Increase In Uplift Rate Research Articles

Transient uplift of a long-term quiescent coast inferred from raised fan delta sediments

We present the first example of a short-term uplift transient within the long-term stagnant forearc of the Arica Bend, northernmost Chile. We date a storm deposit embedded into fan delta sediments, which were deposited close to sea level and are now located ~40 m higher. The radiocarbon age is ca. 10 ka, yielding an average uplift rate of ~4 mm/yr. During the Holocene, the section has been dissected by the Lluta River, the long profile of which shows geomorphic response to coastal uplift. However, the coast of the Arica Bend is characterized by million- year-scale relative subsidence and current global positioning system surveyed quiescence. We therefore interpret the inferred uplift signal to represent a transient increase in uplift rates throughout the Holocene. Similar short-term transients have been globally documented in relation to various subduction zone settings, although their causes still remain a matter of debate and need further investigation. We propose that periods of increased surface uplift may also occur in long-term quiescent forearcs, possibly due to temporal variations of plate interface properties (i.e., strength and/or seismic locking) that induce thickening beneath the coast.

Post-tectonic landscape evolution in NE Iberia using staircase terraces: Combined effects of uplift and climate

River incision into bedrock resulting from the combined effects of tectonic uplift and climate governs long-term regional landscape evolution. We determined spatial and temporal patterns of post-orogenic stream incision from a sequence of well-preserved staircase terraces developed over the last 1Ma in the Central Pyrenees and its southern foreland Ebro basin (NE Spain). Extensive remnants of ten vertically separated terraces (Qt1 to Qt10, from oldest to youngest) were mapped along 170km of the Cinca River valley, transverse to the Pyrenean mountain belt. Multiple outcrops appear in the upper reach of the valley (Ainsa sector, 50km from headwaters) as well as in the lower reach (Albalate sector, 125km from headwaters). Fluvial incision into bedrock was calculated using (i) differentially corrected GPS measurements of the altitude of straths and (ii) numerical dating of alluvial sediments from the lower terraces (Qt5 to Qt9) by Optically Stimulated Luminescence, previously reported by Lewis et al. (2009), and supplemented with new dates for the upper terraces (Qt1, Qt2 and Qt3) based on paleomagnetism and supported by soil development. Considering altitude differences and the elapsed time between successive well preserved terrace couples (Qt3–Qt7, Qt7–Qt9 and Qt9-Active channel), mean bedrock incision rates ranged from 0.76 to 0.38m ka−1, at the upper reach of the valley (Ainsa section), and from 0.61 to 0.20mka−1, at the lower reach (Albalate section). River incision along the valley produced vertically separated, near-parallel longitudinal terrace profiles evidencing a rapid near-uniform regional uplift as response to (i) the tectonic lithospheric thickening in NE Iberia and (ii) the erosional download rebound related to the Ebro basin exorheism. Moreover, a subtle upstream divergence of strath profiles may have been a consequence of an increase in uplift rate toward the head of the valley. Additionally, incision rates changed over time as indicate results from the lower reach (Albalate section); the maximum rate was 1.48mka−1 between Qt7 (61ka) and Qt8 (47ka), and the minimum rate was 0.11mka−1 between Qt3 (401ka) and Qt5 (178ka). The highest incision rates were produced after the Marine Isotope Stage 4 most likely in response to (i) an increased snowmelt discharge during the subsequent deglaciation related to the last maximum advance of glaciers in the southern Pyrenees, and (ii) a limited width of the valley after Qt7 formation, resulting from the deactivation of the westward river migration. Therefore, incision rates over the last 1Ma in the Cinca River valley were basically controlled by near-uniform bedrock uplift, in the context of climate variability. The results reported in this study represent significant data on fluvial incision in NE Iberia, and provide an assessment of the regional post-tectonic landscape evolution.

Morphotectonic records of neotectonic activity in the vicinity of North Almora Thrust Zone, Central Kumaun Himalaya

The landform development and valley floor morphology of active regions is significantly controlled by tectonic processes. In the present study the upper catchments of Kosi and Gagas river valleys have been investigated with special emphasis on aggradational landforms to explain the spatial and temporal variability of aggradation/incision in response to tectonic activity during the late Quaternary and Holocene in Central Kumaun Himalaya. Major tectonic elements such as the North Almora thrust (NAT), Rasiyari fault (RF), and Gagas fault (GF) are considered to be controlling the development of landforms in the region. The valleys have preserved debris flow terraces, bedrock strath terraces, and fluviolacustrine terraces that provide signatures of tectonic activity. Morphostratigraphy of the terraces reveals that the oldest landforms are preserved in the hanging wall block of the NAT, RF, and along the GF. Reconstructions based on morphostratigraphy through the application of optical chronology suggests multiple phases of fluvial aggradation in the upper catchment of the Kosi and Gagas rivers that were initiated around 34ka. The youngest phase of aggradation is dated to be around early to mid-Holocene (7–1ka). Based on terrace morphology, four major phases of enhanced uplift have been estimated during 34, 15.8, 7, and 3ka. The older uplift event dated to be around 34ka is represented uplift along GF, which accommodated onset of valley-fill aggradation in Gagas valley. The 15.8ka event represents uplift along RF, which led to the generation of debris flow from adjacent hillslopes. The 7ka event represents uplift of a hanging wall block of the NAT and deposition of debris flow terraces. However, the youngest 3ka event represents enhanced uplift, which is responsible for the incision of the older valley fill sediments and bedrock. Following this, a phase of accelerated incision/erosion owing to an increase in uplift rate occurred, as evident from the strath terraces between 7 and 1ka.

Late Quaternary tectonic landforms and fluvial aggradation in the Saryu River valley: Central Kumaun Himalaya

The present study has been carried out with special emphasis on the aggradational landforms to explain the spatial and temporal variability in phases of aggradation/incision in response to tectonic activity during the late Quaternary in the Saryu River valley in central Kumaun Himalaya. The valley has preserved cut-and-fill terraces with thick alluvial cover, debris flow terraces, and bedrock strath terraces that provide signatures of tectonic activity and climate. Morphostratigraphy of the terraces reveals that the oldest landforms preserved south of the Main Central Thrust, the fluvial modified debris flow terraces, were developed between 30 and 45ka. The major phase of valley fill is dated between 14 and 22ka. The youngest phase of aggradation is dated at early and mid-Holocene (9–3ka). Following this, several phases of accelerated incision/erosion owing to an increase in uplift rate occurred, as evident from the strath terraces. Seven major phases of bedrock incision/uplift have been estimated during 44ka (3.34mm/year), 35ka (1.84mm/year), 15ka (0.91mm/year), 14ka (0.83mm/year), 9ka (1.75mm/year), 7ka (5.38mm/year), and around 3ka (4.4mm/year) from the strath terraces near major thrusts. We postulate that between 9 and 3ka the terrain witnessed relatively enhanced surface uplift (2–5mm/year).

Transient fluvial incision as an indicator of active faulting and Plio-Quaternary uplift of the Moroccan High Atlas

One of the challenges facing Earth Scientists is to determine the extent to which geomorphic features can be used to extract tectonic signals from landscapes. Here, we quantitatively analyse the long profiles of rivers that drain southwards across the South Atlas Fault (SAF), a thrust fault that forms the southern margin of the High Atlas Mountains in Morocco, to derive new data on the Late Cenozoic activity of this fault system. River long profiles were extracted for 32 major rivers flowing southwards into the Ouarzazate Basin. Of these, eleven exhibit concave-up river profiles with a mean concavity of 0.64 and normalized steepness indices in the range 47.5–219.0m0.9. By contrast, 21 rivers exhibit at least one knickpoint upstream of the thrust front. Knickpoint height varies from 100 to 1300m, with calculated incision at the range bounding fault ranging from 80 to 900m, despite the drainage areas upstream of the knickpoint ranging over several orders of magnitude. In map view, knickpoint locations generally plot along sub-parallel lines and there are no obvious relationships with lithological units for knickpoints exhibiting slope-break morphology. Channel reaches below slope-break knickpoints have higher mean concavities (0.76) than above the knickpoint indicative. This observation combined with a lithological or river-capture origin for the knickpoints having been ruled out suggests that an increase in uplift rate along a planar fault zone during the Plio-Quaternary caused the initiation of the transient response (i.e., knickpoint formation) to a change in base-level observed in the river profiles. This uplift event can be correlated to the convective removal of the lithospheric root to the Atlas Mountains resulting in the anomalously high topography at the present day.

Active forearc shortening in Tohoku, Japan: Constraints on fault geometry from erosion rates and fluvial longitudinal profiles

Convexities in the longitudinal profiles of actively incising rivers are typically considered to represent the morphologic signal of a transient response to external perturbations in tectonic or climatic forcing. Distinguishing such knickzones from those that may be anchored to the channel network by spatial variations in rock uplift, however, can be challenging. Here, we combine stream profile analysis, 10Be watershed-averaged erosion rates, and numerical modeling of stream profile evolution to evaluate whether knickzones in the Abukuma massif of northeast Japan represent a temporal or spatial change in rock uplift rate in relation to forearc shortening. Knickzones in channels that drain the eastern flank of the Abukuma massif are characterized by breaks in slope–area scaling and separate low-gradient, alluvial upper-channel segments from high-gradient, deeply-incised lower channel segments. Average erosion rates inferred from 10Be concentrations in modern sediment below knickzones exceed erosion rates above knickzones by 20–50%. Although profile convexities could be interpreted as a transient response to an increase in rock uplift rate associated with slip on the range-bounding fault, geologic constraints on the initiation of fault slip and the magnitude of displacement cannot be reconciled with a recent, spatially uniform increase in slip rate. Rather, we find that knickzone position, stream profile gradients, and basin averaged erosion rates are best explained by a relatively abrupt spatial increase in uplift rate localized above a flat-ramp transition in the fault system. These analyses highlight the importance of considering spatially non-uniform uplift in the interpretation of stream profile evolution and demonstrate that the adjustment of river profiles to fault displacement can provide constraints on fault geometry in actively eroding landscapes.

Testing fluvial erosion models using the transient response of bedrock rivers to tectonic forcing in the Apennines, Italy

The transient response of bedrock rivers to a drop in base level can be used to discriminate between competing fluvial erosion models. However, some recent studies of bedrock erosion conclude that transient river long profiles can be approximately characterized by a transport‐limited erosion model, while other authors suggest that a detachment‐limited model best explains their field data. The difference is thought to be due to the relative volume of sediment being fluxed through the fluvial system. Using a pragmatic approach, we address this debate by testing the ability of end‐member fluvial erosion models to reproduce the well‐documented evolution of three catchments in the central Apennines (Italy) which have been perturbed to various extents by an independently constrained increase in relative uplift rate. The transport‐limited model is unable to account for the catchments' response to the increase in uplift rate, consistent with the observed low rates of sediment supply to the channels. Instead, a detachment‐limited model with a threshold corresponding to the field‐derived median grain size of the sediment plus a slope‐dependent channel width satisfactorily reproduces the overall convex long profiles along the studied rivers. Importantly, we find that the prefactor in the hydraulic scaling relationship is uplift dependent, leading to landscapes responding faster the higher the uplift rate, consistent with field observations. We conclude that a slope‐dependent channel width and an entrainment/erosion threshold are necessary ingredients when modeling landscape evolution or mapping the distribution of fluvial erosion rates in areas where the rate of sediment supply to channels is low.

The Pleistocene terrace staircase of the River Thame, central-southern England, and its significance for regional stratigraphic correlation, drainage development, and vertical crustal motions

The Thame is one of the principal left-bank affluents of the Thames, the largest river in southern England; it joins the Upper Thames at Dorchester, ∼20 km downstream of Oxford. Its terraces include a younger group of four, which date from the late Middle Pleistocene and Late Pleistocene, are disposed subparallel to the modern river, and represent drainage within the modern catchment. At higher levels there are three older terraces, the Three Pigeons, Tiddington and Chilworth terraces, which are assigned to MIS 16, 14 and 12. With much gentler downstream gradients, these are fragmentary remnants of much more substantial fluvial deposits, indicating a much larger river that was disrupted by the Anglian (MIS 12) glaciation. This interpretation supersedes an earlier view that the glacigenic deposits in the Thame headwaters correlate with the Blackditch terrace, the highest of the younger group, which has hitherto provided an argument that the glaciation in this region occurred in MIS 10. It is suggested that the headwaters of the pre-Anglian ‘Greater Thame’ river were located near Northampton and that the Milton Sands of that area represent an upstream counterpart of the Chilworth terrace deposits. It is envisaged that this early Middle Pleistocene drainage geometry, located between the Jurassic limestone and Chalk escarpments, developed as a result of the increase in uplift rates that followed the Mid-Pleistocene Revolution (MPR). It is suggested that before this time, including during the Early Pleistocene, the modern Thame catchment and adjacent regions drained southeastward through the Chalk escarpment, but these small rivers lacked the erosional power to cut through the Chalk in pace with the faster uplift occurring in the early Middle Pleistocene, and so became diverted to the southwest, subparallel to the Chalk escarpment, to form the pre-Anglian ‘Greater Thame’ tributary of the Upper Thames. The post-MPR uplift is estimated to decrease northwestward from 90 m in the Middle Thames to 75 m near the Thame–Thames confluence and to 65 m upstream of Oxford. The post-Anglian (post-450 ka) component of uplift decreases northward from 33 m near the Thame–Thames confluence to an estimated ∼20 m in the Northampton area; the relative stability of the latter area makes feasible the proposed correlation between the Milton Sands and the pre-Anglian River Thame. Limited post-Anglian uplift in the Northampton area is also inferred from the upstream convergence of the terraces of the modern rivers Nene and Great Ouse. These observed lateral variations in vertical crustal motions reflect lateral variations in crustal properties (including heat flow, crustal thickness, and thickness of underplating at the base of the crust) that are known independently. This study thus provides, for the first time, an integrated explanation of the Pleistocene drainage development across a large region of central-southern England.

Late Quaternary fluvial aggradation and incision in the monsoon‐dominated Alaknanda valley, Central Himalaya, Uttrakhand, India

AbstractThe present study aims to explain the spatial and temporal variability in phases of aggradation/incision in response to changes in climate and seismicity during the late Quaternary in the Alaknanda River valley (a major tributary of the river Ganges or Ganga). Geomorphology, stratigraphy and optical dating of the fluvial sediment reveal that the oldest fluvial landforms preserved in the south of the Main Central Thrust are debris flow terraces developed during the early part of pluvial Marine Isotopic Stage 3. Following this, a period of accelerated incision/erosion owing to an increase in uplift rate and more intense rainfall occurred. In the Lesser Himalaya, three phases of valley fill aggradation around 26 ± 3 ka, 18 ± 2 ka and 15 ± 1 ka and 8 ± 1 ka occurred in response to changes in monsoon intensity and sediment flux. The last phase was regionally extensive and corresponds to a strengthening of the early Holocene Indian Summer Monsoon. A gradual decline in the monsoon strength after 8 ± 1 ka resulted in reduced fluvial discharge and lower sediment transport capacity of the Alaknanda River, leading to valley fill incision and the development of terraces. The study suggests that fluvial dynamics in the Alaknanda valley were modulated by monsoon variability and the role of tectonics was subordinate, limited to providing accommodation space and post‐deposition modification of the fluvial landforms. Copyright © 2010 John Wiley & Sons, Ltd.

Geomorphological, stratigraphic and geochronological evidence of fast Pleistocene coastal uplift in the westernmost part of the Corinth Gulf Rift (Greece)

AbstractRapid extension and active normal faulting in the western extremity of the Corinth Gulf are accompanied by fast coastal uplift. We investigate Pleistocene uplift west of Aigion, by attempting to date remains of marine terraces and sedimentary sequences by calcareous nannoplankton and U‐series analyses. Net uplift initiated recently, due to abandonment of an older rift‐bounding fault zone and increase in activity on the presently active, coastal fault zone. This change apparently coincides with an abrupt slow down (or, termination) of secondary fault block tilting within the broader hangingwall block of the older zone, indicated by an angular unconformity that dates in the early part of MIS10 (∼390–350 ka BP, preferably, in the earlier part of this period). Net uplift driven by the coastal zone resulted in the formation of MIS9c (330 ka) and younger terraces. The formation of the unconformity and the initiation of net uplift coincide temporally with a ∼300–400 ka unconformity recognized by recent studies in a wide area offshore Aigion i.e. they could be part of an evolutionary event that affected the entire western part of the Corinth Rift or, a large area therein. Uplift rate estimates at four locations are discussed with reference to the morphotectonic context of differential uplift of secondary fault blocks, and the context of possible increase in uplift rate with time. The most reliable and most useful estimate for uplift rate at the longitude of the studied transect is 1.74–1.85 mm/year (time‐averaged estimate for the last ∼240 ka, based on calcareous nannoplankton and sequence‐stratigraphic interpretation). Copyright © 2009 John Wiley & Sons, Ltd.

Modes and times of caldera resurgence: The < 10 ka evolution of Ischia Caldera, Italy, from high-precision archaeomagnetic dating

Ischia is a well exposed and densely populated late Quaternary caldera in the Campanian magmatic province of Italy. Ischia Caldera experienced an average uplift rate of 3.3 cm/year in the last ca. 30 ka and is still actively resurging. During the last 10 ka, coeval with the resurgence, a volcanic field of alkali-trachytic to trachyandesitic lava domes, lava flows, tuff and scoria rings, and pumice cones developed, mainly on the eastern sector of the caldera, along both resurgence-related faults and regional NNW- and NE-striking faults. In order to improve both our understanding of the recent volcanic history and the evaluation of future risks on Ischia Island, a high-precision archaeomagnetic dating method was applied to the products of 12 volcanic centres with probable age < 10 ka. Accurate paleofield directions with a median 95% confidence angle of 1.47 were measured on 277 lava and spatter samples (25 sites). Coupled with the reference curves for secular variation in the western Mediterranean sea (during the last 3000 years) and eastern Europe (from 3000 to 8000 years ago), ages of between 4100 BC and 355 AD were obtained. These archaeomagnetic data were supported by a volcanologic and stratigraphic survey and are consistent with written sources, archaeological findings, and previous isotopic (K/Ar and 14C) ages. Archaeomagnetic and other geochronological data, as well as stratigraphic constraints, show that, during the studied time interval, Ischia volcanism occurred in five periods separated by phases of quiescence and coeval with earthquake and landslide events. This fact suggests a pulsating mode of uplifting and deformations of the Ischia resurgence. During the two oldest periods of activity (7200–6800 BC and 4100–2300 BC), resurgence probably produced a dome-shaped structure. Location and geometry of vents suggest the occurrence of magma uprise along the fractures produced by bending of the overburden crustal block. Most of magma was emplaced as intrusions at the interior of the resurgent block, whereas volcanism was represented by very viscous, differentiated, and crystallized lavas that emplaced as domes and high aspect-ratio flows. The resurgent dome caused recurrent lateral collapses that removed about 2.5 km 3 of rocks. During the three youngest periods of activity (1800–1000 BC; 650 BC –355 AD; and 1302 AD), resurgence affected a fault-bounded, asymmetric block. This resulted from both (a) hydrostatic rebound of the crustal block after removal of material involved into huge slope instability triggering an increase in uplift rate, and (b) new influxes of less evolved magma batches into the shallow reservoir that, in turn, favoured the intense volcanism of the last 4000 years.

Late Cenozoic surface uplift revealed by incision by the River Euphrates at Birecik, southeast Turkey

Incision by the River Euphrates has created a dramatic erosional landscape in the northern Arabian Platform in SE Turkey. We obtain accurate heights of its terrace deposits in the Birecik area using differential GPS, and summarise evidence regarding heights of Late Miocene basalt flows and low-altitude wetland sediments. We thus estimate ∼600 m of uplift since the early Late Miocene (∼9 Ma), represented by ∼270 m of fluvial incision, the difference reflecting downstream channel-lengthening as the coastline has retreated to the Persian Gulf, ∼1400 km away. The oldest recognised sediments, inset below this level, form a thick aggradation between ∼100 and ∼130 m above the present level of the Euphrates. We estimate that this accumulated during ∼5–3 Ma when uplift rates were low, such that aggradation was necessary to maintain a downstream channel gradient in response to the coastal retreat. Palaeolithic artefacts have been reported locally in Euphrates deposits up to ∼80 m above present river level, which we tentatively date to ∼1.8 Ma. The Euphrates subsequently incised to within ∼10 m of its present level before aggrading ∼45 m higher in the late Early Pleistocene. The final ∼55 m of incision reflects the increase in uplift rates observed worldwide in the early Middle Pleistocene, following the Mid-Pleistocene Revolution.

Predictions of steady state and transient landscape morphology using sediment‐flux‐dependent river incision models

Recent experimental and theoretical studies support the notion that bed load in mountain rivers can both enhance incision rates through wear and inhibit incision rates by covering the bed. These effects may play an important role in landscape evolution and, in particular, the response of river channels to tectonic or climatic perturbation. We use the channel‐hillslope integrated landscape development (CHILD) numerical model with two different bedrock incision models that include the dual role of the sediment flux to explore the transient behavior of fluvial landscapes. Both models predict that steady state channel slopes increase in landscapes with higher rock uplift rates. However, the incision models predict different transient responses to an increase in uplift rate, and the behavior of each incision model depends on both the magnitude of change in uplift rate and the local drainage area. In some cases, the transient channel behavior is indistinguishable from that predicted for transport‐limited alluvial rivers. In other cases, knickpoints form in some or all of the drainage network, as predicted by the detachment‐limited stream power model. In all cases the response in the lower parts of the network is highly dependent on the response in the upper parts of the network as well as the hillslopes. As the upper parts of the network send more sediment downstream, channel incision rates may rise or fall, and slopes in the lower parts of the channel may, in fact, decrease at times during the transient adjustment to an increase in rock uplift rate. In some cases, channel incision in the upper parts of the network ceases during the transient while the hillslopes adjust to the new uplift rate; drainage density may also change as a function of uplift rate. Our results suggest that if the sediment flux strongly controls bedrock incision rates, then (1) the transient fluvial response will take longer than predicted by the detachment‐limited stream power model, (2) changes in channel slope may be much more complex than predicted by the detachment‐limited stream power model, and (3) changes in the fluvial system will be closely tied to sediment delivery from the hillslopes. Importantly, our results outline quantitative differences in system behavior produced by competing models and provide a framework for identifying locations in natural systems where differences in channel morphology can be used to discern between competing fluvial erosion models.

Dating Quaternary volcanism and incision by the River Tigris at Diyarbakır, southeast Turkey

AbstractNew high‐precision Ar‐Ar dating of basalt underlying the city of Diyarbakır, southeast Turkey, constrains the Quaternary incision history of the River Tigris, strengthening the pre‐existing chronology based on magnetostratigraphy and K‐Ar dates. The basalt, which overlies Tigris terrace gravel 70 m above the river, is part of the Karacadağ shield volcano complex centred ca. 60 km to the southwest. The reverse‐magnetised Diyarbakır basalt, one of relatively few flow units to reach the Tigris valley axis, has been dated to 1196 ± 19 ka (±2σ). Two lower terraces have been recognised in the Diyarbakır area, 46 and 32 m above the present river, with at least one other further downstream. From this evidence the rate of Middle‐Late Pleistocene incision by the Tigris can be estimated; it probably reached ca. 0.07 mm a−1, reflecting the characteristic global increase in uplift rates observed at that time. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Late Pleistocene to Holocene record of changing uplift rates in southern Calabria and northeastern Sicily (southern Italy, Central Mediterranean Sea)

A combination of published and new radiometric dates on uplifted Holocene fossil beaches from northeastern Sicily and southern Calabria (southern Italy) is compared with the altitude of the inner margin of the Last Interglacial (LIg) (Late Pleistocene, ∼124 ka) and older marine terraces in order to gain a regional-scale outline of uplift rates and their temporal changes in a region which is one of the fastest uplifting sectors of the Central Mediterranean Sea. Late Holocene radiocarbon dates from Ioppolo (southern Calabria) and Ganzirri (northeast Sicily), two newly discovered sites are here presented for the first time. The Holocene uplift rates are highest at St. Alessio and Taormina in eastern Sicily (2.4 mm/y) and at Scilla in southwestern Calabria (2.1 mm/y), two sites located across the Messina Straits and which separate the island of Sicily from mainland Italy. Uplift rates decrease towards the south and north from this centre of uplift. Late Holocene uplift rates show an apparent increase of between 64 and 124% when compared with the longer-term uplift rates calculated from the LIg highstand terraces. Furthermore, we discovered that the locations of fastest Late Pleistocene and Late Holocene uplift rates spatially coincide. To what extent the Holocene increase in uplift rates results from incomplete elastic strain release along the major extensional faults which frame the seismotectonic of the area, or indicate a true change in regional tectonic processes, is not resolved. Nonetheless, the heterogeneity of uplift, with a well-defined centre that crosses the Messina Straits, and its persistence at different time-scales indicates a tight connection between wider regional processes and fault-related displacement in controlling crustal instability in this area.

Holocene marine terraces on two salt diapirs in the Persian Gulf, Iran: age, depositional history and uplift rates

AbstractSlightly inclined Holocene marine terraces cover parts of two circular salt diapirs (Hormoz and Namakdan) in the Persian Gulf. Their relative altitude above present sea level results from a combination of general marine transgression/regression affecting the whole area, and of local uplift related to salt diapirism. Differential uplift rate of the studied diapirs in centre‐to‐rim profiles was calculated from results based on: (i) radiocarbon ages of skeletal remains of benthic faunas (19 samples), which originally grew close to sea level; (ii) original altitude of samples, estimated from general sea‐level oscillation curves for the last 10 kyr, and (iii) present sample altitude measured in the field.Calculated uplift rates increase from rim to centre on both diapirs in the range from: 2 mm yr−1 at the rim to 5–6 mm yr−1 at the interior of Hormoz, and 1–3 mm yr−1 at the rim to 3–5 mm yr−1 at the interior of Namakdan. Such uplift rate distributions fit into the parabolic profile of Newtonian fluid rather than to profiles typical for pseudoplastic fluids. The increase in uplift rate with distance from rim to centre of diapirs is gradual as demonstrated also by generally smooth surface of marine terraces. No tectonic dissections were found. The depositional history on both salt diapirs is similar although they are situated more than 100 km apart. Marine sedimentation started at about 9.6k cal. yr BP on Hormoz and at 8.6k cal. yr BP on Namakdan. Owing to rapid transgression, the sea partially truncated both salt diapirs and rapidly deepened, and carbonate mud was deposited on the peripheries of both salt diapirs. Between 7 and 5k cal. yr BP beach deposition replaced carbonate mud. Soon after 5k cal. yr BP, the sea retreated from most of the marine terraces on both salt diapirs. Copyright © 2006 John Wiley &amp; Sons, Ltd.

The effects of rock uplift and rock resistance on river morphology in a subduction zone forearc, Oregon, USA

AbstractRelationships between riverbed morphology, concavity, rock type and rock uplift rate are examined to independently unravel the contribution of along‐strike variations in lithology and rates of vertical deformation to the topographic relief of the Oregon coastal mountains. Lithologic control on river profile form is reflected by convexities and knickpoints in a number of longitudinal profiles and by general trends of concavity as a function of lithology. Volcanic and sedimentary rocks are the principal rock types underlying the northern Oregon Coast Ranges (between 46°30′ and 45°N) where mixed bedrock–alluvial channels dominate. Average concavity, θ, is 0·57 in this region. In the alluviated central Oregon Coast Ranges (between 45° and 44°N) values of concavity are, on average, the highest (θ = 0·82). South of 44°N, however, bedrock channels are common and θ = 0·73. Mixed bedrock–alluvial channels characterize rivers in the Klamath Mountains (from 43°N south; θ = 0·64). Rock uplift rates of ≥0·5 mm a−1, mixed bedrock–alluvial channels, and concavities of 0·53–0·70 occur within the northernmost Coast Ranges and Klamath Mountains. For rivers flowing over volcanic rocks θ = 0·53, and θ = 0·72 for reaches crossing sedimentary rocks. Whereas channel type and concavity generally co‐vary with lithology along much of the range, rivers between 44·5° and 43°N do not follow these trends. Concavities are generally greater than 0·70, alluvial channels are common, and river profiles lack knickpoints between 44·5° and 44°N, despite the fact that lithology is arguably invariant. Moreover, rock uplift rates in this region vary from low, ≤0·5 mm a−1, to subsidence (&lt;0 mm a−1). These observations are consistent with models of transient river response to a decrease in uplift rate. Conversely, the rivers between 44° and 43°N have similar concavities and flow on the same mapped bedrock unit as the central region, but have bedrock channels and irregular longitudinal profiles, suggesting the river profiles reflect a transient response to an increase in uplift rate. If changes in rock uplift rate explain the differences in river profile form and morphology, it is unlikely that rock uplift and erosion are in steady state in the Oregon coastal mountains. Copyright © 2006 John Wiley &amp; Sons, Ltd.

River terraces of the Vltava and Labe (Elbe) system, Czech Republic, and their implications for the uplift history of the Bohemian Massif

The record of river terraces and related fluvial deposits from the Vltava and Labe (Elbe) rivers in the Czech Republic is reassessed. Field relations between the Labe terraces and sediments derived from Scandinavian ice sheets, together with biostratigraphy, provide the basis for correlation with the oxygen isotope record from the oceans. The resulting scheme allows an internally consistent correlation of these terraces with their more fragmentary counterparts downstream in Germany. It is also possible to use the terraces to estimate Late Cenozoic uplift in this region, which appears to have followed a similar pattern to other parts of Europe, in particular with a marked increase in uplift rates after ∼0.9 Ma. Previous schemes may, thus, have overestimated the ages of earlier parts of this Middle Pleistocene fluvial sequence in the Bohemian Massif. Analysis of the biostratigraphy reveals many similarities between interglacial faunas from southern England and the Czech Republic and raises questions about some of the previously published correlations.

Geomorphological consequences of weak lower continental crust, and its significance for studies of uplift, landscape evolution, and the interpretation of river terrace sequences

AbstractEffects of flow in the lower continental crust have often been ignored in the geomorphological literature on the growth of topography during the Quaternary. However, the ability of the lower crust to flow in response to horizontal pressure gradients, caused by lateral variations in the depth of the base of the brittle upper crust, results in two mechanisms for the growth of topography, which can occur either separately or in combination. First, an increase in the rate of erosion in a region will result in a progressive reduction in the depth of the base of the brittle layer, which will drive inflow of lower crust to beneath the region, which will increase the crustal thickness and thus the altitude of the Earth’s surface. It is important to note that this mechanism can increase the mean altitude of the Earth’s surface, not just the altitude of summits formed of erosion-resistant rock or other features that are not eroding, which will rise faster than the surrounding eroding landscape. Second, repeated cyclic surface loading by ice sheets or fluctuations in global sea-level will cause net flow from areas of relatively cool lower crust to beneath areas of warmer crust. This process will thus usually result in net flow of lower crust from beneath offshore areas to beneath land areas, thinning the crust and increasing the bathymetry offshore but adding to the crustal thickness and so uplifting the land surface onshore. Although these two processes have different mechanisms, the time scale over which both operate is governed by the time required for heat diffusion, resulting from lower-crustal flow (which is concentrated near the Moho), to affect the position of the base of the brittle layer. As a result, the uplift responses for both processes can be very similar. This means that to resolve the physical cause of uplift at any locality requires knowledge of the regional conditions before uplift began, not just evidence (such as river terrace sequences) from during the course of uplift. This study illustrates the complexities and practical difficulties that can result from these issues, using case studies of localities that have been modelled in detail. It also points out that, although the ability to carry out quantitative calculations involving lower-crustal flow is new, the idea that such flow provides a general mechanism for the growdi of topography was first suggested in the early 19th century, but was later abandoned - apparently mistakenly. An early Middle Pleistocene increase in uplift rates is widely-recognised from river terrace records, and typically marks a transition from broad valleys in areas of low relief to narrower, more deeply incised gorges. It is suggested that the isostatic response to cyclic surface loading, caused by the growth and decay of continental ice sheets and the associated sea-level fluctuations, is the main cause of this change, following the increase in scale of ice sheet development from oxygen isotope stage 22 (~0.9 Ma) onwards. The less well resolved earlier increase in uplift rates, evident in some river terrace records at ~3 Ma, is more likely to result from the isostatic response to increased rates of erosion linked to the contemporaneous deterioration in climate.

Long-term river terrace sequences: Evidence for global increases in surface uplift rates in the Late Pliocene and early Middle Pleistocene caused by flow in the lower continental crust induced by surface processes

AbstractLong-term river terrace sequences reveal that many regions have uplifted by several hundred metres since the Middle Pliocene. They indeed provide evidence of a global increase in uplift rates in die Late Pliocene, followed by a calm period then a renewed increase around the Early-Middle Pleistocene boundary. It is suggested that this uplift pattern has resulted from thickening of the continental crust caused by flow in the lower crust which has been induced by cyclic surface loading caused by growth and decay of ice sheets and the associated global sea-level fluctuations. Observed uplift histories are modelled using a technique which incorporates increases in the strength of forcing of this process caused by step changes in the intensity of glaciations starting at~3.1,~2.5,~1.2, and~0.9 Ma.