Lateral Extrusion Research Articles

Eocene–Miocene tectonic reworking of the southeastern Tibetan Plateau: Geochronological and geochemical records from granitoids along the Ailao Shan–Red River Shear Zone

The convergence of the Indian Plate and Eurasia shaped the Earth’s broadest and highest-elevation collisional system and topographic growth, which is not simply illustrated by ongoing contractional deformation. The Cenozoic magmatism along the Ailao Shan-Red River Shear Zone (ARSZ), linking surface geology with deep lithospheric processes, provides additional information regarding the age and progressive deformation of the India-Asia collision. In this study, we present new zircon U–Pb geochronological, elemental geochemical and zircon Hf isotopic data for the Eocene–Miocene granitoid to resolve the southeastern Tibetan Plateau deformation history. The Jinping granitoids in the southern ARSZ were dated at 37.5–33.6 Ma. They show similar geochemical characteristics (whole-rock major oxides, trace element, Sr–Nd–Pb isotopic and zircon in-situ Hf isotopic data) with respect to coeval potassic granitoids in NW Yunnan, suggesting their origination from partial melting of thickened lower crust with additional contribution from enriched mantle-derived magmas. Leucogranite samples within the ARSZ yielded distinctive zircon U–Pb ages of 28.3 Ma and 18.5 Ma. Geochemical features suggest that they were derived from a heterogeneous crustal source consisting of metasediments and metabasites. Our results, along with those previous studies, suggest that the widespread potassic magmatism in the central and southeastern Tibetan Plateau is related to lithospheric mantle upwelling, which resulted in the partial melting of the thickened lower crust and a short-lived crustal extension during a period of decreased convergence rate. Subsequently, the orogenic-scale sinistral strike-slip movement provided shearing heat for generating leucogranites within the shear zones during the late Oligocene–Miocene. The plateau-wide shift in stress reorganization and tectonic reworking led to the lateral extrusion of the SE Tibetan Plateau and its attainment of modern high elevation.

Labral Support Shelf Acetabuloplasty for Late Presenting Perthes Disease: Outcomes in Indian Patients.

The treatment of late-presenting Perthes disease with extrusion is controversial and debatable. One of the options available is the labral shelf acetabuloplasty (LSA). The aim of the study was to evaluate the results of LSA in late-presenting Perthes disease in terms of clinic-radiological outcome measures. A retrospective analysis of prospectively collected data of patients with late presenting Perthes disease (Elizabethtown stage 2B onwards) treated by LSA by 2 experienced paediatric orthopaedic surgeons was performed. Data was collected of clinical parameters such as hip range of motion(ROM) and Harris Hip score and radiological parameters such as acetabular height, width and volume, shelf width, Centre Edge angle(CEA) and the lateral extrusion. Thirty-five patients (28 males and 7 females) treated between 2012 to 2019 were analyzed. Majority were in Elizabethtown stage 3A (23) followed by 2B and 3B (12 each). At a mean follow up of 36months, the hip ROM and the Harris Hip Score (from 65 ± 3.5 to 81.33 ± 7.12) improved significantly and there was a statistically significant improvement in terms of all radiological parameters. Majority of the hips were in Stulberg grade 3 (20) followed by grade 1 and 2 (7 each) and Stulberg 4 (1). There were no major complications in any of the patients of the series. Labral support shelf acetabuloplasty is a valuable surgery for late presenting Perthes disease and helps in maintenance of good coverage and allows restoration of range of motion over time.

The Cenozoic spatiotemporal exhumation of the SE Tibetan Plateau: insight from the data mining and modeling of low-temperature thermochronology

Introduction: The SE Tibetan Plateau is distinct from other margins due to its high elevation, long wavelength, and low relief. A clear understanding of the Cenozoic exhumation history of this region is the key to understanding the special geomorphological process and the associated mechanisms. Previous thermochronological studies have either focused on vertical sections or horizontal variations in the local regions. However, the spatiotemporal exhumation pattern of the entire SE Tibetan Plateau is enigmatic.Methods: In this article, we have compiled 1,202 thermochronological data using joint kernel density estimation (KDE) and linear inversion approaches to address the exhumation process.Results: The results reveal that at least six episodes of rapid cooling have occurred since the Cenozoic, which include ∼61–58 Ma, 38–35 Ma, 32–23 Ma, 18–13 Ma, 11–6 Ma, and 4–3 Ma. Furthermore, the entire SE Tibetan Plateau underwent spatially inhomogeneous exhumation throughout the main episodes.Discussion: We infer that all cooling episodes may be attributed to the lateral extrusion and continuous convergence between the Indian and Eurasian continents. Meanwhile, climate changes (e.g., intensified Asian summer monsoon and glacial processes) have also played a non-negligible role in shaping the landscape since the Miocene. Our results will provide new insights into geodynamic mechanisms of the exhumation processes throughout the whole SE Tibetan Plateau since the Cenozoic.

Present–day crustal deformation in the southeastern Tibetan Plateau: Insights from three–dimensional finite–element modeling

Quantifying the present–day kinematics of southeastern Tibet is important for understanding the tectonic evolution of the Tibetan Plateau. In this study, we established a three–dimensional finite element model to estimate fault slip and strain rates in the southeastern Tibetan Plateau based on Global Navigation Satellite Systems (GNSS) observations. Our model yields an overall good fit to the GNSS velocity field with residuals of 0.83 mm/yr and 0.85 mm/yr for the east and north components, respectively, comparable to the uncertainties of 0.67 mm/yr and 0.61 mm/yr. The model–derived strike–slip rates of the Xiaojiang fault, Nanhua–Chuxiong fault, Qujiang fault, central segment of Red River fault, southeastern segment of Red River fault and Dien Bien Phu fault are 13.7 ± 0.7 mm/yr, 1.9 ± 0.6 mm/yr, 3.4 ± 1.1 mm/yr, 1.5 ± 0.5 mm/yr, 1.2 ± 0.4 mm/yr and 1.6 ± 0.4 mm/yr, respectively, which are consistent with previous geological results, implying that the short–term fault motions estimated from GNSS data can reflect the long–term fault slip in the southeastern Tibetan Plateau. The shear strain rate is localized along the Xiaojiang fault, which agrees with predictions of the tectonic extrusion model (i.e., the lateral extrusion of internally rigid blocks along strike-slip faults). Strain orientations in the western part of southeastern Tibetan Plateau are characterized by clockwise rotation. Comparison of the surface strain rates, crustal stress, and seismic anisotropy reveals general consistency between deformation of the surface and upper crust. The eastern part of southeastern Tibetan Plateau has small residuals, and the present–day crustal deformation can be explained by rigid block motion. However, the western part of the study area has relatively large residuals, implying that other processes, such as gravitational spreading, may influence deformation in the area. We propose a hybrid model that combines rigid block motion with continuum deformation to explain the contemporary crustal deformation of southeastern Tibetan Plateau.

Poster 279: A NOVEL MAGNETIC RESONANCE IMAGING MARKER FOR ANTERIOR INSTABILITY OF THE LATERAL MENISCUS

Objectives: Anterior instability (AI) of the lateral meniscus is potentially debilitating in young patients. The lack of reliable and accurate Magnetic Resonance Imaging (MRI) findings limits the ability to preoperatively recognize AI. The goal of this study is to present a novel MRI finding (phantom sign) for AI of the lateral meniscus. Methods: Two independent blinded reviewers performed retrospective review of preoperative MRI for all patients who underwent lateral meniscus repair between 7/1/2019 and 3/31/2022 by a single fellowship trained surgeon. Positive phantom sign was defined as attenuated or decreased signal intensity along the anterior horn of the lateral meniscus on a single coronal cut at the anterior tibial insertion of the ACL. The lateral meniscus was also evaluated on coronal MRI for lateral extrusion, posterior displacement and posterior “megahorn” on sagittal cuts, and increased fluid signal adjacent to the anterior horn or a radial tear at the anterior root on axial cuts. Intraclass Pearson correlations were calculated for inter and intra-rater reliability in assessment for presence of pathology on MRI. Predictive ability of each imaging finding with at least good (ICC 0.6 or above) reliability was also evaluated by comparing MRI and documented intraoperative findings. Results: 55 patients with preoperative MRIs of arthroscopically repaired lateral meniscus tears were included. 21 patients (38.2%) were female with median age 15.0 years (range 10.5-16.0). 26 patients (47.3%) had a discoid lateral meniscus (DLM). Lateral meniscus AI was present on arthroscopy for 25 knees (45.5%) and among these 18 had concurrent DLM. Intraclass correlation was good for phantom sign (ICC 0.75, 95% CI 0.60-0.84), lateral extrusion (ICC 0.60 95% CI 0.41-0.75), posterior displacement (ICC 0.71, 95% CI 0.55-0.82), and posterior “megahorn” (ICC 0.76, 95% CI 0.62-0.85). On predictive analysis, phantom sign was 95.0% sensitive and 74.2% specific for lateral meniscal anterior instability. Posterior displacement and posterior megahorn were specific for anterior instability (both 93.3%) though with limited sensitivity (32.0% and 28.6%, respectively). Conclusions: Phantom sign is a reliable and sensitive MRI finding for AI, even in the absence of frank displacement of the meniscus, and should prompt thorough arthroscopic evaluation for this pathology. It may help guide treatment decision-making and preoperative planning, which may in turn improve patient outcomes and reduce complications of untreated anterior instability. [Table: see text][Table: see text][Table: see text]

Room-Temperature Single-Step Production of Ultrafine-Grained Bulk Metallic Sheets From Al Powder

Abstract Obtaining fully dense products with high strength in one step at room temperature by powder metallurgy (PM) is generally not possible. However, doing so would reduce manufacturing and energy costs substantially. In this work, we have attempted to achieve this on commercially pure aluminum by utilizing the friction-assisted lateral extrusion process (FALEP), which has the capability of producing sheets from bulk or powder metal in a single step, by applying large shear strain. The texture, microstructure, and mechanical properties of the fully compacted powder sample were examined and compared to the bulk sheet’s properties obtained also by FALEP. The powder-FALEP sample showed a smaller grain size and significantly higher strength. Simulations carried out by the Taylor-type lattice curvature-based polycrystal model shed light on the texture characteristics of the obtained materials and were in good agreement with the experiments.

(414) Double Distal Fixation Stitch

Abstract Introduction Distal cylinder extrusion is not a common complication of penile prothesis surgery when compared to mechanical device failure but can have very serious complications if not addressed properly. Reported revision surgery for IPP implantation is more commonly required for cosmetic and erosion problems as opposed to mechanical failure. Complications such as distal crossovers, delayed distal crossovers, and impending lateral extrusion can be treated with a double distal fixation stitch. This technique addresses stabilization issues and keeps penile implant secure and in place. Objective We demonstrate the Double Distal Fixation Stitch for the meandering distal cylinder Methods 62 y/o white male Hx of erectile dysfunction and Penile implant No significant past medical history Presents with pending left lateral cylinder extrusion Implant functional Patient underwent a corporoplasty and double distal fixation stich in order to ensure distal aspect of cylinder remains in place Results A lateral sub-coronal incision is used on the affected side where cylinder should be positioned. Dissection is carried through the Buck fascia, followed by a transverse incision of the tunica albuginea, where the distal aspect of the affected cylinder is delivered. Two 2-0 Ethibon sutures are threaded through the distal cylinder islet of the implant. A new, properly positioned intracorporal channel is created using a longitudinal capsulotomy and dilation with a 9 Hagar. Two 2-0 Ethibon sutures are passed through the distal islet of the implant cylinder. The first suture is passed through the glans with the cylinder in the native corpora position, a second suture is passed through about .25cm apart from first suture creating a small bridge. A small incision is made connecting the two suture exit sites. Both sutures are then tied down individually leaving one suture on each side to be tied across the bridge. The knots are then buried in the glans by elevating the skin and closing the skin with 3-0 chromic. Conclusions This technique ensured that the cylinder remained in the newly created, appropriately positioned channel. The double distal fixation stitch is safe and effective in securing the distal cylinder of an inflatable penile prosthesis and preventing recurrence. This technique is a useful surgical adjunct to treating patients with prosthetic lateral extrusions or crossovers that can be applied in almost all cases and can ensure distal stabilization of a migrating distal cylinder in weak tunica. Disclosure Any of the authors act as a consultant, employee or shareholder of an industry for: coloplast, boston scientific.

Three-dimensional kinematics of the India–Eurasia collision

The collision between India and Eurasia mobilizes multiple processes of continental tectonics. However, how deformation develops within the lithosphere across the Tibetan Plateau is still poorly known and a synoptic view is missing. Here, we exploit an extensive geodetic observatory to resolve the kinematics of this diffuse plate boundary and the arrangement of various mechanisms down to upper-mantle depths. The three-dimensional velocity field is compatible with continental underthrusting below the central Himalayas and with delamination rollback below the western syntaxis. The rise of the Tibetan Plateau occurs by shortening in the Indian and Asian crusts at its southern and northwestern margins. The subsidence of Central Tibet is associated with lateral extrusion and attendant lithospheric thinning aided by the downwelling current from the opposite-facing Indian and Asian collisions. The current kinematics of the Indian-Eurasian collision may reflect the differential evolution of the inner and outer Tibetan Plateau during the late Cenozoic.

Laboratory Test Study on Pile Jacking Penetration Mechanism Considering Different Diameter and Length Based on Photoelectric Integration Technology

Model tests are carried out on the jacked single piles of different diameters and pile lengths under the model pile of different diameters and pile lengths in clayey soil, which aims to investigate the penetration mechanical mechanism. How to accurately test the pile end resistance and pile side resistance during jacked pile sinking is particularly important. In this paper, a full-section spoke-type pressure sensor, a double diaphragm temperature self-compensating fiber Bragg grating (FBG) earth pressure sensor and a sensitized miniature FBG strain sensor are jointly applied to a single pile penetration model test to test a single pile driving force, pile end resistance and pile body stress during penetration. The test results show that the load transfer performance of test piles will be affected by different diameters, and the axial force transfer capability of a large diameter in the depth direction is better than that of a small diameter since the compacting effect is more obvious. The unit skin friction of the pile increases gradually as the depth increases, which is larger due to the lateral extrusion force increasing as the diameter increases. At the same depth, the unit skin friction of two different diameter piles demonstrates “friction fatigue”, which also decreases obviously as the depth increases. Under the conditions of this test, the maximum frictional resistance of the pile TP1 pile side is about 27.7% higher than that of the test pile TP2. In the static pile sinking process of three test piles in cohesive soil, 50% is end bearing; therefore, there is 50% friction, and the diameter influences the end bearing and the length influences the friction.

Northeastward expansion of the Tibetan Plateau: Topographic evidence from the North Qinling Mts.–Weihe Graben Coupling system, Central China

Mountain-Basin Coupling systems form during late stages of the collisional orogeny when mountains rise compared to the adjacent sedimentary basins. In this study, we focus on the spatial and temporal pattern of uplift, subsidence and related topographic features of the transtensional Weihe Graben–North Qinling Mountain Coupling system, at the northeastern extension of the Tibetan Plateau. The opening of the Weihe Graben and the ongoing high seismicity in the Weihe Graben result from the lateral extrusion of the Qinling Block away from the Tibetan Plateau. We quantify the influence of uplift gradients on topography and drainage divides by (a) carrying out field surveys, (b) conducting a digital elevation model-based analysis of topographic properties, and (c) compiling and integrating data on exhumation, uplift, subsidence, and denudation of numerous previous studies. Together, these morphological analyses show that the influence of the uplift of the Tibetan Plateau on the study area gradually decreases from the westernmost Taibai Mts. to the easternmost Huashan Mts. We found large χ gradients across the drainage divide of the Huashan Mts. and river piracy implying a southward migration of the drainage divide in the Huashan Mts. We conclude that the initial exhumation of the North Qinling Mts. occurred during the Lower Eocene to Oligocene (cycle 1). As peneplanation surfaces formed, this implies discontinuous exhumation and surface uplift. A second stage of uplift occurred in the Oligocene to Middle Miocene (cycle 2), followed by the third stage from Middle to Upper Miocene (cycle 3). The final period of uplift occurred from latest Miocene to Holocene times (cycle 4). The uppermost Miocene to Holocene geomorphological features imply surface uplift within the westernmost North Qinling Mts. and are controlled by the inflation of a mid-crustal magma layer, prograding from the eastern Tibetan Plateau towards the Taibai Mts., associated with lateral extrusion of the Qinling Block away from the rising Tibetan Plateau. The high seismicity of the Weihe Graben implies ongoing tectonic activity influenced by deep-seated lithospheric motion.

Measurements of Rayleigh wave ellipticity anisotropy and implications for distinct crustal deformation styles across the SE Tibet margin

Plateau growth of southeastern Tibet is often understood by applying the end-member conceptual models, such as ductile material flow in the deep crust, and lateral extrusion of coherently deforming or rigid blocks. Though recent studies suggest the local crustal flow and movements of rigid blocks may be compatible in a way, ambiguities still persist in the formation of the gently sloping margin of southeastern Tibet and interactions between the plateau and the outer forelands. Here we present a 3D crustal deformation model in southeastern Tibet and adjacent regions, by summarizing different types of seismic anisotropy measurements including a newly obtained continuous map of uppermost crustal anisotropy, which are derived from observations of the directionally dependent teleseismic Rayleigh wave ellipticity. Layered deformation is observed within the crust beneath the southeastern Tibet Plateau, while the outer foreland shows an overall coherently deforming mode. The conspicuous change of the crustal deformation styles across the southeastern Tibet margin is against the idea that the low topographic gradients of the plateau margin were produced by leakage of ductile deep crust beneath the plateau. We argue that both the crustal flow beneath the plateau interior and lateral extrusion of the outer foreland accommodate the intracontinental convergence at southeastern Tibet.

Pulsed counterclockwise rotation of the southwestern Sichuan Basin in response to the India-Asia convergence during 128-42 Ma

Deciphering the building history of the eastern Tibetan Plateau since the India-Asia convergence is crucial to understand geodynamic models of plateau formation. The Sichuan Basin adjacent to the eastern Tibetan Plateau margin potentially provides valuable records of when and how the Tibetan Plateau expanded eastward. In this study, we reconstruct ∼128–42 Ma rotation history of the southwestern Sichuan Basin using detailed paleomagnetic declination data from the Early Cretaceous to Eocene sedimentary sequence of ∼2400 m thick. A total of 78 sampling sites (774 samples) yield primary remanent magnetization confirmed by positive paleomagnetic tests. Our results show that the southwestern Sichuan Basin experienced a long-term clockwise rotation during 128–42 Ma which was interrupted by three transient and rapid counterclockwise rotation pulses. The earlier two pulses (86–82 Ma and 66–60 Ma) coincide with two India Plate acceleration episodes which are probably induced by the Marion and/or Réunion mantle plume upwelling, whereas the later pulse (53–46 Ma) temporally corresponds to the sudden slowdown of the India Plate caused by the India-Asia collision. Our findings indicate that the eastern Tibetan Plateau experienced prolonged and episodic building processes since the Late Cretaceous, long predating the traditional view of the late Miocene. The Tibetan Plateau was built synchronously from the Himalayas to its eastern margin. Lateral extrusion, block rotation, and strike-slip deformation collectively contribute to the mountain building in the eastern Tibetan Plateau region.

Ladoga-Lake-Area Lambeosaurus Capture by A Falling Black Hole, and Its Re-Emergence Through Hydrated Lateral Extrusion, Explanation of The Scandinavian Peninsula and of the Finnish Lakes as Well as of the Low-Ob’s Delta and of the Novaya Zemlya

Two Lambeosauridae in a homosexual act were gobbled into the Earth by a small black hole descending along the axis of the Moon to the Earth. These animals killed into the mantle explain the shape of Scandinavia and of lakes in Central and Eastern Finland as well as topographical structures in Russia’s North. They were shattered into the small black hole and their bodies triggered rerise of corresponding structures explaining the peculiarities of the area in a topographical way and geological way as well, from the olivines of the Scandinavian Alps to the pattern of the Urals and of Novaya Zemlya. The construction from eruptive processes within the mantle explains also the still-ongoing counter-subsidence of the Turun Saaristo and of the Kvarken archipelagoes, altogether with oil reserves on the Norwegian side.

Investigation on the Stratigraphic Response and Plugging Effect Induced by Press-In Open Caisson in Mucky Soil

Press-in open caisson method, which sinks into soil with the aid of mechanical pressure without excavation, is becoming an important method of construction in extremely soft foundations. However, many engineering problems are prone to occur in the construction of caissons, such as sinking and large deformation. In this study, the stratigraphic response model, such as soil migration during the press-in process of open caisson, is focused on. Through model tests of soil and caissons, the migration patterns of each part of the soil were carried out, and the mechanism underlying the stratigraphic response pattern was investigated. The three primary conclusions are as follows: 1) The bottom edge foot of caisson produces lateral extrusion on surrounding soil in the range of 10 cm (20%H) below, resulting in lateral movement of soil. 2) two kinds of plug, i.e., “wedged” and “unwedged” plugs exist, and the driven forces for the soil migration can be divided into three stages, i.e., caused by soil plug with active arch, by soil plug with passive arch and caused by soil squeezing effect. 3) there is a significant linear correlation between the soil plug changes, the horizontal movement of the deep soil body, and the displacement of the surface uplift. The soil plugging effect and squeezing effect are interacted and enhanced with each other. This study provides a new understanding of the stratigraphic response in the construction of press-in open caissons, and guides safety control of open caissons construction.

Geodetic and seismic constraints on contemporary deformation on the northeastern Tibetan plateau: Velocity and strain rate tensor analysis

The study of the velocity and strain rate of the northeastern Tibetan Plateau can explain its crustal deformation characteristics. Previous investigations have relied on mathematical interpolation in GPS velocity to obtain a continuous strain field; these approaches can be applied well to regions where the deformation is relatively smooth and are not entirely suitable for the northeastern Tibetan Plateau that is cut by several large strike-slip faults. Thus, this study incorporates seismic constraints into the GNSS velocity and strain analysis to obtain a more realistic deformation field and compares it with the uplift rate and the stress results in depth, it will provide a comprehensive view of contemporary deformation on the northeastern Tibetan Plateau. It shows that there are two prominent deforming zones localized on the middle segments of the Haiyuan and East Kunlun faults, which may result from the different strengths in fault segmentation. The velocity and strain rate in the four profiles show that the convergence strain obviously decay from ∼15.5 mm/yr and ∼ 15.48 nstrain/yr in the western Qilian Mountains to ∼5.73 mm/yr and ∼ 5.76 nstrain/yr in the eastern Long Zhong Basin, respectively. The comparison between the vertical uplift rate, SKS result, and strain rate field indicates that the northeastern Tibetan Plateau is not only dominated by crustal shortening corresponding to the topography and surface fault geometry, but also by the strain transfer of motion along these significant strike-slip faults accounting for the extrusion and growth of the Tibetan Plateau. Combined with recent seismological results, our results indicate that the deformation of the study region resembles that of the Tibetan Plateau and can be viewed as a combined process linking both NE- or NEE-directed crustal shortening, vertical uplift, lateral extrusion along strike-slip faults and partial melting or mantle upwelling at depth.

Oblique lateral extrusion during dextral transpression along the Beardmore-Geraldton belt, Canada

The Beardmore-Geraldton belt (BGB) is a boundary zone between the Wabigoon and Quetico subprovinces of the Archean Superior Province, Canada. It consists of interleaved metavolcanic and metasedimentary panels that were imbricated during an early D1 thrusting event and regionally folded during a D2 event. D1 and D2 steepened stratigraphy and produced contact-parallel shear zones and a regional bedding-parallel cleavage, which controlled the development of a D3 dextral transpression zone. During D3, the panels deformed by pure shear and were overprinted by a second regional cleavage. Bedding and the early cleavage acted as subvertical planes of anisotropy that were reactivated during D3 dextral shearing. Z-shaped drag folds and pre-existing D1 and D2 folds are parallel to a shallowly-plunging (20°–30°) stretching lineation, which together with the fold axes rotated towards the extensional flow apophysis during D3 shearing. As the lineation and fold axes are oblique to the sub-horizontal shear direction expressed by the intersection between synthetic shear bands and the early cleavage, the reactivated shear zones are transpression zones with shallowly-plunging lateral extrusion and triclinic symmetry. Thus, the shear zones are transpressive within a regional tectonic transpression zone that extends across the BGB and over 500 km along the Quetico-Wabigoon boundary.

Crustal azimuthal anisotropy in the lateral collision zone of the SE margin of the Tibetan Plateau and its tectonic implications

SUMMARYResolving the detail of crustal deformation of the seismically and technically active Sichuan-Yunnan block located in the southeast margin of the Tibetan Plateau is crucial for understanding the lateral extrusion of the Tibetan Plateau. Using seismic data from two regional seismograph stations of the China Earthquake Networks Center and twenty recently deployed broad-band temporary stations from the MCD Array (Middle ChuanDian block seismic array), we constrained crustal thickness and seismic anisotropy at the west boundary of the Sichuan-Yunnan block, near the east Himalayan syntaxis by receiver function techniques including CCP stacking and sinusoidal moveout of PmS conversions. Crustal thickness varies significantly across major faults inside the block which increases by >20 km from west to east. East of the Chenghai Fault, a clear positive phase lies above the Moho and is interpreted to be caused by the magma intrusion related to a late Permian Emeishan large igneous province (ELIP). The crustal anisotropy shows strong zoning features and clockwise rotation with respect to due north. To the west of the Lijiang-Xiaojinhe and the Honghe Faults, anisotropic fast orientations are near NS, indicating strong crustal deformation and movement of southward crustal flow along the Nujiang-Lancangjiang Fault; to the east of this fault, anisotropic fast orientations are near NE. The variation of the anisotropic fast orientations indicates that the southeastward crustal flow from the Tibetan Plateau may be prevented by fast velocity anomalies beneath the ELIP. These observations provided new evidence for strong crustal deformation along the tectonic boundary in the southeastern Tibetan Plateau with the expansion of the Tibetan Plateau.

Paleomagnetic constraint on the formation of the Eastern Himalayan Syntaxis: A new late Eocene result from the Mangkang area of the eastern Tibetan Plateau

The Eastern Himalayan Syntaxis (EHS) is a major structural and geomorphological unit in Eastern Tibet; however, its mechanism of formation and the extent of its northward effect of compression remain unclear. To address these questions, we conducted a paleomagnetic and geochronologic study of the late Eocene trachyte intrusive mass in the Mangkang area of Tibet. The U-Pb SHRIMP II age estimate of the trachyte intrusive mass is ∼40 Ma, and the estimated mean paleomagnetic direction we obtained is D=27.0°, I=36.8°, k=42.9, α95=1.3°, and n=282 specimens. These results indicate that the Mangkang area was located at a paleolatitude of 20.5 ± 0.9° N during the late Eocene. A comparison of paleolatitudes of the Qiangtang block with those inferred from the apparent polar wander paths (APWP) of India and Eurasia suggests that the Eastern Himalayan Syntaxis was formed after 35 Ma when Eurasia was penetrated by ∼600 km by northward-moving India. This is consistent with geologic evidence of intra-continental shortening in the foreland of the Eastern Himalayan syntaxis. The northward counterclockwise penetration of the corner of India also resulted in the large right-lateral strike-slip system of the Sagaing fault (SGF), with ∼400-600 km of displacement between the EHS and the Shan-Thai block. Thus, the northward wedging of the EHS indicates minimal southward displacement of the Shan-Thai block. Our results link the tectonic activity of the EHS to the lateral extrusion of the southeastern margin of the Tibetan Plateau, providing a new perspective on the evolution of this region since the late Eocene.

WEIGHT BEARING 3-D JOINT SPACE WIDTH DISTRIBUTION AT THE KNEE VARIES ACCORDING TO LOCATION AND EXTENT OF MENISCAL EXTRUSION: A MOST INVESTIGATION

Quantitative 3-D measurement of joint space width (JSW) from weight bearing computed tomography (WBCT) allows analysis of knee tibiofemoral compartments in a loaded stance without subregional constraints on analysis. The relationship between 3-D JSW distribution and meniscal extrusion is yet to be established. To demonstrate topographic variations in 3-D JSW distribution at the knee derived from WBCT imaging data according to meniscal extrusion location and extent. WBCT knee imaging was acquired at the 144-month visit of the Multicenter Osteoarthritis Study (MOST). A subset of individuals had medial and lateral meniscal extrusion grading according to the MRI Osteoarthritis Knee Score to identify medial extrusion of the medial meniscus (MMmed), anterior extrusion of the medial meniscus (MMant), anterior extrusion of the lateral meniscus (LMant), and lateral extrusion of the lateral meniscus (LMlat). The extent of extrusion was graded as 0: <2 mm; 1: 2 to 2.9 mm, 2: 3 to 4.9 mm; and 3: >5 mm. After semi-automatic segmentation of 663 baseline knees, joint space mapping was performed to create 3-D JSW maps for each knee registered to a template surface. Statistical parametric mapping (SPM) was performed using a general linear model to test the dependence of 3-D JSW distribution on each of meniscal extrusion type in turn. SPM results were plotted on the canonical joint surface, with unmasked regions demonstrating the topographic difference in JSW for each increment in extrusion score. 568 knees from 330 participants had meniscal extrusion grading available. Participants (178 female) had mean ± SD age 62.0 ± 9.3 years, body mass 82.2 ± 17.4 kg, height 170 ± 9.5 cm, and BMI 28.3 ± 4.8 kg/m2. Analysed knees were bilateral for 238 participants and unilateral for 92 participants. For MMmed (cohort score range 0-3), the central-to-posterior medial compartment had significantly lower JSW by up to 0.4 mm for each increment in meniscal extrusion grade. Both MMmed and MMant (score range 0-3) were associated with a small region of similarly significantly lower JSW along the medial margin of the lateral compartment, but also with significantly greater JSW at the anterior medial compartment of up to 0.6 mm per grade. The posterior medial joint space was significantly narrower (up to 3 mm) comparing knees with LMant scores of 0 vs 1. This same magnitude of narrowed JSW was detected along the periphery of the lateral joint space comparing grades in the lateral tibiofemoral compartment, but this was not statistically significant. LMlat (score range 0-2) was associated with a trend for greater reduction in JSW in the central lateral compartment of up to 0.5 mm per grade, but this was not statistically significant. Direction of meniscal extrusion is represented by the black arrowhead in the figure. Both medial and anterior extrusion of the medial meniscus are associated with significant reduction in JSW at the medial margin of the lateral joint space and widening of the anterior margin, likely from a posteromedial shift of the femoral condyles with respect to the tibia. Medial extrusion of the medial meniscus is also associated with narrowed JSW in the central to posterior medial joint space, likely as a direct effect of the loss of meniscal hoop tension. Anterior (but not lateral) extrusion of the lateral meniscus is associated with significantly lower JSW in the posterior medial joint space. These findings confirm that the extent and location of meniscal extrusion are key factors in weight bearing 3-D JSW distribution.

Mantle structure and dynamics at the eastern boundary of the northern Cascadia backarc

The tectonics of southwestern Canada are dominated by the Cascadia subduction zone. The northern Cascadia backarc encompasses a > 400 km wide region of the Southern Canadian Cordillera. Geophysical observations, including seismic tomography and surface heat flow, show that the backarc is characterized by a hot, thin lithosphere (60–70 km). The eastern limit of the backarc approximately underlies the Rocky Mountain Trench, where there is an abrupt eastward increase in lithosphere thickness to the ∼250 km thick North American (Laurentian) Craton. Seismic tomography studies show that the transition in lithosphere thickness occurs over a horizontal distance of 50–100 km, resulting in a subvertical to west-dipping lithosphere step, with a dip angle of 75–90°. Using numerical models, we show that such a structure can be readily destabilised by internal buoyancy forces, edge-driven convection, and shearing by regional mantle flow. To maintain a subvertical step for > 50 Myr, the lowermost craton mantle lithosphere must be both dry and moderately chemically depleted. The observed westward dip may reflect partial lateral extrusion of the lowermost craton lithosphere, as well as shearing from west-directed mantle flow associated with the Cascadia subduction zone. The models also show that the backarc mantle must be relatively weak, such that vigorous convection maintains the hot, thin lithosphere. This also provides a mechanism to explain the observed lateral seismic gradient between the low-velocity backarc mantle and high-velocity craton. Our models demonstrate that the eastern limit of the Cascadia backarc is a region of active mantle flow, including possible slow deformation of the craton edge.