Active deformation and landscape evolution in North Chilean forearc involve multiscale tectonic processes, such as crustal thickening causing orogenic-scale uplift and faulting modulating the mountain-front landscape. In the Central Depression, faults redirecting Quaternary drainages are poorly understood due to their subtle surface expressions and limited structural data acquisition. To address this, we combined remote-sensing analysis of high-resolution DEMs, satellite and UAV imagery, new geomorphic mapping, structural data, and morphometric analysis with available surface age dating to identify and give temporal constraints on previously unmapped faults impacting drainages across this region. Our findings reveal the reactivation of east-vergent NNW-SSE reverse to transpressive and NW-SE strike-slip faults over approximately 100 km of latitude. Faults movement can be summarized into two main stages (a) A Late Miocene-Pliocene stage, dominated by east-vergent reverse faults inverting the Andean piedmont in the northern study area. (b) A Pliocene-Quaternary stage, characterized by transpressive activity of these east-vergent faults extending southward, alongside structures of the West Vergent Thrust System. The tectonic evolution of the east-vergent structures relates to the ongoing deformation of the coastal forearc, encroaching into the Central Depression. Minimum vertical and strike-slip displacement rates since the Quaternary are 12 m/Ma and 90 m/Ma, respectively, with the potential for higher rates depending on the onset of displacement. Drainage pattern modification, driven by incremental vertical displacement rates, provides insights to qualitatively evaluate individual fault activity rates. Numerous recently detected structures represent previously unknown sources of seismic hazard, requiring further dating of geomorphic markers and high-resolution monitoring.

The shape of soil‐mantled hillslopes is typically attributed to erosion rate and the transport efficiency of the various processes that contribute to soil creep. While climate is generally hypothesized to have an important influence on soil creep rates, a lack of uniformity in the measurement of transport efficiency has been an obstacle to evaluating the controls on this important landscape parameter. We addressed this problem by compiling a data set in which the transport efficiency has been calculated using a single method, the analysis of hilltop curvatures using 1‐m LiDAR data, and the erosion rates have also been determined via a single method, in‐situ cosmogenic 10Be concentrations. Moreover, to control for lithology, we chose sites that are only underlain by resistant bedrock. The sites span a range of erosion rates (6–922 mm/kyr), mean annual precipitation (39–320 cm/yr), and aridity index (0.08–1.38). Surprisingly, we find that hilltop curvature varies with the square root of erosion rate, whereas previous studies predict a linear relationship. In addition, we find that the inferred transport coefficient also varies with the square root of erosion rate but is insensitive to climate. We explore various mechanisms that might link the transport coefficient to the erosion rate and conclude that present theory regarding soil‐mantled hillslopes is unable to explain our results and is, therefore, incomplete. Finally, we tentatively suggest that processes occurring in the bedrock (e.g., fracture generation) may play a role in the shape of hillslope profiles at our sites.

Active intraplate deformation as a far-field effect of the India-Eurasia convergence has led to four Mw∼8 earthquakes in western and southern Mongolia in the past century. Palaeoseismological and morphotectonic studies have shown that these earthquakes are characteristic events along transpressive fault systems with cumulative offsets. The tectonically active Gobi Altai and Hangay mountains are separated by the seismically quiescent Valley of Gobi Lakes, which consists of major perennial rivers draining into endorheic lakes. Despite the scarcity of recorded earthquakes, Quaternary deposits in the Valley of Gobi Lakes are crosscut by multiple fault scarps with significant, landscape-altering displacements. To assess past earthquakes and the potential seismic hazard of this area, we apply remote sensing, tectono-morphometric techniques and cosmogenic nuclide dating to estimate the amount of deformation the faults in the Valley of Gobi Lakes are accommodating, and to determine the effect of these faults on local landscape evolution. The Tuyn Gol (gol = Mongolian for river) is crosscut by four E-W to NE-SW trending fault scarps that display variable fault kinematics due to scarp orientation differences relative to a stable NE-SW principle stress direction. Mapping of the >40–90 km long Valley of Gobi Lake faults shows that they can accommodate M ∼ 7 earthquakes. Offset measurements of the Tuyn Gol deposits allow Middle Pleistocene to modern vertical slip rate estimates and M ∼ 7 earthquake recurrence intervals of 0.012 ± 0.007–0.13 ± 0.07 mm/yr and 5.24 ± 2.61–81.57 ± 46.05 kyr, respectively. Cumulative vertical displacement amounts to 0.27 ± 0.08 mm/yr, which is similar to that of major tectonic structures such as the Bogd fault in the Gobi Altai. This implies that the total active deformation accommodated by southern Mongolian faults may be larger than previously expected and distributed across more faults between the Hangay and the (Gobi) Altai mountain ranges. Geomorphological observations and surface exposure dating indicate that the Tuyn Gol drainage system developed over four to five 100 kyr climate cycles, during which active deformation played an important role in drainage reorganization. Our results demonstrate the dominant role of tectonics on local landscape dynamics, indicating the importance of studying regional geomorphology to adequately estimate the earthquake potential of faults that were considered inactive.

Concentrations of cosmogenic nuclides in rocks at the Earth’s surface are routinely measured by AMS (accelerator mass spectrometry) to obtain exposure ages. One particular application of this technique has been to derive ages for the formation of marine terraces, thus allowing constraints to be placed on rates of tectonic uplift. However, multiple rock samples from the same terrace surface have typically shown an amount of scatter in cosmogenic nuclide concentrations in excess of analytical uncertainties, potentially undermining the confidence in the resulting uplift rates. In addition, earlier works have considered the application of bedrock samples for exposure age dating marine terraces, but little attention has been directed towards the validity of sampling clasts exposed on terrace surfaces. Here, we use 10Be and 26Al measurements from a flight of marine terraces in northern Chile to compare the results from pebbles with those from bedrock samples and to investigate assumptions that sample pre-exposure and burial are negligible. We examine the influence that processes of surface deflation and pebble erosion will have on cosmogenic nuclide concentrations using a Monte Carlo model, the results of which are compared to our 10Be measurements. We find good agreement of 10Be concentrations between bedrock and pebble samples. At the sampled location, sample burial is rare and 10Be inheritance in pebble samples is low relative to the age of the terraces. Our modelling suggests that in arid environments, such as northern Chile, pebble erosion will be a secondary effect compared to surface deflation and that multiple periods of deflation will result in multimodal distributions of 10Be concentrations. More broadly, our findings show that measuring multiple surface clasts from a single marine terrace reveals the geomorphological processes influencing exposure ages, and may also be used to help identify the operation of past surface processes such as episodic deflation.

Preserved remnants of fluvial activity in deserts constitute evidence for changing boundary conditions. The Atacama Desert of northern Chile is the global end-member for aridity, so the history of relict stream networks in this region is a record of how landscapes develop under extreme conditions. On Pampa de Tana in northern Chile (19.4 degrees S), a series of channel forms that are presently inactive but in the past flowed westward are incised into the surface of a fault bounded, topographically elevated portion of the El Diablo Formation, a regionally extensive, relict pediment. We measure cosmic-ray produced Be-10, Al-26 and Ne-21 in fluvial deposits to date the timing of abandonment of three channels and couple this with topographic profile information from a SPOT-6 derived, 2 m resolution digital elevation model. We find two of the channels were abandoned approximately > 5.6 Myr and 2.0 Myr ago. One channel is still capable of flow and has ages suggesting it was fluvially active within the last few hundred thousand years. Using the paleochannel ages measured here and published ages for the end of aggradation of the El Diablo Formation we estimate the rates of fluvial channel incision before channel abandonment, and uplift rates on the faults after channel abandonment. Maximum uplift rates of similar to 12 m/Myr over the last 2 Myr are found. In general, while rates of uplift are relatively low they are several-fold more rapid than the rates of fluvial incision prior to channel abandonment. This implies that westward channel flow was interrupted by uplift of topography above a blind NW-SE striking reverse fault that affects the Central Depression, an alluvial forearc basin. We consider also that shrinkage of the upstream catchment area by stream capture, promoted via headward erosion and lateral expansion of adjacent canyons (quebradas) could be a factor in the abandonment of the channels on Pampa de Tana. Our results highlight the polygenetic nature of this landscape and show that relatively minor amounts of fault displacement in hyperarid regions can have implications for stream network evolution. Even subtle topographic uplift upstream should be taken into account when fluvial deposits are used as proxies for long-term environmental conditions.

There is growing interest in geochronological applications of terrestrial in situ-produced cosmogenic nuclides, with the most commonly measured being Be-10 and Al-26 in quartz. To extract and then separate these radio-nuclides from quartz and prepare them in the oxide form suitable for accelerator mass spectrometry (AMS) requires extensive and careful laboratory processing. Here we discuss the suitability of a crushed, sieved and etched, sub-aerially exposed vein quartz specimen (CoQtz-N) to act as a reference material for chemical laboratory preparation and AMS measurements. Splits of CoQtz-N were distributed to eleven target preparation laboratories. The CoQtz-N Be-10 targets were then measured at seven different AMS facilities and five of the preparation laboratories had their Al-26 targets measured at four different AMS facilities. We show that CoQtz-N splits are sufficiently homogeneous with regard to nuclide concentrations, that it has been cleaned of any atmospheric derived (i.e. meteoric) Be-10 and that it has low concentrations of the major elements that can interfere with Be and Al extraction chemistry and AMS measurements. We derive preliminary concentrations for Be-10 and Al-26 in CoQtz-N as 2.53 +/- 0.09 x 10(6) at/g and 15.6 +/- 1.6 x 10(6) at/g, respectively, at the 95% confidence limit.

Many areas of the Earth’s crust deform by distributed extensional faulting and complex fault interactions are often observed. Geodetic data generally indicate a simpler picture of continuum deformation over decades but relating this behaviour to earthquake occurrence over centuries, given numerous potentially active faults, remains a global problem in hazard assessment. We address this challenge for an array of seismogenic faults in the central Italian Apennines, where crustal extension and devastating earthquakes occur in response to regional surface uplift. We constrain fault slip-rates since ~18 ka using variations in cosmogenic 36 Cl measured on bedrock scarps, mapped using LiDAR and ground penetrating radar, and compare these rates to those inferred from geodesy. The 36 Cl data reveal that individual faults typically accumulate meters of displacement relatively rapidly over several thousand years, separated by similar length time intervals when slip-rates are much lower, and activity shifts between faults across strike. Our rates agree with continuum deformation rates when averaged over long spatial or temporal scales (10 4 yr; 10 2 km) but over shorter timescales most of the deformation may be accommodated by <30% of the across-strike fault array. We attribute the shifts in activity to temporal variations in the mechanical work of faulting.

Terrestrial cosmogenic nuclide exposure ages for the Wohlthat Massif (Antarctica), have previously been determined. This was done with Be-10 and Al-26 measurements by accelerator mass spectrometry (AMS) at the AMS facility at the ETH Zurich.In order to determine the extent to which the results from the Wohlthat Massif are of regional significance, additional samples were collected during the 2007 BGR-expedition "Queenmet". Two of the Steingarden Nunataks (isolated mountain peaks) were chosen as sampling locations, approximately 100 km south-east of the Wohlthat Massif/Queen Maud Land, at the edge of the Polar Plateau. Quartz rich samples were collected at different elevations on the nunataks to reconstruct an elevation-dependent exposure history. The in situ produced cosmogenic nuclides Be-10 and Al-26 in these samples were measured by AMS. During sample processing the quartz separates were prepared by two different methods (Kohl and Nishiizumi, 1992, Altmaier, 2000) and measurements were performed at two different facilities (CologneAMS und Zurich AMS) to confirm the reproducibility of the results.The new results of rock surface exposure ages reveal that the exposure of the lower nunatak to cosmic radiation started between 0.65 and 1.1 Ma ago, while the more elevated regions of the second nunatak were apparently above the ice 3-4 Ma ago. (C) 2015 Elsevier B.V. All rights reserved.

We present results of thermal neutron flux measurements in experimental granite piles that were tailored to study the effect of hydrogen-rich covers on that flux. We find that hydrogen-rich covers (polyethylene, water), used as proxies for snow, dead and/or live plant matter, increase the thermal neutron flux in an underlying rock surface significantly, as compared to the state without cover. The rock serves as the main source for thermal neutrons, the hydrogen-rich cover as a neutron reflector. In situations where the thickness of such a cover would be negligible in terms of high-energy neutron (>10 MeV) attenuation, e.g. 2-3 cm water equivalent cover, a significant enhancement of the thermal neutron flux (factor >2.5 +/- 0.5) can be achieved. This increase is made up of three components (Masarik et al., 2007): (1) reflected thermal neutrons (albedo neutrons), (2) moderated fast neutrons from the ground, and (3) moderated fast neutrons from the atmospheric cascade (Masarik et al., 2007). The higher thermal neutron flux increases the production rates of those cosmogenic nuclides that have a significant thermal neutron production pathway (He-3, Cl-36, Ca-41) Ignoring this effect in situations where target nuclei (Li-6, Cl-36, Ca-40) are abundant will severely underestimate production rates. The effect of hydrogenrich ground cover on the thermal neutron flux has the potential to be used for studies that are aimed at reconstructing the persistence of past plant/snow cover. Isotopic ratios of spallogenic versus predominantly thermal neutron produced nuclides, would reveal the presence or absence of hydrogen-rich cover in the past as compared to the present-day situation. (C) 2013 Elsevier B.V. All rights reserved.

Rates of bedrock denudation are central to theories of landscape evolution and our understanding of geomorphic processes. Advances in several fields of geochronometry over the last few decades have enabled scientists to measure denudation rates over spatial scales that range from a single outcrop to the largest continental basins, and over timescales that may encompass a few hundred to tens of millions of years. This unprecedented ability to readily obtain numerical constraints has helped redefine ideas of how topography develops in a variety of different settings. However, deliberation continues over the processes responsible for controlling rates of denudation, in particular, the significance of climate and tectonics. In this chapter, we consider the definitions of denudation and some of the techniques used to measure it. We outline the debate over what the first-order controls of denudation rates are and stress how understanding the temporal and spatial scales on which measurements are made is crucial when it comes to studying landscape development.

Cosmogenic Cl-36 analysis by accelerator mass spectrometry (AMS) is a valuable environmental and geological sciences research tool. Overcoming the stable nuclide S-36 isobar interfering with measurement is challenging, however. Traditionally this has required large accelerators, but following recent technical advances it is now possible with similar to 30 MeV ion energies. Consequently 5 MV or even smaller modern bespoke spectrometers are now Cl-36-capable, increasing accessibility and promoting wider and more varied Cl-36 use.However, the technical ability to identify Cl-36 ions is quite distinct from demonstrated high-performance AMS. Such is the theme of this paper. We present a systematic analysis of the accurate measurement of sample radioisotope relative to the stable chlorine, the normalisation of the measured ratio and correction for remaining S-36 interference, all combined with the use of stable-isotope dilution to determine sample Cl concentration to begin with. We conclude by showing that repeated analyses support our claims for routine 3% Cl-36-AMS data. Accordingly, the modest SUERC spectrometer well competes with the performance of larger longer-established instruments, and the results may be quite generic for modern bespoke instruments. (C) 2012 Elsevier B.V. All rights reserved.

We have undertaken the first paleoseismological study on an upper plate fault in Chile. The selected structure was the Mejillones Fault, which is marked by a conspicuous fault‐scarp. Using cosmogenic 10Be and OSL dating and detailed sedimentary logging of trenches, we have constrained the abandonment of two alluvial surfaces by fault activity at ca. 35 ka and ca.14 ka. Based on stratigraphic observation we characterized the fault evolution in four intervals over the last ca. 35 ka. During the first three intervals the fault had a steady slip rate of 0.61 ± 0.26 m/ka. The fourth interval is delineated by the last vertical fault slip and the accumulation of un‐deformed hillslope deposits after ca. 3.3 ka and has a slip rate of 0.22 ± 0.06 m/ka. The younger surface abandonment was caused by two Mw ∼ 7 paleoearthquakes with a recurrence interval of 5.0 ± 3.5 ka. The third interval is characterized by the interaction of hillslope deposits and aseismic slip and/or centimeter scale seismic slip events. At ca. 3.5 ka, a last large (Mw ∼ 6.6) earthquake took place. The recurrence intervals of large (Mw > 8.5) subduction earthquakes do not appear to be the same as the recurrence intervals of the Mw ∼ 7 events on the upper plate Mejillones Fault.