Steep Topographic Gradients Research Articles

Impact of climate change on mountain environment dynamics

Mountains are specific ecosystems, characterised by their diversity and complexity (Messerli and Ives, 1997). Steep topographic, climatic and biological gradients combined with sharp seasonal contrasts favour the triggering of extreme climatic and geomorphic events, which may in turn strongly affect ecological and human environments (Ives and Messerli, 1989; Price, 1999; Beniston, 2002; Viviroli et al., 2007; Huggel et al., 2010; Korner, 2013). Mountain populations are quite diverse in their ...

Formation mechanism of steep convergent intracontinental margins: Insights from numerical modeling

AbstractThe margins surrounding the Tibetan Plateau show some diversity in topographic gradient. The most striking example is the eastern Tibetan margin bordered by the Longmen Shan range, which is characterized by a remarkably steep topography transition between eastern Tibet and the Sichuan Basin. There is significant uncertainty over whether this margin was formed by crustal shortening or lower‐crustal flow. To investigate the formation mechanism of steep convergent intracontinental margins, we conducted petrological‐thermomechanical numerical simulations based on the lithospheric structure and thermal state of the eastern Tibetan margin. Our numerical experiments demonstrate that a very steep topographic gradient, such as the eastern Tibetan margin, is an inherent characteristic of convergence between a hot and weak lithosphere with thick crust and a cold and strong lithosphere with thin crust. Although lower‐crustal flow has potentially contributed to the crustal thickness difference between the two convergent blocks, it is not a prerequisite for the growth of steep convergent intracontinental margin. Rather, the topography at the margin can be explained by a near isostatic response to crustal thickening resulting from shortening.

Stratigraphy and 40Ar/39Ar geochronology of the Santa Rosa basin, Baja California: Dynamic evolution of a constrictional rift basin during oblique extension in the Gulf of California

AbstractThe Santa Rosa basin of northeastern Baja California is one of several transtensional basins that formed during Neogene oblique opening of the Gulf of California. The basin comprises Late Miocene to Pleistocene sedimentary and volcanic strata that define an asymmetric half‐graben above the Santa Rosa detachment, a low‐angle normal fault with ca. 4–5 km of SE‐directed displacement. Stratigraphic analysis reveals systematic basin‐scale facies variations both parallel and across the basin. The basin‐fill exhibits an overall fining‐upward cycle, from conglomerate and breccia at the base to alternating sandstone‐mudstone in the depocentre, which interfingers with the fault‐scarp facies of the detachment. Sediment dispersal was transverse‐dominated and occurred through coalescing alluvial fans from the immediate hanging wall and/or footwall of the detachment. Different stratigraphic sections reveal important lateral facies variations that correlate with major corrugations of the detachment fault. The latter represent extension‐parallel folds that formed largely in response to the ca. N‐S constrictional strain regime of the transtensional plate boundary. The upward vertical deflection associated with antiformal folding dampened subsidence in the northeastern Santa Rosa basin, and resulted in steep topographic gradients with a high influx of coarse conglomerate here. By contrast, the downward motion in the synform hinge resulted in increased subsidence, and led to a southwestward migration of the depocentre with time. Thus, the Santa Rosa basin represents a new type of transtensional rift basin in which oblique extension is partitioned between diffuse constriction and discrete normal faulting. 40Ar/39Ar geochronology of intercalated volcanic rocks suggests that transtensional deformation began during the Late Miocene, between 9.36 ± 0.14 Ma and 6.78 ± 0.12 Ma, and confirms previous results from low‐temperature thermochronology (Seiler et al., 2011). Two other volcanic units that appear to be part of a conformable syn‐rift sequence are, in fact, duplicates of pre‐rift volcanics and represent allochthonous, gravity‐driven slide blocks that originated from the hanging wall.

Numerical modeling of eastern Tibetan-type margin: Influences of surface processes, lithospheric structure and crustal rheology

The eastern Tibetan margin is characterized by a steep topographic gradient and remarkably lateral variations in crustal/lithospheric structure and thermal state. GPS measurements show that the surface convergence rate in this area is strikingly low. How can such a mountain range grow without significant upper crustal shortening? In order to investigate the formation mechanism of the eastern Tibetan-type margins, we conducted 2D numerical simulations based on finite difference and marker-in-cell techniques. The numerical models were constrained with geological and geophysical observations in the eastern Tibetan margin. Several major parameters responsible for topography building, such as the convergence rate, the erosion/sediment rate, and the presence of partially molten crust, were systematically examined. The results indicate that the presence of partially molten material in the middle/lower crust can make a positive contribution to the formation of steep topography, but it is not a necessary factor. A steep topographic gradient may be a characteristic feature when a thin lithosphere with thick crust converges with a thick lithosphere with thin crust. In the context of a high erosion rate, the Longmen Shan range still gains and maintains its steep high topography to the present. This could be explained by exerting a large push force on Tibet side. Our numerical experiments suggest that topographic characteristic across the eastern Tibetan-type margins is mainly derived from isostatic equilibration forces and intensive convergence between two continental lithospheres with totally different rheological properties.

Sensitivity of hydrogeomorphological hazards in the Qinling Mountains, China

Abstract Investigations into hydrogeomorphological hazards in the Qinling Mountains were conducted by consideration of orographic characteristics and climatic fluctuations. Climatic-orographic-hydrologic processes in the Qinling Mountains were assessed through model simulation from proposed hydrodynamic gradient model and hydrological hazard index model combined with MM5 model (PSU/NCAR, 2005) and HEC-HMS/RAS ( USACE, 2008 ) model. Results show that climate change is key contributor to flood extreme events, and the Qinling Mountains has increased risk of rainstorm-flood extreme events. The hydrogeomorphological gradient has strong north–south gradient distribution due to orographic effects of the Qinling Mountains. Climate change impacts on hydrogeomorphological processes are dominated by short and severe rainstorms. High-frequency heavy rain regions are located in steep topographic gradients, such as the north–south valleys and windward slopes of higher elevations in the Qinling Mountains. The hydrogeomorphological consequences intensify the occurrence of localized extreme flash floods and accelerated erosion, altering flooding routing in the upper Hanjiang River basin. Flood routing is likely to be controlled by topographic rainfall for shorter periods (5-year and 10-year), and by climate change for longer periods (50-year and 100-year). Rainstorms and sediment gravity accumulation are the main causes of hazards.

Defining and characterizing high-rainfall Mediterranean climates

The world's five Mediterranean-climate regions are typified by similarities in climate reflected in structural vegetation patterns. We developed a methodology to define high-rainfall (annual precipitation ≥800 mm) Mediterranean climates and contrast patterns of between- and within-region climatic heterogeneity. Climatic indices describing the seasonality of the climate were computed in Geographic Information Systems and included in a numerical analysis, in addition to selected bioclimatic variables, using the pattern analysis program PATN. Selected locations from high-rainfall Mediterranean-climate regions were grouped according to climatic similarity, and within-region climatic heterogeneity was compared between regions. Areas traditionally not considered as Mediterranean-climate regions emerged in the classification. Only the Mediterranean-type systems within the Pacific north-west of North America and south-central Chile comprised a substantial extent of high-rainfall Mediterranean climate. Both of these regions were characterized by a high climatic diversity and steep topographic gradients. The numerical analysis demonstrated the high diversity of the high-rainfall Mediterranean-climate type and differences in within-region climatic heterogeneity between the world's high-rainfall Mediterranean-climate regions.

Crustal and uppermost mantle velocity structure beneath northwestern China from seismic ambient noise tomography

SUMMARY In this paper, we conduct ambient noise seismic tomography of northwestern China and adjacent regions. The data include 9 months (2009 January to 2009 September) three-component continuous data recorded at 146 seismic stations of newly upgraded China Provincial Digital Seismic Networks and regional Kyrgyzstan and Kazakhstan networks. Empirical Rayleigh and Love wave Green's functions are obtained from interstation cross-correlations. Group velocity dispersion curves for both Rayleigh and Love waves between 7 and 50 s periods were measured for each interstation path by applying the multiple-filter analysis method with phase-matched processing. The group velocity maps show clear lateral variations which correlate well with major geological structures and tectonic units in the study region. Shear wave velocity structures are inverted from Rayleigh wave and love wave dispersion maps. The results show that the Tibetan Plateau has a very thick crust with a low-velocity zone in its mid-lower crust. Along the northern margin of the plateau where a steep topographic gradient is present, the low-velocity zone does not extend to the Tarim basin which may indicate that crustal materials beneath the Tarim basin are colder and stronger than beneath the plateau, therefore inhibit the extension of mid-lower crustal flow and deformation of the Tibetan Plateau, resulting in very sharp topography contrasts. In the northeastern margin with a gentle topographic gradient toward the Ordos platform, the low-velocity zone diminishes around the eastern KunLun fault. Meanwhile, our results reveal obvious lateral velocity changes in the crust beneath the Tarim basin. In the upper crust, the Manjaer depression in the eastern Tarim basin is featured with very low velocities and the Bachu uplift in the western Tarim basin with high velocities; in the mid-lower crust, the northern Tarim basin in general displays lower velocities than the southern part along latitude ∼40° N with an east–west striking, which is consistent with the high magnetic anomaly zone and may be related to the central suture belt connecting the south and north of Tarim basement blocks together in Pre-Sinian.

Ionospheric structures correlated with Anatolian surface features

[1] A UHF/VHF beacon receiver located in Helwan, Egypt, frequently observes structures in ΔTEC/Δt measurements (where TEC is total electron count), where the F region (300 km) intercept of the radio rays crosses the steep topographic gradients associated with the Anatolian Plateau. There are three classes of structures: bumps, ripples and waves. A bump is defines as a single spatial ΔTEC/Δt peak with a peak-to-trough amplitude of at least 0.01 TECU/s (1 TEC unit (TECU) = 1016 electrons/m2) that is at least 1° wide in F region latitude. A ripple is a bump with smaller structures on either side of the central bump. Finally, waves have amplitudes ≥0.01 TECU/s with several roughly equal peaks. These features were observed repeatedly in a number passes from 31 August to 30 November 2008. Over half of passes had either a bump (34.6%), a ripple (18.2%) or a wave (6.3%). Most of these structures occur near areas with large orographic gradients. The prevailing surface wind blows across the mountains when bumps and ripples are observed. These correlations suggest that the local ionosphere is affected by the ground topography, most likely through the orographic lifting and the associated gravity waves.

Evidence of slumping/sliding in Krishna–Godavari offshore basin due to gas/fluid movements

The Krishna–Godavari (KG) offshore basin is one of the promising petroliferous basins of the eastern continental margin of India. Drilling in this basin proved the presence of gas hydrate deposits in the shallow marine sediments beyond 750 m water depths, and provided lithologic and stratigraphic information. We obtained multibeam swath bathymetry covering an area of about 4500 km 2 in water depths of 280–1800 m and about 1260 line km of high resolution seismic (HRS) records. The general lithology of midslope deposits is comprised of nannofossil-rich clay, nannofossil-bearing clay and foraminifera-bearing clay. The HRS records and bathymetry reveal evidence of slumping and sliding of the upper and midslope sediments, which result in mass transport deposits (MTD) in the northwestern part of the study area. These deposits exhibit 3–9.5 km widths and extend 10–13 km offshore. The boundaries of the MTDs are often demarcated by sharp truncation of finely layered sediments (FLS) and the MTDs are characterized by acoustically transparent zones in the HRS data. Average thickness of recent MTDs varies with depth, i.e., in the upper slope, the thickness is about 45 m, while in the lower slope it is about 60 m, and in deeper offshore locations they attain a maximum thickness of about 90 m. A direct indication for slumping and mass transportation of deposits is provided by the age reversal in 14C AMS dates observed in a sediment core located in the midslope region. Seismic profiling signatures provide indications of fluid/gas movement. We propose that the presence of steep topographic gradients, high sedimentation rates, a regional fault system, diapirism, fluid/gas movement, and neotectonic activity may have facilitated the slumping/sliding of the upper slope sediments in the KG offshore basin.

A estrutura de impacto do Cerro do Jarau, Quaraí, RS

The Cerro do Jarau is located in Quaraí region, in the southwestern portion of the RS state and represent one of the rare astrobleme of the south region of Brazil. The structure presents circular and structurally complex form, with a diameter of approximately 14 km. The central core of the structure is uplifted and has around shape with 7 km of diameter, with steep-topographic gradients of up to 160 meters between the high portions of its edge until the levels more deep in the external portion. The circular features showed by its structure is compound by a morphological group of concentric rings, constituted by topographic depressions in the external and central areas, separated in the north and central portion of the structure by an elongated topographic elevation in half moon shape. These features are reminiscent of weathering and erosive process of an impact crater generated by collision of a meteoritic body of large dimensions. These impact effects under the sandstones of Botucatu Formation (Triassic-Jurassic) and basaltic lava flows of Serra Geral Formation (Jurassic-Cretaceous) generated a group of extensional normal faults and transcurrent ruptile shear zones moreover the structural uplift of the sedimentary rocks in central portion of the Cerro do Jarau structure. The observed features of impact metamorphism are PDF’s and PF’s structures in quartz and plagioclase of impacted sandstones and basalts, as well, probably and rare impact breccias and injection breccias. The circular shape seems today to represent the weathering and erosion remnants of the original crater.

Coupling volcanism and tectonics along divergent plate boundaries: Collapsed rifts from central Afar, Ethiopia

Research Article| September 01, 2010 Coupling volcanism and tectonics along divergent plate boundaries: Collapsed rifts from central Afar, Ethiopia Valerio Acocella Valerio Acocella † Dipartimento Scienze Geologiche Università Roma Tre Largo San Leonardo Murialdo, 1, Roma, 00146, Italy †E-mail: acocella@uniroma3.it Search for other works by this author on: GSW Google Scholar GSA Bulletin (2010) 122 (9-10): 1717–1728. https://doi.org/10.1130/B30105.1 Article history received: 08 Jun 2009 rev-recd: 25 Oct 2009 accepted: 27 Oct 2009 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Valerio Acocella; Coupling volcanism and tectonics along divergent plate boundaries: Collapsed rifts from central Afar, Ethiopia. GSA Bulletin 2010;; 122 (9-10): 1717–1728. doi: https://doi.org/10.1130/B30105.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Magma along divergent plate boundaries is erupted from fissures or vents from central volcanoes, with limited impact on rift architecture. Here I summarize the geological and structural features accompanying the eruption of part of a km-thick volcanic sequence (“stratoids”) along the Red Sea divergent boundary in central Afar, in the area of Tendaho and Dobi grabens. More than 4700 km3/Ma (per 100 km of rift length) of magma have been produced by repeated fissure eruptions from within Tendaho graben. The graben sides show distinctive structural features, as steep topographic gradients, coinciding with inward-tilted blocks forming dominoes coeval to the emplacement of the km-thick volcanic sequence. Similar features are observed also in the Dobi graben. This allows proposing an original mechanism, where the distinctive structure of the grabens results from the collapse at the surface induced by magma withdrawal during the emplacement of the volcanic sequence. This portion of Afar shows how rift architecture is shaped by voluminous fissure eruptions, forming collapsed rifts. These occur in continental domains, during the breakup stage (central Afar) and in oceanic domains, where rifts narrow (East Pacific Rise). Collapsed rifts represent an end-member type of volcano-tectonic activity, where the width of the erupting reservoir balances that of the active rift zone. Along divergent boundaries, the width of the reservoir influences the style of surface deformation: a progressively higher ratio of the width of the reservoir emptied (Re) to that of the active rift zone (Ri) generates, in sequence, axial grabens, calderas, and collapsed rifts. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Well-balanced compressible cut-cell simulation of atmospheric flow

Cut-cell meshes present an attractive alternative to terrain-following coordinates for the representation of topography within atmospheric flow simulations, particularly in regions of steep topographic gradients. In this paper, we present an explicit two-dimensional method for the numerical solution on such meshes of atmospheric flow equations including gravitational sources. This method is fully conservative and allows for time steps determined by the regular grid spacing, avoiding potential stability issues due to arbitrarily small boundary cells. We believe that the scheme is unique in that it is developed within a dimensionally split framework, in which each coordinate direction in the flow is solved independently at each time step. Other notable features of the scheme are: (i) its conceptual and practical simplicity, (ii) its flexibility with regard to the one-dimensional flux approximation scheme employed, and (iii) the well-balancing of the gravitational sources allowing for stable simulation of near-hydrostatic flows. The presented method is applied to a selection of test problems including buoyant bubble rise interacting with geometry and lee-wave generation due to topography.

Modeling the influence of horizontal advection, deformation, and late uplift on the drainage development in the India‐Asia collision zone

We present results of a new model in which we couple a mechanical model to describe continental indentation in plan view with a landscape evolution model to describe drainage development. We apply the model to investigate aspects of the development of drainage systems, erosion, and exhumation in the India‐Asia collision zone. It is shown that the orogen‐scale distribution of erosion and exhumation can be well matched by the model. However, the elevation of the Tibetan Plateau and the steep topographic gradients at its margins are difficult to reproduce. They appear to require an additional cause of uplift. We show that some of the major observed river capture events can be reproduced by invoking such a late uplift in much of the orogen, except the region of the Sichuan Basin. This includes the inferred capture of the Yarlong‐Tsangpo by the Brahmaputra and that of the Upper Yangtze by the Lower Yangtze (thereby deserting the Red River).

Lithosphere structure underneath the Tibetan Plateau inferred from elevation, gravity and geoid anomalies

The Tibetan Plateau is the product of crustal thickening caused by collision between India and Asia. Plate tectonic reconstructions suggest continuous northward movement of the Indian plate relative to stable Eurasia at nearly 50 mm/yr for the last 50 My. The plateau is now at ~5 km elevation with steep topographic gradients across the southern and northern margins. These steep topographic gradients are also related to large lateral variations in the geoid and gravity anomalies. In a SSW to NNE cross section, the Bouguer gravity anomaly decreases over a distance of 500 km from about 0 mGal in the India plate to ~−500 mGal on the Tibetan Plateau. The geoid anomaly also presents steep gradients on both the Himalayan front and the northern margin, reaching values between 20–30 m on the plateau, suggesting a pronounced thinning of the lithospheric mantle. Uplift late in the tectonic evolution of the plateau, the widespread extension, and the associated magmatism have been attributed to convective removal of the lower part of lithospheric mantle and its replacement by hotter and lighter asthenosphere. Here we present a two-dimensional lithospheric thermal and density model along a transect from the Indian plate to Asia, crossing the Himalaya front and the Tibetan Plateau. The model is based on the assumption of local isostatic equilibrium, and is constrained by the topography, gravity and geoid anomalies and by thermal data within the crust. Our results suggest that the height of the Tibetan Plateau is compensated by thick crust in the south and by hot upper mantle to the north. The Tibetan Plateau as a whole cannot be supported isostatically only by thickened crust; a thin and hot lithosphere beneath the northern plateau is required to explain the high topography, gravity, geoid and crustal temperatures. The lithosphere reaches a maximum depth of ~260 km beneath the southern Plateau, and thins abruptly northward to ~100 km under the central and northern Plateau. The lithosphere depth increases again beneath the Qaidam basin and the Qilian Shan to ~160 km.

Effect of ocean conditions on the cross‐shelf distribution of walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus)

AbstractAcoustic trawl surveys were conducted in 2000 and 2001 in two troughs located off the eastern coast of Kodiak Island in the Gulf of Alaska as part of a multiyear, multidisciplinary experiment to examine the influence of environmental conditions on the spatial distribution of adult and juvenile walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus). Continuous underway sea surface temperature samples and water column profiles collected in 2000 and 2001 showed the presence of a sharp shelf‐break front in Chiniak Trough and a mid‐trough front in Barnabas Trough. At distances <22 km from shore, the water column was well mixed, whereas a well‐defined mixed layer was present beyond approximately 22 km from shore. Satellite drifter tracks in Barnabas Trough entered along the upstream edge of the trough and appeared to follow the frontal boundary across the middle portion of the trough. A storm in 2001 weakened stratification and cooled surface water temperature by 1.6–2.1°C. Wind mixing associated with the storm event mixed subsurface chlorophyll a to the surface and enhanced nutrients in the surface waters. The storm event revealed spatial partitioning of summer production in Barnabas Trough, with production concentrated in regions inside the mid‐trough front. In contrast, post‐storm summer production was distributed throughout Chiniak Trough. The spatial distribution of walleye pollock and capelin differed and appeared to be related to differences in habitat characteristics. Acoustic survey data identified four acoustic sign types: age‐1 pollock, adult pollock, capelin, capelin–age‐0 pollock mix. The spatial distribution of these four sign types appears to be influenced by the oceanographic and topographic features of the two troughs. Adult pollock were broadly distributed throughout Chiniak Trough, whereas adult pollock were aggregated on the coastal side of the frontal system in Barnabas Trough. In 2000, capelin occurred with age‐0 pollock. In Chiniak Trough, capelin were most abundant along steep topographic gradients at the edges of the trough and in a deep region near Cape Chiniak, whereas the capelin–age‐0 mix (2000) or capelin (2001) concentrations were observed in slope water intrusions over the outer shelf in Barnabas Trough. Results suggest that habitat selection of walleye pollock and capelin are controlled by different processes. Capelin distributions appear to be limited by oceanographic conditions while other factors appear to be more important for pollock.

Distribution of fallout radionuclides ( 7Be, 137Cs, 210Pb and 239,240Pu) in soils of Taiwan

Depth profiles and cumulative deposition of four fallout radionuclides ( 7Be, 137Cs, 210Pb and 239,240Pu) were determined in presumably undisturbed soils in Taiwan. Inventories of these radionuclides in different areas correlate significantly with each other (except 7Be) and with mean annual rainfall, providing a necessary condition for the development of soil erosion studies in Taiwan. However, the data show very large spatial variability between and within landscape units, reflecting the steep topographic and meteorological gradients in the island. Thus, the application of fallout radionuclides to study soil conservation in Taiwan is expected to be a demanding task; it will call for dense sampling even at undisturbed reference sites.

Mesozoic fluvial deposits on a rifted continental margin near Vaalputs, Namaqualand, South Africa

Scattered outcrops of faulted, silicified and kaolinitised (saprolitised) sediments comprising conglomerate and cross-bedded sandstone, form prominent mesas and inliers near Banke, south of Vaalputs, the national radioactive waste disposal site in Namaqualand. Although their geomorphologic expression is similar, these sediments are lithologically and texturally distinct from the equally silicified soils, diatreme infills and hillside screes of the palaeo-weathering profile type commonly occurring between the farm Banke and the Vaalputs site. Sedimentary and structural features identified in this study show that the Dasdap sediments may represent an alluvial fan sequence with a source area in the west, and deposited on a continental margin marked by relatively steep topographic gradients. The Dasdap beds could not be dated conclusively, but there is evidence their deposition was likely coeval with the emplacement of melilititic and kimberlitic diatremes in the Late Cretaceous. The robust fluvial system inferred by the Dasdap sediments is compatible with the humid climate responsible for the siliceous saprolites widely associated to the erosional African surface across southern Africa. Subsequent to these events, widespread faulting and jointing of the sediments provide striking evidence for tectonic reactivation of the continental margin of southwestern Africa in the Cenozoic. On the basis of these distinguishing features, the Dasdap sediments may deserve the formation status.

Large‐scale crustal deformation of the Tibetan Plateau

The topography, velocity, and strain fields calculated from a three‐dimensional Newtonian viscous model for large‐scale crustal deformation are generally in good agreement with results from geological, geodetic and earthquake studies in and around the Tibetan Plateau, provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau (equivalent to a viscosity of 1012 Pa s within a 250‐m‐thick channel or 1018 Pa s within a 15‐km‐thick channel). Model studies and observations show a plateau at ∼5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins. Rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins (at rates ∼20 mm/yr). Model results show north‐south shortening (∼10 mm/yr) in reasonable agreement with GPS data (5–8 mm/yr of north‐south shortening across the northern two thirds of the plateau) and east‐west stretching (10–15 mm/yr) across the eastern half of the high plateau, in reasonable agreement with seismic, geologic, and GPS data. Upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east. Clockwise rotation of material around the east Himalayan syntaxis (at rates up to ∼10 mm/yr) occurs partly as a result of dextral shear between Indian and Asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east. There is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust, except in southern and northern Tibet where margin‐perpendicular extension occurs only for the case of an extremely weak lower crust. Our results suggest that the Tibetan Plateau is likely to have gone through a two‐stage development. The first stage produced a long, narrow, high orogen whose height may have been comparable to the modern plateau. The second stage produced a plateau that grew progressively to the north and east. East‐west stretching, eastward plateau growth and dextral rotation around the east Himalayan syntaxis probably did not begin until well into the second stage of plateau growth, perhaps becoming significant after ∼20 m.y. of convergence.

The hydrology of ore formation in the Southeast Missouri District; numerical models of topography-driven fluid flow during the Ouachita Orogeny

A growing body of evidence indicates a relationship in the genesis of the Pb-Zn-Cu ores of the Southeast Missouri district to sedimentary brines mobilized during Late Paleozoic tectonism along the Appalachian-Ouachita erogenic belt. An effective mechanism for this brine mobilization would have been the steep topographic gradients created in the Ozark region during the Ouachita orogeny. The present study is an effort to reconstruct the paleohydrology of the flow system and explore its implications for ore formation.Transient finite element simulations of fluid flow, heat transport, and solute transport were used to describe the evolution of the fluid velocity, temperature, and salinity fields as a result of uplift and erosion of Ozark topography. A vigorous south to north flow regime was predicted, characterized by strong recharge near the southern boundary of the Arkoma basin and strong discharge over the crest of the Ozark dome. Ground-water temperature and velocity were found to increase with time during the early stages of uplift, before reaching a maximum and thereafter declining to somewhat lower values that remained steady over time. Continuous meteoric recharge led to the development of a freshwater plume that gradually migrated through the flow system, displacing the more saline pore fluids that were present initially. The calculations showed further that the onset of fluid velocities and temperatures optimal for forming the ores to coincide very closely with the onset of low salinities unsuitable for forming the ores. Unless salinity was somehow replenished, a relatively short period of time, on the order of hundreds of thousands of years or perhaps less, was available for mineralization in Southeast Missouri. Temperature variation in the Cambrian sediments was found to be minimal south of the Viburnum Trend after the earliest stages of uplift. Within the Viburnum Trend, however, a temperature gradient of about 25 degrees C over the length of the trend was predicted. The magnitude of the temperatures generated in the modeling also agreed well with the fluid inclusion data, matching most of the range of measured values.Once uplift had ceased, erosion would have gradually diminished topographic relief and hence, the driving mechanism for fluid flow and much of the heat transfer. The modeling results showed that ore-forming temperatures in Southeast Missouri would have been maintained for no more than a few million years once uplift had ceased, whereas ore-forming fluid velocities would have been maintained much longer, on the order of tens of millions of years.

Low‐frequency oscillations in precipitation, temperature, and runoff on a west facing mountain front: A hydrogeologic interpretation

This paper examines the space‐time patterns of annual, interannual, and decadal components of precipitation, temperature, and runoff (P‐T‐R) using long‐record time series across the steep topographic gradient of the Wasatch Front in northern Utah. This region forms the major drainage area to the Great Salt Lake. The approach is to use multichannel singular spectrum analysis as a means of detecting dominant oscillations and spatial patterns in the data and to discuss the relation to the unique mountain and basin hydrologic setting. Results of the analysis show that high‐elevation runoff is dominated by the annual and seasonal harmonics, while low‐elevation runoff exhibits strong interannual and decadal oscillations. For precipitation and temperature, only the annual/seasonal spectral peaks were found to be significantly different from the underlying noise floor, and these components increase with altitude similar to the mean orographic pattern. Spectral peaks in runoff show a more complex pattern with altitude, with increasing low‐frequency components at intermediate and lower elevation. This pattern is then discussed in terms of basin storage effects and groundwater‐stream interaction. A conceptual hydrogeologic model for the mountain and basin system proposes how losing streams and deep upwelling groundwater in the alluvial aquifer could explain the strong low‐frequency component in streams entering the Great Salt Lake. The phase‐plane trajectories of the dominant components for P‐T‐R are reconstructed as a function of altitude showing the relation of hydrogeologic conditions to the strongest oscillations in mountain runoff and discharge to the Great Salt Lake. The paper shows that weak interannual and decadal oscillations in the climate signal are strengthened where groundwater discharge dominates streamflow.