Subduction of trench-fill sediments beneath an accretionary wedge: Insights from sandbox analogue experiments

Sandy trench-fill sediments at accretionary margins are commonly scraped off at the frontal wedge and rarely subducted to the depth of high-pressure (HP) metamorphism. However, some ancient exhumed accretionary complexes are associated with high-pressure–low-temperature (HP-LT) metamorphic rocks, such as psammitic schists, which are derived from sandy trench-fill sediments. This study used sandbox analogue experiments to investigate the role of seafloor topography in the transport of trench-fill sediments to depth during subduction. We conducted two different types of experiments, with or without a rigid topographic high (representing a seamount). We used an undeformable backstop that was unfixed to the side wall of the apparatus to allow a seamount to be subducted beneath the overriding plate. In experiments without a seamount, progressive thickening of the accretionary wedge pushed the backstop down, leading to a stepping down of the décollement, narrowing of the subduction channel, and underplating of the wedge with subducting sediment. In contrast, in experiments with a topographic high, the subduction of the topographic high raised the backstop, leading to a stepping up of the décollement and widening of the subduction channel. These results suggest that the subduction of stiff topographic relief beneath an inflexible overriding plate might enable trench-fill sediments to be deeply subducted and to become the protoliths of HP-LT metamorphic rocks.

Coupling of sea level changes and neotectonic activities in the Pearl River Delta─insight from stratigraphic profile and chronology

<p>        Despite several tens of Pleistocene eustatic oscillations, it is surprised that only two marine sequences were preserved in the PRD (Pearl River Delta), the third biggest delta along the mainland China coast. The younger marine sequence (SQ1) has been consensus on the age of Holocene, i.e., MIS1, whereas the chronology of the older marine sequence (SQ2) is still in debate, i.e. belongs to MIS3 or MIS5. Those favor younger transgression suggest rapid uplift following the SQ2 deposited according to <sup>14</sup>C and early luminescence dating, while the others argued that the current depth of SQ2 is affected by tectonic subsidence and better match the sea-level altitude in MIS5. To address this problem, it is significant to investigate a complete spatial distribution of SQ2 prior to dating. We applied 250 boreholes to acquire 5 Quaternary stratigraphic profiles throughout the PRD. These profiles reveal that the deposition area of SQ2 with current depth at -15 - -35m a.s.l. only reach the southern part of PRD, showing a much less area than SQ1. Sediments synchronous to SQ2 in the northern part of PRD present coarse grain in fluvial or piedmont environment, implying an erosional state. Preliminary OSL dating on SQ2 in boreholes in southern PRD yielded 85.5±5 ka, suggesting the SQ2 probably deposited in MIS5, here we infer to the high sea-level in MIS5a with altitude at ca. -20m. Moreover, we estimate the isostasy by erosion of granite highland in/around the PRD via hypsometric integral curve. We find that the modern average altitude of the highland is ca. 100-150m lower than the estimated isostatic altitude, suggesting tectonic subsidence in PRD. Overall, we interpret that the PRD was an eroding highland and keep subsiding since MIS5. Because of the topographic high, transgression occurred in MIS5 did not extend northward to modern delta area and led to absence of SQ2 in northern PRD. Subjected to tectonic subsidence, the once topographic high subsided beneath the modern sea-level but still higher the sea-level in MIS3. Marine sequence did not develop in PRD until transgression occurred in Holocene.</p>

Guadalupian (Permian) onset of subduction zone volcanism and geodynamic turnover from passive- to active-margin tectonics in southeast China

New stratigraphic, geochemical, and geochronological data from the late Paleozoic depositional record in Anhui Province, China, signal the onset of active-margin magmatism in East Asia. Chert-shale sequences of the Gufeng Formation are part of a Carboniferous–Permian carbonate platform that developed along the passive margin of the South China block. Thin tuffaceous interlayers in these sequences represent distal ash deposits, marking discrete volcanic events. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dating of the stratigraphically bottom and near-top tuffaceous interlayers has revealed crystallization ages of 270 Ma and 264 Ma, respectively, constraining the time span of subaerial eruptions to ∼6 m.y. during the Guadalupian Epoch. High SiO2 and Al2O3 contents, enrichments in large ion lithophile and light rare earth elements, and depletion patterns of high field strength and heavy rare earth elements indicate a calc-alkaline magma source in an arc setting for the origin of these volcanic tuff deposits. Detrital zircon geochronology of sandstones in the overlying Longtan Formation shows two prominent age populations of 290–250 Ma and 1910–1800 Ma. The former age cluster overlaps with the tightly constrained zircon ages obtained from the Gufeng Formation. The latter age group is compatible with the known magmatic-metamorphic ages from Cathaysia in the South China block, and it points to the existence of a NE-SW–trending topographic high as a major sediment source. We interpret this topographic high and silicic volcanism to represent an Andean-type active margin, developed above a north-dipping paleo-Pacific slab. Our tightly constrained Guadalupian eruption ages indicate the inception of magmatic arc construction and mark a major switch from passive- to active-margin tectonics along SE Asia.

Tectonostratigraphy and provenance analysis to define the edge and evolution of the eastern Wuyi-Yunkai orogen, South China

We report three Palaeozoic sedimentary successions in northeastern South China that display markedly different tectonostratigraphic characteristics: the Jiangshan section exhibits an angular unconformity between the Upper Ordovician and Carboniferous stratra; the Shuangming section exhibits a disconformity between the lower Silurian and Upper Devonian strata; and the Xinqiao section exhibits a disconformity between the upper Silurian and Upper Devonian strata. The Shuangming and Xinqiao sections are interpreted to represent the remnant Nanhua foreland basin, whereas the Jiangshan section is in the fold-and-thrust zone of the Wuyi-Yunkai orogen. The Lizhu-Changshan thrust fault in between is interpreted to be the frontal thrust and the boundary of the orogen. Detrital provenance analysis of the Ordovician–Devonian sandstones from the Shuangming and Xinqiao sections shows that the Ordovician–Silurian, mid- to late-orogenic sandstones contain dominantly 860–780 Ma zircon populations and subordinate 2.5 Ga, 1.89–1.78 Ga, 980–950 Ma, 630–540 Ma and 430 Ma populations, indicating nearby sources including the early Neoproterozoic Sibao orogen, inverted Neoproterozoic rift basins and related plutons, recycled Ediacaran–Cambrian strata and, increasing with time, exposed Cathaysia basement and minor syn- to late-orogenic plutonic intrusions. The Devonian post-orogenic sandstones exhibit a dominant 440 Ma population with minor 2.5 Ga, 1.89–1.78 Ga, 860–780 Ma and 630–540 Ma populations, suggesting a dominant contribution from now widely exposed, mid- to late-orogenic plutonic intrusions (with or without contributions from rare volcanism of similar ages) in a residual topographic high SE of the Lizhu-Changshan fault. This residual topographic high of the Wuyi-Yunkai orogen had completely perished by early Carboniferous time, c. 60Myr after the end of the orogenic event.

Nanoscale Elastic and Tribological Properties of Poly(Acrylic Acid) Superabsorbent Gels

ABSTRACTUltrasonic Force Microscopy and Friction Force Microscopy have been applied to the characterization of the elastic and tribological responses of poly(acrylic acid) hydrogels at ambient conditions. The gels were prepared by free radical polymerization of acrylic acid monomers partially neutralized by sodium hydroxide, using N, N'-methylen-bis-acrylamide as a crosslinker. Nanoscale domains with different stiffness and friction are observed. Increasing the amount of crosslinker leads to the formation of smaller, more densely packed domains. The domains with higher stiffness also exhibit higher friction and lower topographic high. The results can be understood by assuming that (i) neutralization by sodium hydroxide leads to the formation of both acrylic acid and sodium acrylate polymeric strands (ii) the observed domains differ in their acrylic acid / sodium acrylate content. In the acrylic acid rich domains, hydrogen bonding among the polymeric strands explains a higher stiffness and lower topography. In the sodium acrylate rich domains, lubrication by water molecules linked by solvation to the sodium counterions accounts for a lower friction.