Active present faults of the Western segment of the Qilian Mountains (Northern Tiber)

The Qilian Mountains, as the northeastern margin of the Tibetan Plateau, absorbed the crustal shortening and accommodated the left-lateral displacement of the Altun Tagh fault. Detailed geomorphologic study of river valleys on the northern margin of the Qilian Mountains showed that since the late Pleistocene the crustal uplift rate of the northern Qilian Mountains has been greater than the central part. Due to the extension of the Tibetan plateau between the Changma fault and the Yumen fault the latest belt of faults and folds was formed on the northern margin of the Qilian Mountains. The study of the height of river terraces over the past 60 thousand years shows that the rate of vertical displacement along the Changma fault is 0,31±0,06 mm/a and its horizontal crustal shortening rate is 0,11±0,02 mm/a. The rate of vertical displacement along the northernmost Yumen fault is 0,33±0,02 mm/a and its horizontal crustal shortening rate is 0,53±0,03 mm/a. Active faults in the western segment of the northern Qilian Mountains account for 12% of the total crustal shortening in the Qilian Mountains. In addition, the crustal shortening rate of faults in the northern Qilian Mountains is much greater than the crustal shortening rate of faults inside the Qilian Mountains, which further indicates that since the Late Pleistocene the crustal uplift rate of the northern Qilian Mountains has been greater than the central Qilian Mountains.

Tectonic evolution of the East Vietnam-Southwest Borneo margin breakup

<p>We investigate the extensional processes occurring during the rifting of a marginal basin to use long-streamer multichannel seismic transects across the entire southwestern South China Sea (SCS). The basin is characterized by space and time propagating breakup followed by seafloor spreading during Cenozoic. Stretching and thinning of the continental crust were accompanied by ubiquitous large extensional detachment faults. In the proximal E Vietnam margin, rifted basins are filled with lower syn-rift sedimentation (syn-rift I). These sediments pinch out towards the distal margin. On the conjugate NW Borneo margin, the same coeval syn-rift I is truncated at the top, suggesting a period of crustal uplift affecting solely the southern margin. To illustrate the pre-breakup geometries of the southwestern SCS margins, we restore two conjugate sections near the first oceanic magnetic anomaly (20.1 Ma, C6n). The result highlights a thick pre-breakup succession (syn-rift II) offset slightly by several seaward-dipping normal faults above the exhumed basement. The magmatism intruded this hyper-extended basin and proceeded to break up the continental lithosphere eventually. The COT configuration not only illustrates asymmetrical hyper-extension but also appears in map view to have a rhombic shape controlled by N-S abrupt segments and E-W hyper-extended ones. The spatial variation of the crustal structures suggests an initial N-S extension contemporaneous with the first phase of seafloor spreading in the eastern SCS. The extensional direction significantly changed later (circa 23Ma) to NW-SE in relation to a well-documented ridge jump. Interestingly, this change in the direction of opening is coeval with the collision and the counterclockwise rotation in Borneo, thus suggesting that those events are linked. Therefore, we propose that collision was responsible for significant change in the far-field stress and influenced the extensional regime in the SCS.</p>

Estimating the changes in the Moho interface beneath the Tibetan Plateau based on GRACE data

<p>         The Tibetan Plateau (TP) experiences complex mass transfer and redistribution due to the effects from internal earth dynamics and external climate change, such as, land water change, crustal uplift, surface denudation, and Moho interface change. These phenomenas are accompanied by the gravity field change and could be observed by the Gravity Recovery and Climate Experiment (GRACE). This study applies GRACE data to estimate the corresponding mass changes expressed by water equivalent height (EWH) anomaly of the TP. In addition, we use ICESat data and hydrological models to estimate the effects of hydrological factors (lake, glaciers, snow, soil moisture, and groundwater), to separate them from the comprehensive mass field to obtain the tectonic information. The total hydrological contribution to the average EWH change is -0.30±0.21 cm/yr. We further estimate the rates of tectonic uplift and denudation based on GNSS and denudation, with results of 0.71±0.46 mm/yr and 0.38±0.10 mm/yr, respectively. Removing the effects of hydrological change, surface displacements and GIA from the GRACE data, we obtain the EWH change contributed from interior mass change of 0.21±0.27 cm/yr, which is equivalent to a mean Moho interface uplift rate of 3.63±4.32 mm/yr. Final results show that the crustal thickness of the northern TP is thinning because of the upwelling of Moho interface and the southern TP is thickening along with Moho deepening, coinciding with the tomographic results.</p><p>Key words: the Tibetan plateau, mass transfer, land water change, Moho interface change, GRACE</p>

Origin of the southernmost Arctic tundra of continental North America

The Arctic tundra extends beyond the treeline north of 58°N in eastern North America and north of 66°N in western North America and Eurasia. A marked exception to this distribution is the azonal tundra situated as far south as 54°30′–45′N, in the Pointe-Louis-XIV area (JABA), along the fast-rising coasts of James Bay–Hudson Bay. The unusual position of JABA calls into question the influence of climate as the main causal factor for its existence. Macrocharcoal remains extracted from tundra and forest soils were used along a 105 km transect to date the onset of the boreal environment based on past occurrence of conifer fires. Assuming crustal uplift 1.3 m 100 year−1 and 2.4 m 100 year−1 over and before the last 1000 years, and after correcting site elevation at the time the oldest conifer fires occurred, trees established along the coast before 4000 cal. BP. Given charcoal distribution suggesting boreal vegetation in sites ≤13 m a.s.l., JABA was created after 4000 cal. BP when the flat, elongated peninsula emerged above marine waters. It is concluded that JABA origin was most likely caused by the synergistic impact of geophysical factors, isostatic uplift and topography, on a coastal environment already influenced by cold, wind-exposed conditions.