Tectono-sedimentary framework of the Gachsaran Formation in the Zagros foreland basin

Continental collision between Iranian and Arabian plates resulted in the formation of the Zagros fold–thrust belt and its associated foreland basin. During convergence, pre-existing faults in the basement were reactivated and the sedimentary cover was shortened above two different types of basal decollement (viscous/frictional). This led to heterogeneous deformation which segmented not only the Zagros fold–thrust belt but also its foreland basin into different compartments resulting in variation in facies, thickness and age of the sediment infill.Based on this concept, a new tectono-sedimentary model is proposed for one of the most important syn-tectonic sedimentary unit, the Gachsaran salt in the Zagros foreland basin. In this proposed model, it is argued that differential propagation of the deformation front above decollements with different mechanical properties (viscous versus frictional) results in along-strike irregularity of the Zagros deformation front whereas movement along pre-existing basement faults leads to development of barriers across the Zagros basin. The irregularity of the deformation front and the cross-basin barriers divided the Zagros foreland basin into six almost alternating sub-basins where Gachsaran salt and its non-salt equivalents are deposited. In the salt sub-basins, two different processes were responsible for the deposition of Gachsaran salt: (1) evaporation, and (2) dissolution of extruding Hormuz salt and its re-precipitation as Gachsaran salt. Re-precipitation was probably the most significant process responsible for the huge deposit of Gachsaran salt in the extreme south-east part of the Zagros foreland basin.     

Urban form study: the sprawling city—review of methods of studying urban sprawl

There is a large body of research on urban forms. This review paper focuses on the urban sprawl using new and old approaches and new techniques from the subfield of urban Geography. There has been a debate among researchers about the definition of the sprawl, which makes it difficult to suggest a reliable model of the urban sprawl. This review is extremely selective. Numerous papers have focused on measuring the in terms of its physical cost or the environmental and transportation measures linked to this phenomenon. Several attempts have been made to improve the measurement of the sprawl using new approaches. Furthermore, this paper show that certain researchers did not attempt to explore new methods to investigate the urban sprawl in the past and present overlay layers for many years, researchers may use cartography techniques, for example, thereby collecting 20 years of data to match layers and show the differences in any metropolitan area. Several problems remain like differences in the understanding of the sprawl among planners and between other researchers interested in this issue. Also, it is problematic to find one measure, such as mixing different measures together to distinguish between car density and resident density. This study evaluated and reviewed old and recent studies to show that the most important issue in the urban sprawl relates to measurements. Incorporates inform and which measure is best. This article argues that urban spatial expansion reviews some of articles written on urban sprawl and showing the different approaches to studying.

     

Regional Frequency Analysis of Observed Sub-Daily Rainfall Maxima over Eastern China

Based on hourly rainfall observational data from 442 stations during 1960–2014, a regional frequency analysis of the annual maxima(AM) sub-daily rainfall series(1-, 2-, 3-, 6-, 12-, and 24-h rainfall, using a moving window approach) for eastern China was conducted. Eastern China was divided into 13 homogeneous regions: Northeast(NE1, NE2), Central(C), Central North(CN1, CN2), Central East(CE1, CE2, CE3), Southeast(SE1, SE2, SE3, SE4), and Southwest(SW).The generalized extreme value performed best for the AM series in regions NE, C, CN2, CE1, CE2, SE2, and SW, and the generalized logistic distribution was appropriate in the other regions. Maximum return levels were in the SE4 region, with value ranges of 80–270 mm(1-h to 24-h rainfall) and 108–390 mm(1-h to 24-h rainfall) for 20- and 100 yr, respectively.Minimum return levels were in the CN1 and NE1 regions, with values of 37–104 mm and 53–140 mm for 20 and 100 yr,respectively. Comparing return levels using the optimal and commonly used Pearson-III distribution, the mean return-level differences in eastern China for 1–24-h rainfall varied from-3–4 mm to-23–11 mm(-10%–10%) for 20-yr events, reaching-6–26 mm(-10%–30%) and-10–133 mm(-10%–90%) for 100-yr events. In view of the large differences in estimated return levels, more attention should be given to frequency analysis of sub-daily rainfall over China, for improved water management and disaster reduction.     

同构造沉积速率对先存被动盐底辟挤压变形的影响:库车坳陷西秋里塔格构造带盐构造分析及物理模拟

文中构建了两组构造物理实验,对存在先存被动盐底辟构造的含盐盆地的厚皮挤压构造演化特征进行模拟,研究揭 示了库车坳陷西段秋里塔格构造带新生代盐相关构造演化过程。实验结果表明,同构造沉积速率对库车坳陷西段秋里塔格 构造带博孜-却勒区域（西段） 和秋里塔格-克拉苏区域（东段） 盐相关构造横向分段差异变形具有重要的控制影响作 用。在挤压过程中,博孜-却勒区域慢速同构造沉积使得先存被动盐底辟北翼（拜城凹陷南翼） 向南逆冲到底辟南翼之 上,并促使却勒盐推覆体和米斯坎塔克盐背斜形成;而秋里塔格-克拉苏区域快速同构造沉积使得先存被动盐底辟北翼快 速下沉,而其南翼在挤压应力作用下向北逆冲到底辟北翼（拜城凹陷南翼） 之上,形成南秋里塔格盐背斜。     

Numerical modelling of rise and fall of a dense layer in salt diapirs

Numerical models are used to study the entrainment of a dense anhydrite layer by a diapir. The anhydrite layer is initially horizontally embedded within a viscous salt layer. The diapir is down-built by aggradation of non-Newtonian sediments ( n = 4, constant temperature) placed on the top of the salt layer. Several parameters (sedimentation rate, salt viscosity, perturbation width and stratigraphic position of the anhydrite layer) are studied systematically to understand their role in governing the entrainment of the anhydrite layer. High sedimentation rates during the early stages of the diapir evolution bury the initial perturbation and, thus, no diapir forms. The anhydrite layer sinks within the buried salt layer. For the same sedimentation rate, increasing viscosity of the salt layer decreases the rise rate of the diapir and reduces the amount (volume) of the anhydrite layer transported into the diapir. Model results show that viscous salt is capable of carrying separate blocks of the anhydrite layer to relatively higher stratigraphic levels. Varying the width of the initial perturbation (in our calculations 400–800 m), from which a diapir triggers, shows that wider diapirs can more easily entrain an embedded anhydrite layer than the narrower diapirs. The anhydrite layer is entrained as long as rise rate of the diapir exceeds the descent rate of the denser anhydrite layer. We conclude that the four parameters mentioned above govern the ability of a salt diapir to entrain an embedded dense layer. However, the model results show that the entrained blocks inevitably sink back if the rise rate of the diapir is less than the rate of descent of the anhydrite layer or the diapir is permanently covered by a stiff overburden in case of high sedimentation rates.     

Implications of channel flow analogue models for extrusion of the Higher Himalayan Shear Zone with special reference to the out-of-sequence thrusting

The Higher Himalayan Shear Zone (HHSZ) contains a ductile top-to-N/NE shear zone—the South Tibetan detachment system-lower (STDS_L) and an out-of-sequence thrust (OOST) as well as a top-to-N/NE normal shear at its northern boundary and ubiquitously distributed compressional top-to-S/SW shear throughout the shear zone. The OOST that was active between 22 Ma and the Holocene, varies in thickness from 50 m to 6 km and in throw from 1.4 to 20 km. Channel flow analogue models of this structural geology were performed in this work. A Newtonian viscous polymer (PDMS) was pushed through a horizontal channel leading to an inclined channel with parallel and upward-diverging boundaries analogous to the HHSZ and allowed to extrude to the free surface. A top-to-N/NE shear zone equivalent to the STDS_U developed spontaneously. This also indirectly connotes an independent flow confined to the southern part of the HHSZ gave rise to the STDS_L. The PDMS originally inside the horizontal channel extruded at a faster rate through the upper part of the inclined channel. The lower boundary of this faster PDMS defined the OOST. The model OOST originated at the corner and reached the vent at positions similar to the natural prototype some time after the channel flow began. The genesis of the OOST seems to be unrelated to any rheologic contrast or climatic effects. Profound variations in the flow parameters along the HHSZ and the extrusive force probably resulted in variations in the timing, location, thickness and slip parameters of the OOST. Variation in pressure gradient within the model horizontal channel, however, could not be matched with the natural prototype. Channel flow alone presumably did not result in southward propagation of deformation in the Himalaya.     

The effect of basement step/topography on the geometry of the Zagros fold and thrust belt (SW Iran): an analog modeling approach

Systematic analog models are run to study the variation in deformation across basement steps in the Zagros fold-thrust belt. Our model results demonstrate that basement configuration/topography influences the sedimentation thickness and, hence, the kinematics and geometric evolution of the fold and thrust belt. The greater the difference in thickness between the adjacent cover units across a basement step, the sharper and clearer will be the offset of the deformation front. Based on model results, we conclude that in a fold-thrust belt, where basement step/topography is covered by a layer of ductile salt acting as a decollement, the effect of the salt decollement on the evolution of the belt is far greater than the effect of thickness variation of the cover units.     

The control of salt supply on entrainment of an anhydrite layer within a salt diapir

The influence of four parameters (sedimentation rate, viscosity of salt, stratigraphic location of the anhydrite layer within the salt layer, and the perturbation width) on salt supply to down-built diapirs and its entrainment capacity are studied systematically in numerical models. Model results show that these four parameters affect salt supply, and the evolution history of a salt diapir. As such, these parameters strongly influence the style and the amount of entrainment of dense inclusions into a diapir. In active diapirs (i.e. unburied diapirs), salt supply increases with increasing sedimentation rate whereas it decreases with an increase in salt viscosity. Diapirs initiating from wide perturbation provide more salt supply to feed the diapir. Presence and initial stratigraphic location of any denser layer (e.g. an anhydrite layer) within a salt layer also affects salt supply. When lateral forces are negligible, salt supply into a diapir depends on these four parameters, which directly control the entrainment of any embedded anhydrite layer into the diapir.     

Higher Himalayan Shear Zone, Zanskar Indian Himalaya: microstructural studies and extrusion mechanism by a combination of simple shear and channel flow

Thin-section studies of the Zanskar Shear Zone (ZSZ) rocks reveal a top-to-SW and subsequent primary and secondary top-to-NE ductile shearing; brittle–ductile and brittle extensions; top-to-SW brittle shear, steep normal faulting and fracturing. In the proposed two-phase model of ductile extrusion of the Higher Himalayan Shear Zone (HHSZ), a top-to-SW simple shearing during 22–18 Ma was followed by a combination of top-to-SW simple shear and channel flow at 18–16 Ma. The second phase simulates a thin ZSZ characterized by a top-to-NE shearing. The channel flow component ceased around 16 Ma, the extruding HHSZ entered the brittle regime but the top-to-SW shearing continued until perturbed by faults and fractures. Variation in the extrusion parameters led to variable thickness of the ZSZ. Shear strain after the extrusion is presumably maximum at the boundaries of the HHSZ and falls towards the base of the ZSZ, which crudely matches with the existing data. The other predictions: (1) spatially uniform shear strain after the first stage, (2) fastest extrusion rate at the base of the ZSZ, and (3) a lack of continuation of the ZSZ along the Himalayan trend are not possible to validate due to paucity of suitable data. Non-parabolic shear fabrics of the ZSZ indicate their heterogeneous deformation.     

Higher Himalayan Shear Zone, Sutlej section: structural geology and extrusion mechanism by various combinations of simple shear, pure shear and channel flow in shifting modes

The Higher Himalayan Shear Zone (HHSZ) in the Sutlej section reveals (1) top-to-SW ductile shearing, (2) top-to-NE ductile shearing in the upper- and the lower strands of the South Tibetan Detachment System (STDS_U, STDS_L), and (3) top-to-SW brittle shearing corroborated by trapezoid-shaped minerals in micro-scale. In the proposed extrusion model of the HHSZ, the E₁-phase during 25–19 Ma is marked by simple shearing of the upper sub-channel defined by the upper strand of the Main Central Thrust (MCT_U) and the top of STDS_U as the lower- and the upper boundaries, respectively. Subsequently, the E_2a-pulse during 15–14 Ma was characterized by simple shear, pure shear, and channel flow of the entire HHSZ. Finally, the E_2b-pulse during 14–12 Ma observed simple shearing and channel flow of the lower sub-channel defined by the lower strand of the Main Central Thrust (MCT_L) and the top of the STDS_L as the lower- and the upper boundaries, respectively. The model explains the constraints of thicknesses of the STDS_U and the STDS_L along with spatially variable extrusion rate and the inverted metamorphism of the HHSZ. The model predicts (1) shear strain after ductile extrusion to be maximum at the boundaries of the HHSZ, which crudely matches with the existing data. The other speculations that cannot be checked are (2) uniform shear strain from the MCT_U to the top of the HHSZ in the E₁-phase; (3) fastest rates of extrusion of the lower boundaries of the STDS_U and the STDS_L during the E_2a- and E_2b-pulses, respectively; and (4) variable thickness of the STDS_L and rare absence of the STDS_U. Non-parabolic shear fabrics of the HHSZ possibly indicate heterogeneous strain. The top-to-SW brittle shearing around 12 Ma augmented the ductile extruded rocks to arrive a shallower depth. The brittle–ductile extension leading to boudinage possibly did not enhance the extrusion.