Dendrogeomorphic chronologies of landslides: Dating of true slide movements?          下载免费PDF全文

Karel Šilhán 《地球表面变化过程与地形》2017,42(13):2109-2118

Dendrogeomorphic chronologies of landslide movements are frequently used to investigate past landslide activity. Slide areas are often affected by other slope movements (e.g. creep) simultaneously. Trees growing on landslides record all types of ground movements, which potentially creates significant noise in tree ring based chronologies of landslide movements. The effect of creep movements on dendrogeomorphic landslide chronologies was evaluated in a block‐type landslide in the south‐western foreland of the Orlické hory Mountains. In total, 272 trees (Picea abies and Fagus sylvatica) were sampled (1088 increment cores) on the sub‐horizontal surfaces of rotated slide blocks, which were presumably created only by slide movements, and on the steep internal scarps separating landslide blocks, which were presumably created and affected by a combination of slide and creep movements. Ground movements were dated based on growth disturbances identified in an analysis of eccentric tree growth. The trees growing on the internal landslide scarps separating the rotated blocks exhibited a significantly higher number and frequency of growth disturbances than those growing on the sub‐horizontal block surfaces. All eight dated block surface movements were also identified on the internal scarps. Creep‐based events represented as many as 70% of the dated movement events on the internal scarps. Varying the I_t thresholds did not filter out more than 40% of the noise without significantly reducing the number of true dated slide events. A significant difference was observed between the ability of P. abies and F. sylvatica to record ground movements by eccentric growth. Probably due to its shallower roots (and weaker anchoring of the tree to landslide blocks), P. abies appears to be more sensitive to surficial ground movement, which potentially increases the proportion of dated creep events (noise). Thus, the careful selection of sampled tree species with different physiologies should be considered during dendrogeomorphic field sampling. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

P–T–t–D records of Early Palaeozoic Andean-type shortening of a hot active margin: The Dunhuang block in NW China     

Jérémie Soldner  Pavla Štípská  Karel Schulmann  Chao Yuan  Robert Anczkiewicz  Yingde Jiang  Marta Koziarska  Le Zhang  Yunying Zhang  Xinyu Wang 《Journal of Metamorphic Geology》2023,41(1):59-96

High-pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra-subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure–temperature–time-deformation (P–T–t–D) paths. In the current study, P–T–t–D paths of Early Palaeozoic HP granulite facies rocks, in the form of metabasic lenses enclosed in migmatitic metapelite, from the Dunhuang block, NW China, are investigated in order to constrain the nature of the HP rocks and shed light on the geodynamic evolution of a modern hot orogenic system in an active margin setting. The rocks show a polyphase evolution characterized by (1) relics of horizontal or gently dipping fabric (S1) preserved in cores of granulite lenses and in garnet porphyroblasts, (2) a N-S trending sub-vertical fabric (S2) preserved in low-strain domains and (3) upright folds (F3) associated with a ubiquitous steep E-W striking axial planar foliation (S3). Garnet in the granulites preserves relics of a prograde mineral assemblage M1a equilibrated at ~11.5 kbar and ~770–780°C, whereas the matrix granulite assemblage (M1b) from the S1 fabric attained peak pressure at ~13.5 kbar and ~850°C. The granulites were overprinted at ~8–11 kbar and ~850–900°C during crustal melting (M2) followed by partial re-equilibration (M3) at ~8 kbar and ~625°C. A garnet Lu–Hf age of 421.6 ± 1.2 Ma dates metamorphism M1, while a garnet Sm–Nd age of 385.3 ± 4.0 Ma reflects M3 cooling of the granulites. The mineral assemblage, M1, of the host migmatitic metapelite formed at ~9–12.5 kbar and ~760–810°C, partial melting and migmatization (M2) occurred at ~7 kbar and ~760°C and re-equilibration (M3) at ~5–6 kbar and ~675°C. A garnet Lu–Hf age of 409.7 ± 2.3 Ma dates thermal climax (M2) and a garnet Sm–Nd age of 356 ± 11 Ma constrains M3 for the migmatitic metapelites. The timing of this late phase is also bracketed by an emplacement age of syntectonic granite dated at c. 360 Ma. Decoupling of M1 and M2 P–T evolutions between the mafic granulites and migmatitic metapelites indicates their different positions in the crustal column, while the shared pressure–temperature (P–T) evolution M3 suggests formation of a mélange-like association during the late stages of orogeny. The high-pressure event D1-M1 is interpreted as a result of Late Silurian–Early Devonian moderate crustal thickening of a thermally softened and thinned pre-orogenic crust. The high-temperature (HT) re-equilibration D2-M2 is interpreted as a result of Mid-Devonian shortening of the previously thickened crust, possibly due to ‘Andean-type’ underthrusting. The D3-M3 event reflects Late Devonian supra-subduction shortening and continuous erosion of the sub-crustal lithosphere. This tectono-metamorphic sequence of events is explained by polyphased Andean-type deformation of a ‘Cascadia-type’ active margin, which corresponds to a supra-subduction tectonic switching paradigm.  相似文献