Onshore Uplift Research Articles

Isostatic compensation of Quaternary vertical crustal motions: coupling between uplift of Britain and subsidence beneath the North Sea

ABSTRACTNew data from offshore and onshore regions confirm the view that the Quaternary uplift of Britain and subsidence beneath the North Sea are coupled, mediated by lower‐crustal flow induced by the lateral pressure gradient caused by climate‐driven surface processes. Most measured onshore uplift rates, in both upland and lowland localities, are only applicable since the Mid‐Pleistocene Revolution; beforehand, generally lower rates prevailed. Earlier (?Oligocene–Pliocene) phases of uplift and subsidence had typically even lower rates; furthermore, the western margin of the North Sea depocentre was ∼200 km farther from the modern coastline of north‐east England. In the earliest Cenozoic, complex isostatic adjustments occurred in response to the magmatism associated with the Iceland mantle plume; the contemporaneous land surface (relative to the present‐day rock column) in northern England was typically ∼1500 m OD. However, this activity died out during the Palaeogene; the main effect of these events on the modern isostatic configuration was via the emplacement of thick mafic underplating, which now constricts the mobile lower‐crustal layer, explaining the Late Cenozoic ultra‐stability of Ireland. The evidence enables interpretations of the present‐day topography of Britain as static, caused by the Iceland mantle plume, or actively developing in response to plate motions or plate boundary forces, to be excluded.

Patterns of Quaternary uplift of the Corinth rift southern border (N Peloponnese, Greece) revealed by fluvial landscape morphometry

The Rift of Corinth is a world-class example of young active rifting and, as such, is an ideal natural laboratory of continental extension. However, though much investigated for two decades, several aspects of the mechanisms at work are still poorly understood. The aim of this paper is a detailed morphometric study of the fluvial landscape response to the tectonic uplift of the rift southern shoulder in order to reconstruct the rift's Quaternary evolution, with special attention to timing, location, and intensity of uplift episodes. Based on the use of a large set of catchment and long profile metrics complemented by the new R/SR integrative approach of the regional drainage network, we identified three distinct episodes of uplift of the northern Peloponnese coastal tract, of which the intermediate one, dated around 0.35–0.4Ma, is only recorded in the topography of the central part of the rift shoulder, and the youngest one appears to have propagated from east to west over the last 10–20ka. While net uplift remained minimum in the eastern part of the study area during the whole Quaternary, it shows a clear maximum in the central part of the rift shoulder since 0.4Ma and an eastward shift of this maximum in recent times. Maximum uplift rates calculated from the morphometric data are of >1.05 and 2–5mmyear−1 for, the mid-Middle Pleistocene and Holocene uplift episodes, respectively. The morphometric evidence reveals an onshore uplift history remarkably consistent with the rift evolution reconstructed from other data sets. In the long term, it shows a stable pattern of maximum activity in the central part of the rift, confirming previous conclusions about the absence of rift propagation. In the short term, it sheds light on a possible E–W migration of the zone of recent uplift, suggesting that in the near future fault activity and seismic hazard might concentrate in the Heliki–Aegion area, at the western tip of this uplift wave.

Carbonate platform growth and demise offshore Central Vietnam: Effects of Early Miocene transgression and subsequent onshore uplift

Miocene carbonate platforms cover a large part of the Central Vietnamese South China Sea margin. Early carbonate deposition took place on two regional platforms separated by a narrow depression developed along the trace of the East Vietnam Boundary Fault Zone. West of the East Vietnam Boundary Fault Zone, the Tuy Hoa Carbonate Platform fringes the continental margin between Da Nang and Nha Trang. Here, platform growth initiated during the Early Miocene and continued until Middle Miocene time when regional uplift led to subaerial exposure, termination of platform growth and karstification. East of the fault zone, the Triton Carbonate Platform was also initiated during the Early Miocene. Carbonate growth thrived during Early and part of Middle Miocene time and a thick, clean Lower and Middle Miocene carbonate succession cover the Triton Horst and the Qui Nhon Ridge. During the Middle Miocene, partial drowning resulted in the split-up of the Triton Carbonate Platform. Repeated partial drowning events throughout the Middle and Late Miocene resulted in westwards retreat of platform growth and eventual platform drowning and termination of carbonate deposition. Modern carbonate growth continues on isolated platforms hosting the Paracel Islands farther seawards. The onset of widespread carbonate deposition largely reflects the Early Miocene transgression of the area linked with early post-rift subsidence and the opening of the South China Sea. The mid-Neogene shift in carbonate deposition is interpreted as a consequence of regional uplift and denudation of central and south Indochina starting during Middle Miocene time when the Tuy Hoa Carbonate Platform became subaerially exposed. Stressed carbonate growth conditions on the Triton Carbonate Platform probably resulted from increased inorganic nutrient input derived from the uplifted mainland, possibly enhanced by deteriorated climatic conditions and rapid sea-level fluctuations promoting platform drowning.

Numerical modeling of the development of southeastern Red Sea continental margin

The Red Sea continental margin (RSCM) corresponds to a wide hinge zone between Red Sea and Arabian plate. This margin has been studied through geological and geophysical observations primarily in regard to the evolution of Red Sea rift. This margin is characterized by occurrence of thin sediments, significant onshore uplift, tectonic subsidence of the offshore sedimentary basin, active faulting and seismicity. Studies indicate that sedimentary sequences of the margin are deformed by faults and folds resulting from at least two phases of extension and a phase of uplift. During the two phases of extension due to regional plate stress the sequence was cut by set of extensional faults. While during the phase of uplift the sequence was deformed by folding and faulting. The present paper aims to clear the structural development of RSCM during these tectonic episodes, taken as particular tectonic event, by two-dimensional finite element modeling on plane strain condition. Elastic rheology is assumed for the oceanic, continental and transitional crust along with syntectonic deposits. Stress field, shear stress and fault distribution suggests that mantle plume weakened the crust following rifting due to regional stress and developed the margin. These results are well consistent with those from present seismicity, active faulting and neotectonic studies.

Tectonic and climatic control on growth and demise of the Phanh Rang Carbonate Platform offshore south Vietnam

AbstractThe Phan Rang Carbonate Platform located offshore south Vietnam covers more than 15 000 km2, making it one of the largest carbonate platforms in the South China Sea. Based on 2‐D seismic analysis, this paper outlines the platform evolution and analyzes the regional tectonic, climatic and oceanic factors that controlled the platform growth and demise. This study of the Phan Rang Carbonate Platform therefore provides an analogue to the regions late Neogene carbonates that form important targets for petroleum exploration. Platform growth initiated during the late middle Miocene along the open marine Vietnamese margin and continued throughout the late synrift to early postrift period of the area terminating around Pliocene time. During this period, the structural grain, local and regional tectonics as well as oceanographic effects exerted major controls on carbonate deposition. Optimal growth conditions existed during initial platform deposition and locally accumulation rates reached ca. 230 m Ma−1. Late Miocene regional uplift caused subaerial exposure that interrupted platform growth and caused intense karstification. A gradual reestablishment of marine conditions promoted renewed platform growth. However, carbonate production was stressed by increased terrigenous input caused by onshore uplift and by inorganic nutrification of the surface waters. Nutrification probably occurred in response to increased nutrient influx derived from onshore denudation, enhanced periodically by soil ravinement during transgression. The onset or intensification of summer upwelling along the southern platform margin occurred in response to the onshore uplift and most likely contributed to the nutrification. The deteriorated growth conditions and fast subsidence resulted in platform split‐up, backstepping and local drowning. Subsequently, isolated platforms nucleated on structural highs as transgression continued. The remaining platforms thrived for a period but eventually failed to keep pace with subsidence, backstepped and drowned. The longest surviving platform now crops out at the seafloor at ca. 500 m depth.

The transition from the continent to the ocean: a deeper view on the Norwegian margin

We present a regional, crustal-scale, 3D structural model of the Norwegian continental margin integrating sedimentary and crustal layers from the continental and the oceanic domain. The model includes six sedimentary units, underlain on the continental side by a thinned crystalline crust and a lower-crustal high-velocity body. In the oceanic domain, three crustal layers (2AB, 3A and 3B), thickened at the continent–ocean transition (COT), are modelled below the post-breakup deposits. Two major rift phases with different rift axes (Late Jurassic–Early Cretaceous and Late Cretaceous–Early Tertiary) have caused post-Jurassic subsidence and post-depositional deformation of the pre-Cretaceous units. The modelled COT suggests that the pre-breakup rifting event was related to differential stretching focused at the outer margin and that breakup took place in a ‘base-up’ magmatic process as a continuation of underplating. For the earlier rift event, stretching was distributed over the entire margin and led to accumulation of up to 12 km of Cretaceous deposits. The large sediment thickness of the Cretaceous units requires deep-water conditions and abundant sediment supply and thus coeval offshore subsidence and onshore uplift. All layers indicate a sinistral offset along the Jan Mayen Fracture Zone and its continentward continuation.

South Atlantic continental margins of Africa: A comparison of the tectonic vs climate interplay on the evolution of equatorial west Africa and SW Africa margins

Africa displays a variety of continental margin structures, tectonic styles and sedimentary records. The comparative review of two representative segments: the equatorial western Africa and the SW Africa margins, helps in analysing the main controlling factors on the development of these margins. Early Cretaceous active rifting south of the Walvis Ridge resulted in the formation of the SW Africa volcanic margin that displays thick and wide intermediate igneous crust, adjacent to a thick unstretched continental crust. The non-volcanic mode of rifting north of the Walvis ridge, led to the formation of the equatorial western Africa margin, characterised by a wide zone of crustal stretching and thinning, and thick, extensive, syn-rift basins. Contrasting lithologies of the early post-rift (salt vs shale) determined the style of gravitational deformation, whilst periods of activity of the decollements were controlled by sedimentation rates. Regressive erosion across the prominent shoulder uplift of SW Africa accounts for high clastic sedimentation rate during Late Cretaceous to Eocene, while dominant carbonate production on equatorial western Africa shelf suggests very little erosion of a low hinterland. The early Oligocene long-term climate change had contrasted response in both margins. Emplacement of the voluminous terrigenous Congo deep-sea fan reflects increased erosion in equatorial Africa, under the influence of wet climate, whereas establishment of an arid climate over SW Africa induced a drastic decrease of denudation rate, and thus reduced sedimentation on the margin. Neogene emplacement of the African superswell beneath southern Africa was responsible for renewed onshore uplift on both margins, but it accelerated erosion only in the Congo catchment, due to wetter climatic conditions. Neogene high sedimentation rate reactivated gravitational tectonics that had remained quiescent since late Cretaceous.

Fault-related fold growth and isostatic subsidence, California Channel Islands

In the Northern Channel Islands of California, preservation of paleo-shoreline features both as uplifted landforms onshore and subsided depositional features offshore has allowed precise measurement of late Quaternary fold growth relative to sea level and has helped to disentangle the relative contributions of tectonic forcing and isostatic response. We precisely measured several hundred points on uplifted Pleistocene coastal terraces of Santa Cruz and Anacapa islands, the elevations of which document anticlinal growth of the islands since at least 400 ka. We also interpreted a dense grid of seismic-reflection profiles that image a complex of eustatic lowstand deltas that record regional subsidence surrounding the uplifting structure. We combined onshore uplift with offshore subsidence measurements to produce a quantitative mass balance of structural growth since 400 ka. The data outlined above were then used to test the hypothesis that subsidence around the Northern Channel Islands represents the flexural isostatic response to tectonic loading of the islands. Calculations of cumulative uplifted mass and cumulative subsided mass across the structure closely match the values predicted for density-driven isostatic compensation and local flexural support, although the wavelength of flexure predicted for the study area suggests that an additional component of flexural support may extend distally 10s of kilometers into the surrounding offshore basins. In the Northern Channel Islands, isostatic subsidence is explicitly manifested because erosional mass removal is small, because the crust here is relatively thin and weak, and most importantly because sea level provides an absolute datum against which to measure vertical deformation. An explicit treatment of the interplay between crustal thickening, erosional denudation, surface uplift, and isostatic response helps to illustrate the conditions under which isostatic subsidence may or may not be a significant process. Where surface denudation rates approach the rate of rock uplift, for example, isostatic subsidence will approach zero. In contrast, failure to recognize isostatic subsidence in earthquake-hazard studies can lead to order-of-magnitude errors in slip-rate and hazard estimates.

Neotectonics seismicity in the south-eastern Beaufort Sea polar continental margin of north-western Canada

A two-dimensional numerical model is presented that links inferences about recent rates of onshore uplift in the area of the Mackenzie Delta to rapid subsidence of the offshore Beaufort–Mackenzie Basin and the present-day upper lithospheric structure across the continent-ocean transition in this region of north-western Canada. The model results comprise a self-consistent set of internal displacements (velocities) and boundary conditions which predict a concentration of shear stress in the oceanic crust (and overlying sedimentary basin) in a region of the south-eastern Beaufort Sea featuring a concentration of earthquake epicentres. The mode of faulting implied by the calculated stress tensors is normal, consistent with a published fault-plane solution and in-situ stress measurements suggesting the axis of maximum compression to be perpendicular to the model cross-section. The results indicate that the uplift⧹subsidence pattern and seismicity are tectonically related but do not support models invoking tectonic compression, which have been previously proposed here and on other seismically active continental margins with rapidly subsiding sedimentary basins. Rather, a key role for dynamic processes in the upper mantle (including the asthenosphere), related to the continent-ocean lithosphere transition at passive continental margins, is implied.

Recent crustal deformation in the Antofagasta region(northern Chile) and the subduction process

New neotectonic observations, along with a detailed aerial photograph analysis, allow a new interpretation of the recent tectonic behaviour of the outer forearc in northern Chile between 22.5°S and 24.5°S (Antofagasta region). Both the Coastal Cordillera and the Mejillones Peninsula are under E–W extension. Normal faults dipping east with an almost N–S orientation are predominant. Large-scale Neogene to Recent deformation is characterized by vertical uplift and subsidence related to normal faulting. Recent kinematics along the Atacama Fault System, including the Atacama Fault itself, are controlled by the regional extensional stress regime, and the predominant component of recent displacement along the Atacama Fault is vertical (normal). However, strike-slip components are also observed, but with moderate offsets, no larger than one to two tens of metres. The sense of shear, and the ratio between vertical and horizontal offsets, vary coherently with the fault azimuth, compatible with E–W extension. Oblique-slip faulting, with normal and left-lateral offsets of similar amplitude, is observed on the most northeasterly oriented segment of the Atacama Fault. This small left-lateral component of displacement and the strike-slip motion predicted by models of strain partitioning of the convergence along the Atacama Fault are in opposite senses. This occurs because the stress regime in the coastal region is not compressional but extensional. Very fresh recent ruptures are numerous, especially between 23°S and 24°S, and provide clear indications of the occurrence of moderate to large continental earthquakes during the Late Quaternary. As pointed out by several authors, indirect lines of evidence suggest active subduction erosion and underplating along the margin of northern Chile. We propose that part of the long-term extension is due to the broad-scale flexure of the outer forearc. This flexure may be controlled by offshore subsidence caused by subduction erosion near the trench, and by onshore uplift related to the underplating of eroded low-density material beneath the Coastal Cordillera. Neotectonic observations show that the state of stress in the outer forearc remains extensional (in a deviatoric sense) in the long term. However, it is proposed that the state of stress has a strong time-dependent component, which is intimately linked to the subduction seismic cycle. The Mw=8.0 Antofagasta earthquake of 1995 July 30 showed that large subduction earthquakes produce E–W extension in the coastal region. The overall E–W deviatoric extension in the outer forearc should be reduced by interseismic contraction in the period separating two subduction earthquakes. This reduction means that continental faults will remain locked and aseismic in the interseismic period of the subduction cycle, in agreement with microseismic observations, which indicate an absence of shallow crustal seismicity. We assume that the amount of extension produced by subduction earthquakes in the coastal area is larger than the cumulative interseismic contraction. This occurs because the displacement field generated by subduction earthquakes is concentrated in the outer forearc. The E–W extension should grow over prepeated subduction seismic cycles and lead to large continental earthquakes occurring within the overriding block, simultaneous with large subduction events. We show new surface ruptures associated with normal faulting along the Atacama Fault, which were most probably triggered by the 1995 Antofagasta subduction earthquake.