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We investigate rifting during continental collision in southern Tibet by testing kinematic models for two classes of rifts: Tibetan rifts are defined as >150 km in length and crosscut the Lhasa Terrane, and Gangdese rifts are <150 km long and isolated within the high topography of the Gangdese Range. Discerning rift kinematics is a crucial step towards understanding rift behavior and evolution that has been historically limited. We evaluate spatiotemporal trends in fault displacement and extension onset in the Tangra Yumco (TYC) rift and several nearby Gangdese rifts and examine how contraction and rift exhumation relate to evolution of the Gangdese drainage divide. Igneous U-Pb and zircon (U-Th)/He (ZHe) results indicate rift footwall crystallization between ~59-49 Ma and cooling between ~60-4 Ma, respectively, with ZHe ages correlating with sample latitude. Samples from Gangdese latitudes (~29.4-29.8°N) yield predominantly Oligocene-early Miocene ages, whereas samples north of ~29.8°N yield both late Miocene-Pliocene ages and Paleocene-Eocene ages. Thermal history models indicate two-stage cooling, with initially slow cooling followed by accelerated cooling during late Miocene-Pliocene time. From spatial distributions of ZHe ages we interpret: (1) ~28-16 Ma ages from Gangdese latitudes reflect exhumation along contractional structures, (2) ~8-4 Ma ages reflect rift-related exhumation, and (3) ~60-48 Ma ages indicate these samples experienced lesser rift exhumation. Our data are consistent with a segment linkage evolution model for the TYC rift, with interactions between rifts and contractional structures likely influencing the evolution of topography and location of the Gangdese drainage divide since Miocene time

 

Suture zones located across the Tibetan region clearly demarcate the rift-and-drift and continental accretion history of the region. However, the intraplate responses to these marginal plate-tectonic events are rarely quantified. Our understanding of the Paleo-Tethyan orogenic system, which involved ocean opening and closing events to grow the central Asian continent, depends on the tectonic architecture and histories of major late Paleozoic−early Mesozoic orogenic belts. These opening and collision events were associated with coupled intracontinental deformation, which has been difficult to resolve due to subsequent overprinting deformation. The late Paleozoic−early Mesozoic Zongwulong Shan−Qinghai Nanshan belt in northern Tibet separates the Qilian and North Qaidam regions and is composed of Carboniferous−Triassic sedimentary materials and mantle-derived magmatic rocks. The tectonic setting and evolutional history of this belt provide important insight into the paleogeographic and tectonic relationships of the Paleo-Tethyan orogenic system located ∼200 km to the south. In this study, we integrated new and previous geological observations, detailed structural mapping, and zircon U-Pb geochronology data from the Zongwulong Shan−Qinghai Nanshan to document a complete tectonic inversion cycle from intraplate rifting to intracontinental shortening associated with the opening and closing of the Paleo-Tethyan Ocean. Carboniferous−Permian strata in the Zongwulong Shan were deposited in an intracontinental rift basin and sourced from both the north and the south. At the end of the Early−Middle Triassic, foreland molasse strata were deposited in the southern part of the Zongwulong Shan during tectonic inversion in the western part of the tectonic belt following the onset of regional contraction deformation. The Zongwulong Shan−Qinghai Nanshan system has experienced polyphase deformation since the late Paleozoic, including: (1) early Carboniferous intracontinental extension and (2) Early−Middle Triassic tectonic inversion involving reactivation of older normal faults as thrusts and folding of pre- and synrift strata. We interpret that the Zongwulong Shan−Qinghai Nanshan initiated as a Carboniferous−Early Triassic intracontinental rift basin related to the opening of the Paleo-Tethyan Ocean to the south, and it was then inverted during the Early−Middle Triassic closing of the Paleo-Tethyan Ocean. This work emphasizes that pre-Cenozoic intraplate structures related to the opening and closing of ocean basins in the Tethyan realm may be underappreciated across Tibet. 

The northwest-trending Altai Mountains of central Asia expose a complex network of thrust and strike-slip faults that are key features accommodating intracontinental crustal shortening related to the Cenozoic India-Asia collision. In this study, we investigated the Quaternary slip history of the Fuyun fault, a right-lateral strike-slip fault bounding the southwestern margin of the Altai Mountains, through geologic mapping, geomorphic surveying, and optically stimulated luminescence (OSL) geochronology. At the Kuoyibagaer site, the Fuyun fault displaces three generations of Pleistocene–Holocene fill-cut river terraces (i.e., T3, T2, and T1) containing landslide and debris-flow deposits. The right-lateral offsets are magnified by erosion of terrace risers, suggesting that river course migration has been faster than slip along the Fuyun fault. The highest Tp2 terrace was abandoned in the middle Pleistocene (150.4 ± 8.1 ka uppermost OSL age) and was displaced 145.5 +45.6/–12.1 m along the Fuyun fault, yielding a slip rate of 1.0 +0.4/–0.1 mm/yr since the middle Pleistocene. The lower Tp1 terrace was abandoned in the late Pleistocene and aggraded by landslides and debris flows in the latest Pleistocene–Holocene (36.7 ± 1.6 ka uppermost OSL age). Tp1 was displaced 67.5 +14.2/–6.1 m along the Fuyun fault, yielding a slip rate of 1.8 +0.5/–0.2 mm/yr since the late Pleistocene. Our preferred minimum slip rate of ~1 mm/yr suggests the Fuyun fault accommodates ~16% of the average geodetic velocity of ~6 mm/yr across the Altai Mountains. Integration of our new Fuyun slip rate with other published fault slip rates accounts for ~4.2 mm/yr of convergence across the Chinese Altai, or ~70% of the geodetic velocity field.

 

Problem-solving is a typical type of assessment in engineering dynamics tests. To solve a problem, students need to set up equations and find a numerical answer. Depending on its difficulty and complexity, it can take anywhere from ten to thirty minutes to solve a quantitative problem. Due to the time constraint of in-class testing, a typical test may only contain a limited number of problems, covering an insufficient range of problem types. This can potentially reduce validity and reliability, two crucial factors which contribute to assessment results. A test with high validity should cover proper content. It should be able to distinguish high-performing students from low-performing students and every student in between. A reliable test should have a sufficient number of items to provide consistent information about students’ mastery of the materials. In this work-in-progress study, we will investigate to what extent a newly developed assessment is valid and reliable. Symbolic problem solving in this study refers to solving problems by setting up a system of equations without finding numeric solutions. Such problems usually take much less time. As a result, we can include more problems of a variety of types in a test. We evaluate the new assessment's validity and reliability. The efficient approach focused in symbolic problem-solving allows for a diverse range of problems in a single test. We will follow Standards for Educational and Psychological Testing, referred to as the Standards, for our study. The Standards were developed jointly by three professional organizations including the American Educational Research Association (AERA), the American Psychological Association (APA), and the National Council on Measurement in Education (NCME). We will use the standards to evaluate the content validity and internal consistency of a collection of symbolic problems. Examples on rectilinear kinematics and angular motion will be provided to illustrate how symbolic problem solving is used in both homework and assessments. Numerous studies in the literature have shown that symbolic questions impose greater challenges because of students’ algebraic difficulties. Thus, we will share strategies on how to prepare students to approach such problems. 

Eclogite bodies exposed across Tibet record a history of subduction-collision events that preceded growth of the Tibetan Plateau. Deciphering the time-space patterns of eclogite generation improves our knowledge of the preconditions for Cenozoic orogeny in Tibet and broader eclogite formation and/or exhumation processes. Here we report the discovery of Permo-Triassic eclogite in northern Tibet. U-Pb zircon dating and thermobarometry suggest eclogite-facies metamorphism at ca. 262–240 Ma at peak pressures of ∼2.5 GPa. Inherited zircons and geochemistry show the eclogite was derived from an upper-plate continental protolith, which must have experienced subduction erosion to transport the protolith mafic bodies to eclogite-forming conditions. The Dabie eclogites to the east experienced a similar history, and we interpret that these two coeval eclogite exposures formed by subduction erosion of the upper plate and deep trench burial along the same ∼3000-km-long north-dipping Permo-Triassic subduction complex. We interpret the synchroneity of eclogitization along the strike length of the subduction zone to have been driven by accelerated plate convergence due to ca. 260 Ma Emeishan plume impingement. 

The Proterozoic−Phanerozoic tectonic evolution of the Qilian Shan, Qaidam Basin, and Eastern Kunlun Range was key to the construction of the Asian continent, and understanding the paleogeography of these regions is critical to reconstructing the ancient oceanic domains of central Asia. This issue is particularly important regarding the paleogeography of the North China-Tarim continent and South China craton, which have experienced significant late Neoproterozoic rifting and Phanerozoic deformation. In this study, we integrated new and existing geologic field observations and geochronology across northern Tibet to examine the tectonic evolution of the Qilian-Qaidam-Kunlun continent and its relationships with the North China-Tarim continent to the north and South China craton to the south. Our results show that subduction and subsequent collision between the Tarim-North China, Qilian-Qaidam-Kunlun, and South China continents occurred in the early Neoproterozoic. Late Neoproterozoic rifting opened the North Qilian, South Qilian, and Paleo-Kunlun oceans. Opening of the South Qilian and Paleo-Kunlun oceans followed the trace of an early Neoproterozoic suture. The opening of the Paleo-Kunlun Ocean (ca. 600 Ma) occurred later than the opening of the North and South Qilian oceans (ca. 740−730 Ma). Closure of the North Qilian and South Qilian oceans occurred in the Early Silurian (ca. 440 Ma), whereas the final consumption of the Paleo-Kunlun Ocean occurred in the Devonian (ca. 360 Ma). Northward subduction of the Neo-Kunlun oceanic lithosphere initiated at ca. 270 Ma, followed by slab rollback beginning at ca. 225 Ma evidenced in the South Qilian Shan and at ca. 194 Ma evidenced in the Eastern Kunlun Range. This tectonic evolution is supported by spatial trends in the timing of magmatism and paleo-crustal thickness across the Qilian-Qaidam-Kunlun continent. Lastly, we suggest that two Greater North China and South China continents, located along the southern margin of Laurasia, were separated in the early Neoproterozoic along the future Kunlun-Qinling-Dabie suture. 

Recent advancements in quantitatively estimating the thickness of Earth's crust in the geologic past provide an opportunity to test hypotheses explaining the tectonic evolution of southern Tibet. Outstanding debate on southern Tibet's Cenozoic geological evolution is complicated by poorly understood Mesozoic tectonics. We present new U‐Pb geochronology and trace element chemistry of detrital zircon from modern rivers draining the Gangdese Mountains in southern Tibet. Results are similar to recently published quantitative estimates of crustal thickness derived from intermediate‐composition whole rock records and show ~30 km of crustal thinning from 90 to 70 Ma followed by thickening to near‐modern values from 70 to 40 Ma. These results extend evidence of Late Cretaceous north–south extension along strike to the west by ~200 km, and support a tectonic model in which an east–west striking back‐arc basin formed along Eurasia's southern margin during slab rollback, prior to terminal collision of India with Eurasia.

 

North-trending rifts throughout south-central Tibet provide an opportunity to study the dynamics of synconvergent extension in contractional orogenic belts. In this study, we present new data from the Dajiamang Tso rift, including quantitative crustal thickness estimates calculated from trace/rare earth element zircon data, U-Pb geochronology, and zircon-He thermochronology. These data constrain the timing and rates of exhumation in the Dajiamang Tso rift and provide a basis for evaluating dynamic models of synconvergent extension. Our results also provide a semi-continuous record of Mid-Cretaceous to Miocene evolution of the Himalayan-Tibetan orogenic belt along the India-Asia suture zone. We report igneous zircon U-Pb ages of ~103 Ma and 70–42 Ma for samples collected from the Xigaze forearc basin and Gangdese Batholith/Linzizong Formation, respectively. Zircon-He cooling ages of forearc rocks in the hanging wall of the Great Counter thrust are ~28 Ma, while Gangdese arc samples in the footwalls of the Dajiamang Tso rift are 16–8 Ma. These data reveal the approximate timing of the switch from contraction to extension along the India-Asia suture zone (minimum 16 Ma). Crustal-thickness trends from zircon geochemistry reveal possible crustal thinning (to ~40 km) immediately prior to India-Eurasia collision onset (58 Ma). Following initial collision, crustal thickness increases to 50 km by 40 Ma with continued thickening until the early Miocene supported by regional data from the Tibetan Magmatism Database. Current crustal thickness estimates based on geophysical observations show no evidence for crustal thinning following the onset of E–W extension (~16 Ma), suggesting that modern crustal thickness is likely facilitated by an underthrusting Indian lithosphere balanced by upper plate extension.