Drainage integration and sediment dispersal in active continental rifts: A numerical modelling study of the central Italian Apennines

Progressive integration of drainage networks during active crustal extension is observed in continental areas around the globe. This phenomenon is often explained in terms of headward erosion, controlled by the distance to an external base‐level (e.g. the coast). However, conclusive field evidence for the mechanism(s) driving integration is commonly absent as drainage integration events are generally followed by strong erosion. Based on a numerical modelling study of the actively extending central Italian Apennines, we show that overspill mechanisms (basin overfilling and lake overspill) are more likely mechanisms for driving drainage integration in extensional settings and that the balance between sediment supply vs. accommodation creation in fault‐bounded basins is of key importance. In this area drainage integration is evidenced by lake disappearance since the early Pleistocene and the transition from internal (endorheic) to external drainage, i.e. connected to the coast. Using field observations from the central Apennines, we constrain normal faulting and regional surface uplift within the surface process model CASCADE (Braun & Sambridge, 1997, Basin Research, 9, 27) and demonstrate the phenomenon of drainage integration, showing how it leads to the gradual disappearance of lakes and the transition to an interconnected fluvial transport system over time. Our model results show that, in the central Apennines, the relief generated through both regional uplift and fault‐block uplift produces sufficient sediment to fill the extensional basins, enabling overspill and individual basins to eventually become fluvially connected. We discuss field observations that support our findings and throw new light upon previously published interpretations of landscape evolution in this area. We also evaluate the implications of drainage integration for topographic development, regional sediment dispersal and offshore sediment supply. Finally, we discuss the applicability of our results to other continental rifts (including those where regional uplift is absent) and the importance of drainage integration for transient landscape evolution.     

Modern and ancient fluvial megafans in the foreland basin system of the central Andes, southern Bolivia: implications for drainage network evolution in fold-thrust belts

ABSTRACT Fluvial megafans chronicle the evolution of large mountainous drainage networks, providing a record of erosional denudation in adjacent mountain belts. An actualistic investigation of the development of fluvial megafans is presented here by comparing active fluvial megafans in the proximal foreland basin of the central Andes to Tertiary foreland‐basin deposits exposed in the interior of the mountain belt. Modern fluvial megafans of the Chaco Plain of southern Bolivia are large (5800–22 600 km²), fan‐shaped masses of dominantly sand and mud deposited by major transverse rivers (Rio Grande, Rio Parapeti, and Rio Pilcomayo) emanating from the central Andes. The rivers exit the mountain belt and debouch onto the low‐relief Chaco Plain at fixed points along the mountain front. On each fluvial megafan, the presently active channel is straight in plan view and dominated by deposition of mid‐channel and bank‐attached sand bars. Overbank areas are characterized by crevasse‐splay and paludal deposition with minor soil development. However, overbank areas also contain numerous relicts of recently abandoned divergent channels, suggesting a long‐term distributary drainage pattern and frequent channel avulsions. The position of the primary channel on each megafan is highly unstable over short time scales. Fluvial megafans of the Chaco Plain provide a modern analogue for a coarsening‐upward, > 2‐km‐thick succession of Tertiary strata exposed along the Camargo syncline in the Eastern Cordillera of the central Andean fold‐thrust belt, about 200 km west of the modern megafans. Lithofacies of the mid‐Tertiary Camargo Formation include: (1) large channel and small channel deposits interpreted, respectively, as the main river stem on the proximal megafan and distributary channels on the distal megafan; and (2) crevasse‐splay, paludal and palaeosol deposits attributed to sedimentation in overbank areas. A reversal in palaeocurrents in the lowermost Camargo succession and an overall upward coarsening and thickening trend are best explained by progradation of a fluvial megafan during eastward advance of the fold‐thrust belt. In addition, the present‐day drainage network in this area of the Eastern Cordillera is focused into a single outlet point that coincides with the location of the coarsest and thickest strata of the Camargo succession. Thus, the modern drainage network may be inherited from an ancestral mid‐Tertiary drainage network. Persistence and expansion of Andean drainage networks provides the basis for a geometric model of the evolution of drainage networks in advancing fold‐thrust belts and the origin and development of fluvial megafans. The model suggests that fluvial megafans may only develop once a drainage network has reached a particular size, roughly 10⁴ km²– a value based on a review of active fluvial megafans that would be affected by the tectonic, climatic and geomorphologic processes operating in a given mountain belt. Furthermore, once a drainage network has achieved this critical size, the river may have sufficient stream power to prove relatively insensitive to possible geometric changes imparted by growing frontal structures in the fold‐thrust belt.     

Drainage spacing regularity on a fault-block: A case study from the eastern Ruahine Range

Abstract:  Recent research has indicated river basin outlets draining linear sections of large, uplifting mountain belts often show a regularity of spacing, transverse to the main structural trend. A morphometric analysis of part of the Ruahine Range, on the North Island was undertaken to test whether drainage regularity may exist in smaller, younger mountain ranges. The ratio, R , of the half-width of the mountain belt, W , and the outlet spacing, S , was used to characterize drainage networks on the eastern side of the range. The spacing ratio for the range of 1.31 is lower than R results from studies of larger mountain belts ( R  = 1.91–2.23). We suggest the cause of this lower ratio is related to eastward migration of the Ruahine drainage divide.     

Sediment routing in the Zagros foreland basin: Drainage reorganization and a shift from axial to transverse sediment dispersal in the Kurdistan region of Iraq

In the northwestern sector of the Zagros foreland basin, axial fluvial systems initially delivered fine-grained sediments from northwestern source regions into a contiguous basin, and later transverse fluvial systems delivered coarse-grained sediments from northeastern sources into a structurally partitioned basin by fold-thrust deformation. Here we integrate sedimentologic, stratigraphic, palaeomagnetic and geochronologic data from the northwestern Zagros foreland basin to define the Neogene history of deposition and sediment routing in response to progressive advance of the Zagros fold-thrust belt. This study constrains the depositional environments, timing of deposition and provenance of nonmarine clastic deposits of the Injana (Upper Fars), Mukdadiya (Lower Bakhtiari) and Bai-Hasan (Upper Bakhtiari) Formations in the Kurdistan region of Iraq. Sediments of the Injana Formation (~12.4–7.75 Ma) were transported axially (orogen-parallel) from northwest to southeast by meandering and low-sinuosity channel belt system. In contrast, during deposition of the Mukdadiya Formation (~7.75–5 Ma), sediments were delivered transversely (orogen-perpendicular) from northeast to southwest by braided and low-sinuosity channel belt system in distributive fluvial megafans. By ~5 Ma, the northwestern Zagros foreland basin became partitioned by growth of the Mountain Front Flexure and considerable gravel was introduced in localized alluvial fans derived from growing topographic highs. Foredeep accumulation rates during deposition of the Injana, Mukdadiya and Bai-Hasan Formations averaged 350, 400 and 600 m/Myr respectively, suggesting accelerated accommodation generation in a rapidly subsiding basin governed by flexural subsidence. Detrital zircon U-Pb age spectra show that in addition to sources of Mesozoic-Cenozoic cover strata, the Injana Formation was derived chiefly from Palaeozoic-Precambrian (including Carboniferous and latest Neoproterozoic) strata in an axial position to the northwest, likely from the Bitlis-Puturge Massif and broader Eastern Anatolia. In contrast, the Mukdadiya and Bai-Hasan Formations yield distinctive Palaeogene U-Pb age peaks, particularly in the southeastern sector of the study region, consistent with transverse delivery from the arc-related terranes of the Walash and Naopurdan volcano-sedimentary groups (Gaveh-Rud domain?) and Urumieh-Dokhtar magmatic arc to the northeast. These temporal and spatial variations in stratigraphic framework, depositional environments, sediment routing and compositional provenance reveal a major drainage reorganization during Neogene shortening in the Zagros fold-thrust belt. Whereas axial fluvial systems initially dominated the foreland basin during early orogenesis in the Kurdistan region of Iraq, transverse fluvial systems were subsequently established and delivered major sediment volumes to the foreland as a consequence of the abrupt deformation advance and associated topographic growth in the Zagros.     

近50年来山东城市体系的演化过程--基于城市中心性的分析

在前人工作的基础上,在简单讨论已有的相对中心性测度方法的基础上,根据研究目的对Tietz公式进行了一定的补充。利用1955年私营商业调查,1982年和2000年人口普查数据,通过NegF法和SumF法分析了近50年山东省中心城市的分布,及其紧密腹地、松散腹地的变动,勾勒出了1955年到2000年山东城市体系的动态演化过程。认识到山东省城市体系的发展是一个逐步复杂化、有序化的过程,由分别以青岛和济南为中心城市的两个子体系逐步演化至以青岛为中心城市的单一共同体系。在这一过程中,中心城市的等级性逐渐显化。     

Drainage patterns and tectonic forcing: a model study for the Swiss Alps

ABSTRACT A linear surface process model is used to examine the effect of different patterns of rock uplift on the evolution of the drainage network of the Swiss Alps. An asymmetric pattern of tectonic forcing simulates a phase of rapid retrothrusting in the south of the Swiss Alps (‘Lepontine’‐type uplift). A domal pattern of tectonic forcing in the north of the model orogen simulates the phase of the formation of the ‘Aar massif’, an external basement uplift in the frontal part of the orogenic wedge (‘Aar’‐type uplift). Model runs using the ‘Lepontine’‐type uplift pattern result in a model mountain chain with a water divide in the zone of maximum uplift and orogen‐normal rivers. Model runs examining the effect of ‘Lepontine’‐type uplift followed by ‘Aar’‐type uplift show that the initially formed orogen‐normal river system and the water divide are both very stable and hardly affected by the additional uplift. This indifference to changes in tectonic forcing is mainly due to the requirement of a high model erosion capacity for the river systems in order to reproduce the exhumation data (high‐grade rocks in the south of the Swiss Alps point to removal of a wedge‐shaped nappe stack with a maximum thickness of about 25 km). The model behaviour is in agreement with the ancestral drainage pattern of the Alps in Oligocene and Miocene times and with the modern pattern observed in the Coast Range of British Columbia; in both cases river incision occurred across a zone of rapid uplift in the lower course of the rivers. The model behaviour does not, however, explain the modern drainage pattern in the Alps with its orogen‐parallel rivers. When the model system is forced to develop two locally independent main water divides (simultaneous ‘Lepontine’‐ and ‘Aar’‐type uplift), a zone of reduced erosional potential forms between the two divides. As a consequence, the divides approach each other and eventually merge. The new water divide remains fixed in space independent of the two persisting uplift maxima. The model results suggest that spatial and temporal changes in tectonic forcing alone cannot produce the change from the orogen‐normal drainage pattern of the Swiss Alps in Oligocene–Miocene times to the orogen‐parallel drainage observed in the Swiss Alps today.     

Characterization of active fault scarps from LiDAR data: a case study from Central Apennines (Italy)

A high-resolution digital elevation model (DEM, 1 ms spacing) derived from an airborne light detection and ranging (LiDAR) campaign was used in an attempt to characterize the structural and erosive elements of the geometry of the Pettino fault, a seismogenic normal fault in Central Apennines (Italy). Four 90- to 280 m-long fault scarp segments were selected and the surface between the base and the top of the scarps was analyzed through the statistical analysis of the following DEM-derived parameters: altitude, height of the fault scarp, and distance along strike, slope, and aspect. The results identify slopes of up to 40° in faults lower reaches interpreted as fresh faces, 34° up the faces. The Pettino fault maximum long-term slip rate (0.6–1.1 mm/yr) was estimated from the scarp heights, which are up to 12–19 m in the selected four segments, and the age (ca. 18 ka) of the last glacial erosional phase in the area. The combined analysis of the DEM-derived parameters allows us to (a) define aspects of three-dimensional scarp geometry, (b) decipher its geomorphological significance, and (c) estimate the long-term slip rate.     

Drainage basin object-based method for regional-scale landform classification: a case study of loess area in China

Landform classification is one of the most important procedures in recognizing and dividing earth surface landforms. However, topographical homogeneity and differences in regional-scale landforms are often ignored by traditional pixel- and object-based landform classification methods based on digital elevation models (DEMs). In this work, a drainage basin object-based method for classifying regional-scale landforms is proposed. Drainage basins with least critical areas are first delineated from DEMs. Then, terrain derivatives of mean elevation, mean slope, drainage density, drainage depth, and terrain texture are employed to characterize the morphology of the drainage basins. Finally, a decision tree based on the topographical characteristics of the drainage basins is constructed and employed to determine the landform classification law. The experiment is validated in the landform classification of regional-scale loess areas in China. Results show that clear boundaries exist in different landforms at the regional scale. Landform type in a specific region shows significant topographical homogeneity under its specific regional geomorphological process. Classification accuracies are 87.3 and 86.3% for the field investigation and model validation, respectively. Spatial patterns of classified landforms and their proximity to sediment sources and other factors can be regarded as indicators of the evolutionary process of loess landform formation.     

A major seismogenic fault in a 'silent area': the Castrovillari fault (southern Apennines, Italy)

Large historical earthquakes in Italy define a prominent gap in the Pollino region of the southern Apennines. Geomorphic and palaeoseismological investigations in this region show that the Castrovillari fault (CF) is a major seismogenic source that could potentially fill the southern part of this gap. The surface expression of the CF is a complex, 10–13 km long set of prominent scarps. Trenches across one scarp indicate that at least four surface-faulting earthquakes have occurred along the CF since Late Pleistocene time, each producing at least 1 m of vertical displacement. The length of the fault and the slip per event suggest M =6.5-7.0 for the palaeoearthquakes. Preliminary radiocarbon dating coupled with historical considerations imply that the most recent of these earthquakes occurred between 380 BC and 1200 AD, and probably soon after 760 AD; no evidence for this event has been found in the historical record. We estimate a minimum recurrence interval of 1170 years and a vertical slip rate of 0.2-0.5 mm yr^-1 for the CF, which indicates that the seismic behaviour of this fault is comparable to other major seismogenic faults of the central-southern Apennines. The lack of mention or the mislocation of the most recent event in the historical seismic memory of the Pollino region clearly shows that even in Italy, which has one of the longest historical records of seismicity, a seismic hazard assessment based solely on the historical record may not be completely reliable, and shows that geological investigations are critical for filling possible information gaps.     

Headless submarine canyons and fluid flow on the toe of the Cascadia accretionary complex

Headless submarine canyons with steep headwalls and shallowly sloping floors occur on both the second and third landward vergent anticlines on the toe of the Cascadia accretionary complex off central Oregon (45 °N, 125° 30′W). In September 1993, we carried out a series of nine deep tow camera sled runs and nine ALVIN dives to examine the relationship between fluid venting, structure and canyon formation. We studied four canyons on the second and third landward vergent anticlines, as well as the apparently unfailed intercanyon regions along strike. All evidence of fluid expulsion is associated with the canyons; we found no evidence of fluid flow between canyons. Even though all fluid seeps are related to canyons, we did not find seeps in all canyons, and the location of the seeps within the canyons differed. On the landward facing limb of the second landward vergent anticline a robust cold seep community occurs at the canyon’s inflection point. This seep is characterized by chemosynthetic vent clams, tube worms and extensive authigenic carbonate. Fluids for this seep may utilize high-permeability flow paths either parallel to bedding within the second thrust ridge or along the underlying thrust fault before leaking into the overriding section. Two seaward facing canyons on the third anticlinal ridge have vent clam communities near the canyon mouths at approximately the intersection between the anticlinal ridge and the adjacent forearc basin. No seeps were found along strike at the intersection of the slope basin and anticlinal ridge. We infer that the lack of seepage along strike and the presence of seeps in canyons may be related to fluid flow below the forearc basin/slope unconformity (overpressured by the impinging thrust fault to the west?) directed toward canyons at the surface.     

江苏省产业带建设效应与演化动力

在综述国内外产业带相关概念及其形成机制研究的基础上,对江苏省产业带建设过程及其区域效应进行了分析,并从产业带的基本内涵出发,分析产业带形成与发展的主要驱动因素,构建出产业带形成与发展动力的综合评价指标体系,运用熵值法,对产业带形成与发展的演化动力进行了定量比较.结果表明:江苏省通过产业带建设布局与发展,使南北区域差异扩...     

Behind the scenes: the evolving urban networks of film production in China

ABSTRACT

Urban networks in the modern economy have become a focus of research in geography and other related disciplines. However, the network of the motion picture industry has remained an underdeveloped topic in the relevant literature. Drawing on data from film production projects, this paper presents a pilot study of the changing inter-city networks that are created by the motion picture industry in mainland China over the past 15 years. Combining the perspectives of both the urban network and economic geography literature, this study not only advances the research on urban networks by offering new insights into the project-based, inter-firm cooperation networks of an important cultural industry sector but also enriches our knowledge of the functioning of local clusters and trans-local networks in new economic activities. In addition, the paper also contributes to our understanding of the evolution of modern urban networks in a transitional economic system.     

Land cover and land use change in the Italian central Apennines: A comparison of assessment methods

Analyses of land-use cover changes (LUCC) are fundamental to the understanding of numerous social, economical and environmental problems and can be carried out rapidly, using either cartographic or census data. However, the trends of the two methods differ in direction and quantity.For this study, a historical and a recent remote sensing-derived map were homogenized to reduce misleading changes and to assess spatial aggregation errors. This was carried out by means of a data integration procedure based on landscape metrics, allowing cartographic and census trends to be compared. Discrepancies between data were thus highlighted, both in absolute surface value and in evolution.The methodology presented, and the results obtained, could be employed to evaluate and improve LUCC analyses aimed at assessing landscape identity, both in the case of analyses based only on LU census data, or of those based only on LC cartographic data. This could lead to benefits for both biodiversity conservation and environmental planning on a large scale.     

阿尔金山-祁连山山地植被垂直带谱分布及地学分析

本文揭示阿尔金-祁连山区垂直带谱空间分布模式并进行地学解释。研究表明:北坡荒漠草原带的上限具有随经度变化的二次曲线分布模式,草原带的上限分布为线性模式,亚冰雪带的分布与7月份气温的零度层分布模式基本相同;北坡镜铁山以西的带谱中没有森林带,以东含有森林带,分布在2500³300米的高度范围内。利用气象台站的地面及高空数据,计算了湿度———生长季平均温度露点差随海拔变化的梯度、多年平均温度、1月多年均温、多年平均降水指标以解释带谱分布,结果表明:由东向西,湿度梯度增大,以酒泉为界,西侧的露点温度差梯度大于0.12℃/100米,东部小于0.09℃/100米,与北坡山地森林带分布基本吻合。     

长江上游山地垂直带谱及其空间分布模式

长江上游包括青藏高原东南部、秦巴山地、四川盆地与云贵高原部分地区,在地理、地貌、气候、生物多样性方面都表现得极为复杂和丰富多彩,在世界山地中也占有举足轻重的位置。特别是复杂多样的山地垂直带谱更是欧亚大陆乃至世界山地垂直带研究中至关重要的组成部分。在地学信息图谱和数字山地垂直带体系的基础上,本文系统地收集和分析了长江上游共50个山地垂直带谱所体现的空间规律,河源区、横断山区、秦巴山区及贵州高原的垂直带谱类型多样并各具特色,且在经度和纬度方向又具有统一的分布规律,如雪线、林线、针叶林及阔叶林等的分布界线变化规律比较符合二次曲线规律,验证了大陆尺度上山地垂直带二次曲线模式假说。另外,山地垂直带分布规律又具有尺度效应,中小尺度上地形的影响作用表现得极为显著。     

Lithological control on thrust-related deformation in the Sassa-Guardistallo Basin (Northern Apennines hinterland, Italy)

The Sassa‐Guardistallo Basin (SGB) is located close to the Tyrrhenian Sea and represents one of the most internal Neogene–Quaternary hinterland basins of the Northern Apennines fold‐and‐thrust belt. Its sedimentary succession consists of ca. 400‐m‐thick Late Tortonian–Messinian continental – largely conglomeratic – units overstepping a mainly shaly substratum (Palombini Shales) and overlain by Late Messinian evaporites and marine to continental Pliocene–Pleistocene sediments. This stratigraphic succession can be approximated to a composite rheological multilayer that dictated the style of basin deformation. Detailed geological mapping and structural analysis revealed that basin deposits were affected by compressional deformations that can be found both at map and outcrop scales. Decametric splay thrusts emanating from the substratum–conglomerate interface locally double the continental succession and are bounded by a roof thrust along the Late Messinian evaporite décollement, defining a deformation pattern consistent with a duplex‐like structure. The time–space structural evolution of the basin inferred from the fieldwork was addressed and tested by analogue modelling that approximated the rheological stratification of the study area to a layered brittle–ductile system. The model results support the hypothesis that the evolution of the thrust system affecting the SGB started as an early floor imbricate fan thrust system that successively evolved to a duplex structure as the link thrusts propagated into the upper décollement layer that resulted from the deposition of the Late Messinian evaporites. Models display many structural features that may be compared with the natural prototype, and highlight the importance of syntectonic sedimentation in the development and evolution of tectonic structures. The results of this study retain relevant implications for the Neogene evolution of the Tyrrhenian Basin–Northern Apennines system. This study also supports that combining between field structural analyses and analogue modelling can give useful hints into the evolutionary history of tectonically complex areas.     

Experimental study of the anisotropy of longitudinal and transverse waves from local earthquake records

Summary. The paper gives the results of a study of the anisotropy of seismic wave velocities within the Ashkhabad test field in Central Asia. The anisotropy was studied by analysing variations in the values of apparent velocities of first arrivals for epicentral distances ranging from 30 to 130 km and by analysing the delays (Δ t_s1-_s2 ) between the arrival times of shear waves with different polarizations.
The velocities of P -waves vary with azimuth from 5.3 to 6.27 km s^-1 and the velocities of S -waves vary from 3.15 to 3.5 km s^-1.
The delay times Δ t_S1 - _S2 depend on the direction of the propagation. The character of the variation of the propagation velocity of the longitudinal wave, the presence of two differently polarized shear waves S 1 and S 2 propagating at different velocities, and the character of the distribution of Δ t_S1 - _S2 on the stereogram suggest that the symmetry of the anisotropic medium is close to hexagonal with a nearly horizontal symmetry axis coinciding with the direction of maximal velocity. The azimuth of the symmetry axis of the medium is 140° and coincides with the direction of geological faults.     

Initiation and growth of salt-based thrust belts on passive margins: results from physical models

Scaled sandbox models simulated primary controls on the kinematics of the early structural evolution of salt‐detached, gravity‐driven thrust belts on passive margins. Models had a neutral‐density, brittle overburden overlying a viscous décollement layer. Deformation created linked extension–translation–shortening systems. The location of initial brittle failure of the overburden was sensitive to perturbations at the base of the salt. Salt pinch‐out determined the seaward limit of the thrust belt. The thrust belts were dominated by pop‐up structures or detachment folds cut by break thrusts. Pop‐ups were separated by flat‐bottomed synclines that were partially overthrust. Above a uniformly dipping basement, thrusts initiated at the salt pinch‐out then consistently broke landward. In contrast, thrust belts above a seaward‐flattening hinged basement nucleated above the hinge and then spread both seaward and landward. The seaward‐dipping taper of these thrust belts was much lower than typical, frictional, Coulomb‐wedge models. Towards the salt pinch‐out, frictional resistance increased, thrusts verged strongly seawards and the dip of the taper reversed as the leading thrust overrode this pinch‐out. We attribute the geometry of these thrust belts to several causes. (1) Low friction of the basal décollement favours near‐symmetric pop‐ups. (2) Mobile salt migrates away from local loads created by overthrusting, which reduces the seaward taper of the thrust belt. (3) In this gravity‐driven system, shortening quickly spreads to form wide thrust belts, in which most of the strain overlapped in time.