Isostatic Response Research Articles

Deep subsidence of the early catagenesis zones of organic matter in the sedimentary rocks of Northeastern Sakhalin shelf at high thermal regime of the basin

Heat flux measurements in the shelf zone of Northern Sakhalin reveal strong heating of the sedimentary complexes with a temperature gradient of the order of 36 ° C/km along the entire length of the Okhotsk Sea margin of the North Sakhalin basin [2]. However, despite the high thermal regime of the lithosphere in the region, the fact that initial stages of catagenesis gradations PC3‐MC1 ( R o = 0.50‐0.64%) are confined to great depths (3800‐5000 m) is a characteristic property of the deep distribution of the organic matter catagenesis of sedimentary rocks in the boreholes of Northern Sakhalin. It is likely that this fact is related to a high rate of sediment accumulation in the Neogene-Quaternary time. In order to check this supposition and estimate the rate of variations in the degree of transformation of organic matter (OM) in the deeper layers of the basin, we applied the modeling system of GALO basins and reconstructed numerically the history of subsidence and temperature evolution of the rocks of the sedimentary cover in the northeastern part of the Sakhalin shelf in Kaigano-Vasyukanskii area of Sakhalin-5 region (Fig. 1). In addition to the traditionally considered Cenozoic complex, our model of the sedimentary section of the basin includes Upper Cretaceous formations, whose thicknesses were estimated from the seismic data (table). The peculiarity of the shelf region considered here was in the fact that it was located on the one hand within the continental margin of the Okhotsk Sea Plate and on the other hand near the complex of the accretion prism of Eastern Sakhalin [1]. Although the close location of the accretion cline (and consequently the underthrust zone) predetermines the domination of compression stresses over the territory of the basin and consequently the nonisostatic response of the basin lithosphere to the load of water and sediments, there is sufficient evidence of active fracture tectonics in the northeastern shelf of Sakhalin with notable weakening of the lithosphere in the Late Cretaceous and Cenozoic (see below). Therefore, we considered two limiting versions of the basin development. In the first of them, we assumed that numerous fractures and displacements along the fractures significantly weakened the lithosphere strength in the region studied. Under these conditions, the local isostatic response of the lithosphere to the load of water and sediments existed during the entire history of the basin, which we consider started in the Late Cretaceous (and possibly excluded a few relatively short periods). In the second alternative version of the basin formation, we assumed that the local isostatic response of the basin lithosphere to the load of water and sediments existed only during the last 34 Ma starting from the time of the Kuril arc formation, which absorbed the main part of the regional compression stresses. The initial heat flux in the basin in the second version was assumed notably lower than in the first one. Of course, in both versions of the basin development, modern temperature distributions and OM maturity with depth (the values of the reflecting property of vitrinite, R o , %) calculated in the model were close to the values measured in the regions of the basin considered here and in the neighboring regions (Fig. 2). The main modeling results of the thermal history of the basin and variation in the OM maturity are shown in Fig. 3 for the first version of the basin development, which assumed an isostatic response of the lithosphere to the load of water and sediments and consequently high heat flux (approximately 160 mW/m 2 ) in the beginning of the Late Cretaceous. In such a model, intense subsidence of isotherms in the Pre-Cenozioc period of the basin development (Fig. 3) is determined

Erosion-driven uplift of the modern Central Alps

We present a compilation of data of modern tectono-geomorphic processes in the Central European Alps which suggest that observed rock uplift is a response to climate-driven denudation. This interpretation is predominantly based on the recent quantification of basin-averaged Late Holocene denudation rates that are so similar to the pattern and rates of rock uplift rates as determined by geodetic leveling. Furthermore, a GPS data-based synthesis of Adriatic microplate kinematics suggests that the Central Alps are currently not in a state of active convergence. Finally, we illustrate that the Central Alps have acted as a closed system for Holocene redistribution of sediment in which the peri-Alpine lakes have operated as a sink for the erosional products of the inner Central Alps. While various hypotheses have been put forward to explain Central Alpine rock uplift ( e.g. lithospheric forcing by convergence, mantle processes, or ice melting) we show with an elastic model of lithospheric deformation, that the correlation between erosion and rock uplift rates reflects a positive feedback between denudation and the associated isostatic response to unloading. Thus, erosion does not passively respond to advection of crustal material as might be the case in actively converging orogens. Rather, we suggest that the geomorphic response of the Alpine topography to glacial and fluvial erosion and the resulting disequilibrium for modern channelized and associated hillslope processes explains much of the pattern of modern denudation and hence rock uplift. Therefore, in a non-convergent orogen such as the Central European Alps, the observed vertical rock uplift is primarily a consequence of passive unloading due to erosion.

Dynamic cycles, ice streams and their impact on the extent, chronology and deglaciation of the British–Irish ice sheet

We present results from a suite of forward transient numerical modelling experiments of the British and Irish Ice Sheet (BIIS), consisting of Scottish, Welsh and Irish accumulation centres, spanning the last Glacial period from 38 to 10 ka BP. The 3D thermomechanical model employed uses higher-order physics to solve longitudinal (membrane) stresses and to reproduce grounding-line dynamics. Surface mass balance is derived using a distributed degree-day calculation based on a reference climatology from mean (1961–1990) precipitation and temperature patterns. The model is perturbed from this reference state by a scaled NGRIP oxygen isotope curve and the SPECMAP sea-level reconstruction. Isostatic response to ice loading is computed using an elastic lithosphere/relaxed asthenosphere scheme. A suite of 350 simulations were designed to explore the parameter space of model uncertainties and sensitivities, to yield a subset of experiments that showed close correspondence to offshore and onshore ice-directional indicators, broad BIIS chronology, and the relative sea-level record. Three of these simulations are described in further detail and indicate that the separate ice centres of the modelled BIIS complex are dynamically interdependent during the build up to maximum conditions, but remain largely independent throughout much of the simulation. The modelled BIIS is extremely dynamic, drained mainly by a number of transient but recurrent ice streams which dynamically switch and fluctuate in extent and intensity on a centennial time-scale. A series of binge/purge, advance/retreat, cycles are identified which correspond to alternating periods of relatively cold-based ice, (associated with a high aspect ratio and net growth), and wet-based ice with a lower aspect ratio, characterised by streaming. The timing and dynamics of these events are determined through a combination of basal thermomechanical switching spatially propagated and amplified through longitudinal coupling, but are modulated and phase-lagged to the oscillations within the NGRIP record of climate forcing. Phases of predominant streaming activity coincide with periods of maximum ice extent and are triggered by abrupt transitions from a cold to relatively warm climate, resulting in major iceberg/melt discharge events into the North Sea and Atlantic Ocean. The broad chronology of the modelled BIIS indicates a maximum extent at ∼20 ka, with fast-flowing ice across its western and northern sectors that extended to the continental shelf edge. Fast-flowing streams also dominate the Irish Sea and North Sea Basin sectors and impinge onto SW England and East Anglia. From ∼19 ka BP deglaciation is achieved in less than 2000 years, discharging the freshwater equivalent of ∼2 m global sea-level rise. A much reduced ice sheet centred on Scotland undergoes subsequent retrenchment and a series of advance/retreat cycles into the North Sea Basin from 17 ka onwards, culminating in a sustained Younger Dryas event from 13 to 11.5 ka BP. Modelled ice cover is persistent across the Western and Central Highlands until the last remnant glaciers disappear around 10.5 ka BP.

Ophiolites in ocean–continent transitions: From the Steinmann Trinity to sea-floor spreading

Before the theory of plate tectonics took hold, there was no coherent model for ocean–continent transitions that included both extant continental margins and fragmentary ancient examples preserved in orogenic belts. Indeed, during the early 1900, two strands of thought developed, one relying on the antiquity and permanence of continents and oceans, advocated by the mainstream of the scientific community and one following mobilist concepts derived from Wegener's hypothesis (1915) of continental drift. As an illustration of the prevailing North-American view, the different composition and thickness of continental and oceanic crust and the resulting isostatic response showed “how improbable it would be to suppose that a continent could founder or go to oceanic depth or that ocean floor at ± 3000 fathoms could ever have been a stable land area since the birth of the oceans” [H.H. Hess, Trans. R. Soc. London, A 222 (1954) 341–348]. Because of the perceived permanence of oceans and continents, mountain chains were thought to originate from narrow, elongated, unstable belts, the geosynclines, circling the continents or following “zones of crustal weakness” within them, from which geanticlines and finally mountain belts would develop. This teleological concept, whereby a geosyncline would inevitably evolve into a mountain chain, dominated geological interpretations of orogenic belts for several decades in the mid-twentieth century. However, the concept of permanence of oceans and continents and the concept of the geosyncline had already met with the critiques of Suess and others. As early as 1905, Steinmann considered the association of peridotite, “diabase” (basalt/dolerite) and radiolarite (a typical ocean–continent transition assemblage), present in the Alps and Apennines, as characteristic of the deep-ocean floor and Bailey (1936) placed Steinmann's interpretation into the context of continental drift and orogeny. Indeed, in both authors’ writings, the concept of ophiolites as ocean crust is apparent. Between 1920 and 1930, the stage was thus potentially set for modern mobilist concepts that were, however, to prove attractive to only a small circle of Alpine and peri-Gondwanian geologists. After the Second World War, the 1950s saw the rapid progress of the geophysical and geological exploration of oceans and continental margins that provided the data for a reevaluation of the geosynclinal concept. Actualistic models now equated the former preorogenic miogeosyncline of Stille (1940) and Kay (1951) with passive continental margins [C.L. Drake, M. Ewing, G.H. Sutton, Continental margin and geosynclines: the east coast of North America, north of Cape Hatteras, in: L. Ahrens, et al. (Eds.), Physics and Chemistry of the Earth 3, Pergamon Press, London, 1959, pp. 110–189], the (American version of the) eugeosyncline and its igneous rocks with “collapsing continental rises” [R.S. Dietz, J. Geol. 71 (1963) 314–333] and the ophiolites, the Steinmann Trinity, of the (European) eugeosyncline with fragments of oceanic lithosphere [H.H. Hess, History of ocean basins, in: Petrologic Studies: a Volume to Honor A.F. Buddington, Geol. Soc. Am., New York. 1962, pp. 599–620]. The concept of sea-floor spreading [H.H. Hess, History of ocean basins, in: Petrologic Studies: a Volume to Honor A.F. Buddington, Geol. Soc. Am., New York. 1962, pp. 599–620; H.H. Hess, Mid-oceanic ridges and tectonics of the sea-floor, in: W.F. Whittard, R. Bradshaw (Eds), Submarine Geology and Geophysics, Colston Papers 17, Butterworths, London, 1965, pp. 317–333] finally eliminated the weaknesses in Wegener's hypothesis and, with the coming of the “annus mirabilis” of 1968, the concept of the geosyncline could be laid to rest. Ocean–continent transitions of modern oceans, as revealed by seismology and deep-sea drilling, could now be compared with the remnants of their ancient counterparts preserved in the Alps and elsewhere.

Regional isostatic response to Messinian Salinity Crisis events

The salinity crisis of the Mediterranean during Messinian time was one of the most dramatic episodes of oceanic change of the past 20 or so million years, resulting in the deposition of kilometer thick evaporitic sequences. A large and rapid drawdown of the Mediterranean water level caused erosion and deposition of non-marine sediments in a large ‘Lago Mare’ basin. Both the surface loading by the Lower Messinian evaporites, and the removal of the water load resulted in isostatic/flexural rebound that significantly affected river canyons and topographic slopes. We use flexure models to quantitatively predict possible signatures of these events, and verify these expectations at well-studied margins. The highly irregular shape of the reconstructed basin calls for a three-dimensional model. Near basin margins, plate-bending effects are most pronounced which is why flexure is particularly important for a relatively narrow basin like the Mediterranean. We focus on one specific sea level scenario for the Messinian Salinity Crisis, where most of the evaporite load was deposited during a sea level highstand, followed by a rapid desiccation. Evaporite loading at current sea level is expected to cause subsidence of the deep basins by hundreds of meters and simultaneous uplift of continental parts of the margins. Differential uplift may lead to significant slope angle changes and thus gravity flows. The relative scarcity of Lower Evaporite sequences along the margins may be a result of these phenomena. Normal faulting of Lower Evaporite and older sediments and rocks is expected on the margins. Desiccation enhances erosion of the freshly exposed continental shelf and slope. Subsidence and riverbed sedimentation occurs on the continental margins, and significant uplift towards the basin center. Reverse faulting is predicted at the margins. Finally, regional isostatic uplift following Zanclean flooding is predicted to destabilize margin slope deposits, and to cause marginal uplift, river down-cutting, and normal faulting.

Channel profiles around Himalayan river anticlines: Constraints on their formation from digital elevation model analysis

We present a comparison between measured and numerically modeled channel profiles of rivers in two important drainage basins of Central Nepal: the Kali‐Gandaki and the Arun drainage basins. Modeled channel profiles are based on a simple stream power approach using best fit exponents defining the nonlinearities in the relative contributions of local channel gradient and water flux to erosion rate. Our analysis of the stream power in the whole river network confirms the work of other authors that a 50‐ to 80‐km‐wide zone, roughly corresponding to the High Himalayan topography, is subjected to rapid rock uplift. We suggest a model where the uplift of this zone is driven by erosion and isostatic response, so that centers of maximum uplift are located within the main channels of the north–south draining rivers. We also suggest that the rate of uplift slows down with increasing distance to the main channels. Such a spatial distribution of the uplift leads ultimately to the formation of river anticlines as observed along all major Himalayan rivers. We propose that the formation of river anticlines along south draining Himalayan rivers was accelerated by a sudden increase of the drainage area and discharge when the rivers captured orogen‐parallel drainages on the north side of the range. This may follow successive headward cutting into the Tibetan Plateau. The model is confirmed by differences between main channels and east–west running tributaries. Time‐dependent numerical models predict that capture events cause strongly elevated erosion rates in the main channel.

Flexural isostatic response of the Alps to increased Quaternary erosion recorded by foreland basin remnants, SE France

AbstractWe test the hypothesis that flexural isostatic compensation of the mass removed by enhanced Quaternary erosion is responsible for uplift of the Western European Alps and their forelands. We use two well‐preserved and well‐dated (1.8 Ma) abandonment surfaces of foreland basin remnants in SE France (the Chambaran and Valensole plateaux) as passive benchmarks for tilting of the foreland. Estimating their initial slope from morphometric scaling relationships, we determine bulk post‐depositional tilting of 0.5–0.8% for these surfaces. The calculated isostatic response of the Alpine lithosphere to erosional unloading, using the method recently proposed by Champagnac et al.[Geology 35 (2007) 195–198], yields a predicted tilting of 0.3–0.4% in the considered areas, explaining approximately half of the determined post‐depositional tilting. Such long‐term deformation being insensitive to cyclic loading/unloading because of glaciations, we suspect the other half to be related to as yet undetermined long‐wavelength and long‐lived tectonic process(es).

Fast and partitioned postglacial rebound of southwestern Iceland

Located both on the Mid‐Atlantic Ridge and above a mantle plume, Iceland is subject to horizontal and vertical motions. Many studies described these deformations in terms of rifting episodes that have combined both extensional tectonics and magmatism. However, few studies have described the glacio‐isostatic response induced by the retreat of the Weichselian ice cap. The melting of this ice cap induced a postglacial rebound for the whole of Iceland that may be controlled by the geodynamic setting and the rheological layering of the lithosphere. This study is devoted to (1) understanding the Holocene rebound on the southwestern coast and (2) estimating the asthenosphere viscosity and depth beneath Iceland. Two stages of holocene evolution were determined by means of GPS profiles, morphological observations, and data compilation. The first stage corresponds to a vertical uplift of 67.5 to 157.5 m. It started at 10,000 years BP and ended at 8500 years BP implying uplift rates between 4.5 and 10.5 cm/a. It was a quick isostatic response to the fast ice retreat. The second stage had vertical motion of tens of meters with a probable tectonic origin and started at 8500 years BP. The uplift rate is 1 to 2 orders of magnitude slower than the one during the first stage. Uplift partitioning during the first stage was controlled by the thermal state of the lithosphere, the highest geothermal flux inducing the maximum uplift rates. The relaxation time for uplift provides a viscosity estimate of 5.4–5.8 × 1019 Pa s for the asthenosphere. This value is similar to those determined for glacial areas in different continental contexts. However, the flexural wavelength indicates a shallower asthenosphere than that occurring in continental domains. Therefore this study highlights a coupling between the thermal structure of the Icelandic asthenosphere and the glacial rebound.

Fluvial dissection, isostatic uplift, and geomorphological evolution of volcanic islands (Gran Canaria, Canary Islands, Spain)

Digital analysis of torrential gullies (‘barrancos’) deeply incised into the volcanic Island of Gran Canaria (Canary Islands) allows us to extract the longitudinal profiles and pre-incision surfaces for individual basins, from which morphometric parameters (length, elevation, area, slope) have been calculated. Other derived parameters, such as ridgeline profiles, maximum incision values, volume removed by fluvial erosion, geophysical relief and isostatic uplift, have also been computed. Based on K/Ar ages for the island, well-constrained incision-uplift rates have been calculated by means of the combination of different methodological approaches commonly used in orogens and large mountain ranges. The geomorphological and morphometric analyses reveal that the island is clearly divided into four environmental quadrants determined by the combination of a couple of key-factors: the age of the volcanic surfaces and the climatic conditions. These factors determine a young sector covered with Plio-Quaternary platform-forming lavas (finished at 1.9–1.5 Ma) evolving under contrasting wet (NE) to dry (SE) climates, and an older sector, conserving the residual surfaces of the Miocene shield building (14.5–8.7 Ma) at the ridgelines, also subjected to wet (NW) and dry (SW) climates. Incision is related to the age zonation of the island. Maximum incisions (< 1200 m) are logically recorded in the older SW sector of the island, but incision rates are directly related to the climatic zonation, with maximum mean values in the wet Northern quadrants (0.18–0.12 mm/yr). The evaluation of the material removed by fluvial erosion for individual basins allows us to assess the consequent theoretical isostatic response in the different sectors of the island. The obtained uplift rates indicate that water availability (by drainage area and elevation) is a relevant controlling factor: the records from the wet Northern sectors show uplift values of between 0.09 and 0.03 mm/yr, whereas in the drier Southern sectors, the maximum values are under 0.015 mm/yr. Mean uplift rates obtained in this study are within the range of those inferred from stratigraphical markers, as is the case for horizons of raised Pliocene pillow-lavas (c.a. 4 Ma) uplifted between 46 and 143 m (0.014–0.024 mm/yr). The estimation of the bulk uplift promoted by fluvial unloading is of + 143 m for the entire island, and of + 71 m for the wet NE sector. These data explain 73% to 99% of the maximum uplift recorded by the raised Pliocene sea-level markers. This reflects that erosional unloading is a critical control factor in the uplift of the oceanic island, but is not capable of explaining the full recorded uplift in Gran Canaria. Additional sources of uplift, such as gravitational unloading, lithospheric flexure induced by adjacent islands, and/or volcanic underplating, are required. The theoretical onset of lithospheric bulging beneath Gran Canaria, as exerted by Tenerife, promoted a broad westwards tilting of the former from 3.8–3.5 Ma ago. This overall tilting accelerated fluvial incision, erosional unloading, and, therefore, the sustained differential uplift on the Eastern slope of the island over its last erosional stage. Considering mean uplift rates for the East and West sectors, Eastern values (0.024 mm/yr) are double than those in the West (0.011 mm/yr), supporting the role of lithospheric flexure of adjacent islands as an additional source of uplift. Complex feedback between fluvial unloading, differential uplift, orographic effect, lithospheric flexure, and volcanic underplating, seems to control the geomorphological development of hot-spot volcanic islands, after the gravitational collapse of stratovolcanos during their rejuvenation stage.

Isostatic response of the Laccadive Ridge from admittance analysis of gravity and bathymetry data

The Laccadive Ridge (L-R), trending roughly parallel to the west coast of India, is an intriguing segment of the northernmost Chagos-Laccadive Ridge (C-L-R) system. Although crustal nature and isostatic response of the southern C-L-R is well known, there are no similar studies on the L-R. In the present study, the isostatic response of the lithosphere beneath the L-R is estimated so as to characterize its crustal nature, total crustal as well as effective elastic plate thickness and mode of compensation. Twelve gravity and bathymetry profiles across the ridge were analyzed using linear transfer function and forward model techniques. The observed admittance function within the diagnostic waveband of 250 < λ > 80 km (0.025 < k > 0.080 km−1) fits well with (i) the Airy model whose average crustal thickness (Tc) and density are 17 ± 2 km and 2.7 × 103 kg m−3, respectively, and (ii) the thin plate flexure model of isostasy with an effective elastic plate thickness (Te) of 2–3 km. The estimated average crustal thickness and density are in good agreement with published seismic refraction results over the ridge. The results of the present study support an Airy model of isostasy for the L-R. The low Te value, in view of other published results in the study area, suggests stretched and loaded continental lithosphere of the L-R during the evolution of the western continental margin of India.

Pliocene-quaternary orogeny in the Central Tien Shan

Formation of mountains of the Central Tien Shan is usually explained by the isostatic response of the crust to its shortening caused by the India-Eurasia collision. The rise of the region in the period from Oligocene to Late Pliocene (2 myr ago) reached ∼700 m on average, which corresponds to the isostatic response. For the last 2 myr (Late Pliocene–Quaternary), the rate of rise increased by an order of magnitude. This is proved by the coarsening of Cenozoic molasse up the sections, acceleration of cutting of drainage systems into ridges, and formation of new ridges within basins. In the Quaternary, most of intermontane basins underwent uplifting, though not so intense. The average rate of lateral crustal shortening increased ∼2–2.5 times only, and the contribution of this process to the Late Pliocene-Quaternary orogeny was no more than 10%. The acceleration of rise was caused mainly by the convective replacement of the mantle lithosphere by the less dense asthenosphere. This was due to the quick softening of the mantle lithosphere as a result of the infiltration of active fluid from the lower mantle. Such accelerations of crustal uplifting took place in the Pliocene and Quaternary in many continental regions. This evidences that mantle processes, first of all, the full or partial replacement of the mantle lithosphere by the asthenosphere, played an important role in the formation of mountains.

Quantifying transient mantle convective uplift: An example from the Faroe‐Shetland basin

The Icelandic mantle plume generated maximum uplift in early Paleogene times. The Faroe‐Shetland basin, which fringes the North Atlantic margin of Europe, was close to the center of early Paleogene Icelandic plume activity. Three‐dimensional seismic reflection data from the Faroe‐Shetland basin reveal sedimentary geometries that allow a phase of transient uplift to be accurately reconstructed and quantified. Close to the Paleocene/Eocene boundary (circa 56 Ma), rapid uplift resulted in fluvial incision into marine sediments. This unconformity was buried by nonmarine sediments, recording the decay of transient uplift. Relief on the unconformity was ∼550 m, constraining the minimum amount of surface uplift. Some 60 m of this uplift can be attributed to the isostatic response to erosional unloading. Tectonic uplift of over 490 m peaked and decayed within 3 Ma. Rates of water‐loaded tectonic subsidence following peak uplift are several times greater than maximum expected postrift subsidence rates. The amplitude and duration of this transient effect is best explained by a mantle convective phenomenon. We suggest that a region of hot plume material flowed laterally beneath the lithosphere, producing transient uplift which decayed when plume material was advected farther away. Our analysis suggests that under certain circumstances, stratigraphic records can yield valuable quantitative information about aspects of mantle convective circulation.

Quaternary erosion-induced isostatic rebound in the western Alps: COMMENT: COMMENT

[Champagnac et al. (2007)][1] deduced that rock uplift in the western Alps is occurring mainly as the isostatic response to erosion and is thus being largely driven by climate rather than by other potential causes such as plate motions. They reported that spatial average erosion rates within the

Vertical motions of the fjord regions of central East Greenland: Impact of glacial erosion, deposition, and isostasy

Research Article| July 01, 2008 Vertical motions of the fjord regions of central East Greenland: Impact of glacial erosion, deposition, and isostasy Sergei Medvedev; Sergei Medvedev 1Physics of Geological Processes, University of Oslo, 0316 Oslo, Norway Search for other works by this author on: GSW Google Scholar Ebbe H. Hartz; Ebbe H. Hartz 2Physics of Geological Processes, University of Oslo, 0316 Oslo, Norway, and Aker Exploration, Haakon VII'gt 9, 4005 Stavanger, Norway Search for other works by this author on: GSW Google Scholar Yuri Y. Podladchikov Yuri Y. Podladchikov 3Physics of Geological Processes, University of Oslo, 0316 Oslo, Norway Search for other works by this author on: GSW Google Scholar Author and Article Information Sergei Medvedev 1Physics of Geological Processes, University of Oslo, 0316 Oslo, Norway Ebbe H. Hartz 2Physics of Geological Processes, University of Oslo, 0316 Oslo, Norway, and Aker Exploration, Haakon VII'gt 9, 4005 Stavanger, Norway Yuri Y. Podladchikov 3Physics of Geological Processes, University of Oslo, 0316 Oslo, Norway Publisher: Geological Society of America Received: 20 Nov 2007 Revision Received: 14 Mar 2008 Accepted: 22 Mar 2008 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2008 Geological Society of America Geology (2008) 36 (7): 539–542. https://doi.org/10.1130/G24638A.1 Article history Received: 20 Nov 2007 Revision Received: 14 Mar 2008 Accepted: 22 Mar 2008 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Sergei Medvedev, Ebbe H. Hartz, Yuri Y. Podladchikov; Vertical motions of the fjord regions of central East Greenland: Impact of glacial erosion, deposition, and isostasy. Geology 2008;; 36 (7): 539–542. doi: https://doi.org/10.1130/G24638A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The origin of the vertical motions of East Greenland is a longstanding enigma. The area is characterized by a mountain chain >1000 km long, wherein the high peaks (2.5–3.7 km elevation) reside above a relatively thin crust (30–35 km). These mountains contain Mesozoic marine sediments uplifted to 1.2 km during the tectonic quietness of the middle to late Cenozoic. This uplifted area is cut by some of the world's biggest fjords. Scoresbysund fjord is as wide as 60 km, and cuts 400 km into the land and more than 4 km down from the peaks of the region. We test the potential amount of regional uplift and subsidence caused by the fjords' incision and associated deposition of sediments offshore. These tests are based on modeling the erosion process backward in time. The fjords are filled back to the summit surface while similar weights of sediments are removed from the shelf. The model considers the isostatic response of the lithosphere due to the loading and unloading of bedrock, sediments, water, and ice. Our estimates show that an average of almost 1.2 km of bedrock was eroded in the region from the middle Cenozoic summit surface. Most of the erosional products were deposited on the continental shelf outside the mouth of the fjords. Our calculations demonstrate that rocks in the central Fjord Mountains may be uplifted as much as 1.1 km due to the erosional unloading and flexural isostatic effects. Thus these effects should present a main part of the mechanisms responsible for the Cenozoic uplift in central East Greenland. Because the North Atlantic is rimmed by young glacially carved mountain chains, the model may be applicable to other parts of the area. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Norse in Greenland and late Holocene sea-level change

ABSTRACTNorse immigrants from Europe settled in southern Greenland in around AD 985 and managed to create a farming community during the Medieval Warm Period. The Norse vanished after approximately 500 years of existence in Greenland leaving no documentary evidence concerning why their culture foundered. The flooding of fertile grassland caused by late Holocene sea-level changes may be one of the factors that affected the Norse community. Holocene sea-level changes in Greenland are closely connected with the isostatic response of the Earth's crust to the behaviour of the Greenlandic ice sheet. An early Holocene regressive phase in south and west Greenland was reversed during the middle Holocene, and evidence is found for transgression and drowning of early-middle Holocene coast lines. This drowning started between 8 and 7ka BP in southern Greenland and continued during the Norse era to the present. An average late Holocene sea level rise in the order of 2–3 m/1000 years may be one of the factors that negatively affected the life of the Norse Greenlanders, and combined with other both socio-economic and environmental problems, such as increasing wind and sea ice expansion at the transition to the Little Ice Age, may eventually have led to the end of the Norse culture in Greenland.

Quaternary glaciation history and glaciology of Jakobshavn Isbræ and the Disko Bugt region, West Greenland: a review

The Disko Bugt region in central West Greenland is characterised by permanent ice streams, of which Jakobshavn Isbræ is by far the most important. The first thorough studies on the glaciology of the region were conducted over 150 years ago by H.J. Rink, who introduced the terms 'ice streams' and 'Inland Ice'. Rink's work inspired new field work, which has continued to the present, and the long series of observations are unique for an Arctic region.&#x0D; Cooling during the Cenozoic led to ice-sheet growth in Greenland. A number of interglacial occurrences have been reported from the Disko Bugt region, and during the penultimate glacial stage, the Greenland ice-sheet margin extended to the shelf break. During the last glacial maximum, the ice margin probably extended only to the inner part of the banks on the continental shelf, and large floating glaciers may have been present at this time. During the Younger Dryas cold period, the ice margin may have been located at a marked basalt escarpment west of Disko Bugt.&#x0D; Disko Bugt was deglaciated rapidly in the early Holocene, around 10 500–10 000 years before present (10.5–10 ka B.P.), but when the ice margin reached the eastern shore of the bay, recession paused, and major moraine systems were formed. With renewed recession, the present ice-margin position was attained around 8–6 ka B.P., and by c. 5 ka B.P. the ice margin was located east of its present position. The subsequent Neoglacial readvance generally reached a maximum during the Little Ice Age, around AD 1850. This was followed by recession that has continued to the present day.&#x0D; The relative sea-level history shows a rapid sea-level fall in the early Holocene, and a slow rise in the late Holocene. This development mainly reflects a direct isostatic response to the ice-margin history.&#x0D; Jakobshavn Isbræ is the main outlet from the Greenland ice sheet. It drains c. 6.5% of the present Inland Ice, and produces c. 35–50 km3 of icebergs per year, corresponding to more than 10% of the total output of icebergs from the Inland Ice. The velocity of the central part of the ice stream at the front has been around 7 km/year since records began, but has nearly doubled in recent years. Other calf-ice producing glacier outlets in Disko Bugt produce c. 18 km3 per year. The large calf-ice production of Jakobshavn Isbræ may have been initiated at about 8 ka B.P. when the glacier front receded from the iceberg bank (Isfjeldsbanken) near Ilulissat. Ice streams in inner and outer Egedesminde Dyb may have been active during the early Holocene and during the last glacial maximum.

Sedimentation record in the Konkan–Kerala Basin: implications for the evolution of the Western Ghats and the Western Indian passive margin

ABSTRACTThe Konkan and Kerala Basins constitute a major depocentre for sediment from the onshore hinterland of Western India and as such provide a valuable record of the timing and magnitude of Cenozoic denudation along the continental margin. This paper presents an analysis of sedimentation in the Konkan–Kerala Basin, coupled with a mass balance study, and numerical modelling of flexural responses to onshore denudational unloading and offshore sediment loading in order to test competing conceptual models for the development of high‐elevation passive margins. The Konkan–Kerala Basin contains an estimated 109 000 km3 of Cenozoic clastic sediment, a volume difficult to reconcile with the denudation of a downwarped rift flank onshore, and more consistent with denudation of an elevated rift flank. We infer from modelling of the isostatic response of the lithosphere to sediment loading offshore and denudation onshore infer that flexure is an important component in the development of the Western Indian Margin. There is evidence for two major pulses in sedimentation: an early phase in the Palaeocene, and a second beginning in the Pliocene. The Palaeocene increase in sedimentation can be interpreted in terms of a denudational response to the rifting between India and the Seychelles, whereas the mechanism responsible for the Pliocene pulse is more enigmatic.

Late Holocene sea-level change around Newfoundland

Differential sea-level change in formerly glaciated areas is predicted owing to variability in extent and timing of glacial coverage. Newfoundland is situated close to the margin of the former Laurentide ice sheet, and the orientation of the shoreline affords the opportunity to investigate variable rates and magnitudes of sea-level change. Analysis of salt-marsh records at four sites around the island yields late Holocene sea-level trends. These trends indicate differential sea-level change in recent millennia. A north–south geographic trend reflects submergence in the south, very slow sea-level rise in the northeast, and a recent transition from falling to rising sea-level at the base of the Northern Peninsula. This variability is best explained as a continued isostatic response to deglaciation.

Late Cenozoic uplift of the Amanos Mountains and incision of the Middle Ceyhan river gorge, southern Turkey; Ar–Ar dating of the Düziçi basalt

Using the Ar–Ar technique, we have obtained the first numerical dates for the Pleistocene volcanism along the valley of the River Ceyhan in the Düziçi area of southern Turkey, in the western foothills of the Amanos Mountains. Our six dates indicate a single abrupt episode of volcanism at ∼ 270 ka. We have identified a staircase of 7 fluvial terraces, at altitudes of up to ∼ 230 m above the present level of the Ceyhan. Using the disposition of the basalt as an age constraint, we assign these terraces to cold-climate stages between marine oxygen isotope stages 16 and 2, indicating rates of fluvial incision, equated to surface uplift, that increase upstream through the western foothills of this mountain range at between 0.25 and 0.4 mm a − 1 . Extrapolation of these uplift rates into the axis of the range suggests that the entire ∼ 2300 m of present-day relief has developed since the Mid-Pliocene, a view that we confirm using numerical modelling. Since ∼ 3.7 Ma the Amanos Mountains have formed a transpressive stepover along the northern part of the Dead Sea Fault Zone, where crustal shortening is required by the geometry. Using a physics-based technique, we have modelled the overall isostatic response to the combination of processes occurring, including crustal thickening caused by the shortening, erosion caused by orographic precipitation, and the resulting outward flow of mobile lower-crustal material, in order to predict the resulting history of surface uplift. This modelling suggests that the effective viscosity of the lower crust in this region is in the range ∼ 1–2 × 10 19 Pa s, consistent with a Moho temperature of ∼ 590 ± 10 °C, the latter value being in agreement with heat flow data. This modelling shows that the nature of the active crustal deformation is now understood, to first order at least, in this key locality within the boundary zone between the African and Arabian plates, the structure and geomorphology of which have been fundamentally misunderstood in the past.

Gyre‐scale atmospheric pressure variations and their relation to 19th and 20th century sea level rise

Most of the long tide gauge records in the North Atlantic and North Pacific commonly used to estimate global sea level rise and acceleration display a marked difference in behavior in the late 1800's – early 1900's compared to the latter half of the 20th century. The rates of sea level rise tend to be lower in the 19th compared to 20th century. We show this behavior may be related to long‐term, gyre‐scale surface pressure variations similar to those associated with the Northern Annular Mode. As sea level pressure increases (decreases) at decadal and longer timescales at the centers of the subtropical atmospheric gyres, sea level trends along the eastern margins in each ocean basin decrease (increase). This is not an isostatic response; the scaling between local surface pressure and sea level at interannual and longer timescales is 3 to 6 times greater than expected by that mechanism. Rather, it appears to be the result of large, possibly gyre‐scale changes in ocean circulation. Some evidence is also presented indicating slow, ∼2 cm/sec, westward propagation of sea level changes in the Atlantic from the west coast of Europe to the east coast of the U.S. which produce the decadal variability seen there.