Terrace Treads Research Articles

A fluvial record of late Quaternary climate changes and tectonic uplift along the Marche Piedmont Zone of the Apennines: New insights from the Tesino River (Italy)

Fluvial terraces are a pre-eminent continental record of climate changes and tectonics. Although the use of fill terraces for incision rate derivation is complicated due to their cyclical nature relative to strath terraces, they remain a relatively complete record of incision and deposition, thus allowing one to derive interpretations about both tectonics and climate. The Adriatic piedmont of the Apennines (central Italy) offers a well-preserved staircase of such terraces, caused by the cyclical alternation of fluvial aggradation and incision over the Middle and Late Pleistocene in response to a combination of low-rate tectonic uplift and climate changes. Although the staircase has been already widely described with examples from several valleys of the Adriatic Piedmont, its geochronological constraints are limited and must be enhanced to extract reliable climatic and tectonic signals from terrace deposits—especially for the older terrace levels.This research provides new Optically Stimulated Luminescence (OSL) data on the terrace chronology in combination with new semi-automatically extracted data on the altimetric and along-valley terrace distributions, as well as the sedimentological characteristics of fill terraces within the Tesino River basin in the Marche piedmont zone of the Apennines. The semi-automatic extraction of the terrace tread allowed a preliminary level classification as a function of the terrace height above the modern channel thalweg. The results demonstrate that, within the slowly uplifting Adriatic piedmont of the Apennines, the main phases of fluvial aggradation occurred during cold and full-glacial conditions followed by river incision and floodplain-channel abandonment during warmer periods. This finding confirms previous results from the last fluvial aggradation-incision cycle (Late Pleistocene). The results also show that the same formation mechanism is valid for the older (Middle Pleistocene) terraces. Indeed, this study provides new geochronological constraints for the regional-scale paradigm of cyclical formation and development of Quaternary fill terrace staircases along this sector of the Apennines. The use of a dated fill terrace staircase allowed estimation of a Middle Pleistocene long-term bedrock incision rate ranging from 0.3 to 0.5 mm yr−1 in agreement with previous studies in the central-northern sector of the Adriatic Apennines. The resulting uplift rate supports the presence of differently uplifted blocks due to the fragmentation of the peri-Adriatic region caused by the activity of left-lateral faults.

A technique for quantification of quaternary ground tilting as recorded by alluvial fill terraces: An example from the northern Gulf of Mexico coastal plain

To quantify the degree of Quaternary structural tilting, we examined topographic slopes of treads of alluvial fill terraces. Like strath terrace treads, fill terrace treads can serve as a structural datum if restored to their pre-erosion geometry. Their slope degree and direction can then be compared to the slope of the modern floodplain to assess neotectonic tilting. On the northern Gulf of Mexico coastal plain, a ~45km-wide flight of seven Pleistocene alluvial fill terraces of the Ouachita River records a preferred southwestward migration of the river, and previous authors attribute this lateral migration to Quaternary structural tilting. The original geometries of Ouachita River terrace treads are needed as structural datums, but the treads are dissected by tributary streams. On younger treads, we observe concordant elevations of tributary divides and stronger soil development on divides than on tributary valley flanks. This indicates that divides are minimally eroded and preserve parts of the original treads. In order to restore terrace treads to their pre-dissection geometries, we constructed a topographic envelope model using elevations of tread remnants on tributary divides. This model effectively ‘fills’ the tributary valleys on the treads. We extracted each restored tread from the model and determined the tread's best-fit plane. We used three planes with R2 values >0.9 as datums. Consistent with westward tilting, directions of topographic slope of middle/late Pleistocene treads are rotated 80o toward the south-southwest, and treads have slopes that increase with terrace age from 0.017° to 0.09° (0.3 to 1.6 m/km). This quantitative estimate of late Quaternary westward regional tilting reveals a 10-fold increase in slope relative to the Holocene floodplain (0.14 m/km).

Rainfall infiltration and slope stability of alpine colluvial terraces subject to storms (NE Italy)

In the alpine environment, rainfall-induced shallow landslides can involve thin covers of colluvial soil (50–300 cm) on terraced belts that were formed as a result of fill-and-cut sedimentary processes that followed the deglaciation of the alpine valleys. Notably, research on shallow slope failures involving alpine terraces consisting of a near-flat upper ground surface (“tread”) and a moderately steep scarp (“riser”) is lacking in the literature. This paper describes the engineering geological characteristics and failure mechanisms of a large number of shallow landslides (soil slips or slide-debris flows) that were activated on alpine stratified colluvial terraces due to a rainstorm that hit the mountain area of the Friuli Venezia Giulia Region (NE Italy) on 21–22 June 1996. The paper reports data on the geomorphological and engineering geological characteristics of the soil slips acquired through extensive fieldwork and shows the outcomes of some two-dimensional seepage and slope stability analyses that were carried out in order to investigate the critical hydrological conditions and mechanisms that were responsible for the soil slip activation during and after rainfall. The soil slip activation can occur at two different stages during the infiltration process, based on the interacting water flows through the terrace tread and riser. The first critical stability condition is reached during the phases of greater precipitation intensity or at the end of the rainstorm because of the saturation of the top soil layer on the terrace riser and the subsequent formation of an ephemeral water table accompanied by a seepage sub-parallel to the slope face. The second critical condition is achieved some hours after the end of rainfall as a result of a tread-to-riser water outflow that is supplied by the water amount stored within the near-flat terrace tread during the peak rainfall stages (reservoir-like effect). This study also shows that a critical value of rainfall intensity of about 40–45 mm/h can cause the activation of soil slips in mountain basins characterised by a humid continental climate and by the occurrence of colluvial deposits with a high content of fine fraction. This critical value of rainfall intensity should be considered as a rainfall threshold for a basin-scale under geomorphological and geological conditions similar to those investigated in this paper.

Pedogenesis of Fluvial Terrace Soils Related to Geomorphic Processes in Central Taiwan

Pedogenetic features of the soils could be a proxy for revealing the landform surface processes. Our work first analyzed the particle size distributions and lithological discontinuities (LDs) of the soils in the midstream of the Zoushui River, central Taiwan. The results showed that the parental materials of the soils derive from mixed sediments of the Zoushui River and its tributaries, and the LDs of some soils suggested multi-depositional events with homogeneous lithology. Then, we proposed a soil chronosequence of Inceptisols, Ultisols, and Oxisols, over the Pleistocene timescale. There was a very well-defined semilogarithmic relationship between weighted profile development index (WPDI) values and soil age with correlation coefficients (r) greater than 0.9. The age of the soils did not certainly agree with the interglacials of the main marine isotope stage (MIS). However, the soils started to develop only after being aggraded by relatively warm and humid periods or by extreme rain events in cool and dry periods. Irrespective of whether the soils had started to develop, tectonic downcutting, providing clear altitudinal separation of the terrace treads, could inevitably happen later (or almost synchronologically) to ensure stabilities of the tread surfaces.

Internal deformation of continental blocks within converging plates: insights from the Ovacık Fault (Anatolia, Türkiye)

The active tectonics of Anatolia is mostly characterized by its westward motion with respect to Eurasia between the Hellenic subduction in the west and Arabia-Eurasia continental collision in the east. Although most of the deformation is suggested to be confined along Anatolia?s boundary elements, viz. the North and East Anatolian shear zones, recent studies indicate a higher magnitude of internal strain accumulation, especially along the parallel/subparallel strike-slip faults of its central province. We present the first morphochronology-based slip rate estimate for one of these strike-slip structures, the Ovacık Fault, by using cosmogenic 36Cl dating of offset fluvial deposits. At the Köseler Site (39.3643°N, 39.1688°E), two faulted risers, bounding the alluvial fan with its subplanar surface (NF1/NF1?) and the inset terrace tread (NF1/T2), are offset 19?24 and 15?22 m, respectively. The scattered surface ages and variability of 36Cl concentrations in depth profiles suggest strong evidence for inheritance in alluvial fan and terrace deposits; thus, we used modelled depth-profile ages for both surfaces. The modelled ages 8?10 ka for NF1 and 6?8 ka for T2 yield slip-rate estimates 2.4 +0.5/?0.4 mm/a and 2.8 +0.7/?0.7 mm/a, respectively, for the upper-tread reconstruction of the NF1/NF1?and the lower-tread reconstruction of the NF1/T2. Our results together with previous slip-rate estimates for other structures show a significant internal deformation for Anatolia, especially along its subparallel strike-slip faults. These secondary faults slice Anatolia into several pieces giving rise to the formation of the Malatya-Erzincan, Cappadocian, and Central Anatolian slices, where the geometry is strongly controlled by the distribution of the Tethyan accretionary complexes.

Topographic evidence of long wavelength active folding in the New Madrid seismic zone, intraplate North America

The northeast-striking Axial and Bootheel faults (AF and BHF) in the central USA are components of the New Madrid seismic zone (NMSZ) fault complex, source of M > 7 earthquakes in 1811–1812. Previous work shows Holocene displacement of the AF and BHF to be primarily right-lateral strike-slip movement west and south of the NMSZ restraining bend. In this study, treads of two Late Pleistocene alluvial terraces of the Mississippi River, the 14–16 ka Kennett and the 12 ka Morehouse terraces, were used as structural datums to assess any long-wavelength vertical component of deformation across these two faults. Using 10 m-resolution LiDAR, we constructed a best-fit polynomial trend surface of these terrace treads. Our results reveal a subtle ≤4 m-deep, 40 km-wide, 190 km-long, topographic trough. In the south, it corresponds to the southern segment of the AF and parallels the BHF for 50 km. Farther north it corresponds to a syncline in subsurface Paleozoic strata 10 to 20 km northwest of and parallel to the BHF.The trough axis crosses obliquely down-valley from the younger Morehouse terrace to the older Kennett terrace and does not correspond to major drainages. Significantly, the Malden-Bernie fluvial scarp separating the two terraces continues into the trough but is down-warped, inconsistent with an erosional origin of the trough. Along the southern segment of the trough, the older Kennett tread is topographically lower than the Morehouse tread, but relict Kennett braided channels were not buried during deposition of younger Morehouse alluvium. This suggests the Kennett terrace tectonically subsided along the trough after Morehouse terrace deposition. We interpret this trough as a subtle syncline along the western margin of the NMSZ restraining bend that developed in response to down-on-the-northwest fault movement after deposition of Morehouse alluvium.

Chronology of terraces in the Rio Grande rift, Socorro basin, New Mexico: Implications for terrace formation

AbstractThe Rio Grande rift hosts a remarkable record of Quaternary river incision preserved in an alluvial terrace sequence that has been studied for more than a century. However, our understanding of Rio Grande incision history in central New Mexico since the end of basin filling ca. 0.78 Ma remains hampered by poor age control. Robust correlations among Rio Grande terrace sequences in central and southern New Mexico are lacking, making it difficult to address important process-related questions about terrace formation in continental-scale river systems. We present new age controls using a combination of 40Ar/39Ar, 36Cl surface-exposure, and 14C dating techniques from alluvial deposits in the central New Mexico Socorro area to document the late Quaternary incision history of the Rio Grande. These new age controls (1) provide constraints to establish a firm foundation for Socorro basin terrace stratigraphy, (2) allow terrace correlations within the rift basin, and (3) enable testing of alternative models of terrace formation. We identified and mapped a high geomorphic surface interpreted to represent the end of basin filling in the Socorro area and five distinct, post–Santa Fe Group (ca. 0.78 Ma) alloformations and associated geomorphic surfaces using photogrammetric methods, soil characterization, and stratigraphic descriptions. Terrace deposits exhibit tread heights up to 70 m above the valley floor and are 5 to >30 m thick. Their fills generally have pebble-to-cobble bases overlain by fine-to-pebbly sand and local thin silt and clay tops. Alluvial-fan terraces and associated geomorphic surfaces grade to former valley levels defined by axial terrace treads. Carbon-14 ages from detrital charcoal above and below a buried tributary terrace tread show that the most recent aggradation event persisted until ca. 3 ka during the transition from glacial to modern climate conditions. Drill-log data show widespread valley fill ∼30 m thick that began aggrading after glacial retreat in northern New Mexico and southern Colorado (ca. 14 ka). Aggradation during this transition was likely due to hillslope destabilization, increased sediment yield, decreased runoff, and reduced stream competence. Chlorine-36 ages imply similar controls on earlier terraces that have surface ages of ca. 27–29, 64–70, and 135 ka, and suggest net incision during glacial expansions when increased runoff favored down-cutting and bedload mobilization. Our terrace chronology supports existing climate-response models of arid environments and links tributary responses to the axial Rio Grande system throughout the central Rio Grande rift. The terrace chronology also reflects a transition from modest (60 m/m.y.) to rapid (300 m/m.y.) incision between 610 and 135 ka, similar to patterns observed throughout the Rio Grande rift and the western United States in general.

Terrestrial 10Be and electron spin resonance dating of fluvial terraces quantifies quaternary tectonic uplift gradients in the eastern Pyrenees

The 120 km -longTêt River flows out of the Pyrenees to the Mediterranean. By displaying a mappable sequence of Quaternary alluvial units between the Pleistocene frontal moraines of the high range and the offshore sedimentary depocentres, its 1400 km2 watershed is well suited to quantifying gradients of topographic uplift. Five main generations of terrace treads had previously been inferred from constrasts in regolith weathering features, but here we present the first radiometric age constraints based on 15 ESR sediment burial ages covering the full sequence, and 3 vertical TCN age profiles restricted to three mid-sequence terraces. Analytically robust results were obtained for the oldest and uppermost terrace T5 (ESR age: 1099 ± 179 ka), for T3b (ESR age: 374 ± 47 ka, ∼MIS 10), and for T2 (ESR age: 174 ± 44 ka, ∼MIS 6). These results are consistent with the contrasts in weathering grade of the deposits. The TCN profiles only yielded minimum exposure ages but provided precise post-depositional denudation rates for the fluvial terrace treads. The land-to-sea geometry of the chronosequence also provided clues about valley incision rates in response to topographic uplift during the last ∼1 Ma. Based on a critical review of similar data obtained for other Mediterranean and Atlantic watersheds in the Pyrenees, the full regional correlation reveals that post-orogenic topographic uplift was substantial and relatively uniform throughout the entire mountain range. Patterns and magnitudes suggest a shared, probably subcrustal driving mechanism of Neogene and Quaternary mountain growth, with only subsidiary influence from isostasy, and climatic forcing.

Eolian sand and loess deposits indicate west-northwest paleowinds during the Late Pleistocene in western Wisconsin, USA

AbstractOur study adds to the Quaternary history of eolian systems and deposits in western Wisconsin, USA, primarily within the lower Chippewa River valley. Thickness and textural patterns of loess deposits in the region indicate transport by west-northwesterly and westerly winds. Loess is thickest and coarsest on the southeastern flanks of large bedrock ridges and uplands, similar in some ways to shadow dunes. In many areas, sand was transported up and onto the western flanks of bedrock ridges as sand ramps, presumably as loess was deposited in their lee. Long, linear dunes, common on the sandy lowlands of the Chippewa valley, also trend to the east-southeast. Small depressional blowouts are widespread here as well and often lie immediately upwind of small parabolic dunes. Finally, in areas where sediment was being exposed by erosion along cutbanks of the Chippewa River, sand appears to have been transported up and onto the terrace treads, forming cliff-top dunes. Luminescence data indicate that this activity has continued throughout the latest Pleistocene and into the mid-Holocene. Together, these landforms and sediments paint a picture of a locally destabilized landscape with widespread eolian activity throughout much of the postglacial period.

Fluvial terraces and their implications for Weihe River valley evolution in the Sanyangchuan Basin

Fluvial terraces are important geomorphic markers for modern valley development. When coupled with numeric ages, terraces can provide abundant information about tectonic, climatic, paleohydrological and the paleoenvironmental changes. On the basis of the paleomagnetic, electron spin resonance (ESR) and optically stimulated luminescence (OSL) dating, in addition to an investigation of local loess-paleosol sequences, we confirmed that 13 fluvial terraces were formed, and then preserved, along the course of the Upper Weihe River in the Sanyangchuan Basin over the past 1.2 Ma. Analyses of the characteristics and genesis of these terraces indicate that they resulted from the response of this particular river system to climate change over an orbital scale. These changes can further be placed within the context of local and regional tectonic uplift, and represent an alternation between lateral migration and vertical incision, dependent upon the predominance of climatic and tectonic controls during different periods. Most of the terraces are strikingly similar in that they have several meters of paleosols which have developed directly on top of fluvial deposits located on the terrace treads, suggesting that the abandonment of terraces due to river incision occurred during the transitions from glacial to interglacial climates. The temporal and spatial differences in the distribution patterns of terraces located on either side of the river valley indicate that a tectonic inversion occurred in Sanyangchuan Basin at ~0.62 Ma, and that this was characterized by a transition from overall uplift to depression induced by fault activity. Synthesized studies of the Basin’s terraces indicate that formation of the modern valley of the Upper Weihe River may have begun in the late Early Pleistocene between 1.4–1.2 Ma.

Climatic and tectonic controls on strath terraces along the upper Weihe River in central China

The Weihe River in central China is the largest tributary of the Yellow River and contains a well-developed strath terrace system. A new chronology for the past 1.11 Ma for a spectacular flight of strath terraces along the upper Weihe River near Longxi is defined based on field investigations of loess–paleosol sequences and magnetostratigraphy. All the strath terraces are strikingly similar, having several meters of paleosols that have developed directly on top of fluvial deposits located on the terrace treads. This suggests that the abandonment of each strath terrace by river incision occurred during the transition from glacial to interglacial climates. The average fluvial incision rates during 1.11–0.71 Ma and since 0.13 Ma are 0.35 and 0.32 m/ka, respectively. These incision rates are considerably higher than the average incision rate of 0.16 m/km for the intervening period between 0.71 and 0.13 Ma. Over all our results suggest that cyclic Quaternary climate change has been the main driving factor for strath terrace formation with enhanced episodic uplift.

Rates and kinematics of active shortening along the eastern Qilian Shan, China, inferred from deformed fluvial terraces

AbstractIn the eastern Qilian Shan, a flight of fluvial terraces developed along the Jinta River valley are deformed across the Nanying anticline. Four individual fluvial terraces are preserved at different elevations above the river, and higher terrace treads are draped by systematically thicker aeolian loess. Optically stimulated luminescence dating of deposits at the base of the loess provides constraints on the timing of surface abandonment; terraces were abandoned at 69 ± 4 ka B.P. (T4), 57 ± 4 ka B.P. (T3), and between 34 ± 3 ka B.P. (T2), respectively. Differential GPS measurement of the terrace profile across the anticline allows reconstruction of subsurface fault geometry; we model terrace deformation above a listric thrust fault with a tip line at 2.2 ± 0.1 km depth and whose dip shallows systematically to 23 ± 3° at depth of 5.8 ± 1.1 km. Combining terrace ages with this model of fault geometry, we estimate a shortening rate of 0.8 ± 0.2 mm/a across the Nanying fold and a shortening rate of ~0.1 mm/a across the mountain front fault since ~70 ka B.P. This rate suggests that the frontal fault system along the eastern Qilian Shan accomplishes crustal shortening at rates of approximately 0.9 ± 0.3 mm/a during late Pleistocene time.

Spatial slip behavior of large strike‐slip fault belts: Implications for the Holocene slip rates of the eastern termination of the North Anatolian Fault, Turkey

AbstractWe present new data on Holocene slip rates for the eastern end of the North Anatolian Fault (NAF) by using the optically stimulated luminescence ages of the offset terrace deposits at two sites, where a total of four displaced landforms was studied. Each offset feature was analyzed independently, and three different assumptions were made for all the offsets, depending on whether the age of the upper tread (upper tread reconstruction), the lower tread (lower tread reconstruction), or all bounding surfaces (intermediate solution) were used in dating of the terrace risers. The deflected geometry of the risers strongly suggests the use of either the intermediate solution or the upper tread reconstruction. The joint slip rate distributions for the upper tread reconstructions and the intermediate solutions were modeled as 13.0 + 1.8 / −1.4 and 14.3 + 5.8 / −2.4 mm/yr (2σ), respectively. Although the intermediate solution covers the full range of ages for the measured displacements, the curved geometry of the terrace risers suggests that the initiations of the riser offsets are most probably close to the abandonment ages of the upper terrace treads. Therefore, we accepted the joint slip rate of the intermediate solution but suggested that the average rate for the main displacement zone of the eastern NAF should be close to its lower limits. This slower rate with respect to previous estimates suggests that the total deformation is not only accommodated on the main displacement zone but is also distributed along the secondary faults to the south of the easternmost segments of the NAF.

Constraints on Pleistocene glaciofluvial terrace age and related soil chronosequence features from vertical 10Be profiles in the Ariège River catchment (Pyrenees, France)

Sequences of alluvial fill-terraces record the long-term variation of fluvial transport regimes in response to Quaternary climatic changes. Through a production of increasingly precise chronologies, the broadening range of sediment dating methods in recent years has improved our understanding of correlations between alluvial dynamics and external forcing mechanisms. However, results in this field have mostly focused on periglacial and other mid-latitude settings. These may not readily apply to glaciofluvial environments, where alluvial regimes are narrowly dependent on the behavior of the adjacent ice-field. Here we constrain the ages of three glaciofluvial fill terraces based on five vertical 10Be age profiles in the Ariège River catchment, and produce a chronofunction of related contrasts in terrace tread topsoil characteristics. Each profile consists of 9 to 11 gravel samples collected over depths of 4.6 to 15m. Terrace T1 (+15m above the current riverbed) was sampled at ~4, 22 and 53km downstream from the Last Glacial Maximum terminal moraine system of the Ariège outlet glacier. The profiles of T2 (+30m) and T3 (+50m) complete the data set. Beryllium-10 age profiles date the age of terrace tread abandonment as a result of fluvial incision. Results show that the incision of T1 occurred over a protracted period during the Last Termination, beginning ca. 17.5–3.5+2.0ka. Prior to this, aggradation occurred from MIS 4 to the global Last Glacial Maximum (LGM), i.e.,under maximum ice extent conditions, and lasted a few thousand years thereafter. The protracted influx of debris after the LGM was sustained by paraglacial storage release in the catchment and promoted by strong coupling between slope and channel processes. Fluvial incision of T1 occurred during the post-LGMcold-to-warm transition after a time (supported by paleobotanical evidence) when the vegetation cover was capable of ensuring catchment-wide slope stability. The wave of incision propagated downstream between 17.5–3.5+2.0ka (proximal profile) and 13.0–0.5+3.5ka (distal profiles). The exposure-age confidence intervals for T2 and T3 are much larger than for T1 (60–145ka and 204–226ka, respectively) but the data nonetheless allow the aggradation of T2 and T3 to be attributed to MIS 6 and MIS 8, respectively. The soil chronosequence permitted by the fill-terrace age sequence indicates that the Luvic Cambisols capping T1 formed after ~13–16ka, i.e.,during the Holocene; (ii) the Haplic Luvisols capping T2 required at least ~130ka, i.e.,two interglacials; and (iii) the deeply rubified, highly eluviated Luvisols capping T3 required at least ~250ka, i.e.,at least three interglacials.

Late Quaternary faulting on the Manas and Hutubi reverse faults in the northern foreland basin of Tian Shan, China

The Tian Shan Range lies in the actively deforming part of the India–Asia collision zone. In the northern foreland basin of Tian Shan, the strata were intensively deformed by Cenozoic folding and faulting. Slip rate studies along these faults are important for understanding the dynamics of crustal deformation and evaluating the seismic hazards in the region. Two reverse faults (the Manas and Hutubi faults) in the northern foreland basin were investigated. Due to past faulting events along these faults, the terrace treads along the Manas River were ruptured, forming fault scarps several meters in height. Loess deposits were trapped and preserved at the surface ruptures along these scarps. The thickness of the trapped loess is dependent on the size of the ruptures. The minimum and maximum ages of these scarps are constrained by dating the loess preserved at the surface ruptures and the terrace treads, respectively, using the quartz optically stimulated luminescence (OSL) dating technique. Our dating results suggest that the loess trapped at the ruptures was deposited from the early to mid-Holocene at the Hutubi Fault, and from the mid- to late-Holocene at the Manas Fault. The vertical displacements of the faults were evaluated by measuring the topographic profiles across the investigated fault scarps using the differential global position system (DGPS). Our results suggest that, during the late Quaternary in the studied region, the vertical slip rates of the Manas Fault were between ∼0.74 mm/yr and ∼1.6 mm/yr, while the Hutubi Fault had a much lower vertical slip rate between ∼0.34 mm/yr and ∼0.40 mm/yr. The tectonic implications of our results are discussed.

Late Quaternary climatic and tectonic mechanisms driving river terrace development in an area of mountain uplift: A case study in the Langshan area, Inner Mongolia, northern China

The Langshan Range is located in the western Yin Mountain orogenic belts and the western Hetao fault-depression zone in Inner Mongolia, northern China. This area is on the northwestern margin of the East Asian monsoon region. The fluvial terraces in the transverse drainage of the Langshan Range represent a primary geomorphic response to local tectonic uplift and climatic changes. The terrace evolution was reconstructed using a combination of optically stimulated luminescence (OSL) dating and terrace tread measurements. The terraces, designated T4 through T1, were abandoned at about 58.00, 46.25, 32.19, and 15.79ka BP, respectively. Their aggradation occurred primarily during cold periods of the last glacial stage, and incision occurred primarily during shifts from cold to warm climate stages. Geomorphic analysis showed the terrace heights were controlled by the tectonic uplift in the area. Differences in river incision rates and terrace geomorphic features indicate that the uplift of the Langshan Range included a component of tilting north to south during the period of 58.00–41.28ka BP, whereas the uplift of the Langshan area tended to be equal on a regional scale after 32.19ka BP.

The evolution of a terrace sequence along the Manas River in the northern foreland basin of Tian Shan, China, as inferred from optical dating

The Tian Shan range lies in the actively deforming part of the India–Asia collision zone. The uplift rate and deformation pattern of the Tian Shan are important for understanding the dynamics of crustal deformation in the region. The river terraces in northern Tian Shan provide key records of past changes in climate and/or regional tectonics. In this study, a terrace sequence along the Manas River in a tectonically active zone in the northern foreland basin of Tian Shan is investigated. Six river terraces were identified and dated using optically stimulated luminescence (OSL). The results show that the six terraces were abandoned at ~0.5ka, ~1.4ka, ~3.1ka, ~4.0ka, ~12.4ka and ~19.9ka, respectively. Together with high resolution Global Positioning System (GPS) measurements on the terrace treads, the fluvial history of Manas River is reconstructed. From ~20ka to ~4.8ka, the height of the fluvial bed of Manas River decreased at an average rate of 2.2±0.6mm/yr. From ~4.8ka to the present, the height of the fluvial bed decreased at an average rate of 13.5±0.6mm/yr, corresponding to intensified incision of Manas River during the late Holocene. This accelerated incision is very likely caused by the tectonic forces rather than climatic influences alone, suggesting that the tectonic uplift activity was significantly intensified since ~4.8ka in the northern piedmont of Tian Shan. Other controlling factors on the incision of Manas River are also discussed.

Geoarchaeological dating of Holocene stream terraces along the San Pedro River, southeastern Arizona, USA

Three Pleistocene and four Holocene floodplain terraces were differentiated along the San Pedro River in southeastern Arizona during a geological mapping project conducted by the Arizona Geological Survey. We used archaeological chronometric data to refine age estimates for the Holocene surfaces that had been initially based on only geomorphic and pedologic criteria. Eight archaeological sites deeply buried in the fill of the most extensive Holocene terrace map unit (Qy2r) yielded 10 radiocarbon dates and temporally diagnostic San Pedro complex (3150–1900 cal BP) artifacts. Twenty sites recorded on the surface of Qy2r terrace treads contained numerous temporally diagnostic Hohokam (1950–500 cal BP) ceramic sherds and features. The age constraints provided by these archaeological materials suggest that the Qy2r terrace fill was deposited primarily between 4500 and 1000 cal BP, with only a thin veneer of flood deposits added between 1000 and 100 cal BP. This late Holocene to historical (4500–100 cal BP) age estimate is consistent with other alluvial histories in the greater Gila River drainage basin and supplants our initial geologically based latest Holocene to historical (<2000 cal BP) Qy2r age estimate. The archaeologically derived maximum age estimate for Qy2r also provides a minimum age for the topographically higher and older Qy1r terrace, prompting us to revise our original, early to late Holocene age estimate for Qy1r to an early to middle Holocene age range of 11,500–4500 cal BP. The post-A.D. 1850 historical age we originally proposed for the Qy3r terrace deposits inset into Qy2r, however, is consistent with the presence of four historical period surface sites and a concordant absence of surface and buried prehistoric sites.

Interpretation and downstream correlation of bedrock river terrace treads created from propagating knickpoints

Derivation of an analytical model for the slope of a strath terrace created following an upstream propagating wave of incision reveals that for detachment‐limited river incision the exponent on channel slope, n, governs terrace tread slope. Terrace elevations can increase upstream (n &gt; 1), downstream (n &lt; 1), or can be horizontal (n = 1). Numerical modeling confirms these results for temporally evolving knickpoint geometries and for incision due to sudden base level fall and an increase in rock uplift rate. Except in the case of bedrock river incision with a slope threshold only exceeded during knickpoint propagation, a terrace created from transient headward incision contains no information about the paleo‐channel gradient. Gradients in rock uplift rate along channels potentially complicate the interpretation of terraces by altering the primary tilt on strath terraces. In particular, monotonic gradients in rock uplift can produce terrace treads that are apparently folded. Because gradients in rock uplift rate are common, where terraces are not longitudinally traceable, care is warranted in terrace correlation. In simple tectonic settings where terraces are longitudinally traceable, the slope of a strath terrace created from headward incision may provide a means of estimating the dependency of river incision rate on channel slope. Terraces in four locations argued to have formed from headward incision are parallel or close to parallel with the active channel, implying a slope exponent on river incision rate that is much greater than one or a threshold slope for incision that is only exceeded during knickpoint propagation.

Climatic, geomorphic, and archaeological implications of a late Quaternary alluvial chronology for the lower Salt River, Arizona, USA

Recent archaeological excavations along the lower Salt River, Arizona resulted in the unexpected discovery of buried late Pleistocene soils and cultural features dating 5800–7100cal YBP (Early Archaic), the latter representing the earliest evidence of human activity in the lower Salt River floodplain thus far identified. Because the lower Salt River floodplain has been heavily impacted by recent agriculture and urbanization and contains few stratigraphic exposures, our understanding of the river's geological history is limited. Here we present a late Quaternary alluvial chronology for a segment of the lower Salt River based on 19 accelerator mass spectrometry 14C and four optically stimulated luminescence ages obtained during two previous geoarchaeological investigations. Deposits are organized into allostratigraphic units and reveal a buried late Pleistocene terrace inset into middle-to-late Pleistocene terrace deposits. Holocene terrace fill deposits unconformably cap the late Pleistocene terrace tread in the site area, and the lower portion of this fill contains the Early Archaic archaeological features. Channel entrenchment and widening ~900cal YBP eroded much of the older terrace deposits, leaving only a remnant of fill containing the buried latest Pleistocene and middle-to-late Holocene deposits preserved in the site area. Subsequent overbank deposition and channel filling associated with a braided channel system resulted in the burial of the site by a thin layer of flood sediments. Our study confirms that the lower Salt River is a complex mosaic of late Quaternary alluvium formed through vertical and lateral accretion, with isolated patches of buried soils preserved through channel avulsion. Although channel avulsion is linked to changes in sediment load and discharge and may have climatic linkages, intrinsic geomorphic and local base level controls limit direct correlations of lower Salt River stratigraphy to other large rivers in the North American Southwest.