Anisotropy Of Magnetic Susceptibility Research Articles

Building an inversely zoned post-orogenic intrusion in the Neoproterozoic-Cambrian Araçuaí orogen (Brazil)

Post-orogenic plutons are excellent recorders of the last stages of collapsing orogens. In the Neoproterozoic-Cambrian Araçuaí orogen (Brazilian Southeast), one of the Brasiliano orogenies (Western Gondwana), voluminous amounts of Cambrian magmatism crop out. The Venda Nova pluton (VN) is located at the southern portion of the AO, exhibiting a norite/charnockitic west-ring in contact with a syenomonzonite that envelops an alkali-gabbro core with distinct geochemical signatures. To explore the tectonic significance of this post-orogenic pluton in the AO, anisotropy of magnetic susceptibility (AMS) together with a microstructural analysis were performed. AMS data reveals, majorly, a concentric magnetic foliation/lineation pattern, implying that buoyancy forces controlled the magma emplacement of syenomonzonites and gabbros, while a previous pulse of a charnockitic intrusion was deformed by their intrusion. Restrict solid-state deformation (following the regional trend) inside the pluton could result of the reactivation of an emplacement structure in the last stages of VN crystallization. The emplacement of inversely zoned post-orogenic plutons through deep conduits in the colder forelands of the AO, such as the VN, might record different stages of a large collapsing orogen in a single plutonic body.

Cenozoic deformation in the Tethyan Himalaya, SE Tibet: Insights from magnetic fabrics and structural analysis of Upper Triassic flysch

Anisotropy of magnetic susceptibility (AMS) analyses and structural observations of Triassic flysch in the Tsetang area, SE Tibet, were carried out to assess the Cenozoic tectonic evolution and kinematics of deformation of the eastern Tethyan Himalaya Thrust Belt. We analysed a total of 202 standard-sized specimens from four sample sites across the Renbu–Tsetang Thrust from north to south. Rock magnetism and the detailed mineralogy demonstrated that ferromagnetic and paramagnetic minerals both contribute to the AMS. However, the paramagnetic contribution is dominant in the study area. The AMS results, together with field-based structural investigations and analyses of microstructures, revealed that layer-parallel shortening (LPS) was accommodated mainly by isoclinal folds, cleavage, and thrust faults. Shear fabrics (K1 axes oriented parallel to the direction of local tectonic transport) were found mainly in the north thrust fault zone and were linked to fault movement. Microstructural analyses of quartz and metamorphic minerals such as muscovite (sericite) and chlorite in slates and sandstones imply that the peak temperature during the deformation did not exceed 300 °C. The shear AMS fabric in north Qonggyai, near the thrust zone, shows that the strain variation is related to the main activity (11–19 Ma) of the Renbu–Tsetang Thrust. The magnetic lineation that plunges towards the east in the southern Qonggyai area indicates extension that was linked to an E–W extensional event during the early Miocene. Based on the available geochronological data, two stages of tectonic deformation can be defined: (a) syncollisional layer-parallel shortening at ~40 Ma ago, and (b) E–W extension and continuing compaction resulting in shearing at 11–19 Ma.

A multiple-pulse emplacement model for the Shonkin Sag laccolith, Montana, USA

The Shonkin Sag laccolith (Montana) is a differentiated mafic hypabyssal intrusion that was concordantly emplaced into undeformed sandstones. Erosional processes have produced a cross-section-like exposure of the laccolith, revealing its base, roof, and eastern contact, where it transitions to fringing sills. We present evidence for a multi-pulse emplacement model for the Shonkin Sag laccolith by showing that the rapidly cooled margin of the laccolith and fringing sills preserve features formed during emplacement. These features include internal contacts associated with variably deformed zones at the margin of the laccolith and normal and thrust faults within the surrounding sedimentary rocks. Bulk magnetic susceptibility measurements, anisotropy of magnetic susceptibility and thin section analyses support evidence for strain accumulation in pulses. Thermal models suggest that the pulses emplaced over ~3 years, while total cooling and differentiation of the laccolith to the solidus occurred over ~21 years. From these results, a three-stage emplacement model was determined. The model provides insight into ground deformation and associated hazards caused by the rapid emplacement of shallow magma bodies similar in size to the Shonkin Sag laccolith, such as the 1980 Mt. St. Helens cryptodome and the 2011 Cordón Caulle laccolith.

Magnetic fabric of the Parashi stock and related dyke swarm, Alta Guajira (Colombia): The Caribbean-South American plates oblique convergence

Anisotropy of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) were used to evaluate the emplacement history of the Parashi stock and related dyke swarm situated in NW Colombia. The average magnetic susceptibility of 4.5×10-2 SI, in conjunction with low-coercivity components provided by the isothermal remanence and thermomagnetic curves with net Verwey and Curie transitions, indicates that multidomain magnetite records the anisotropy directions. The similar orientation and shape of the AMS and AAR ellipsoids indicate the absence of very fine magnetite with an inverse fabric. The magnetic foliation is the best-defined fabric element in these rocks and outlines a concentric structure, elongated parallel to the NE-SW direction of the pluton. Crystallisation age of the stock and dykes (51-47 Ma), along with pressure of emplacement determination indicate that the stock and the dyke swarm probably formed simultaneously, and they were emplaced in the shallow crust (

Application of anisotropy of magnetic susceptibility (AMS) fabrics to determine the kinematics of active tectonics: examples from the Betic Cordillera, Spain, and the Northern Apennines, Italy

Abstract. The anisotropy of magnetic susceptibility (AMS) technique provides an effective way to measure fabrics and, in the process, interpret the kinematics of actively deforming orogens. We collected rock fabric data of alluvial fan sediments surrounding the Sierra Nevada massif, Spain, and a broader range of Cenozoic sediments and rocks across the Northern Apennine foreland, Italy, to explore the deformation fabrics that contribute to the ongoing discussions of orogenic kinematics. The Sierra Nevada is a regional massif in the hinterland of the Betic Cordillera. We recovered nearly identical kinematics regardless of specimen magnetic mineralogy, structural position, crustal depth, or time. The principal elongation axes are NE–SW in agreement with mineral lineations, regional GPS geodesy, and seismicity results. The axes trends are consistent with the convergence history of the Africa–Eurasia plate boundary. In Italy, we measured AMS fabrics of specimens collected along a NE–SW corridor spanning the transition from crustal shortening to extension in the Northern Apennines. Samples have AMS fabrics compatible only with shortening in the Apennine wedge and have locked in penetrative contractional fabrics, even for those samples that were translated into the actively extending domain. In both regions, we found that specimens have a low degree of anisotropy and oblate susceptibility ellipsoids that are consistent with tectonic deformation superposed on compaction fabrics. Collectively, these studies demonstrate the novel ways that AMS can be combined with structural, seismic, and GPS geodetic data to resolve orogenic kinematics in space and time.

AMS and rock magnetism in the Caviahue-Copahue Volcanic Complex (Southern Andes): Emission center, flow dynamics, and implications to the emplacement of non-welded PDCs

Pyroclastic deposits can cover significant areas and register major geological events. Despite their importance, understanding depositional dynamics of pyroclastic density currents (PDCs) and linking explosive deposits to their emission centers is still a challenge, especially in the case of non-welded, massive ignimbrites. Located in the Southern Andes, the Caviahue Copahue Volcanic Complex (CCVC) comprises one of the most active volcanic centers in the Andean Belt. This volcanic complex hosts massive ignimbrites with both source emplacement poorly constrained, currently grouped in the Riscos Bayos Ignimbrites (RBI). In this contribution, we perform a full magnetic characterization and anisotropy of magnetic susceptibility (AMS) study on the massive RBI of the CCVC. The magnetic characterization was performed using magnetic experiments including isothermal remanet magnetization, thermomagnetic curves, hysteresis loops, first-order reversal curves, and scanning electron microscopy. Magnetic experiments indicate primary, multi-domain, high Curie temperature titanomagnetites as the AMS carriers. Ellipsoids are predominately oblate, with a low degree of anisotropy and east-southeastward imbrication. This fabric arrangement is consistent with PDC sedimentary fabrics deposited under laminar flow conditions. Despite RBI massive structure AMS data reveals changes in transport capacity of the PDC and particle organization. These changes are marked by increasing AMS dispersion and decreasing degree of anisotropy up-section within flow units. Directional statistics of AMS data implies the Las Mellizas Caldera as the emission center of RBI. The reconstructed flow path also suggests the PDC overrun of the Caviahue Caldera topographic rim. This study highlights the application of AMS to the identification of emission centers of explosive deposits, featuring its application to massive ignimbrites.

Magnetic fabric analysis of meta-ultramafic rocks and associated gneisses from the Moyar Shear Zone, Southern Granulite Terrane, India

The magnetic fabric of the meta-ultramafic rocks and the associated gneisses in the Wayanad district, in the Moyar Shear Zone (MOSZ) of the Southern Granulite Terrane (SGT) of the state of Kerala, in India has been investigated using Anisotropy of Magnetic Susceptibility (AMS) and its low-field variation, as well as temperature variation of magnetic susceptibility vs temperature (Tc). The dominant magnetic minerals that control and contribute to the magnitude of the AMS were quantified and the regional deformation in the area and its effects on the rock types deciphered. The mean bulk susceptibility of the two rock types indicates the ferromagnetic character in the area. Ferromagnetic and ferrimagnetic minerals – magnetite and maghemite – are the major magnetic mineral contributors of AMS in the meta-ultramafic rocks of Wayanad whereas the mafic minerals – garnet, hornblende, biotite, and pyroxene – influence the magnetic bulk susceptibility in the gneisses. The development of fabric in the meta-ultramafics of Wayanad has an intimate evolutionary relationship with the gneisses and the shear zone itself. This has been demonstrated by magnetic and field-based studies. Both the magnetic foliation of the meta-ultramafics and the gneisses have orientations that fit well with the occurrence of the meta-ultramafic body in the field within the orientation of the Moyar Shear Zone.

A Practical Approach for Identification of Magnetic Fabric Carriers in Rocks

AbstractMagnetic fabric analyses of rocks by anisotropy of magnetic susceptibility (AMS) are a robust petrofabric tool that has been used in varied geological environments and tectonic settings. A fundamental difficulty of this method is to define the dominant magnetic phases and their resulting geological interpretation. We study the magnetic behavior of rocks by simulating data of mixed magnetic phases (i.e., diamagnetic, paramagnetic, and ferromagnetic). We show that it is possible to recognize the dominant magnetic phases by measuring the mean susceptibility at room temperature (kmRT) and at low temperature (kmLT). Distinct regions of magnetic phase dominancy are demonstrated in kmLT/kmRT versus kmRT and kmLT versus kmRT plots. We present a comprehensive approach by coupling the magnetic phase dominancy with possible magnetic fabrics, which are obtained from AMS measured at room and low temperatures (RT‐AMS and LT‐AMS) and anisotropy of anhysteretic remanent magnetization, into a scenario table. Application of this table allows a robust procedure for determining which magnetic phases are dominant, and permits a fast and reliable geological interpretation in complex settings.

Magnetic anisotropy of an ancient volcanic system: Flow dynamics of post-collisional Ediacaran volcanism in southernmost Brazil

Knowledge about flow dynamics of volcanic sequences is fundamental for understanding their emplacement and consequently the evolution of the associated volcanic terrain. Despite this importance, studies that apply different approaches to ancient volcanic systems are still rare. In this paper, we study the case of silicic volcanic sequences in southernmost Brazil, contributing to the interpretation of the post-collisional Ediacaran volcanic settings of the Sul-riograndense shield. Rock magnetism analyses, anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM), were performed on 32 sites of silicic volcanic rocks integrated with fieldwork observations. Magnetic mineralogy data indicate that magnetite or Ti-poor magnetite and high-coercivity phases (e.g., hematite) are the main magnetic carriers for the studied volcanic deposits. AARM results reveal an inverse magnetic fabric when single-domain grains are present, strongly affecting the interpretation of flow directions of lavas and ignimbritic deposits. AMS scalar results integrated with ignimbrite lithofacies analyses showed different fabric imbrication styles between stratified lower units and rheomorphic upper ignimbrites, allowing their separation in the emplacement model. Flow directions based on AMS, AARM data and field observations show a potential correlation of these volcanic deposits with an intrusive complex located on the southeastern border of the ignimbritic plateau. The emplacement of pyroclastic flow deposits was probably associated with a complex fissure system, where discontinuities within the basement plateau border may have served as feed conduits for these deposits. Our results highlight the importance of applying a regionally distributed AMS sampling coupled with a strong mineralogical and field control to the study of ancient volcanic systems.

Paleowind directions from the magnetic fabric of loess deposits in the western Chinese Loess Plateau and implications for dust provenance

AbstractThe aeolian loess-paleosol sequences in the Chinese Loess Plateau (CLP) are an excellent archive of variations in atmospheric circulation in the geological past. However, there is no consensus regarding the roles of the East Asian winter monsoon and westerly winds in transporting the dust responsible for loess deposition during glacial and interstadial periods. We conducted detailed measurements of the anisotropy of magnetic susceptibility (AMS) on two parallel loess profiles covering the most recent 130 ka in the western CLP to determine paleowind directions. Results show that the magnetic lineations of the loess and paleosol units in both sections are significantly clustered along the northwest to southeast direction. These observations demonstrate that the prevailing wind system responsible for dust transport in the western CLP was the northwesterly winter monsoon, rather than the westerly winds. The AMS-derived dust-bearing wind direction was relatively stable during the last glacial and interglacial cycle in the western CLP, consistent with sedimentary and AMS evidence from the eastern CLP. Accordingly, it is reasonable to conclude that large areas of deserts and Gobi deserts areas located in the upwind direction were the dominant sources for the aeolian deposits of the Loess Plateau.

Focal Mechanisms of Intraslab Earthquakes: Insights From Pseudotachylytes in Mantle Units

AbstractDevastating seismic events occur mainly in subduction zones, and a significant percentage of them are intraslab earthquakes. The geologic record of these events holds valuable information that needs to be investigated for a comprehensive seismic risk assessment. Here we investigate pseudotachylytes formed in oceanic peridotites and that are interpreted to result from intraslab seismic rupture. Each vein has recorded the seismic slip direction and slip sense of a single coseismic shear‐heating event. The well‐preserved exposures, showing individual veins up to 7 m in length and about 3 cm in width, of Cima di Gratera, in the Schistes Lustrés ophiolitic units of Corsica, offer unparalleled opportunities to investigate intraslab rupture kinematics in mantle rocks. The principal ferromagnetic phase in these rocks is a Ti‐poor magnetite. We use the anisotropy of magnetic susceptibility (AMS) recorded in pseudotachylyte generation veins (bulk susceptibilities range from 600 to 20,000 × 10−6 [SI] volume, with P′ ranging from 1.05 to 2.5) to reconstruct the co‐seismic deformation parameters, that is, fault plane attitude, direction and sense of slip. These new results, internally consistent at the vein level, span across oblate and prolate symmetries and reveal that seismic deformation recorded in these veins was kinematically diverse and included mostly normal mechanisms acting along the same subduction zone. In addition, our investigations show that the magnetic fabric of peridotite‐hosted pseudotachylytes provides key information bearing on the complex dynamics of frictional melts at a unprecedently high spatial resolution.

Internal structure of the Paleoarchean Mt Edgar dome, Pilbara Craton, Western Australia

The Paleoarchean East Pilbara Terrane of Western Australia is a dome-and-keel terrane that is often highlighted as recording a vertically convective tectonic regime in the early Earth. In this model, termed 'partial convective overturn', granitic domes diapirically rose through a dense, foundering mafic supracrustal sequence. The applicability of partial convective overturn to the East Pilbara Terrane and to other Archean dome-and-keel terranes is widely debated and has significant implications for early Earth geodynamics. A critical data gap in the East Pilbara Terrane is the internal structure of the granitic domes. We present field-based, microstructural, and anisotropy of magnetic susceptibility (AMS) data collected within the Mt Edgar dome to understand its internal structure and assess its compatibility with existing dome formation models. Field and microstructural observations suggest that most fabric development occurred under submagmatic and high-temperature solid-state conditions. The AMS results reveal a coherent, dome-wide structural pattern: 1) Sub-vertical lineations plunge radially inward towards the center of the dome and foliations across much of the dome consistently strike northwest; 2) Shallowly plunging lineations define an arch that extends from the center of the dome to the southwest margin; and 3) Migmatitic gneisses, which represent the oldest granitic component of the dome, are folded and flattened against the margin of the dome in two distinct lobes. The structural relationships between rocks of different ages indicate that units of different crystallization ages deformed synchronously during the last major pulse of granitic magmatism. These data are broadly consistent with a vertical tectonics model, and we synthesize our structural results to propose a three-stage diapiric evolution of the Mt Edgar dome. The critical stage of dome development was between 3.3 and 3.2 Ga, when widespread, melt-assisted flow of the deep crust led to the formation of a steep-walled, composite dome. These data suggest that diapiric processes were important for the formation of dome-and-keel terranes in the Paleoarchean.

Rock magnetism of ejected basaltic boulders from Lonar crater, India: Implications for the existence of a short‐lived impact‐generated weak magnetic field

AbstractThe continuous ejecta deposit around the rim of Lonar impact crater, central India, contains angular basaltic boulders of size ≤5 m. These boulders experienced varying level of shock between 2–30 GPa due to impact, as indicated by the extreme fracturing of these basaltic boulders, fragmentation of plagioclase and titanomagnetite constituents of these ejected boulders, and the presence of maskelynite in them. We measure some rock magnetic properties, e.g., NRM/χ (natural remanent magnetization [NRM]/bulk magnetic susceptibility [χ]), REM (=NRM/saturation isothermal remanent magnetization [SIRM] ratio expressed in %), and anisotropy of magnetic susceptibility (AMS) on 53 subsamples from 18 oriented drill cores of the shocked ejected basaltic boulders from the eastern half of ejecta deposit in the present study. The measured data are similar in many respects to our previous observations on Lonar crater rim shocked basalts (Arif et al. 2012b). For example, a small population of the ejected basaltic boulder samples show very high NRM/χ (between 378 and 989 Am−1; n = 7) and REM (between 1.5 and 7%; n = 4) and the AMS axes of these ejected basaltic boulders show triaxial distributions in stereographic projections. Moreover, some of the ejected basaltic boulders show higher values of squareness ratio (Mrs/Ms) and median destructive field (MDF) suggesting permanent changes in the intrinsic magnetic properties due to impact shock pressure. In stereographic plot, the high coercivity and high temperature (HC_HT) magnetization component of these ejected basaltic boulders are distributed in discrete clusters on the periphery of a small enveloping circle whose center (D = 108.0°, I = +69.2°) lies close to the HC_HT cluster of the crater rim shocked basalts. The center of this enveloping circle and the average HC_HT component of Lonar crater rim shocked basalts have the same statistical orientation, although the former has steeper dip. This distribution suggests the possibility that the ejected basaltic boulders, which were deposited during the modification stage of Lonar crater evolution, were magnetized in an impact‐induced magnetic field that was rapidly decaying just after the impact. Our present study suggests that the ejected basaltic boulders and Lonar crater rim shocked basalts experienced high shock pressure (≥2 GPa) magnetization during impact.

Anisotropy of magnetic susceptibility study to locate the feeder zone and lava flow directions of the Rajmahal Traps (India): Implications to Kerguelen mantle plume interaction with Indian Plate

Rajmahal Traps (RT) represents the earliest eruption from the Kerguelen hotspot activity over Indian Plate. The results of anisotropy of magnetic susceptibility (AMS) fabrics depict distribution anisotropy with the maximum susceptibility axes (Kmax) predominantly clustered NE-SW with plunge in NE. It infers that the feeder zone for RT was located in the NE quadrant of the volcanic province and along the Narmada-Son-Brahmaputra Transform (NSBT) fault. Our results further suggest that the NSBT system provide the major topographic control for the lava flow channel distribution in the Rajmahal province. It implies that the fracture zone was available during the lava emplacement, which records the earliest Kerguelen plume activity. Comparison between AMS fabrics and field topography suggests the distal horizontal magma flow and fissural mode in the absence of volcanic vent in the region, when the plume centre was located in the Comei-Cona region of Tibet.

Magnetostratigraphic study of a Late Cretaceous–Paleogene succession in the eastern Xining basin, NE Tibet: Constraint on the timing of major tectonic events in response to the India-Eurasia collision

Abstract Cretaceous-Cenozoic basins developed in the NE Tibetan Plateau contain key archives to unravel the growth history of the plateau in response to the India-Eurasia collision. Here we present magnetostratigraphic results of a Late Cretaceous to Paleogene succession of the Zhongba section outcropping at the southern margin of the eastern Xining basin. This succession consists of three lithological units punctuated by two stratigraphic unconformities, which best recorded the deformation history of this foreland basin. Detailed magnetostratigraphic investigation show that the lower terrestrial sedimentary rock unit, the Minhe Group, was deposited in latest Cretaceous in the time span of ca. 74.5–69.2 Ma; the middle unit was deposited in Paleogene in the time span of ca. 49.3–22 Ma; and the upper conglomeratic unit, not dated, possibly was deposited in early Miocene. Accordingly, the Cretaceous–Paleogene unconformity, widely observed in the foreland basins of NE Tibet, represents a sedimentary hiatus duration of ∼19.9 m.y., from ca. 69.2 Ma to ca. 49.3 Ma, which possibly recorded the far-field response to the tectonic transition from Neo-Tethys oceanic plate subduction to the India-Eurasia collision in southern Tibet. Changes in provenance, sedimentary accumulation rate, and mean susceptibility value at ca. 33–30 Ma, and the total prolate anisotropy of magnetic susceptibility (AMS) ellipsoids and provenance shifting since ca. 23–19 Ma, point to the pulsed growth of West Qinling, and rapid uplift of Laji Shan, respectively, indicating an enhanced effect of the India-Eurasia collision in Oligocene and early Miocene. AMS results show a clockwise rotation of the shortening direction from NEN-SWS in latest Cretaceous to NE-SW in Paleogene.

Highly anisotropic excitons and multiple phonon bound states in a van der Waals antiferromagnetic insulator.

Two-dimensional (2D) semiconductors enable the investigation of light-matter interactions in low dimensions1,2. Yet, the study of elementary photoexcitations in 2D semiconductors with intrinsic magnetic order remains a challenge due to the lack of suitable materials3,4. Here, we report the observation of excitons coupled to zigzag antiferromagnetic order in the layered antiferromagnetic insulator NiPS3. The exciton exhibits a narrow photoluminescence linewidth of roughly 350 μeV with near-unity linear polarization. When we reduce the sample thickness from five to two layers, the photoluminescence is suppressed and eventually vanishes for the monolayer. This suppression is consistent with the calculated bandgap of NiPS3, which is highly indirect for both the bilayer and the monolayer5. Furthermore, we observe strong linear dichroism (LD) over a broad spectral range. The optical anisotropy axes of LD and of photoluminescence are locked to the zigzag direction. Furthermore, their temperature dependence is reminiscent of the in-plane magnetic susceptibility anisotropy. Hence, our results indicate that LD and photoluminescence could probe the symmetry breaking magnetic order parameter of 2D magnetic materials. In addition, we observe over ten exciton-A1g-phonon bound states on the high-energy side of the exciton resonance, which we interpret as signs of a strong modulation of the ligand-to-metal charge-transfer energy by electron-lattice interactions. Our work establishes NiPS3 as a 2D platform for exploring magneto-exciton physics with strong correlations.

Synchronous Sedimentation in Gonjo Basin, Southeast Tibet in Response to India‐Asia Collision Constrained by Magnetostratigraphy

AbstractThe India‐Asia collision has been the object of vigorous debate for decades, with ages of the start ranging from Late Cretaceous to Oligocene. Sedimentary records preserved in both near‐field and far‐field settings provide critical evidence concerning the age and mechanism of the collision. Gonjo Basin, one of a series of fault‐controlled basins developed along the eastern margin of Tibetan Plateau, contains well‐exposed sedimentary successions, providing great potential for understanding depositional, tectonic‐geomorphological and climatic history associated with India‐Asia collision and plateau uplift. Lithofacies analysis reveals that the sequences consist of sandstone, mudstone and conglomerate, formed in fluvial‐lacustrine systems. High‐resolution magnetostratigraphy, constrained by U‐Pb dating of zircon from a volcaniclastic layer which has been found in the central Gonjo Basin for the first time, indicates that sedimentation started at 69 Ma and continued in this locality until 50 Ma. Anisotropy of magnetic susceptibility (AMS) interpretation based on the age framework reveals a post‐depositional compression after 50 Ma, responding simultaneously to the early uplift of southeast Tibetan Plateau driven by India‐Asia collision.

Late Triassic contractional tectonics in the overriding plate of the Sulu orogenic belt, Eastern China

It is well known that the Dabie-Sulu orogenic belt resulted from the northward continental subduction of the South China Block (SCB) beneath the North China Block (NCB) during the Triassic. Correlating the deformation events recorded in the subducted and overriding plates is essential to understand its tectonic evolution, but related deformation is poorly documented in the overriding plate. In this contribution, we present detailed structural analysis, anisotropy of magnetic susceptibility (AMS), and geochronological data in the Linsishan unit of the Jiaobei massif, the overriding plate of the Sulu orogenic belt. The Linsishan unit preserves a significant ductile deformation characterized by NNW-, NE-, or SE-dipping foliations with NE–SW trending mineral lineations. Coupling mesoscopic and magnetic fabrics augment the NE–SW linear structures. The NE-dipping monoclinal structures with top-to-SW kinematics could be interpreted as a SW-directed thrusting, the deformation temperature of which is 350–450 °C according to the quartz c-axis fabrics. The timing of deformation is constrained to be 223–221 Ma based on 40Ar/39Ar dating of the deformed rocks and linked with a syn-collisional stage in the Sulu orogenic belt. Combining these new results with previous works, we propose a three-stage evolution model for the Sulu orogenic belt, namely (1) subducted SCB experienced ultra-high pressure (UHP) metamorphism before 230 Ma; (2) SW-directed thrusting was developed in the overriding plate, coeval with the syn-collisional exhumation of UHP rocks during 230–220 Ma; (3) post-collisional migmatization, magmatism, extension, and UHP rocks exhumation took place at 220–205 Ma.

Reconstruction of subglacial depositional conditions based on the anisotropy of magnetic susceptibility: an example from Dębe (central Poland)

ABSTRACTThe analysis of low‐field anisotropy of magnetic susceptibility (AMS) was used to reconstruct the subglacial deposition conditions during the Main Stadial of the Odranian Glaciation (MIS 6) in till deposits from a site in Dębe (central Poland). Based on the AMS parameters, six till beds were identified (intervals 1–6). The declination of the maximum magnetic susceptibility axis (k1) indicates that the ice sheet was moving in from the northwest. The obtained results confirm the thesis about the preferred direction of ice‐sheet transgression during the Odranian Glaciation (MIS 6) in this part of Poland. This interpretation is also confirmed by data obtained from measurements of the long axis of clasts, which agree with the orientation of k1. Based on the AMS results, a significant part of the profile was deformed through simple shear and direct interaction of the ice sheet with the underlying sediment (beds 2–5). The lowest part of the till (bed 6) may have been deposited on a southeast‐trending slope or post‐depositional deformed by uneven loading of the ice cover. The upper part of the profile (especially in interval 1) could be deposited with an impact of pore water.

The Late Jurassic extensional event in the Yanshan fold and thrust belt (North China): New insights from an integrated study of structural geology, geophysics, and geochemistry of the Siganding granitic pluton

The Yanshan fold and thrust belt (YFTB) was built up by multiple contractional events, namely the “Yanshan Movement”, which was traditionally subdivided into the “Yanshan-A” and “Yanshan-B”. In contrast with the better constraint on “Yanshan-B” (ca.140 Ma), the timing of “Yanshan-A” is still in dispute. The Middle Jurassic contraction (172–160 Ma) was widely accepted, however, whether this contraction can last into 150 Ma is still unclear. In this article, to clarify the Late Jurassic tectonic regime in the YFTB, a multidisciplinary study, including structural geology, Anisotropy of Magnetic Susceptibility (AMS), gravity modeling, geochronology, and geochemistry, has been carried out to investigate the emplacement mechanism of the Siganding pluton (ca. 161–157 Ma) in the northern Beijing area. The magma was predominantly derived from the Precambrian continental lower crust. The magnetic foliations are pluton margin-parallel and outward dipping, resembling the fabric pattern of magmatic dome. The NE-SW striking magnetic lineations and NW-SE linear feeder zones revealed by gravity modeling indicate that the emplacement of the Siganding pluton was controlled by a weak NE-SW crustal stretching interpreted as a regional tectonic extension. Considering that the Late Jurassic (164–153 Ma) NE-SW oriented extension structures are not only documented in the YFTB, but also the entire eastern NCC, hereby we propose that the “Yanshan-A” is constrained at 172–164 Ma, instead of a wide range from the Middle Jurassic to Late Jurassic (172–150 Ma) as argued by previous workers. The extension could be ascribed to the oblique subduction of the Izanagi plate.