perturbation maps from the measured group delays with a transdimensional Bayesian tomographic method. We further perform an S-wave velocity inversion to image depth-varying velocity structures. Our high-resolution data allowed us to image S-wave velocities down to 15 km depth and reveal weak crustal zones along the NAFZ, as indicated by low shear wave velocities. We find a low-velocity zone along the Main Marmara Fault, linked with aseismic slip and a deep creep mode. Furthermore, we identify a high-velocity anomaly associated with the unruptured section that defines the boundaries of the locked zone in the crust, which can potentially trigger a destructive earthquake in the future.

Onset of Iberian-European plate convergence: Late Cretaceous flexural response of a hot lithosphere (Aquitaine Basin, France)

The Aquitaine Basin is closely linked to the evolution of the Pyrenees, providing precious evidence of the early stages of the Pyrenean orogeny in the Late Cretaceous. Although the timing and geometry of the Early Cretaceous rifting stage is well constrained in the Pyrenees and surrounding north Pyrenean basins, the subsequent early inversion stage has resisted characterization. In this study, we used well log correlations and interpretation of seismic reflection profiles to determine the Late Cretaceous evolution of the sedimentary record in the Aquitaine Basin, spanning the transition from the postrift stage to the beginning of convergence. We identified two main deformation stages during the Late Cretaceous with radically different basin geometries in the western and eastern portions of the Aquitaine Basin. The western part appears to remain under a thermal subsidence regime, inherited from the postrift stage initiated in the Turonian, with little to no deformation. In contrast, the eastern part records early inversion, as indicated by the presence of a well-defined Campanian flexural basin. Between them, Coniacian and Santonian strata indicate a sharply increasing distinction between a very slowly subsiding North Aquitaine Domain (condensed sedimentation) and a strongly subsiding South Aquitaine Domain. We interpret this development as a consequence of lithospheric buckling resulting from far-field compressive strain between the Eurasia and Africa plates. During the Campanian, northward convergence of the Iberia plate led to the formation of a flexural basin with a narrower, steeper geometry than the subsequent Eocene basin that formed in nearly the same region during the Pyrenean orogeny. We attribute this difference to the development of the foreland over a lithosphere that was not thermally reequilibrated in Campanian time, roughly 10 Myr after rifting. This thermal inheritance resulted in lithosphere of very low rigidity. Sedimentary basins formed during both the Coniacian-Santonian and Campanian to early Maastrichtian were thus accommodated by short-wavelength flexural deformation of the lithosphere, a distinctive deformation pattern that represents an immediate response to the onset of compressive strain.

Recurrent seismicity in Rajasthan State in the tectonically stable NW Indian Craton

Indian peninsular region comprises several Archean cratonic blocks (Dharwar, Bastar, Singhbhum, Aravalli – Bundelkhand), bordered by Proterozoic mobile belts. Therefore, this region is considered as tectonically stable and designated as the least vulnerable region to earthquake hazard except the still active Central Indian Tectonic Zone (CITZ). The latter is a major suture between southern and northern Indian blocks. Seismicity in India is common along its northern and northeastern (Himalayan) region defining the Indian Plate margin, in collision with the Eurasian (Tibetan) Plate. Being tectonically active, this region has a documented record of frequent earthquakes including some high magnitude and devastating ones. The northwestern Indian block is amongst the relatively stable Precambrian regions of India and categorized under Zone – II by the Indian Meteorology Department, one of the seismically least vulnerable regions. However, the region has been a site of recurring low to moderate magnitude earthquakes. Compilation of the earthquake data from the Rajasthan State in NW India documents at least 45 earthquakes in this region during the last one and a half decades. Several of these have remained unnoticed because of their low magnitude. The NE –SW trending Aravalli Mountain Region (AMR), running across the eastern half of Rajasthan State represents the most significant tectonomorphic feature of NW India. The AMR is an ensemble of Proterozoic age Aravalli and Delhi Supergroup rocks (metasediments, volcanics and intrusives) that overlie an Archean basement (Banded Gneissic Complex – BGC). These Proterozoic mobile belts have evolved through several episodes of deformation that have shaped its present day geomorphology. The western part of the State, the Marwar Block, is relatively younger in age and was accreted to the AMR during the 1 Ga subduction event. The western margin of AMR, also called as the Western Margin Fault, represents a major suture between the two terranes. However, the entire region was cratonized by end Proterozoic and has remained tectonically stable during the Phanerozoic Eon. The Archean basement and overlying Proterozoic cover rocks are infested with several major and minor faults and shear zones. The most prominent ones are the NE-SW trending major lineaments corresponding with the regional tectonic grain and several minor ones across. The earthquake epicenter distribution pattern shows a close spatial association with these lineaments. In the absence of any significant tectonic activity in the region and rise in pore pressure either due to magmatism or excessive rainfall, we attribute recurrence of earthquakes in the region to reactivation of such old sutures/weaker zones as a response to stress build-up along the northern margin of the Indian Plate resulting from ongoing northward convergence of the Indian Plate. Some of the faults in western part are traceable into the Cambay Basin active faults in the south that may have triggered seismic activity in western Rajasthan.

Lithospheric Structure of the Northern Ordos From Ambient Noise and Teleseismic Surface Wave Tomography

AbstractWe constructed a high‐resolution 3‐D Vs model of the northern Ordos block and its surrounding areas by surface wave tomography, which reveals significant intracratonic heterogeneities. In the western Ordos, the lithosphere thickness is ~200 km with shear velocities comparable to the high velocities of other Archean cratons worldwide. However, the lithosphere thins gradually toward the east and Vs drops by 2–3% in the uppermost mantle beneath the eastern Ordos, coincident with the high surface heat flow (~68 mW/m2 in average) there. This observation suggests that the thick, cratonic keel is only locally preserved beneath the western Ordos, and the eastern part of the Ordos seems to undergo local rejuvenation. At greater depths (>180 km), a low‐velocity channel is observed beneath the high‐velocity keel of the Ordos. Beneath the Datong volcanoes, a low‐velocity anomaly is observed, dipping westward with depth and closely following the slope of the lithosphere beneath the northern Ordos. This prominent low velocity is connected with the low‐velocity zone beneath the northern Ordos, which is further connected with the low‐velocity zone beneath the northeastern Tibetan Plateau (NET). We propose that the asthenosphere beneath the NET flows toward the northern Ordos in response to the continuous northward convergence of the Indian‐Eurasian continents, and the asthenosphere flows upward following the eastward thinning lithosphere which leads to decompression partial melting, which migrates upward to feed the Datong volcanoes. The significant variations of the lithospheric thickness of the Ordos block may control the distribution of the asthenospheric flow.

Block rotation induced by the change from the collision to subduction: Implications for active deformations within the areas surrounding South Caspian Basin

Northern of Iran is experiencing a complex deformation due to the northward convergence of Arabian plate since late Cenozoic. The majority of this complexity is due to the presence of a rigid, old, thickened oceanic crust of the South Caspian Basin (SCB). This rigid block, trapping between Eurasia and Central Iran, makes an inhomogeneous body in the context of deforming zone composed by Alborz, Talesh, Apsheron-Balkhan, Caucasus, and Kopeh Dagh located in south, west, northwest, north, and east of the SCB, respectively. Among them, the formation and evolution of the Alborz orocline are under debates. The presence of sinistral strike-slip faults with normal separation initially understood by NW motion of northern Alborz inferred by GPS measurements raised long debates. In the latest proposition, it is assumed that the SCB is rotating clockwise. This study, however, shows that the model of SCB's rotation and/or NW-ward motion of SCB is not kinematically consistent. Therefore, it aims at constraining the Alborz, northern central Iran, and NW Iran, as a sliver block, are rotating around the SCB. The block rotation is facilitated by geo-sutures inherited since Paleozoic. Therefore, this article specifically pays attention to the active extensional tectonic regime as a signal for the area that is under rotational movement. The block rotation induced by the change from the collision (in Kopeh Dagh) to subduction (SCB and Kura Basin beneath Alborz, Talesh, Apsheron-Balkhan, and Great Caucasus, respectively), and consequently the reactivation of ancient geo-sutures in northern Iran led to a rapid rotation of microcontinents. This assumption may account for the ongoing deformational pattern in Alborz and NW Iran. The disharmonic Arabian plate convergence and SCB's retreating induce forces by the block rotation and cause episodic tectonic activities that are responsible for current tectonic complexity in the north of Iran. If the climax of the collision was ~5 Ma, then the block rotation and related deformation are younger events with respect to the collision and subduction.

The 2016 Mw 6.7 Aketao earthquake in Muji range, northern Pamir: Rupture on a strike-slip fault constrained by Sentinel-1 radar interferometry and GPS

On 25 November 2016, the Aketao, Xinjiang earthquake occurred on the Muji fault, which is located at the northernmost end of the right-lateral Karakorum Fault (KF). This event provides a rare chance to gain insights into how the stress accumulates in Pamir margin as the Indian plate collides with the Eurasian plate. Space geodetic measurements including InSAR and GPS were used to obtain coseismic surface displacements associated with this earthquake. Based on a finite fault model, the coseismic slip distribution inverted by the combined datasets indicates that the 2016 Aketao event is caused by a primary shallow strike-slip with minor normal-slip at a steep-dipping angle. To explore the real structure of Muji fault, listric fault model inferred by relocated aftershocks as well as the planar fault model, were used in our slip distribution inversion. The results suggest that the optimal fault model should be a highly-dipping planar fault with two separated asperities. The large slip zone is beneath the surface near the epicenter with a maximum slip of 1.1 m, while the small one in the east breaks the surface, in a good agreement with the field seismic geological survey. The total geodetic moment is 1.35 × 1019N∙m, equivalent to Mw 6.7. The nearly pure dextral strike-slip Aketao earthquake, and the recent 2015 Mw 7.2 sinistral strike-slip Tajikstan earthquake in this region, to some extent, manifest the extension motion is dominated in northern Pamir Plateau, in response to the northward convergence between Indian and Eurasian collision.

Longitudinal and Temporal Evolution of the Tectonic Style Along the Cyprus Arc System, Assessed Through 2‐D Reflection Seismic Interpretation

AbstractThe Cyprus Arc system constitutes a major active plate boundary in the eastern Mediterranean region. This structure is directly linked with the northward convergence of the African and Eurasian plates since the Late Cretaceous. Two‐dimensional reflection seismic data were utilized, which image the main plate structures and their lateral evolution south of the island of Cyprus. Interpretation of these data allowed the identification of nine tectono‐sedimentary packages in three different crustal domains south of the Cyprus Arc system: (1) the Levant Basin (attenuated continental crust), (2) the Eratosthenes microcontinent (continental crust), and (3) the Herodotus Basin (oceanic crust). Within these domains, numerous tectonic structures were documented and analyzed in order to understand the mechanism and timing of deformation. In the north, south verging thrusting commenced in early Miocene along the Larnaca and Margat ridges, whereas no activity was identified before middle Miocene along the Latakia Ridge. Thus, the deformation front migrated southward and was accompanied by the development of flexural basins and stratigraphic onlaps as in the Cyprus Basin. The acme of deformation occurred in mid‐late Miocene. A regional unconformity of Pliocene age marks the end of the first deformation sequence. In Plio‐Pleistocene time, the westward escape of the Anatolian microplate resulted in the reactivation of existing structures. The evolution of deformation along the plate boundary is identified from the creation of positive flower structures revealing transpressive movements along the Larnaca and Latakia Ridges (eastern domains), whereas in the Eratosthenes domain a flexural basin highlights a compressive regime.

Tectonic amalgamation of the Gaoligong shear zone and Lancangjiang shear zone, southeast of Eastern Himalayan Syntaxis

The Gaoligong shear zone (GLGSZ) and Lancangjiang shear zone (LCJSZ) are two major large-scale shear zones southeast of Eastern Himalayan Syntaxis (EHS) and play important roles in the accommodation of the intracontinental deformation in Southeast Asia. The two shear zones, which are mainly composed of high-grade metamorphic rocks and have long been regarded as the Precambrian basement of this area, are gradually merged northward and become tectonically amalgamated from Fugong to Gongshan area. Structural and kinematic analyses reveal that the amalgamation area has experienced strongly partitioned dextral transpression. N–S trending tight to isoclinal folds, sub-vertical foliation and sub-horizontal lineation are ubiquitously developed both within the shear zones and the Carboniferous flysch sediments of Baoshan block in between. Micro- and macro-kinematic indicators such as CS structures, rotated porphyroclasts, asymmetric shear folds, sheath folds, domino and boudin structures suggest that the shear sense of the GLGSZ and LCJSZ is dextral in the amalgamation area. Comparative analyses of strata reveal that the Baoshan block is aligned with south Qiangtang block and the Lanping–Simao block was aligned with north Qiangtang block prior to the Cenozoic large scale tectonic extrusion and rotation. The amalgamation area, which has progressively switched positions from northeast of EHS to east and southeast of EHS, is just located at the neck of the large scale boudin of south Qiangtang–Baoshan block. A boudin model is proposed for the tectonic amalgamation of the GLGSZ and LCJSZ from Fugong to Gongshan area in response to the continued northward convergence between Indian plate and Eurasian plate since early Eocene, resistance of South China block, southeastward extrusion and clockwise rotation of Baoshan block around EHS.

Crustal thickening and uplift of the Tibetan Plateau inferred from receiver function analysis

The India–Eurasia collision, crustal thickening and northward convergence leading to uplift of the Tibetan Plateau have been topics of multidisciplinary studies. Here we employ the H–k stacking technique and depth domain receiver function to constrain the crustal thickness and P- and S-wave velocity ratio as well as the upper mantle discontinuity. Our results identify the signature of slab break-off and lower crustal delamination in southern Tibet, which might have induced the mantle upwelling and crustal melting. The convergence of the Indian plate inducing northward crustal flow might be a major factor that contributes to the uplift and crustal thickening of Tibet. We also evaluate the E–W trending cumulative compression as another dominant factor that contributes to the increase in crustal thickness in the northern Tibet.

Transverse tectonics in the Sikkim Himalaya: A magnetotelluric study

The tectonics of seismically active Sikkim Himalaya, as inferred by numerous seismological studies, is distinct from the conventional thrust tectonics proposed for the Himalayan collision belt. Here, focal mechanisms of several moderate magnitude earthquakes and composite fault plane solutions of microearthquakes have revealed strike–slip motion along faults transverse to the northward convergence direction of the Indian plate. In the present study, we analyze broadband magnetotelluric data of 12 sites located along an approximately N–S profile cutting across major geological sub-domains of Sikkim to test whether magnetotelluric strikes also support such transverse tectonic nature of the region. We have performed strike analysis of the data by two decomposition approaches as well as by phase tensor method. The study has revealed local variations in the strike directions within the region consistent with the geological and tectonic setup and the presence of transverse tectonic features in the region of Main Central Thrust Zone (MCTZ) where major axis of phase ellipses align in NNW–SSE to NW–SE direction. This trend coincides with the one obtained by microseismic data recorded after the September 18, 2011 earthquake (Mw 6.9). Magnetotelluric strike analysis thus supports the presence of NNW-to-NW trending transverse tectonic zone in MCTZ.

Multifractal analysis of earthquakes in Kumaun Himalaya and its surrounding region

Himalayan seismicity is related to continuing northward convergence of Indian plate against Eurasian plate. Earthquakes in this region are mainly caused due to release of elastic strain energy. The Himalayan region can be attributed to highly complex geodynamic process and therefore is best suited for multifractal seismicity analysis. Fractal analysis of earthquakes (mb ≥ 3.5) occurred during 1973–2008 led to the detection of a clustering pattern in the narrow time span. This clustering was identified in three windows of 50 events each having low spatial correlation fractal dimension (D C ) value 0.836, 0.946 and 0.285 which were mainly during the span of 1998 to 2005. This clustering may be considered as an indication of a highly stressed region. The Guttenberg Richter b-value was determined for the same subsets considered for the D C estimation. Based on the fractal clustering pattern of events, we conclude that the clustered events are indicative of a highly stressed region of weak zone from where the rupture propagation eventually may nucleate as a strong earthquake. Multifractal analysis gave some understanding of the heterogeneity of fractal structure of the seismicity and existence of complex interconnected structure of the Himalayan thrust systems. The present analysis indicates an impending strong earthquake, which might help in better hazard mitigation for the Kumaun Himalaya and its surrounding region.

Genesis of Daba arcuate structural belt related to adjacent basement upheavals: Constraints from Fission-track and (U-Th)/He thermochronology

Fission-track, (U-Th)/He thermochronology, and cooling properties indicate that the southern Daba arcuate zone (SDBAZ) underwent a distinctive phase of rapid cooling in 153–100 Ma at a rate of 1.44–1.90°C/Ma. This rapid uplifting strongly contrasts with (1) the previous, rapid foreland subsidence during Early to Middle Jurassic in response to late-orogenic compression from the Qinling belt, (2) the succeeding long, slow cooling phase and relative thermal stability that occurred during the 100–45 Ma period. This rapid cooling event in the SDBAZ parallels those experienced by two adjacent upheavals of Huangling (HLUZ) and Hannan-Micang (HMUZ), with cooling rates of 2.22–3.17°C/Ma for the HLUZ in 160–126 Ma, 4.91°C/Ma for the southern HMUZ in 150–125 Ma, as well as 2.11°C/Ma for the northern HMUZ in 150–105 Ma. Comparing thermal histories among the SDBAZ, the HLUZ, the HMUZ, and the Wudang metamorphic zone (WDMZ), we infer that the Daba arcuate structural belt formed in 153–100 Ma. The combined dating data support a correlation with a low-angle arcuate south-thrusting of the Qinling orogen triggered by northward convergence of the Yangtze Craton, contemporaneously encountering rigid basement obstructions from the HLUZ and the HMUZ, respectively. Both the SDBAZ and neighboring domains additionally underwent a comparatively fast cooling and uplift since about 45 Ma.

New constraints to the onset of the India–Asia collision: Paleomagnetic reconnaissance on the Linzizong Group in the Lhasa Block, China

To better understand the onset of the India–Asia collision, a paleomagnetic study has been conducted on Paleogene volcanic rocks and sediments of the Linzizong Group (∼ 64–44 Ma) from the Linzhou and Namling basins of the Lhasa Block at the southern margin of the Asian continent. Following detailed rock magnetic investigations and progressive thermal or hybrid thermal and alternating field demagnetization, stable characteristic remanent magnetizations (ChRMs) were successfully defined from the Dianzhong (∼ 64–60 Ma), Nianbo and Pana (∼ 60–44 Ma) formations. These ChRMs are of dual polarity, and pass a fold test with some of them in volcanic rocks carried by oxidation-induced hematite; we thus interpret them as primary remanences. Paleomagnetic results from the bulk of the collection imply that the southern leading edge of Asia, i.e. the Lhasa Block, remained essentially stationary in low latitudes at ∼ 10°N without paleomagnetically-detectable tectonic rotation and latitudinal variation during the formation of the Linzizong Group. Consequently, a large-scale post-collisional northward convergence in the Tibetan plateau and central Asia is very likely to have occurred after formation of the Linzizong Group, and the definitive paleoposition of the Lhasa Block during the period embracing the interval between ∼ 64 and 44 Ma provides further constraints on the latitudinal position of the initial contact between India and Asia. This evidence rules out the model for a younger (∼ 35 Ma) India–Asia collision and implies that the initial India–Asia collision occurred between ∼ 65 and 50 Ma. Given that Greater India comprised the present Indian continent together with a ∼ 950 km extension beyond its northern margin prior to the rifting of Gondwana in the Middle Jurassic, we conclude that the initial India–Asia collision most probably occurred between ∼ 55 and 60 Ma.

Viscoelastic-cycle model of interseismic deformation in the northwestern United States

SUMMARY We apply a viscoelastic cycle model to a compilation of GPS velocity fields in order to address the kinematics of deformation in the northwestern United States. A viscoelastic cycle model accounts for time-dependent deformation following large crustal earthquakes and is an alternative to block models for explaining the interseismic crustal velocity field. Building on the approach taken in Pollitz et al., we construct a deformation model for the entire western United States—based on combined fault slip and distributed deformation—and focus on the implications for the Mendocino triple junction (MTJ), Cascadia megathrust, and western Washington. We find significant partitioning between strike-slip and dip-slip motion near the MTJ as the tectonic environment shifts from northwest-directed shear along the San Andreas fault system to east–west convergence along the Juan de Fuca Plate. By better accounting for the budget of aseismic and seismic slip along the Cascadia subduction interface in conjunction with an assumed rheology, we revise a previous model of slip for the M∼ 9 1700 Cascadia earthquake. In western Washington, we infer slip rates on a number of strike-slip and dip-slip faults that accommodate northward convergence of the Oregon Coast block and northwestward convergence of the Juan de Fuca Plate. Lateral variations in first order mechanical properties (e.g. mantle viscosity, vertically averaged rigidity) explain, to a large extent, crustal strain that cannot be rationalized with cyclic deformation on a laterally homogeneous viscoelastic structure. Our analysis also shows that present crustal deformation measurements, particularly with the addition of the Plate Boundary Observatory, can constrain such lateral variations.

The southern Central America puzzle: Cronology and structure. A review

From northern Costa Rica to eastern Panama oceanic rocks are extensively exposed along the Pacific margin of Isthmic Central America. For the past 30 years, their nature and significance were subject to debate. This study distinguishes numerous units using the following criterions: ages of igneous and sedimentary rocks, petrological and geochemichal features, structural setting and ages of the sedimentary "covers". New field, radiometric and stratigraphical data are compared with available data published in recent papers. Strong disparities are noticed for ages, from Lias-lower Dogger to Eocene and petrological patterns: mantle peridotites, several tholeiitic and alkaline series. Various units are mélange formations made of exotic blocks unknown elsewhere. We propose that these disparate units were emplaced by two contrasting processes: 1) Closing of a Mesozoic northern oceanic basin by northward convergence and emplacement of an E-W 150 km ophiolitic suture; 2) successive accretions of terranes originated in the Pacific plate and corresponding to possible oceanic plateau basalts and seamounts.

Timing of deformation within Old Red Sandstone lithologies from the Dingle Peninsula, SW Ireland

Finite strain analysis of deformed clastic sedimentary rocks below and above the Acadian unconformity on the Dingle Peninsula, SW Ireland, reveals that this boundary represents a significant bulk strain discontinuity. The Late Emsian Acadian event is primarily responsible for penetrative cleavage development and high strain in rocks below the unconformity and not, as previously held, the later Variscan event, which overprinted the peninsula with a weak and localized disjunctive cleavage. The presence of apparent anticlockwise transecting cleavage and implied dextral closure of the Dingle Basin during the Acadian event is compatible with northward convergence of Armorica with respect to Eastern Avalonia.

Paleomagnetic and geochronological study of the Halaqiaola basalts, southern margin of the Altai Mountains, northern Xinjiang: Constraints on neotectonic convergent patterns north of Tibet

A combined geochronological and paleomagnetic study is reported from Miocene basalts from Halaqiaola and Paleocene to Eocene red beds from Fuyun at the southern Altai Mountains, northern Xinjiang. Three new 40Ar/39Ar ages determined by fresh matrix from the Halaqiaola basalts collectively confirm the presence of Cenozoic magmatic activity in the northern Xinjiang. Alternating field and thermal demagnetization identify stable characteristic remanences with single reversed polarity in the basalts and red beds. Rock magnetic analysis identifies pseudosingle‐domain titanomagnetites as carriers of remanence in the basalts. Paleomagnetic results from the Fuyun red beds indicate that inclination shallowing is present in this rock facies as also found in previous red bed studies from central Asia. In contrast, the time‐averaged paleomagnetic direction from the basalts shows that no paleomagnetically discernible northward convergence has occurred north of Junggar since early Miocene times. Analysis of available Cretaceous and Cenozoic paleomagnetic data from volcanic rocks in central Asia shows that Neogene and Quaternary paleolatitudes are statistically concordant with predicted values from Eurasian references, suggesting that no significant northward convergence has occurred north of Tibet during the last 20 Myr. Cretaceous and Paleogene paleolatitudes lie ∼5–6 ± 7° south of predicted values from the Eurasian path and suggest that neotectonic convergence of the order of several hundred kilometers has occurred north of the southwest Tian Shan. It is possible that neotectonic northward convergence north of Tibet will prove to be paleomagnetically undetectable, but more data are required to confirm this.

Active transtension inside central Alborz: A new insight into northern Iran–southern Caspian geodynamics

The tectonic activity in the Alborz mountain range, northern Iran, is due both to the northward convergence of central Iran toward Eurasia, and to the northwestward motion of the South Caspian Basin with respect to Eurasia inducing a left-lateral wrenching along this range. These two mechanisms give rise to a NNE-SSW transpressional regime, which is believed to have affected the entire range for the last 5 6 2 m.y. In this paper, we show that the internal domain of central Alborz is not affected by a transpressional regime but by an active transtension with a WNW-ESE extensional axis. We show that this transten- sion is young (middle Pleistocene). It postdates an earlier N-S compression and may have been initiated when the South Caspian Basin started moving. Consequently, our results suggest that the South Caspian Basin motion may have taken place more recently than previously proposed.

Paleomagnetic and geochronological constraints on the post-collisional northward convergence of the southwest Tian Shan, NW China

A combined paleomagnetic and geochronological study is reported of Paleogene basalt lavas and an intercalated red bed succession, comprising a minimum of 14 basalt flows and 10 red bed horizons in the Tuoyun Basin of the southwest Tian Shan Range, China. Two basalt matrix samples yield 40Ar / 39Ar isochron ages of 58.5 ± 1.3 Ma (2 σ, MSWD = 0.9) and 60.4 ± 1.3 Ma (2 σ, MSWD = 1.7). These compare well with a previously published K–Ar dilution age of 61.7 ± 2.3 Ma for comparable Paleogene basalts and confirm that the younger pulse of magmatism in this basin is represented by both intrusive and extrusive activity. Demagnetization and component analysis identify a stable characteristic remanence (ChRM) with predominantly reversed polarity following removal of secondary remanence by peak demagnetization steps below 250–350 °C or 5 mT. Rock magnetic analysis identifies pseudo-single domain magnetite or titanomagnetite as carriers. The stable ChRM passes a fold test; it was probably acquired at the time of lava emplacement. Results from the bulk of the collection imply that paleomagnetic data from the upper and lower (∼ 115 Ma) basalt series in the Tuoyun Basin are not distinguishable at the 95% significance level and indicate that this tectonic domain remained essentially stationary with respect to the Earth's spin axis for ∼ 50 Ma prior to onset of the India/Asia collision in early Eocene times. It is therefore probable that no paleomagnetically detectable crustal shortening occurred in the southwest Tian Shan prior to collision. Paleomagnetic data sets from the Tuoyun Basin also show that little or no paleolatitude difference is present between the Tian Shan and the reference latitude of Eurasia at ∼ 60 Ma. This supports previous evidence suggesting that central Asian blocks in the vicinity of the Tian Shan are unlikely to have experienced appreciable northward convergence relative to Eurasia since onset of the India/Asia collision and initiation of the Himalaya.