Controlling the formation and alignment of low molecular weight gel ‘noodles’

We show how to control the formation and alignment of gel ‘noodles’. Nanostructure alignment can be achieved reproducibly by extensional deformation as the filaments form. Using a spinning technique, very...

Magmatic record of the Mesozoic geology of Hainan Island and its implications for the Mesozoic tectonomagmatic evolution of SE China: effects of slab geometry and dynamics in continental tectonics

Our field-based geochemical studies of the Triassic, Jurassic and Cretaceous granitoids on Hainan Island indicate that their magmas had different geochemical affinities, changing from alkaline in the Triassic through ocean island basalt (OIB) in the Jurassic, to calc-alkaline in the Cretaceous. We show that these changes in the geochemical affinities of the Mesozoic granitoids on Hainan and in SE China reflect different melt sources and melt evolution patterns through time. Our new geodynamic model suggests that: (1) Triassic geology was controlled by flat-slab subduction of the palaeo-Pacific plate beneath SE China. This slab dynamics resulted in strong coupling between the lower and upper plates, causing push-over tectonics and contractional deformation in SE China. Flat subduction-induced edge flow and aesthenospheric uprising led to the production of high-K granites, syenites and mafic rocks. (2) Slab foundering, accelerated subduction rates and subduction hinge retreat in the Early Jurassic caused rapid rollback of the downgoing slab. Strong decoupling of the upper and lower plates resulted in pull-away tectonics, producing extensional deformation in SE China. Decompression melting of the upwelling aesthenosphere produced OIB-type melts, which interacted with the subcontinental lithospheric mantle (SCLM) to form A- and I-type granitoids. (3) Segmentation of the palaeo-Pacific plate in the Early Cretaceous resulted in steeply dipping slabs and their faster rollback, facilitating lithospheric-scale extension and oceanward migration of calc-alkaline magmatism. This extensional deformation played a significant role in the formation of metamorphic core complexes, widespread crustal melting and development of a Basin and Range-type tectonics and landscape evolution in SE China.

Inheritance processes of active extensional deformation in ocean-continent subduction zones: the example of the northern Andes compressional margin in Ecuador

<p>Over the last 23 Myr, the roughly east-directed subduction of the Nazca Plate beneath South America led to the formation of several mountain ranges associated with the overall northern Andes evolution. Along the active southwestern Ecuadorian margin, the compressional setting involves the Cretaceous-Miocene Chongón-Colonche / Santa Elena terranes, overlain by recent sedimentary basins. This geological setting, generally interpreted as an onshore-offshore forearc system, evolves in close relation with the active tectonic escape of the North Andean Sliver and the opening of the Gulf of Guayaquil. This region is characterised by a widespread extensional deformation in the upper plate that overprints moderate subduction and crustal earthquakes.</p><p>To better document such extensional processes, we specifically explore the offshore shelf and the littoral area of the Santa Elena Peninsula using academic and industrial 2D seismic profiles calibrated with local wells and field observations. We document a trench-parallel fault network, composed of >20km-long normal faults that take place on top of the former Chongón-Colonche accretionary wedge. These faults are linearly-steep along the trench, and are listric toward the continent where they clearly control fault-block rotation. They separate flexural basins developing on the platform ahead the Chongón-Colonche Cordillera, and are associated with immerged terraces most likely formed during the Last Glacial Maximum. They also may link to further onshore marine terraces developing since the Pleistocene across the coastline.</p><p>These observations suggest a peculiar dismantlement of the margin, mainly affected by tectonic erosion involving reactivation of former compressional features. Normal faults are specifically interpreted as a regional syn-orogenic collapse of the Chongón-Colonche Cordillera, which may result from transecting subducting ridges, fracture zones and seamounts controlling, at least partially, the geometry and the nature of the deformation along the southwestern Ecuadorian margin. This deformation pattern is likely linked to a weak interseismic coupling along the subduction interface to which the active opening of the Gulf of Guayaquil overlaps. This project is funded by the project ANR MARACAS ANR-18-CE31-0022 (<em>MARine terraces along the northern Andean Coast as a proxy for seismic hazard ASsessment</em>).</p>

Dating extensional deformation to unravel exhumation patterns in the Internal Dinarides

<p>Ar/Ar-in-situ geochronology by laser ablation NGMS (noble gas mass spectrometry) provides a powerful tool to determine inter- and intra-granular age variations of potassium-bearing minerals while maintaining the structural integrity of a sample. This makes it an excellent method in targeting the understanding of the post-collisional evolution of an orogen by dating different mica generations. In order to investigate the timing of exhumation related to extensional deformation in the Internal Dinarides, we sampled paragneisses from the upper greenschist- to amphibolite-grade mylonitic detachment zones of two metamorphic core complexes (MCC’s). The MCC’s are located at the distal Adriatic passive margin (Cer MCC, central western Serbia) and within the Late Cretaceous suturing accretionary wedge complex (Motajica MCC, northern Bosnia and Herzegovina) that separates Adria-derived units from blocks of European affinity.</p><p>Mica grains were assigned to either pre-kinematic or syn-kinematic growth, according to their structural context, texture and grainsize. Pre-kinematic growth is characterized by large, deformed minerals of up to 3.5 mm in size, while rather fine-grained, recrystallized mineral aggregates that usually formed in the strain shadow of larger clasts represent syn-kinematic growth.</p><p>The ages of pre-kinematic white mica from paragneisses of the Motajica detachment range from approx. 80 to 25 Ma. They partly show a large intra-granular age spread characterized by significantly older core ages becoming progressively younger towards the rim. This pattern likely suggests diffusive loss of radiogenic Ar. Ages between 80-55 Ma in the central parts of the grains, associated with a top-W transport direction, are interpreted as the time interval of mineral growth and subsequent deformation in an accretionary wedge during E-ward subduction of the Adriatic passive margin underneath European units.</p><p>Syn-kinematic white mica from Motajica yielded ages between 22 and 16 Ma, which are interpreted as the time of peak activity of extension. This also corresponds with the time of crustal extension in the Pannonian Basin to the north. At Cer MCC, located roughly 150 km ENE of Motajica MCC and structurally below the accretionary wedge complex, ages of deformed white mica indicate exhumation between 19 and 15 Ma with a top-N directed transport.  </p><p>Our results suggest that the opening of the Pannonian Basin in response to slab-retreat underneath the Carpathian orogen resulted in the extensional reactivation of suturing thrusts that separated Adriatic from European units, leading to exhumation of parts of the accretionary wedge (Motajica MCC). This event was followed by the progressive exhumation of the passive Adriatic margin (Cer MCC) that occupied a structural position below the suturing accretionary wedge.</p>

Chapter 17 Accretionary orogens reworked in an overriding plate setting during protracted continent–continent collision, Sveconorwegian orogen, southwestern Sweden

The Eastern Segment in the Sveconorwegian orogen, southwestern Sweden, is dominated by 2.0–1.8, 1.7 and 1.5–1.4 Ga crust; and the overlying Idefjorden terrane by 1.6–1.5 Ga crust. Assuming reorganization of a subduction system prior to 1.5–1.4 Ga and applying a sinistral transpressive component of disruption during the subsequent Sveconorwegian orogeny (1.1–0.9 Ga), the Idefjorden terrane is inferred to be indigenous outboard rather than exotic with respect to the continental plate Fennoscandia (Baltica). The geological record then records successive westwards shift of accretionary orogens along a convergent plate boundary for at least 500 million years. Sveconorwegian foreland-younging tectonic cycles at c. 1.05 (or older)–1.02 Ga (Idefjorden terrane) and at c. 0.99–0.95 Ga (Eastern Segment) prevailed. Crustal thickening and exhumation during oblique convergence preceded migmatization, magmatic activity and a changeover to an extensional regime, possibly triggered by delamination of continental lithosphere, in each cycle. Convergence after 0.95 Ga involved antiformal doming with extensional deformation at higher crustal levels (Eastern Segment) and continued magmatic activity (Idefjorden terrane). An overriding plate setting is inferred during either accretionary orogeny or, more probably, protracted continent–continent collision. Continuity of the erosional fronts in the Grenville and Sveconorwegian orogens is questioned.