Active Mountain Belts Research Articles

Critical porosity of melt segregation during crustal melting: Constraints from zonation of peritectic garnets in a dacite volcano

The presence of leucogranitic dikes in orogenic belts suggests that partial melting may be an important process in the lower crust of active orogenies. Low seismic velocity and low electrical resistivity zones have been observed in the lower crust of active mountain belts and have been argued to reflect the presence of partial melt in the deep crust, but volcanoes are rare or absent above many of these inferred melt zones. Understanding whether these low velocity zones are melt-bearing, and if so, why they do not commonly erupt, is essential for understanding the thermal and rheologic structure of the crust and its dynamic evolution. Central to this problem is an understanding of how much melt can be stored before it can escape from the crust via compaction and eventually erupt. Experimental and theoretical studies predict trapped melt fractions anywhere from <5% to >30%. Here, we examine Mn growth-zoning in peritectic garnets in a Miocene dacite volcano from the ongoing Betic–Rif orogeny in southern Spain to estimate the melt fraction at the time of large-scale melt extraction that subsequently led to eruption. We show that the melt fraction at segregation, corresponding approximately to the critical melt porosity, was ∼30%, implying significant amounts of melt can be stored in the lower crust without draining or erupting. However, seismic velocities in the lower crust beneath active orogenic belts (southern Spain and Tibet) as well as beneath active magmatic zones (e.g., Yellowstone hotspot) correspond to average melt porosities of <10%, suggesting that melt porosities approaching critical values are short-lived or that high melt porosity regions are localized into heterogeneously distributed sills or dikes, which individually cannot be resolved by seismic studies.

Historical Reconstruction of Submarine Earthquakes Using210Pb,137Cs, and241Am Turbidite Chronology and Radiocarbon Reservoir Age Estimation off East Taiwan

AbstractTaiwan is a young and seismically active mountain belt, where a series of strong earthquakes (M&gt;7) have occurred over the past hundred years. Identifying historical earthquakes around Taiwan is a key to better constrain the geodynamic of this active region. Sedimentological and geochemical analyses of surface sediments from one station offshore east Taiwan revealed the presence of coarse-grained layers interpreted as turbidites. The age of these layers have been determined by210Pb,137Cs, and241Am chronology. Dating of the three most recent turbidites provides ages of AD 2001±3, AD 1950±5, and AD 1928±10. The results show striking temporal correspondence of turbidite layers to large (M≥6.8) earthquakes recorded in the region since the 20th century. The chronologies of sediment layers lead us to believe that turbidites resulted from the 2003 Taitung earthquake (M 6.8), the 1951 Chengkong earthquake (M 7.1), and the 1935 Lutao earthquake (M 7.0), respectively. Such a good correlation between turbidites and high-magnitude (M≥6.8) historical and instrumental seismic events suggests that turbidite paleoseismology constitutes a valuable tool for earthquake assessment in the eastern Taiwan margin. Moreover, the modern reservoir radiocarbon age and the regional marine reservoir correction (ΔR) of the Kuroshio Current off Taiwan were estimated by comparing accelerator mass spectrometry (AMS)14C ages with ages derived from corrected210Pb profiles and historical accounts of identifiable seismic events. Such a determination is important to calibrate the14C ages of marine materials for accurate comparison of marine and continental geological records. Our calculated mean ΔR value of 232±5414C yr (n=2) is higher than its modern value of 86±4014C yr. This high value can be explained by the presence of local upwelling cells and turbulence in the Kuroshio Current, north of Green Island. These upwelling cells bring14C-depleted water to the surface, resulting in an increase of the modern ΔR value in this portion of the Kuroshio Current.

Peak metamorphic temperature and thermal history of the Southern Alps (New Zealand)

The Southern Alps orogen of New Zealand results from late Cenozoic convergence between the Indo-Australian and Pacific plates and is one of the most active mountain belts in the world. Metamorphic rocks carrying a polymetamorphic legacy, ranging from low-greenschist to high-grade amphibolites, are exhumed in the hanging wall of the Alpine Fault. On a regional scale, the metamorphic grade has previously been described in terms of metamorphic zones and mineral isograds; application of quantitative petrology being severely limited owing to unfavorable quartzofeldspathic lithologies. This study quantifies peak metamorphic temperatures (T) in a 300×20km area, based on samples forming 13 transects along-strike from Haast in the south to Hokitika in the north, using thermometry based on Raman spectroscopy of carbonaceous material (RSCM). Peak metamorphic T decreases across each transect from ≥640°C locally in the direct vicinity of the Alpine Fault to less than 330°C at the drainage divide 15–20km southeast of the fault. Thermal field gradients exhibit a degree of similarity from the southernmost to the northernmost transects, are greater in low-grade semischist than high-grade schist, are affected by folding or discontinuous juxtaposition of metamorphic zones, and contain limited information on crustal-scale geothermal gradients. Temperatures derived by RSCM thermometry are slightly (≤50°C) higher than those derived by traditional quantitative petrology using garnet–biotite thermometry and THERMOCALC modeling. The age of RSCM T appears to be mostly pre-Cenozoic over most of the area except in central Southern Alps (Franz Josef–Fox area), where the amphibolite facies schists have T of likely Cenozoic age. The RSCM T data place some constraints on the mode of exhumation along the Alpine Fault and have implications for models of Southern Alps tectonics.

Insights on continental collisional processes from GPS data: Dynamics of the peri‐Adriatic belts

AbstractWe present a new GPS velocity field covering the peri‐Adriatic tectonically active belts and the entire Balkan Peninsula. From the velocities, we calculate consistent strain rate and interpolated velocity fields. Significant features of the crustal deformation include (1) the eastward motion of the northern part of the Eastern Alps together with part the Alpine foreland and Bohemian Massif toward the Pannonian Basin, (2) shortening across the Dinarides, (3) a clockwise rotation of the Albanides‐Hellenides, and (4) a southward motion south of 44°N of the inner Balkan lithosphere between the rigid Apulia and Black Sea, toward the Aegean domain. Using this new velocity field, we derive the strain rate tensor to analyze the regional style of the deformation. Then, we devise a simple test based on the momentum balance equation, to investigate the role of horizontal gradients of gravitational potential energy in driving the deformation in the peri‐Adriatic tectonically active mountain belts: the Eastern Alps, the Dinarides, the Albanides, and the Apennines. We show that the strain rate fields observed in the Apennines and Albanides are consistent with a fluid, with viscosity η ∼ 3×1021 Pa s, deforming in response to horizontal gradients of gravitational potential energy. Conversely, both the Dinarides and Eastern Alps are probably deforming in response to the North and North‐East oriented motion of the Adria‐Apulia indenter, respectively, and as a consequence of horizontal lithospheric heterogeneity.

Chemical weathering in active mountain belts controlled by stochastic bedrock landsliding

In active mountain belts, erosion is driven by bedrock landsliding. River water chemistry in New Zealand’s Southern Alps suggests that stochastic mass wasting processes also enhance chemical weathering in such environments. A link between chemical weathering and physical erosion exists at the catchment scale over a wide range of erosion rates1,2. However, in mountain environments, where erosion rates are highest, weathering may be kinetically limited3,4,5 and therefore decoupled from erosion. In active mountain belts, erosion is driven by bedrock landsliding6 at rates that depend strongly on the occurrence of extreme rainfall or seismicity7. Although landslides affect only a small proportion of the landscape, bedrock landsliding can promote the collection and slow percolation of surface runoff in highly fragmented rock debris and create favourable conditions for weathering. Here we show from analysis of surface water chemistry in the Southern Alps of New Zealand that weathering in bedrock landslides controls the variability in solute load of these mountain rivers. We find that systematic patterns in surface water chemistry are strongly associated with landslide occurrence at scales from a single hillslope to an entire mountain belt, and that landslides boost weathering rates and river solute loads over decades. We conclude that landslides couple erosion and weathering in fast-eroding uplands and, thus, mountain weathering is a stochastic process that is sensitive to climatic and tectonic controls on mass wasting processes.

Late Pleistocene to Holocene alluvial tableland formation in an intra-mountainous basin in a tectonically active mountain belt ― A case study in the Puli Basin, central Taiwan

The morphology in Taiwan is a product of high tectonic activity at the convergent margin and East Asian monsoon climate. Tablelands are prominent geomorphic features in the Puli Basin in central Taiwan. These tablelands provide an archive to understand links between past climatic evolution and tectonic events resulting in the formation of the present-day landforms. To establish a geochronological framework for the alluvium underlying the tablelands in the Puli Basin, optically stimulated luminescence dating was applied to obtain burial ages. The numerical data indicate an accumulation phase of alluvial fans in the Late Pleistocene to Early Holocene transition. The study area in the Taomi River catchment, an obvious longer precursor of the Taomi River, originating from west of the Yuchih Basin, transported the sediments forming the present-day southern tablelands. During the Pleistocene–Holocene transition, the climate changed to wetter and warmer conditions, so that slope processes might have changed and an increasing transport in the fluvial system was stimulated. Fluvial and fan terraces in other river catchments in Taiwan also indicate a period of increased fluvial transport and deposition. A geomorphic evolution model in the Puli Basin is reconstructed on the basis of the chronological framework and of sedimentological features. Fluvial processes controlled by climatic change and accompanied by tectonic activities have created the diverse topography in the Puli Basin.

Large landslides lie low: Excess topography in the Himalaya-Karakoram ranges

Mass wasting is an important process for denuding hillslopes and lowering ridge crests in active mountain belts such as the Himalaya-Karakoram ranges (HKR). Such a high-relief landscape is likely to be at its mechanical threshold, maintained by competing rapid rock uplift, river incision, and pervasive slope failure. We introduce excess topography, Z E , for quantifying potentially unstable rock-mass volumes inclined at angles greater than a specified threshold angle. We find that Z E peaks along major fluvial and glacial inner gorges, which is also where the majority of 492 large (>0.1 km 2 ) rock-slope failures occur in the Himalaya9s largest cluster of documented Pleistocene to Holocene bedrock landslides. Our data reveal that bedrock landslides in the HKR chiefly detached from near or below the median elevation, whereas glaciers and rock glaciers occupy higher-elevation bands almost exclusively. Less than 10% of the area of the HKR is upslope of glaciers, such that possible censoring of evidence of large bedrock landslides above the permanent snow line barely affects this finding. Bedrock landslides appear to preferentially undermine topographic relief in response to fluvial and glacial incision along inner gorges, unless more frequent and smaller undetected failures, or rigorous (peri-)glacial erosion, compensate for this role at higher elevation. Either way, the distinct patterns of excess topography and large bedrock landsliding in the HKR juxtapose two stacked domains of landslide and (peri-)glacial erosion that may respond to different time scales of perturbation. Our findings call for more detailed analysis of vertical erosional domains and their geomorphic coupling in active mountain belts.

Controls on fluvial evacuation of sediment from earthquake-triggered landslides

Large earthquakes in active mountain belts can trigger landslides, which mobilize large volumes of clastic sediment. Delivery of this material to river channels may result in aggradation and flooding, while sediment residing on hillslopes may increase the likelihood of subsequent landslides and debris flows. Despite recognition of these processes, the controls on the residence time of coseismic landslide sediment in river catchments remain poorly understood. Here we assess the residence time of fine-grained ( 5 mm day–1). Together with previous observations from the C.E. 1999 Chi-Chi earthquake in Taiwan, our results demonstrate the importance of landslide density and runoff intensity in setting the duration of earthquake-triggered landslide impacts on river systems.

Landslide erosion associated with the Wenchuan earthquake in the Minjiang River watershed: Implication for landscape evolution of the Longmen Shan, eastern Tibetan Plateau

In tectonically active mountain belts, earthquakes can contribute to surface erosion by generating large-scale landslides. This study focuses on establishing the relationship between surface erosion caused by the earthquake-induced landslides and landscape evolution of the Longmen Shan, eastern Tibet Plateau. The inventory of landslides related to the 2008 Wenchuan earthquake in the Minjiang River watershed was based on high-resolution remote sensing images and field surveys. The estimated landslide erosion rate related to the Wenchuan earthquake is of the order 0.4–0.6 mm year−1 on both sides of the Minjiang River and its tributaries. This erosion is similar to erosion rates of 0.5–0.8 mm year−1 measured by low-temperature thermochronology over a Myr-timescale. The landslides associated with repeated large earthquakes may contribute to this Myr-timescale surface erosion via enhanced erosion efficiency. Post-seismic high-resolution digital elevation models covering the period from 2008 and 2012 were compared to quantify fluvial erosion in the Baisha River, a tributary of the Minjiang River. The volume of eroded materials was approximately 1.9 × 104 m3 over the 4-year period. In addition, the rapid removal of co-seismic knickpoints indicates significant post-seismic river transportation. If large earthquakes, such as the Wenchuan earthquake, have occurred at intervals of 2,000–3,000 years, then the associated rapid landslide erosion together with fluvial erosion during inter-seismic intervals may have played an important role in shaping the present landscape of the Longmen Shan.

The influence of landscape connectivity and landslide dynamics upon channel adjustments and sediment flux in the Liwu Basin, Taiwan

ABSTRACTCatchment‐scale analyzes of spatial and temporal variability in landscape connectivity are critical considerations in appraisals of landscape evolution and disaster mitigation in tectonically active mountain belts such as Taiwan. This study uses historical aerial photographs, flow discharge and seismic data to analyze landslide changes and channel adjustments over a 30 year period in the Liwu Basin. Recurrent earthquakes and typhoon events trigger frequent landslide activity, channel adjustment and sediment reworking in this system. Spatial variability in magnitude–frequency relations of hillslope‐valley floor (lateral) and upstream–downstream (longitudinal) connectivity during the study period are shown to reflect annual reworking in source and accumulation zones, while partly‐confined valleys in the mid‐catchment area trap sediment behind landslide‐induced dams that are formed and breached on an approximately decadal basis. This promotes partial longitudinal connectivity in these areas. Landscape responses to disturbance events were especially pronounced following combinations of seismic and typhoon events prior to the 1998 and 2005 images. Although single high magnitude events and series of moderate events affect patterns of landscape connectivity in the Liwu Basin, residence times for sediment storage are very short in this highly‐connected river system, where confined valley settings extend virtually to the coast. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Evolution of valley-fill terraces in the Alaknanda Valley, NW Himalaya: Its implication on river response studies

The present study attempts to understand the importance of place and local factors in the process of valley filling in an active mountain belt. The accumulation of sediments in a valley stretch depends upon the relationship between sediment carried from upstream by the main river entering into the stretch (qi), locally derived sediments (i.e., from local streams (qt), hillslopes (qh), and glacial and periglacial processes (qg)), and sediment exiting that stretch (qo); these are in turn governed by other processes like tectonic activity, intensity of precipitation, and lithology.As an example a valley stretch in the NW Himalaya is chosen in the Alaknanda River, one of the two uppermost tributaries of the Ganga River. This stretch is called as Pipalkoti Valley after the name of the main town; it is located close to the Main Central Thrust (MCT) and falls in the zone of orographic precipitation. Detailed geomorphic mapping reveals four surfaces and two terraces. Three debris-flow surfaces suggest their deposition by local mass flows and stream flows with very small contributions from the Alaknanda River. Terrace T2 occurs as a strath in some parts and is made up of fluvial and lacustrine sediments in others. The presence of lacustrine sediments suggests local damming of the river. Thick fluvial sediment is deposited in stretches of the rivers where it becomes wide due to the joining of a tributary. Synsedimentary deformation has been noted indicating that tectonic activity has affected sedimentation.The results suggest that local processes played a dominant role in the accumulation of sediments in this valley stretch. A model is proposed showing the role of glacial activity, high intensity meteorological events, river width, active faults and landslide damming in the origin of valley-fill deposits of the Pipalkoti Valley. It is concluded that before correlating valley-fill deposits along the lengths of the Himalayan valleys, it is important to evaluate the role of various local processes that have operated in particular stretches.

A midcrustal ramp‐fault structure beneath the Taiwan tectonic wedge illuminated by the 2013 Nantou earthquake series

AbstractTaiwan's active mountain belt is a spotlight for orogenic studies and was first used to test the critical‐taper wedge mechanics. The concept of an orogenic wedge above a shallow detachment surface has been highly influential on current understanding of orogenic processes in Taiwan. However, the recent ML 6.2 and ML 6.5 2013 Nantou reverse‐faulting earthquakes in central Taiwan have nucleated below the proposed detachment, indicating that active mountain building is occurring below the orogenic wedge. We estimate the coseismic slip distributions and fault geometry using the uniform stress drop slip inversions. The earthquakes occur on essentially the same 30° dipping fault plane ramping up from ~20 km depth near a cluster of 1999 Chi‐Chi earthquake aftershocks to the shallow detachment and the Chi‐Chi fault plane. The fault could be a deep extension of a mature shallow fault or a newly developed deep ramp fault that is not reflected in the surface geology.

Runoff-driven export of particulate organic carbon from soil in temperate forested uplands

We characterise the sources, pathways and export fluxes of particulate organic carbon (POC) in a headwater catchment in the Swiss Alps, where suspended sediment has a mean organic carbon concentration of 1.45%±0.06. By chemically fingerprinting this carbon and its potential sources using carbon and nitrogen elemental and isotopic compositions, we show that it derives from binary mixing between bedrock and modern biomass with a soil-like composition. The hillslope and channel are strongly coupled, allowing runoff to deliver recent organic carbon directly to the stream beyond a moderate discharge threshold. At higher flows, more biomass is mobilised and the fraction of modern carbon in the suspended load reaches 0.70, increased from 0.30 during background conditions. Significant amounts of non-fossil organic carbon are thus transferred from the hillslope without the need for extreme events such as landsliding. Precipitation is key: as soon as the rain stops, biomass supply ceases and fossil carbon again dominates. We use rating curves modelled using samples from five storm events integrated over 29-year discharge records to calculate long-term export fluxes of total POC and non-fossil POC from the catchment of 23.3±5.8 and 14.0±4.4tkm−2yr−1 respectively. These yields are comparable to those from active mountain belts, yet the processes responsible are much more widely applicable. Such settings have the potential to play a significant role in the global drawdown of carbon dioxide via riverine biomass erosion, and their contribution to the global flux of POC to the ocean may be more important than previously thought.

Tracking the relationship between mountain uplift, silicate weathering, and long-term CO2 consumption with Ca isotopes: Southern Alps, New Zealand

Mountain uplift is frequently invoked as a mechanism that accelerates atmospheric CO2 consumption by chemical weathering, thereby cooling climate over geological timescales. Testing the uplift hypothesis requires quantifying whether Ca in rivers draining active mountain belts originates from silicate versus carbonate weathering because only silicate weathering affects long-term climate. Here, we examine whether high-precision Ca isotope measurements (δ44/40Ca) can track Ca sources to rivers draining the Southern Alps of New Zealand, where uplift and erosion rates span three orders of magnitude across a region of silicate bedrock hosting accessory calcite. We quantified the percentages and uncertainties of Ca originating from silicate and carbonate weathering using Ca isotope mixing equations (the δ44/40Ca method), and we compared the results to those obtained from a widely utilized method employing Ca/Na ratios (the Ca/Na method). The δ44/40Ca method is less precise than the Ca/Na method owing to the limited variability of the Ca isotope system. Nonetheless, both methods reveal that riverine Ca largely originates from carbonate weathering and that the fraction of Ca from carbonate weathering increases with increasing tectonic activity, from ~50–60% in regions experiencing the lowest uplift rates to >90% in regions experiencing the highest uplift rates. Results agree within 2% for all non-glacial rivers, but the δ44/40Ca method yields up to 35% more silicate-derived Ca for glacial rivers. We attribute the discrepancy to glacial grinding, which damages mineral grains and causes non-stoichiometric release of silicate Ca.Total chemical weathering rates in the most rapidly uplifting region of the Southern Alps are about one order of magnitude higher than the global mean and rank among the highest chemical weathering rates in the world. However, because the proportion of riverine Ca from carbonate weathering increases as uplift and erosion rates increase, the highest long-term atmospheric CO2 consumption rates for non-glacial rivers are no higher than the global mean. Source apportionment using the δ44/40Ca method suggests that glaciation yields higher atmospheric CO2 consumption rates than previously realized, but we find that the spatial extent of montane glaciation is not large enough to make a difference globally. Extrapolated to the Himalaya Mountains, our data show that atmospheric CO2 consumption in glacial watersheds accounts for <1% of the global CO2 consumption flux, implying that >99% of CO2 consumption occurs in other regions of the world. We therefore conclude that silicate weathering in uplifting mountain ranges does not control long-term climate change.

THEORETICAL AND NUMERICAL STUDY OF WAVE-CURRENT INTERACTION IN STRONGLY-SHEARED FLOWS

The application of wave-current interaction theory in ocean circulation models has been extensively developed over the past decade, with formulations extended to three dimensions and based either on radiation stress formulations or on the Craik-Leibovich formulation. However, few of these studies consider the interaction of waves with relatively strongly sheared current, in which current shear can affect linear wave dynamics at leading order. The problem arises from the study of the evolution of highly concentrated sediment plumes developing at the mouth of small mountainous rivers. Although the annually averaged discharge of these small mountainous rivers is trivial compared to large rivers, during the extreme flooding events triggered by typhoon or tropic cyclones, these rivers, most of which located at tectonically active mountain belts, can carry highly concentrated sediment ( up to several g/l in the river plume) into the ocean. The magnitude of river discharge velocity at the river mouth may reach several m/s, comparable to the wave phase speed in coastal water. In addition, these flooding events usually coincide with very energetic wave conditions induced by the storms. Therefore, the interaction of waves with strongly sheared current becomes a very important dynamic process at this kind of river plumes. In our study, we establish a new framework to describe the interaction of small amplitude surface gravity waves and strongly sheared currents, where shear can exist in both vertical and horizontal directions. To begin with, we limit the derivation to the case of a narrow-banded slowly varying wave train propagating shoreward in the coastal ocean outside of the surf zone. Accordingly, our problem is assumed to be finite depth without wave breaking. Later we can extend the formulation to describe a spectrum of surface waves and include wave energy dissipation. In contrast to existing formulations, where waves at most feel a weighted depth-average current which follows from a weak-current, weak-shear approximation, the present formulation allows for an arbitrary degree of vertical shear, leading to a description of the vertical structure of waves in terms of solutions to the Rayleigh stability equation. The resulting formulation leads to a conservation law for wave action, and forcing terms for the description of mean flow using the Craik-Leibovich vortex force formulation. This new framework of wave-current interaction can be applied to numerical model based on ROMS/SWAN to study dynamics in coastal waters.

Soils as pacemakers and limiters of global silicate weathering

The weathering and erosion processes that produce and destroy regolith are widely recognized to be positively correlated across diverse landscapes. However, conceptual and numerical models predict some limits to this relationship that remain largely untested. Using new global data compilations of soil production and weathering rates from cosmogenic nuclides and silicate weathering fluxes from global rivers, we show that the weathering-erosion relationship is capped by certain ‘speed limits’. We estimate a soil production speed limit of between 320 to 450tkm−2yr−1 and the associated weathering rate speed limit of roughly 150tkm−2yr−1. These limits appear to be valid for a range of lithologies, and also extend to mountain belts, where soil cover is not continuous and erosion rates outpace soil production. We argue that the presence of soil and regolith is a requirement for high weathering fluxes from a landscape, and that rapidly eroding, active mountain belts are not the most efficient sites for weathering.

Floodplains of large rivers: Weathering reactors or simple silos?

Large river sediments are mostly derived from tectonically active mountain belts, but then undergo a series of sedimentation, temporary storage and reworking episodes on their journey to the ocean. The long transfer time of these sediments through active floodplains might result in a significant chemical maturation via weathering reactions, which is of critical importance for biogeochemical cycles at the Earth surface. This study reports the chemical composition of river sediments from different locations throughout the courses of the main tributaries of the Amazon Basin. Sampling along river depth-profiles yields access to the whole grain size and chemical composition range of river sediment. Here, weathering intensities (i.e. losses of Na, K, Mg and Ca) associated with chemical weathering in floodplains are (1) examined as a function of grain size, and (2) integrated over the whole grain size range, for three selected long river reaches flowing through the foreland and the lowland of the Amazon Basin: the upper Marañon, the Beni and the lower Madeira rivers. A relatively small Na loss through plagioclase dissolution is observed in the Madeira reach, while an important Ca loss due to carbonate dissolution occurs in the two foreland reaches (Marañon and Beni). No significant loss of K and Mg is observed in any of the reaches, showing the low alterability of primary K and Mg-bearing minerals and suggesting retention of K and Mg in the particulate phase by secondary minerals. The combination of these findings with previously reported data on the downstream change of dissolved Na, K, Mg and Ca fluxes suggests that chemical weathering of “stable” alluvial deposits could also significantly contribute to theweathering flux generated in foreland and lowland areas. The comparison between the three Amazon reaches and the Gangetic plain tends to show that the features observed in the Amazon are valid on a global scale. Finally, we show that, although resulting in a relatively small change in the chemical composition of the river sediment, silicate weathering in the lower Madeira floodplain can lead to a CO2 drawdown equivalent to ca. 10% of the total CO2 consumption flux of the whole Madeira basin.

Quantifying rainfall controls on catchment‐scale landslide erosion in Taiwan

ABSTRACTLandslide erosion is a dominant hillslope process and the main source of stream sediment in tropical, tectonically active mountain belts. In this study, we quantified landslide erosion triggered by 24 rainfall events from 2001 to 2009 in three mountainous watersheds in Taiwan and investigated relationships between landslide erosion and rainfall variables. The results show positive power‐law relations between landslide erosion and rainfall intensity and cumulative rainfall, with scaling exponents ranging from 2·94 to 5·03. Additionally, landslide erosion caused by Typhoon Morakot is of comparable magnitude to landslide erosion caused by the Chi‐Chi Earthquake (MW = 7·6) or 22–24 years of basin‐averaged erosion. Comparison of the three watersheds indicates that deeper landslides that mobilize soil and bedrock are triggered by long‐duration rainfall, whereas shallow landslides are triggered by short‐duration rainfall. These results suggest that rainfall intensity and watershed characteristics are important controls on rainfall‐triggered landslide erosion and that severe typhoons, like high‐magnitude earthquakes, can generate high rates of landslide erosion in Taiwan. Copyright © 2012 John Wiley &amp; Sons, Ltd.

A new experimental material for modeling relief dynamics and interactions between tectonics and surface processes

We developed a new granular material (MatIV) to study experimentally landscape evolution in active mountain belt piedmonts. Its composition and related physical properties have been determined using empirical criteria derived from the scaling of deformation, erosion-transport and sedimentation natural processes. MatIV is a water-saturated composite material made up with 4 granular components (silica powder, glass microbeads, plastic powder and graphite) whose physical, mechanical and erosion-related properties were measured with different laboratory tests. Mechanical measurements were made on a modified Hubbert-type direct shear apparatus. Erosion-related properties were determined using an experimental set-up that allows quantifying the erosion/sedimentation budget from tilted relaxation topographies. For MatIV, we also investigated the evolution of mean erosion rates and stream power erosion law exponents in 1D as a function of slope. Our results indicate that MatIV satisfies most of the defined criteria. It deforms brittlely according to the linear Mohr-Coulomb failure criterion and localizes deformation along discrete faults. Its erosion pattern is characterized by realistic hillslope and channelized processes (slope diffusion, mass wasting, channel incision). During transport, eroded particles are sorted depending on their density and shape, which results in stratified alluvial deposits displaying lateral facies variations. To evaluate the degree of similitude between model and nature, we used a new experimental device that combines accretionary wedge deformation mechanisms and surface runoff erosion processes. Results indicate that MatIV succeeded in producing detailed morphological and sedimentological features (drainage basin, channel network, terrace, syntectonic alluvial fan). Geometric, kinematic and dynamic similarity criteria have been investigated to compare precisely model to nature. Although scaling is incomplete, it yields particularly informative orders of magnitude. With all these characteristics, MatIV appears as a very promising material to investigate experimentally a wide range of scientific questions dealing with relief dynamics and interactions between tectonics, erosion and sedimentation processes.

Honouring geographers and contemporary exploration: from the archive to the ocean at the RGS-IBG Medals and Awards Ceremony 2011

The Royal Geographical Society (with the Institute of British Geographers) annual Medals and Awards recognise achievements in geographical research, fieldwork, exploration, photography, teaching, and in enthusing public audiences. The speeches and citations are a record of the ceremony of 2011. With comments by Dr Sylvia Earle, Professors David Livingstone, Stuart Elden, John Lowe and Lewis Owen, the speeches provide insights into contemporary geography – from archival research to unexplored oceans, political geography to tectonically active mountain belts. President Michael Palin's introduction provides a background to the awards, which links earlier geographical exploration and its modern counterparts.