Tectonics

Reynolds, S. C., (2005). Geographic variation in selected African mammalian taxa: a comparison of modern and fossil conspecifics. Ph.D. thesis, Liverpool John Moores University, United Kingdom.

by Sally Reynolds

This study examines the nature and extent of intraspecific variation in four African mammalian taxa: the Reck’s... more This study examines the nature and extent of intraspecific variation in four African mammalian taxa: the Reck’s springbok Antidorcas recki (Schwarz, 1932) the impala Aepyceros melampus (Lichtenstein, 1812) the spotted hyaena, Crocuta
crocuta (Erxleben, 1777) and the Plains zebra, Equus burchellii (Gray, 1824). These taxa co-occur in East and southern African Plio-Pleistocene fossil sites. Modern conspecifics are known for all taxa except for the extinct springbok (A. recki) which is here compared to the descendant species Antidorcas marsupialis
(Zimmermann, 1780). The four focal species are investigated for geographic body size differences and for body size changes through time. Multivariate methods are used to characterise cranial and dental variation in modern samples from East (‘East’) and southern African (‘South’) regions. Statistical comparisons are made between samples of modern and Plio-Pleistocene fossil specimens of each species. Results of this study show that modern intraspecific variation in Crocuta, Equus and Antidorcas is primarily size-related, while Aepyceros shows mainly shape differences. The same pattern of geographic variation is not identified in the fossil conspecifics of these taxa. Significant differences between the ‘East’ modern and fossil conspecifics outnumber differences between modern and fossil samples of the South region. The South sample not only shows fewer changes
but these indicate a different pattern of change to the East samples. Body size changes in various mammalian taxa through time have previously been interpreted as indicating climatic and environmental shifts. The size-related changes identified in this study may indicate that climatic and environmental shifts in the East African region were greater than contemporaneous changes in southern Africa. Studies of Plio-Pleistocene climates and environments seem to corroborate this body size interpretation. Evidence suggests that a greater degree of climatic and environmental change occurred in the East African region during this period. This study suggests that climatic and environmental shifts influence mammalian body size changes, both in present and past conspecifics, but that not all species have changed in the same way nor to the same extent.

Alpine metamorphic and tectonic evolution of the Inzecca-Ghisoni area (southern Alpine Corsica, France)

by Florence ReSeT

Francesca Garfagnoli, Francesco Menna, Enrico Pandeli, Gianfranco Principi

In the Inzecca-Ghisoni area (southern Alpine Corsica), a complex assemblage of vertically juxtaposed tectonic units,... more In the Inzecca-Ghisoni area (southern Alpine Corsica), a complex assemblage of vertically juxtaposed tectonic units, affected by Alpine deformations and metamorphism, crops out. Among them, there are some tectonic units (Parautochthonous Units, i. e. parautochtone of previous studies), that represent fragments of the continental Corse basement (Palaeozoic granitoids and associated volcanic and metamorphic pre-Carboniferous rocks) and of its Mesozoic to Tertiary sedimentary cover, that are tectonically sliced between the allochthonous Ligurian-Piedmontese Units (Schistes Lustrés) and the autochthonous basement (Variscan Corsica). The reconstructed polyphase deformation and metamorphic evolution of such units and the finding of high-pressure/low-temperature mineral assemblages in the continental-derived tectonic slices, points to the involvement of the south-eastern border of the European basement of Corsica in the tectonic processes linked to the Alpine subduction

STRATIGRAPHY AND TECTONIC AND METAMORPHIC EVOLUTION OF THE PORTO AZZURRO UNIT IN THE MONTE CALAMITA PROMONTORY (SOUTHEASTERN ELBA ISLAND, TUSCANY)

by Florence ReSeT

Francesca Garfagnoli, Francesco Menna, Enrico Pandeli, Gianfranco Principi

The Elba Island has a key role in the reconstructions of the stratigraphic, tectonic, metamorphic and magmatic... more The Elba Island has a key role in the reconstructions of the stratigraphic, tectonic, metamorphic and magmatic evolution of the Northern Tyrrhenian Sea and of the inner part of the Northern Apennines chain. The Porto Azzurro Unit, cropping out in the SE part of the Island, is the deepest tectonic unit of the central-eastern Elba structural pile of Tuscan, Ligurian and Ligurian-Piedmontese Nappes, which were intruded by Late Tortonian-Lower Pliocene granitoids and mainly acidic dikes. Moreover, in this part of the Island, the relationships between the uplift of the plutonic bodies and the final deformations of the tectonic stack are well exposed. To improve the geological knowledge of SE Elba, the authors carried out a 1:10.000 geological survey of the Calamita Promontory (mostly made up of the Porto Azzurro Unit) and performed petrographic and meso-/micro-structural studies on its rocks. The Porto Azzurro Unit consists of a Paleozoic, likely pre-Carboniferous basement (Mt. Calamita Fm.), which is unconformably overlain by the ?Triassic Verrucano metasiliciclastics (Barabarca Quartzites) and ?Upper Triassic-?Hettangian metacarbonates. In the Mt.Calamita Fm., five main lithofacies were recognized and mapped. In particular, garnet-bearing, albite micaschist (lithofacies a) geometrically underlie a phyllitic-quartzitic unit (lithofacies b); Porphyroids-like rocks (lithofacies e), metabasite bodies (lithofacies d) and graphite-rich siliciclastics (lithofacies c) are also present. The rocks of the lithofacies a are similar to those of the ?pre-Paleozoic-?Paleozoic Micaschist Complex of the Larderello Geothermal Field, whereas the other lithofacies can be probably correletable with the ?Ordovician formations of the Tuscan Metamorphic Units. The complex deformation-metamorphic evolution of the Porto Azzurro Unit consists of the following events: a) a Variscan tectono-metamorphic event (Dx), recognized in the Mt.Calamita Fm., which is defined by pre- Alpine schistosity and mineralogic relics (garnet); b) two Alpine tectono-metamorphic folding events (D1 and D2) in the Greenschists facies, which deformed also the Mesozoic covers; c) a following folding event (D3) which probably occurred during or immediately after the strong thermometamorphic imprint (including the magnetite-rich skarn bodies), due to the Neogene magmatic intrusions; d) Subsequently, the uplift of the magmatic bodies caused low-angle detachments within the Porto Azzurro Unit (between the Mt.Calamita Fm. and the Mesozoic cover) and between the latter and the overlying tectonic Units (e.g. Zuccale Fault between the Porto Azzurro Unit and the Cretaceous Flysch). A final weak antiformal folding (D4) of the whole promontory took place before the development of NW-SE and N-S trending high –angle normal fault systems, locally sealed by hydrothermal, sometimes Fe-rich mineralizations. The lithostratigraphic, tectonic, metamorphic and magmatic evolution of the Porto Azzurro Unit is similar to that defined for the Larderello geothermal region. Thus, the Mt.Calamita area can be considered as a little older, but similar geological model for all the future interpretations of the deep structure of southern Tuscany crossed by the Crop 18 profile.

The Porto Azzurro unit (Mt. Calamita promontory, south-eastern Elba Island, Tuscany) : stratigraphic, tectonic and metamorphic evolution

by Florence ReSeT

Francesca Garfagnoli, Francesco Menna, Enrico Pandeli, Principi Gianfranco

Elba Island has a key role in the reconstructions of the stratigraphic, tectonic, metamorphic and magmatic evolution... more Elba Island has a key role in the reconstructions of the stratigraphic, tectonic, metamorphic and magmatic evolution of the Northern Tyrrhenian Sea and the inner part of the Northern Apennines chain. The Porto Azzurro Unit, cropping out in the south eastern part of Elba Island, is the deepest tectonic unit of the central-eastern Elba structural pile of Tuscan, Ligurian and Ligurian-Piemontese Nappes, which were intruded by Late Tortonian-Lower Pliocene granitoids and mainly acidic dykes. Moreover, in this part of the island, the relationships between the emplacement of the plutonic bodies and the final deformations of the tectonic stack are easily detectable. To improve our geological knowledge of south eastern Elba, the authors carried out 1:10,000 geological mapping of the Calamita promontory (mostly composed of the Porto Azzurro Unit) and performed petrographical and structural studies on its rocks. The Porto Azzurro Unit consists of a Paleozoic, probably pre-Carboniferous basement (Mt. Calamita Metamorphic Complex), which is unconformably overlain by the PTriassic Verrucano metasiliciclastics (Barabarca Quartzites) and ?Upper Triassic-?Hettangian metacarbonates. In the Mt. Calamita Metamorphic Complex, five main lithofacies were recognized and mapped. In particular, garnet-bearing-, albite mica-schists (lithofacies a) geometrically underlie a phyllitic-qurtzitic unit (lithofacies b); porphyroids (lithofacies e), metabasite bodies (lithofacies d) and graphite-rich siliciclastics (lithofacies c) are also present. The rocks of lithofacies a are similar to those of the ?pre-Paleozoic-?Paleozoic Micaschist Complex of the Larderello geothermal field, whereas the other lithofacies can probably be correlated with the ?Ordovician formations of the Tuscan Metamorphic Succession (e.g. Apuan Alps). The complex deformation-metamorphic evolution of the Porto Azzurro Unit consists of the following events: a) a Variscan tectono-metamorphic event (D,), recognized in the Mt. Calamita Metamorphic Complex, which is defined by a pre-Alpine foliation and mineral relicts (garnet); b) two Alpine tectono-metamorphic folding events (D1 and D2) in the greenschist facies, which also deformed the Mesozoic cover; c) a later folding event (D3) which probably occurred during or immediately after the thermometamorphic imprint (including the magnetite-rich skarn bodies), caused by Neogene magmatic intrusions; d) subsequently, the uplift of the magmatic bodies caused low-angle detachments within the Porto Azzurro Unit (between the Ml. Calamita Metamorphic Complex and the Mesozoic cover) and between the latter and the overlying tectonic units (e.g. Zuccale Fault between the Porto Azzurro Unit and the Cretaceous Flysch Unit); e) a final weak antiformal folding (D4) of the whole promontory took place before the development of NW-SE and NE-SW trending high-angle normal fault systems, locally sealed by hydrothermal, sometimes Ferich mineralizations. The lithostratigraphical, tectonic, metamorphic and magmatic evolution of the Porto Azzurro Unit is similar to that detected for the Larderello geothermal region. Thus, the Mt. Calamita area can be considered as an older, but similar geological model for all future interpretations of the deep structure of southern Tuscany crossed by the CROP 18 profile.

Fault array evolution in extensional basins: insights from statistical analysis of gravel deposits in the Cecina River (Tuscany, Italy)

by Florence ReSeT

Andrea Ciampalini, Ilaria Consoloni, Giovanni Sarti

Two statistical analyses of gravel clasts from the Lower Pleistocene deposits in the Lower Cecina Valley (Tuscany,... more Two statistical analyses of gravel clasts from the Lower Pleistocene deposits in the Lower Cecina Valley (Tuscany, Italy) have been combined to unravel changes in the palaeo-drainage system. Data from 16 outcrops were collected and 6400 clasts described. Facies analysis, micro-palaeontology and macro-palaeontology and petrographic characteristics of the gravel deposits have highlighted the presence of three allostratigraphic units. Clast lithology is the main discriminator among these units. Cluster and principal component analyses of the 6400 clasts have improved understanding of the stratigraphy of the Lower Pleistocene deposits and constrain the re-routing of the lower palaeo-Cecina River from a supposedly south-east to north-west direction to the present east to west direction. Short rivers feeding small fan deltas represented by the oldest allostratigraphic units were abandoned in the Lower Pleistocene, when the re-routing of the Cecina River caused the capture of these streams. This evolution suggests a change in the tectonic regime of the area. The fan deltas developed on the hanging wall of normal faults sub-parallel to the coast; a change to a transtensile tectonic regime caused the deviation of the main river channel toward the present coast and the formation of a pull-apart basin, which is now exploited by the Cecina River. This study illustrates the value of lithological analyses of gravel deposits for understanding the tectonic evolution of an area.

The Gippsland Basin Top Latrobe Unconformity, and its expression in other SE Australian Basins

by Stephen Gallagher

Lithospheric drift on early Mars: Evidence in the magnetic field

by Daisuke Kobayashi

Comment on “The distribution of basaltic volcanism on Tenerife, Canary Islands: Implications on the origin and dynamics of the rift systems” by A. Geyer and J. …

by Valentin Troll

Rift zone reorganization through flank instability in ocean island volcanoes: an example from Tenerife, Canary Islands

by Valentin Troll

Cyclic caldera collapse: Piston or piecemeal subsidence? Field and experimental evidence

by Valentin Troll

Elliptical calderas in active tectonic settings: an experimental approach

by Valentin Troll

Archean gravity-driven tectonics on hot and flooded continents: Controls on long-lived mineralised hydrothermal systems away from continental margins

by Patrice Rey

Nicolas Thébaud and Patrice Rey, 2012, Precambrian Geology,    http://dx.doi.org/10.1016/j.precamres.2012.03.001.

We present the results of two-dimensional numerical modelling experiments on the thermal evolution of Archean... more We present the results of two-dimensional numerical modelling experiments on the thermal evolution of Archean greenstones as they sink into a less dense, hot and weak felsic crust. We compare this thermal evolution to that obtained via the analysis of isotopic data and fluid inclusion microthermometry data obtained in the Paleoarchean to Mesoarchean Warrawoona Synform (Eastern Pilbara Craton, Western Australia). Our numerical experiments reveal a two-stage evolution. In the first stage, cooling affects zones of downwelling as greenstone belts are advected downward, whereas adjacent domes become warmer as deep and hot material is advected upward. We show that this is consistent with stable isotopes data from the Warrawoona Synform, which reveal an early episode of seafloor-like alteration (90–160 °C) strongly focused along steeply dipping shear zones. In a second long-lived stage, lateral heat exchanges between domes and basins dominate the system as domes cool down while downwelling zones become increasingly warmer. In the Warrawoona greenstone belt, stable isotopes in gold-bearing quartz veins post-dating the sagduction-related vertical fabrics reveal that rock–fluid interaction occurred at much higher temperatures (234–372 °C) than seafloor-like alteration. We propose that emplacement of thick and dense continental flood basalts, on flooded hot and weak continental plates, led to conditions particularly favourable to hydrothermal processes and the formation of mineral deposits. We further argue that sagduction was able to drive crustal-scale deformation in the interior of continents, away from plate margins. On largely flooded continents, sagduction-related shear zones acted as fluid pathways promoting gold mineralisation far away from active plate boundaries, continental rift zones or collisional mountain belts.

Lower crustal flow kept Archean continental flood basalts at sea level

by Patrice Rey

Flament, N., Rey, P. F., Coltice, N., Olivier, N and Dromart, G. 2011. Geology, v.39, p.1159-1162.

Large basaltic provinces as much as 15 km thick are common in Archean cratons. Many of these flood basalts erupted... more Large basaltic provinces as much as 15 km thick are common in Archean cratons. Many of these flood basalts erupted through continental crust but remained at sea level. Although common in the Archean record, subaqueous continental flood basalts (CFBs) are rare to absent in the post-Archean. Here we show that gravity-driven lower crustal fl ow may have contributed to maintaining Archean CFBs close to sea level. Our numerical experiments reveal that the characteristic time to remove the thickness anomaly associated with a CFB decreases with increasing Moho temperature (TM), from 500 m.y. for TM ≈ 320 °C to 1 m.y. for TM ≈ 900 °C. This strong dependency offers the opportunity to assess, from the subsidence history of CFBs, whether continental geotherms were significantly hotter in the Archean. In particular, we show that the subsidence history of the ca. 2.7 Ga upper Fortescue Group in the East Pilbara Craton, Western Australia, requires Moho temperatures >>700 °C. Applied to eight other unambiguous subaqueous Archean CFBs, our results indicate Moho temperatures >>650 °C at the time of eruption. We suggest that the decrease in the relative abundance of subaqueous CFBs over Earth’s history could reflect the secular cooling of the continental lithosphere due to the decrease in radiogenic heat production.

Reynolds, S.C.; Bailey, G. and King G. C. P. (2011). Landscapes and their relation to hominin habitats: case studies from Australopithecus sites in eastern and southern Africa. Journal of Human Evolution 60(3):281-298.

by Sally Reynolds

See the accompanying paper by Bailey, Reynolds and Kind (2011) for more detail on the theoretical approaches to the study of landscapes and humans.

We examine the links between geomorphological processes, specific landscape features, surface water drainage, and the... more We examine the links between geomorphological processes, specific landscape features, surface water drainage, and the creation of suitable habitats for hominins. The existence of mosaic (i.e., heterogeneous) habitats within hominin site landscape reconstructions is typically explained using models of the riverine and gallery forest settings, or the pan or lake setting.We propose a different model: the Tectonic Landscape Model (TLM), where tectonic faulting and volcanism disrupts existing pan or river settings at small-scales (∼10e25 km). Our model encompasses the interpretation of the landscape features, the role of tectonics in creating these landscapes, and the implications for hominins. In particular, the model explains the underlying mechanism for the creation and maintenance of heterogeneous habitats in regions of active tectonics. We illustrate how areas with faulting and disturbed drainage patterns would have been attractive habitats for hominins, such as ustralopithecus, and other fauna. Wetland areas are an important characteristic of surface water disturbance by fault activity; herefore we examine the tectonically-controlled Okavango Delta (Botswana) and the Nylsvley wetland (South Africa) as modern
examples of how tectonics in a riverine setting significantly enhance the faunal and floral biodiversity. While tectonic landscapes may not have been the only type of attractive habitats to hominins, we propose a suite of landscape, faunal, and floral indicators, which when recovered together suggest that site environments may have been influenced by tectonic and/or volcanic activity while hominins were present. For the fossil sites, we interpret the faulting and landscapes around australopithecine-bearing sites of the Middle Awash (Ethiopia) and Makapansgat, Taung, and Sterkfontein (South Africa) to illustrate these relationships between landscape features and surface water bodies. Exploitation of tectonically active landscapes may explain why the paleoenvironmental signals, anatomy, diets, as well as the fauna associated with Australopithecus appear largely heterogeneous through time and space. This hypothesis is discussed in light of potential preservation and time-averaging effects which may affect patterns visible in the fossil record. The model, however, offers insight into the landscape processes of how such habitats are formed. The landscape features and range of habitat conditions, specifically the wetter, down-dropped plains and drier, uplifted flanks persist in close proximity for as long as the fault motion continues. The Tectonic Landscape Model provides an alternative explanation of why mixed habitats may be represented at certain sites over longer timescales.

The bending mechanics in a dynamic subduction system: Constraints from numerical modelling and global compilation analysis

by Gabriele Morra

published in "Tectonophysics"

Lithospheric plates bend at subduction zones where the vertical motions of the slabs are converted to surface plate... more Lithospheric plates bend at subduction zones where the vertical motions of the slabs are converted to surface plate motions. To understand the mechanics of plate bending we derive scaling laws for the deflection at the margin, i.e. radius and dip, from numerical models of a subducting viscoelastic plate. In such dynamic system we find that the buoyancy and the stiffness of the plates control the radius and the dip, as well as the plate motions toward the trench. This mechanical model successfully predicts the curvature of published three- dimensional laboratory and numerical models. For a thorough comparison with the observable, we have also implemented forces additional to the slab pull, such as the suction force and far-field stresses. By increasing or resisting the torque applied at the trench by the slab, these forces can largely rearrange the dip and the radius of slabs and the inherent plate motions, although they do not alter the observed anticorrelation between radius and dip. Similar inverse correlation relationship and dip-radius ranges are shown by most of the subduction zones analysed from a global compilation. Radii in the range of 100–350 km and dips of 30°–70° for slabs that extends to the bottom of the upper mantle are compatible with the models, and allow estimating an average lith- ospheric viscosity contrast of ~200 in the bending with respect to the ambient mantle. Radii and dips outside of this range are in good agreement with the trends and the magnitudes of models that include suction and far-field forces. In all these subduction zones, the correlation between dip, radius and plate velocity is found to be com- patible with that of the models, showing how relevant bending is for the dynamics of Earth.

Insights on the kinematics of the India-Eurasia collision from global geodynamic models

by Nicolas Flament

Authors: Sabin Zahirovic, R. Dietmar Müller, Maria Seton, and Nicolas Flament, Michael Gurnis, Joanne Whittaker

The Eocene India-Eurasia collision is a first order tectonic event whose nature and chronology remains controversial.... more The Eocene India-Eurasia collision is a first order tectonic event whose nature and chronology remains controversial. We test two end-member collision scenarios using coupled global plate motion-subduction models. The first, conventional model, invokes a continental collision soon after 60 Ma between a maximum extent Greater India and an Andean-style Eurasian margin. The alternative scenario involves a collision between a
minimum extent Greater India and a NeoTethyan back-arc at 60 Ma that is subsequently subducted along southern Lhasa at an Andean-style margin, culminating with continent-continent contact at 40 Ma. Our numerical models suggest the conventional scenario does not adequately reproduce mantle structure related to Tethyan convergence. The alternative scenario better reproduces the discrete slab volumes and their lateral and vertical distribution in the mantle, and is also supported by the distribution of ophiolites indicative of Tethyan intraoceanic subduction, magmatic gaps along southern Lhasa and a two-stage slowdown of India. Our models show a strong component of southward mantle return flow for the Tethyan region, suggesting that the common assumption of near-vertical slab sinking is an oversimplification with significant consequences for interpretations of seismic tomography in the context of subduction reference frames.

Experimental modelling of orogenic wedges: A review

by Fabien Graveleau

Graveleau, F., Malavieille, J. Dominguez, S., Tectonophysics, 2012

Experimental modelling applied to the study of orogenic wedge dynamics has been a subject of fruitful research for... more Experimental modelling applied to the study of orogenic wedge dynamics has been a subject of fruitful research for more than 30 years, although the technique dates back as far as the early XIXth century. On one hand, several first order parameters controlling the structural evolution of mountain belts have been intensively investigated using the classic tectonic “sandbox” models. The main parameters are the properties of the basal décollement, the deforming material, the backstop, and fluxes, kinematics and surface processes. On the other hand, the morphological evolution of a mountain relief subjected to changing tectonic or climatic forcing has been addressed using another kind of approach called “geomorphic” models. Nowadays, the literature is extremely rich, particularly for the sandbox technique, so that it becomes difficult to have an exhaustive view of the effects of the above parameters on mountain evolution. In this article, we propose a detailed review of the main results obtained using both “tectonic” and “geomorphic” approaches. Our goal is to provide an almost complete state-of-the-art in the experimental study of relief dynamics to guide present and future researchers in their understanding of mountain belt evolution.

Geologic History of Utah

by Bart Kowallis

Hintze, L.F. and Kowallis, B.J., 2009, Geologic History of Utah: BYU Geology Studies Special Publication 9, 225 p.

Physical Modeling of Arc–Continent Collision: A Review of 2D, 3D, Purely Mechanical and Thermo‐Mechanical Experimental Models

by David Boutelier

D. Boutelier and A. Chemenda

Contrasting origins of serpentinites in a subduction complex, northern Dominican Republic

by Benoit Saumur

Published in GSA Bulletin; part of M.Sc. at U. Ottawa