Block Margins Research Articles

Late Mesoproterozoic passive continental margin in the northwestern Yangtze Block, South China: Insights from the Huodiya Group in the Hannan‐Micangshan Massif

Whether the Yangtze Block was involved in the global late Mesoproterozoic orogeny remains contentious. The Mesoproterozoic Huodiya Group exposed in the Hannan‐Micangshan Massif on the northwestern margin of the Yangtze Block could provide sufficient evidence to further clarify the regional tectonic affinity. However, quantitative insights into the depositional age, provenance and basin tectonic setting of the Huodiya Group still lack comprehensively constraining. This study presents a combined analysis of zircon U–Pb dating for meta‐sedimentary rocks of the Shangliang Formation of the upper Huodiya Group and intruded gabbroic dykes. Intergraded dating results constrain the late Mesoproterozoic maximum depositional age at ca. 1050 Ma. Ca. 900 Ma formation age for the intruded gabbroic dyke, plus the minimum formation age of the overlying Xixiang Group at ca. 950 Ma, further suggests its deposition should be prior to ca. 950–900 Ma. Concordant detrital zircon ages define pronounced age clusters of ca. 2229–1741 and 2950–2388 Ma and few age populations of ca. 3232–3082 and 1069–1033 Ma, consistent with the tectonothermal events in proximal domains at the northwestern and northern Yangtze Block. Detrital zircon age distribution patterns and cumulative curves, coupled with the stable carbonate platform and shallow‐marine facies sedimentation, suggest a passive continental margin setting for the Huodiya Group on the Yangtze Block margin at the late Mesoproterozoic. Comparative insights into the contemporaneous passive continental margin from north to southwest argue against the existence of a late Mesoproterozoic orogeny along the exterior periphery of the Yangtze Block.

Middle Miocene Onset of the Litang Fault System Records Kinematic Change in Eastern Tibet

AbstractThe ∼400‐km‐long Litang fault system (LTFS) is a major intracontinental strike‐slip fault inside the Chuandian block, eastern Tibet, but its evolution and role in accommodating the India‐Asia convergence remain poorly known. Structural analysis shows that the LTFS splits into 5 strands as a left‐lateral, right‐stepping en‐echelon pattern formed under NW‐directed compression, subsequently reactivated by transtensive faults under NNE‐directed extension. Displaced geological and morphological markers yield a cumulative left‐lateral offset of 28.9–42.8 km. Inverse thermal‐history modeling of thermochronological data of the faulted rocks reveal accelerated cooling at 38–35 Ma, 16–13 Ma, and 7–5 Ma. The late Eocene rapid cooling is ascribed to the reactivation of the Garze‐Litang suture. Rapid cooling events at 16–13 Ma and 7–5 Ma record the onset of transpression and transtension of the LTFS, respectively, yielding a geologic slip rate of 2.6 ± 0.7 mm/yr. Both bifurcated geometry and slow slip rate of the LTFS since 16–13 Ma indicate diffuse deformation inside the Chuandian block, contrasting with strain localized on fast‐slip strike‐slip faults on the block margins. This implies a significant kinematic transition in the middle Miocene, such that the extrusion of the segmented mega‐blocks has been accommodated by both localized and distributed deformation in eastern Tibet. This tectonic transition could be explained by a change in lithospheric rheology from an earlier rigid state to a viscous state underneath the Chuandian block due to thermal weakening of the lower crust. We thus reconcile the end‐member geodynamic models of block extrusion and lower crustal flow in late Cenozoic times.

Cleaning the dose falloff with low modulation in SBRT lung plans

Purpose. This dosimetric study is intended to lower the modulation factor in lung SBRT plans generated in the Eclipse TPS that could replace highly modulated plans that are prone to the interplay effect. Materials and methods. Twenty clinical lung SBRT plans with high modulation factors (≥4) were replanned in Varian Eclipse TPS version 15.5 utilizing 2 mm craniocaudal and 1 mm axial block margins followed by light optimization in order to reduce modulation. A unique plan optimization methodology, which utilizes a novel shell structure (OptiForR50) for R50% optimization in addition to five consecutive concentric 5 mm shells, was utilized to control dose falloff according to RTOG 0813 and 0915 recommendations. The prescription varied from 34–54 Gy in 1–4 fractions, and the dose objectives were PTV D95% = Rx, PTV Dmax < 140% of Rx, and minimizing the modulation factor. Plan evaluation metrics included modulation factor, CIRTOG, homogeneity index (HI), R50%, D2cm, V105%, and lung V8–12.8Gy (Timmerman Constraint). A random-intercept linear mixed effects model was used with a p ≤ 0.05 threshold to test for statistical significance. Results. The retrospectively generated plans had significantly lower modulation factors (3.65 ± 0.35 versus 4.59 ± 0.54; p < 0.001), lower CIRTOG (0.97 ± 0.02 versus 1.02 ± 0.06; p = 0.001), higher HI (1.35 ± 0.06 versus 1.14 ± 0.04; p < 0.001), lower R50% (4.09 ± 0.45 versus 4.56 ± 0.56; p < 0.001), and lower lungs V8–12.8Gy (Timmerman) (4.61% ± 3.18% versus 4.92% ± 3.37%; p < 0.001). The high dose spillage V105% was borderline significantly lower (0.44% ± 0.49% versus 1.10% ± 1.64%; p = 0.051). The D2cm was not statistically different (46.06% ± 4.01% versus 46.19% ± 2.80%; p = 0.835). Conclusion. Lung SBRT plans with significantly lower modulation factors can be generated that meet the RTOG constraints, using our planning strategy.

Matching Protocol and Practice: The Challenge of Meeting Lung and Kidney Total Body Irradiation Constraints for Scleroderma

TBI, a form of immunomodulation, improves treatment outcomes for rapidly progressive scleroderma. The landmark Scleroderma: Cyclophosphamide or Transplantation (SCOT) trial used strict 200 cGy lung and kidney dose restrictions to limit the likelihood of normal tissue toxicity. The protocol, as written, did not specify how or where the 200 cGy limit was to be measured, opening the door to variable techniques and outcomes. Following the SCOT protocol, a validated 18 MV TBI beam model was used to evaluate lung and kidney doses with varying Cerrobend half-value layers (HVLs). Block margins were constructed per the SCOT protocol. Using the 2 HVL SCOT block guidelines, the average central point dose under the lung block center was 353(±27) cGy, almost double the mandated 200 cGy. The mean lung dose was to 629(±30) cGy, triple the mandated 200 cGy. No block thickness could achieve the mandated 2 Gy, due to contribution from unblocked peripheral lung tissue. With 2 HVLs, the average kidney dose was 267(±7) cGy. Three HVLs were needed to reduce it below 200 cGy, meeting the mandated SCOT limit. There is considerable ambiguity (and inaccuracy) in lung and kidney dose modulation for TBI. It is not possible to achieve the mandated lung doses using the protocol-specified block parameters. Future investigators are encouraged to take these findings into account to develop more explicit, achievable, reproducible and accurate TBI methodology.

Interaction of thin-skinned detached faults and basement-involved strike-slip faults on a transform margin: the Moattama Basin, Myanmar

Abstract The Sagaing Fault (SF), one of the world's most active strike-slip faults, defines a plate boundary on the eastern West Burma Block margin, from the Andaman Spreading centre, northwards for &gt;1600 km to the eastern Himalayas. In the Northern Andaman Sea the SF traverses the Late Miocene–Recent Moattama Basin. There, 2D and 3D seismic reflection data show highly unusual fault patterns, that overall resemble a giant (area &gt;33 000 km 2 ) horsetail structure. A horsetail pattern typically implies loss of displacement at a fault tip, which is potentially incompatible with the SF forming a transform margin. In the thinner, northern part of the Late Miocene–Recent basin three branches of the SF can be identified. These become lost in the thickest (&gt;7 km), central part of the basin, and two branches emerge to the south where the basin thins. The fault patterns are interpreted to represent a previously unknown interaction of thin- and thick-skinned styles, where relatively shallow detached structures and sediment loading have interacted with basement-involved strike-slip faults that form a releasing bend geometry at the basement level. The Moattama Basin demonstrates how very thick sedimentary basins can produce fault patterns that differ from classic structural models.

Middle-lower crustal flow in response to the India-Eurasia collision: Structural evidence from the southern Chong Shan belt within the Sundaland block, southeastern Tibetan Plateau

Abstract In this contribution, we present new structural, microstructural, fabrics, and geochronological data from the southern Chong Shan complex, one of the metamorphic complexes in the southeastern Tibetan Plateau that were sheared and exhumed during the India-Eurasia convergence. The NW-SE–striking complex is comprised of a central high-grade metamorphic zone (Unit I) flanked by two low-grade metamorphic zones (units II and III) on the northeastern and southwestern sides, respectively. High-grade metamorphic rocks (e.g., amphibolites, sillimanite-mica schists) of up to amphibolite facies, of the Proterozoic Chong Shan group and granitic intrusions of Permo-Triassic to Cenozoic in age in Unit I are characterized by high-temperature deformation. Units II (i.e., the Wuliangshan group) and III (i.e., the Lancang group) on both sides of the high-grade Unit I consist of metamorphic rocks of low greenschist facies (e.g., phyllites) with low-temperature deformation. The high- and low-grade units possess consistent kinematics, i.e., northwestward motion of the core rocks relative to the two limbs, and they are separated by large scale shear discontinuities. Thereby, the high- and low-grade units are kinematically linked but mechanically decoupled. Zircon laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of syn-shearing granitic dikes reveals that ductile shearing occurred from 29 to 19 Ma. Structural analysis reveals that these units constitute an A-type dome that has long axis parallel to the stretching lineations and fold axes of outcrop-scale A-type folds. It is shown that three stages of deformation contributed to the formation of the southern Chong Shan dome, during which subhorizontal shearing were in connection with regional doming. The events occurred as the consequence of middle to lower crustal flow that led to lateral flow and vertical exhumation of crustal masses. Therefore, the lateral crustal flow was not only limited along the boundary high strain zones of the Sundaland block, but distributed within the southeastern Tibetan Plateau. We would argue that the tectonic extrusion of the Sundaland block occurred through ductile crustal flow of a viscous middle and lower crust in the plate interior combined with concurring channel flow along the block margins.

Effect of total body irradiation lung block parameters on lung doses using three-dimensional dosimetry.

PurposeTotal body irradiation (TBI) is an integral part of stem cell transplant. However, patients are at risk of treatment‐related toxicities, including radiation pneumonitis. While lung dose is one of the most crucial aspects of TBI dosimetry, currently available data are based on point doses. As volumetric dose distribution could be substantially altered by lung block parameters, we used 3D dosimetry in our treatment planning system to estimate volumetric lung dose and measure the impact of various lung block designs.Materials and methodsWe commissioned a TBI beam model in RayStation that matches the measured tissue‐phantom ratio under our clinical TBI setup. Cerrobend blocks were automatically generated in RayStation on thoracic Computed Tomography (CT) scans from three anonymized patients using the lung, clavicle, spine, and diaphragmatic contours. The margin for block edge was varied to 0, 1, or 2 cm from the superior, lateral, and inferior thoracic borders, with a uniform margin 2.5 cm lateral to the vertebral bodies. The lung dose was calculated and compared with a prescription dose of 1200 cGy in six fractions (three with blocks and three without).ResultThe point dose at midplane under the block and the average lung dose are at the range of 73%–76% and 80%–88% of prescription dose respectively regardless of the block margins. In contrast, the percent lung volume receiving 10 Gy increased by nearly two‐fold, from 31% to 60% over the margins from 0 to 2 cm.ConclusionsThe TPS‐derived 3D lung dose is substantially different from the nominal dose assumed with HVL lung blocks. Point doses under the block are insufficient to accurately gauge the relationship between dose and pneumonitis, and TBI dosimetry could be highly variable between patients and institutions as more descriptive parameters are not included in protocols. Much progress remains to be made to optimize and standardize technical aspects of TBI, and better dosimetry could provide more precise dosimetric predictors for pneumonitis risk.

Effect of Lung Blocks Parameters on Lung and Cardiac Doses in Total Body Irradiation Using 3D Dosimetry

Despite being an integral part of stem cell transplant, the technical aspects of classical total body irradiation (TBI) remain relatively crude. In particular, the implementations of lung and cardiac shielding vary widely between institutions. We hypothesize that the block margins away from anatomical landmarks could substantially alter lung and cardiac dose parameters, and thus affecting the likelihood of TBI-related toxicities such as radiation pneumonitis and late cardiotoxicities. This work seeks to utilize a newly commissioned 3D TBI beam model to determine potential dosimetric effects caused by variations in the choice of shielding parameters to better understand the potential for such detrimental effects.We commissioned a TBI beam model in a treatment planning system. CT scans of 3 anonymized patients without gross disease are contoured following typical clinical practice. Patients are planned in the classic standing AP/PA at extended distance technique for a total dose of 1200 cGy in 6 fractions to midline at umbilicus level, with 3/6 fractions delivered with lung blocks. Lung blocks are modeled using automated script in the treatment planning system, with varying superior (from clavicles), inferior (from diaphragms), and lateral margins from 0 to 2 cm, as well as cases for which the cardiac silhouette was blocked or unblocked for the study. Dose to lungs and heart were calculated and compared for the following metrics: mean dose, volume receiving 10 Gy (V10 Gy) and 12 Gy (V12 Gy), and point dose behind the blocks.The results showed that the point dose behind the blocks is approximately 75% of prescription dose for all patients, with insignificant changes with varying block margins. The dose off the block center increases rapidly that much of the tissue under the blocks receives 80% to 90% of prescription dose. Similar results are observed for mean lung dose. In contrast, the lung V10 Gy increases by nearly two-fold, from approximately 30% to 60% over the 0 to 2 cm block margin range. The mean heart dose increased by approximately 7.5% when the cardiac silhouette is unblocked, while the cardiac V12 Gy increased by approximately 25%. The mean dose to left anterior descending artery (LAD) increases significantly in the unblocked case, with as much as 43% difference.This work has shown that margins of lung blocks could affect the dose to lungs and heart substantially, despite the indifference that the commonly used point dose parameter, point dose behind the blocks, indicates. A 3D TBI model could provide more dosimetry data to help future studies to correlate TBI-associated toxicity to the heart and lungs.

On the formation of magmatic sulphide systems in the lower crust by long‐lived mass transfer through the lithosphere: Insights from the Valmaggia pipe, Ivrea Verbano Zone, Italy

AbstractThe lower crustal domain of the Ivrea‐Verbano Zone (NW Italy) hosts five ~300‐m‐wide pipe‐like ultramafic intrusions that are metasomatized and exhibit Ni‐Cu‐PGE sulphide mineralization. To better constrain the role of metasomatism in the ore genesis, we studied the best‐preserved pipe at Valmaggia which was emplaced 249 Myrs ago. Phlogopite 40Ar/39Ar analyses show that the pipe was infiltrated by metasomatic fluids derived from the subcontinental lithospheric mantle (SCLM) in two pulses at ~208 Ma and ~189 Ma which introduced sulphides into the pipe. Consequently, the pipe repeatedly acted as a preferred path for mass transfer from the SCLM into the lower crust over &gt;60 Myrs (i.e., emplacement to second metasomatic pulse). Uplifted block margins, such as the Ivrea‐Verbano Zone, are potentially important exploration targets for magmatic sulphides. We argue that exploration strategies should focus on structures such as pipes that can focus metasomatic agents during ascent through the lithosphere.

Evidence of Localized Failure Along Altered Basaltic Blocks in Tectonic Mélange at the Updip Limit of the Seismogenic Zone: Implications for the Shallow Slow Earthquake Source

AbstractField studies have led to several interpretations on the mechanics behind slow earthquake phenomena downdip of the seismogenic zone. To date, field studies have not examined the shallow subduction interface which may also host slow earthquake phenomena. We examine a subduction mélange exhumed from conditions representing the source of shallow slow earthquake phenomena. The mélange consists of a shale matrix containing rigid blocks, including basalt which is altered along the margins. Cataclasite‐bearing faults attest to localized faulting along the altered margins of basaltic blocks, concurrent with distributed shear in the shale matrix. These cataclasite‐bearing faults link individual blocks. Microstructures show mutually crosscutting tensile and shear veins, consistent with failure having occurred at, or near, lithostatic pore fluid pressures. We model the stress concentrations around the altered margins of basaltic blocks during distributed shear and show that frictional failure of the altered basalt is predicted to occur at lower imposed strain rates than frictional failure of the shale, favoring fault development along block margins. Calculations of critical nucleation lengths for the blocks show they would fail dynamically at hydrostatic pore fluid pressures, producing microearthquakes. At near‐lithostatic pore fluid pressures, block lengths are below the critical nucleation length for dynamic failure and may produce slow earthquake phenomena. Mixing of velocity‐weakening blocks into a viscously flowing, velocity‐strengthening matrix may serve as a common mechanism for slow earthquake phenomena updip and downdip of the seismogenic zone.

Cataclastic deformation and metasomatism in the subduction zone of mafic blocks-in-mélange, San Simeon, California

The primary origin of shale-matrix mélange forming the Central Belt of the Franciscan Complex is still debated. The seacliff exposures near San Simeon display numerous lensoidal blocks of graywacke, greenstone, lesser layered chert, and blueschist that are encased in a matrix of scaly shale. The boudinaging of mafic greenstone and blueschist blocks was accommodated by cataclasis and varied amounts of metamorphic reconstitution that is particularly intense along block margins, zones of necking, and intra-block shear zones. Cataclastic zones in blueschists are greenish where chlorite and pumpellyite have formed. The mineralogic assemblage produced during cataclasis in greenstones is the same as that replacing the interior of the block: albite + chlorite + pumpellyite.The cataclastic margins of many (7 of 12) greenstones gained K, Rb, Cs, and Ba. In contrast, the cataclastic margins of all blueschist blocks lost Na, several (3 of 9) with substantial decreases, and some (3 of 9) had significant increases in K, Rb, Cs, and Ba. This metasomatic alteration is indicative of fluid-rock interaction between mafic blocks and sediments. Alteration of the mafic rocks with the growth of pumpellyite and chlorite occurred when blocks were encased in the shale matrix of the mélange, which was dynamically deforming and heated to temperatures of ~150 °C. As synsubduction temperature/depth gradients were ~10 °C/km, burial of mafic blocks while encased in the mélange matrix to depths approaching 15 km is apparent. Surficial olistostromal movements alone do not account for the incorporation, deformation, and retrograde metasomatic alteration of the exotic blueschist blocks and the metamorphism of the shale matrix at such inferred depths. These attributes indicate chemical exchange with shale and smearing displacement of the margins of mafic blocks occurred in the subduction zone as the shale matrix flowed and exerted shear tractions.

Conodont biostratigraphy and astronomical tuning of the Lower–Middle Ordovician Liangjiashan (North China) and Huanghuachang (South China) marine sections

In this study, systematic conodont biostratigraphy and cyclostratigraphy are carried out on Ordovician sections from the North China and South China blocks. The Liangjiashan section, situated on the North China Block margin, accumulated shallow marine carbonate during the Early Ordovician. The Huanghuachang section (GSSP for the Dapingian Stage), situated on the South China Block margin, accumulated thinly bedded carbonates and minor clastics in a deep shelf environment. Conodont biostratigraphy and 405-kyr long-eccentricity cycles interpreted from the Liangjiashan and Huanghuachang sections are used to establish an Astronomical Time Scale (ATS) for the Early–Middle Ordovician period. The ATS indicates that the duration of the Floian Stage is 7.08 ± 0.405 Myr. The temporal ranges of Lower–Middle Ordovician conodont biozones are also estimated: the durations of the Serratognathus bilobatus, Serratognathus extensus, Oepikodus communis, Oepikodus evae, and Baltoniodus triangularis zones are 1.0 Myr, 5.1 Myr, 0.2 Myr, 0.8 Myr, and 0.2 Myr, respectively. The evolution of the ratio of obliquity to total power and short-eccentricity power to total power depicts periodicities of 1 Myr for the s4–s3 term, and 1.9 Myr shifting to 1 Myr for the g4–g3 term. The shift in the g4–g3 term may indicate a chaotic transition in the Mars and Earth orbital motions from a (s4–s3)–2(g4–g3) to (s4–s3)–(g4–g3) resonance state during the Early Ordovician.

Geochemical evidence for thin syn-collision crust and major crustal thickening between 45 and 32 Ma at the southern margin of Tibet

Geochemical data on widely distributed igneous rocks of southern Tibet are used to reconstruct paleo-crustal thickness during the 50+ million years that have elapsed since the onset of the India-Asia collision. We use two approaches, one based on Nd isotopes and an assimilation-recharge model for granitic magma genesis and another empirical method based on trace element geochemistry (La/Yb). The focus is on granitic rocks of two age ranges in a segment of the southern Lhasa Block between approximately 89.5° and 92.5°E longitude. One age range, 45 to 62 Ma, spans the time of the onset of collision and for which we infer the geochemistry of granitic rocks reflects mainly pre-collision structure. The other age range is 21 to 9 Ma for the Nd isotopic approach, and 32 to 9 Ma for La/Yb, where the geochemistry must reflect post-collision structure. Our results suggest that the pre- and syn-collision southern margin of the Lhasa block, that portion now located within 50–60 km of the Indus-Yarlung suture (IYS) and south of 29.8°N latitude was relatively thin, about 25–35 km thick until 45 Ma. At approximately 29.8°-29.9°N latitude there was a pronounced crustal discontinuity, and north of that latitude (for a distance that we cannot constrain), the inferred crustal thickness was greater, at least 50–55 km, as indicated by latest Cretaceous and Early Tertiary granitoids and ignimbrites that have large fractions of assimilated continental crust and high La/Yb ratios. Post-collision Nd isotopic and La/Yb data from granitoids younger than 32 Ma suggest that the southern margin south of 29.8°N was thickened substantially to at least 55–60 km (based on Nd isotopes) and possibly as much as 70–75 km (based on La/Yb) by Early to mid-Miocene time. These observations require that thickening of the southern Lhasa Block margin in the period 45–32 Ma was non-uniform; the crust now within 60 km of the suture was thickened by approximately 40 km whereas the crust north of 29.9°N latitude was thickened much less, or not at all. The region currently between 29.8°N and the YTS may have been the highest elevation mountain terrane in the period from roughly 30 to 20 Ma. The amount of Miocene denudation reflects this difference, as there is evidence of substantially more denudation near the IYS than in the region north of 29.9°N. Some of the difference in thickening could be due to magmatic additions from the mantle in the region south of 29.8°N, but there is need for at least 30 km of tectonic thickening between 45 and 32 Ma. The non-uniform thickening suggests that the high elevations at the southern margin of the Himalaya-Tibet orogen propagated southward by about 200 km, from north of Lhasa to their present position, during the period from 50 to 20 Ma. Present crustal thickness requires an additional 10–15 km of more uniform post-Miocene thickening.

Explore the Potential Clinical and Dosimetric Improvements for Brain Cancer Proton Beam Treatment Based on the New Delivery and Planning Technique Developments - Collimator Based Intensity Modulated Proton Therapy and Spot-Scanning Proton Arc Therapy

Utilizing Intensity Modulated Proton Therapy (IMPT) for brain cancer patient has been reaching its dosimetric limitation due to the large lateral penumbra of the pencil beam spots and secondary proton scattering in the air and tissue. To improve the plan quality of using IMPT, new merging techniques were under development including collimator base IMPT (co-IMPT) and Spot-Scanning Proton Arc (SPArc) therapy. This is the first study to quantitatively investigate the dosimetric outcome and benefits among these three different treatment modalities (IMPT, co-IMPT and SPArc) for brain cancer treatment. A glioblastoma (GBM) patient CT and structure data set was used in this study whose clinical target volume (CTV) is abutting the brainstem. 3mm expansion from CTV to PTV is used. PTV is prescribed to 66 Gy with acceptable minor deviation 95% PTV covered by 95% of prescription dose (6270 Gy) due to the brainstem location. 3-field IMPT uses single field optimization technique to ensure the uniform PTV coverage through all three beams (PA; LT-LAT; Vertex directions); 3-field co-IMPT uses the same beam arrangement but adding a block margin 5mm for PTV to reduce the lateral penumbra; SPArc used two non-coplanar arcs. All theplans use the same objective functions for the optimization. Final objective values of each plan were analyzed. The lower of objective value means a better agreement to the plan optimization goal. Plan qualities were evaluated via Homogeneity Index (GI = D5%/D95%); Comformity Index (CI = Target volume covered by the prescription/ total volume received prescription dose); Brainstem maximum dose (0.03cc); Brainstem mean dose; mean brain dose. Treatment delivery time is estimated based on 2ms spot switching time, gantry rotation speed = 6 degree per second. Proton therapy systems with different energy layer switching time (ELST) from 0.1s to 4s were simulated as well. Comparing SPArc, co-IMPT and IMPT plans, the brainstem maximum dose received 49.99Gy, 53.41Gy and 54.10Gy; The brainstem mean dose received 16.99Gy, 23.19Gy and 25.18Gy and the mean brain dose received 6.12Gy, 8.12Gy and 9.11Gy respectively. Furthermore, CI of PTV is 0.97 for SPArc, 0.86 for co-IMPT and 0.83 for IMPT respectively. HI of PTV is 0.76 for SPArc; 0.79 for co-IMPT and 0.78 for IMPT. SPArc treatment delivery time is close to 3 field IMPT or co-IMPT when the ELST is close to 0.1s (Table 1). The final plan objective value is 0.1857 for SPArc; 0.2927 for co-IMPT; 0.3041 for IMPT.Abstract TU_26_3150; Table 1Estimated treatment delivery time vs. ELST using IMPT; co-IMPT and SPArcEnergy Layer Switching Time (s)IMPT (s)co-IMPT (s)SPArc (s)0.17878950.295851270.5108108222114514638122202216993295298101743603741335 Open table in a new tab The study shows both co-IMPT and SPArc could improve the plan quality and spare the dose to organs abutting the target. SPArc demonstrated a significant potential to further improve the plan quality. More developments are needed in order to deliver SPArc therapy in a clinical machine.

Dividing the South American continent to fit a Gondwana reconstruction: A model based on continental geology

The South American continental plate has been affected by intraplate deformation since the start of West Gondwana disruption in the Lower Cretaceous (about 140 Ma). That the present shape of South America is not precisely the same as its shape in reassembled Gondwana partly explains the imperfect fits of the conjugate margins of the South Atlantic proposed since the first reconstruction models of the early 20th century. Several attempts at defining subplates within South America have been published but not all take account of existing knowledge of its continental geology. Here a subdivision into eight rigid subplates is proposed, based primarily on geological and tectonic evidence. Our model is tested against three published models of a multi-subplate Africa, as re-shaped to the pre-breakup outline of that continent, by visual fitting and the use of piercing points. The South America blocks were rotated and the Euler poles calculated interactively in reconstruction software. All three proposed fits had overlapping block margins within South America, indicating post-breakup rifting, except for the Transbrasiliano lineament. This NNE-SSW crustal-scale shear zone was used as a boundary for seven of the eight blocks. It is clearly the main intraplate accommodation zone in South America and an important piercing point in relation to the Kandi lineament in West Africa. The other block boundaries correspond to narrow zones with sedimentary basins and/or dyke swarms that have developed since South Atlantic opening. Each fit required a different configuration of the South America subplates since the pre-rift disposition of the African subplates also differ from each other, contributing to the uncertainty in any detailed reassembly.

Geological and geomorphic evidence for activity of the Mengzi fault along the southeastern margin of the Tibetan Plateau

Few studies have been conducted on the activity of the Mengzi fault system, which is located to the southeast of the Sichuan-Yunnan block. The Mengzi fault is distributed along the eastern margin of the Mengzi basin. However, little is known about the location, length, and activity of the fault or the occurrence of paleo-earthquakes during the late Quaternary. This paper studies the geometric distribution and activity of the fault via remote sensing imagery interpretation, fault outcrops in the area, faulted landforms and chronological analyses. The results show that the Mengzi fault extends from Nandong (Kaiyuan basin) in the north through the eastern margin of the Mengzi basin to Pingbian in the southeast, eventually intersecting with the Honghe fault. The Mengzi fault is approximately 250 km long and is characterized as a reverse strike-slip fault. We survey and calculate the offset gullies and terraces dislocated by the fault; combined with sample dating, we calculate the horizontal slip rate of the fault since 30 ka BP to be 0.91–1.56 mm/a. Three paleo-events since 30–40 ka BP are observed with a 10-ka major earthquake recurrence period. We analyze the relationships among the Mengzi fault, southern Xiaojiang fault, and Qujiang-Jianshui fault and note a slip rate loss of ≥8 mm/a from the southern Xiaojiang fault that was absorbed by reverse dextral strike-slip faulting and lateral shortening of the Qujiang-Jianshui fault and the Mengzi fault. The Mengzi fault plays an important role in the adjustment of the large slip rate reduction along the southern Xiaojiang fault. Moreover, the Mengzi fault has a similar tectonic position to the seismogenic faults of the 2008 Wenchuan earthquake, 2013 Lushan earthquake and 2014 Ludian earthquake, and the Mengzi fault and southern Xiaojiang fault both accumulate the energy generated by the southeastward movement of the Sichuan-Yunnan block. Studies on the Mengzi fault are helpful for renewing our understanding of the tectonic patterns of the southeastern Sichuan-Yunnan block and delimiting the boundary of the southeastern block margin. Furthermore, because the cities of Mengzi and Kaiyuan are located along the Mengzi fault, this study has practical significance for earthquake disaster mitigation.

Petrogenesis and geodynamic significance of the ~850 Ma Dongling A-type granites in South China

The ongoing controversy on the timing of amalgamation of the Yangtze and Cathaysia Blocks (either prior to 890 Ma or later than 830 Ma) impedes a proper understanding of geological evolution of the South China Block and its role in the breakup of Rodinia supercontinent. In this study, we report ~850 Ma LA-ICP-MS zircon UPb ages and whole rock geochemistry of the Dongling granites from the southeastern Yangtze Block that did not receive much geoscientific attention as compared to the better-studied 830–760 Ma sedimentary and igneous rocks in the South China Block. The studied Dongling granites have high K2O + Na2O (7.44–9.09 wt%) and low MgO (0.07–0.54 wt%) and CaO (0.16–1.21 wt%) contents. Their high FeOT/(FeOT + MgO) (0.82–0.94) and Ga/Al (> 2.6) values and HFSE abundance (Zr + Nb + Ce + Y = 443–965 ppm) allow their grouping as A-type granite. In the chondrite normalized diagrams, these granites display right-inclined REE patterns and strong to moderate negative Eu anomalies (Eu/Eu* = 0.32–0.70). The Dongling granites have variable and negative whole-rock εNd(t) (−14.6 to −8.6) and zircon εHf(t) (−12.8 to −4.4) with Paleoproterozoic two-stage Nd (2.20–2.69 Ga) and Hf model ages (2.01–2.41 Ga). Zircon saturation parameters (TZr) suggest > 850 °C temperature and other geochemical characteristics reveal low melting pressure (> 350 to < 700 MPa) and 1.9–2.6 wt% H2O content. Geochemical and isotopic characteristics indicate derivation of the Dongling A-type granites by partial melting of an ancient granulitic metasedimentary source, after a previous melt extraction episode. Emplacement of the Dongling A-type granites is inferred in an extensional setting following the arc-continent collision along the southeastern Yangtze Block margin. The oceanic slab continued to subduct underneath the southeastern margin of the Yangtze Block, leading to the extensive 840–820 Ma arc-related magmatism and coeval sedimentation. These arc-related igneous and sedimentary rocks indicate that the continental arc systems started from 850 Ma, and hint at the collision between the Yangtze and Cathaysia blocks after 850 Ma.

Optimal beam margins in linac-based VMAT stereotactic ablative body radiotherapy: a Pareto front analysis for liver metastases

We explored the Pareto fronts mathematical strategy to determine the optimal block margin and prescription isodose for stereotactic body radiotherapy (SBRT) treatments of liver metastases using the volumetric-modulated arc therapy (VMAT) technique. Three targets (planning target volumes [PTVs] = 20, 55, and 101 cc) were selected. A single fraction dose of 26 Gy was prescribed (prescription dose [PD]). VMAT plans were generated for 3 different beam energies. Pareto fronts based on (1) different multileaf collimator (MLC) block margin around PTV and (2) different prescription isodose lines (IDL) were produced. For each block margin, the greatest IDL fulfilling the criteria (95% of PTV reached 100%) was considered as providing the optimal clinical plan for PTV coverage. Liver Dmean, V7Gy, and V12Gy were used against the PTV coverage to generate the fronts. Gradient indexes (GI and mGI), homogeneity index (HI), and healthy liver irradiation in terms of Dmean, V7Gy, and V12Gy were calculated to compare different plans. In addition, each target was also optimized with a full-inverse planning engine to obtain a direct comparison with anatomy-based treatment planning system (TPS) results. About 900 plans were calculated to generate the fronts. GI and mGI show a U-shaped behavior as a function of beam margin with minimal values obtained with a +1 mm MLC margin. For these plans, the IDL ranges from 74% to 86%. GI and mGI show also a V-shaped behavior with respect to HI index, with minimum values at 1 mm for all metrics, independent of tumor dimensions and beam energy. Full-inversed optimized plans reported worse results with respect to Pareto plans. In conclusion, Pareto fronts provide a rigorous strategy to choose clinical optimal plans in SBRT treatments. We show that a 1-mm MLC block margin provides the best results with regard to healthy liver tissue irradiation and steepness of dose fallout.

Preseismic Changes of Water Temperature in the Yushu Well, Western China

We observed abnormal changes of the water temperature in Yushu (YSWT) well, China, most of which were followed by earthquakes. This study statistically analyzes the correlation between the magnitude and duration of the anomalies in YSWT and earthquakes in the Tibetan block and its margins. The effectiveness of using observed YSWT data to predict earthquakes was quantitatively examined by the Molchan error diagram method. The results show that (1) the YSWT underwent several abnormal changes marked by “V”-shaped patterns, which might be related to several earthquakes that occurred in the Tibetan block and its margins. The extent and duration of the abnormal changes in the YSWT were linearly related to the magnitude of the earthquake; i.e., the higher the magnitude, the greater the change in the YSWT, and the shorter the duration. (2) Abnormal changes in the YSWT are somewhat predictive of earthquakes with magnitudes ≥5.5 (≥M5.5) within 800 km of the Yushu well and ≥M6.5 earthquakes in the Tibetan block and its margins. The prediction has a probability gain of approximately 2, and the most likely time period for an earthquake to occur is within approximately 3 months after the occurrence of an YSWT anomaly. Most of the anomalies in YSWT appeared before earthquakes in the thrust of block margins. Notably the larger strains from the earthquake did not produce any response. We speculate that the preseismic responses reflect the regional tectonics, such as the motion of the Indian plate, straining sub-blocks of the Tibetan block.

Late Jurassic–Early Cretaceous episodic development of the Bangong Meso-Tethyan subduction: Evidence from elemental and Sr–Nd isotopic geochemistry of arc magmatic rocks, Gaize region, central Tibet, China

The Bangong Meso-Tethys plays a critical role in the development of the Tethyan realm and the initial elevation of the Tibetan Plateau. However, its precise subduction polarity, and history still remain unclear. In this study, we synthesize a report for the Late Jurassic–Early Cretaceous two-phase magmatic rocks in the Gaize region at the southern margin of the Qiangtang block located in central Tibet. These rocks formed during the Late Jurassic–earliest Cretaceous (161–142Ma) and Early Cretaceous (128–106Ma), peaking at 146Ma and 118Ma, respectively. The presence of inherited zircons indicates that an Archean component exists in sediments in the shallow Qiangtang crust, and has a complex tectonomagmatic history. Geochemical and Sr–Nd isotopic data show that the two-phase magmatic rocks exhibit characteristics of arc magmatism, which are rich in large-ion incompatible elements (LIIEs), but are strongly depleted in high field strength elements (HFSEs). The Late Jurassic–earliest Cretaceous magmatic rocks mixed and mingled among mantle-derived mafic magmas, subduction-related sediments, or crustally-derived felsic melts and fluids, formed by a northward and steep subduction of the Bangong Meso-Tethys ocean crust. The magmatic gap at 142–128Ma marks a flat subduction of the Meso-Tethys. The Early Cretaceous magmatism experienced a magma MASH (melting, assimilation, storage, and homogenization) process among mantle-derived mafic magmas, or crustally-derived felsic melts and fluids, as a result of the Meso-Tethys oceanic slab roll-back, which triggered simultaneous back-arc rifting along the southern Qiangtang block margin.