Thorsten J. Nagel

Generation of Eoarchean tonalite-trondhjemite-granodiorite series from thickened mafic arc crust

The earliest compounds forming Earth9s first continental crust were magmatic rocks with tonalitic-trondhjemitic-granodioritic composition (TTGs). TTGs are widely seen as originating from melting of hydrated oceanic crust in subduction zones. Alternative models argue that they may have formed by melting within thickened mafic oceanic protocrust. To simulate formation of Eoarchean TTGs in different tectonic regimes, we combine for the first time the thermodynamic calculation of residual assemblages with subsequent modeling of trace element contents in TTGs. We compare water-absent partial melting of two hydrated starting compositions, a modern mid-oceanic-ridge basalt (MORB) and a typical Eoarchean arc tholeiite from the Isua Supracrustal Belt that represents the country rock of Earth9s oldest TTGs in southern West Greenland. At 10 kbar, partial melting of MORB-like residues results in modeled TTG compositions that are very different from natural ones. Melting at higher pressures (14 and 18 kbar) leads to a better match, but several key trace element parameters in TTGs are still amiss. A perfect fit for trace element compositions is achieved by melting of Eoarchean arc tholeiites at 10 and 14 kbar. These protoliths contain less Al and Na and more Fe and Mg as compared to present-day MORB and form amphibole-rich and plagioclase-free residues even at low pressures. Formation of Earth9s oldest continental crust is therefore best explained by melting within tectonically thickened mafic island-arc crust.

Symmetric alternative to asymmetric rifting models

We describe the first numerical simulation of continental rifting that reproduces the three major structures observed at magma-poor margins such as the Galicia Margin west of the Iberian Peninsula or the Apulia Margin in the Alps: (1) Distal continental margins consist of fault-bounded blocks separated by oceanward-dipping normal faults. (2) At the tip of the continent, lower crust is scarcely preserved or absent, and upper crust directly overlies exhumed mantle. (3) The base of the rotated crustal blocks is a prominent seismic reflector and represents a high strain zone with a top-to-the-ocean sense of shear. In our model, these structures do not reflect asymmetric rift geometry at a lithospheric scale. Instead, they derive from upper-crustal collapse over a mid-crustal shear zone into the rift center and are present on both sides of the rift axis. The model has a horizontal weak zone in the middle crust on top of strong lower crust and a localized vertical zone of thermal weakness in the rift center. We hypothesize that the development of a thermal perturbation and associated strain localization in the deeper lithosphere may cause the transition from widely distributed faulting and crustal thinning to constricted faulting directed toward a well-defined rift center.

Noble metal nanoclusters and nanoparticles precede mineral formation in magmatic sulphide melts

In low temperature aqueous solutions, it has long been recognized by in situ experiments that many minerals are preceded by crystalline nanometre-sized particles and non-crystalline nanophases. For magmatic systems, nanometre-sized precursors have not yet been demonstrated to exist, although the suggestion has been around for some time. Here we demonstrate by high temperature quench experiments that platinum and arsenic self-organize to nanoparticles, well before the melt has reached a Pt–As concentration at which discrete Pt arsenide minerals become stable phases. If all highly siderophile elements associate to nanophases in undersaturated melts, the distribution of the noble metals between silicate, sulphide and metal melts will be controlled by the surface properties of nano-associations, more so than by the chemical properties of the elements.

A new occurrence of microdiamond-bearing metamorphic rocks, SW Rhodopes, Greece

We describe a new locality with microdiamond-bearing, ultrahigh-pressure metamorphic rocks near the village Sidironero in the Rhodope Metamorphic Province in northern Greece, about 70 km west of the nearest known location at Xanthi. High- and ultrahigh-pressure metamorphic conditions are preserved in an intensely strained melange zone which is sandwiched between upper-greenschist to lower-amphibolite-facies rocks in the footwall (Pangaion-Pirin Complex) and upper-amphibolite-facies rocks in the hanging wall (Rhodope Terrane). The melange zone consists of various paragneisses, orthogneisses and metamafics. A strong overprint at upper-amphibolite-facies conditions associated with migmatisation in the orthogneisses and subsequent intense mylonitisation at lower-amphibolite-facies conditions almost obliterates peak-pressure assemblages. Relics of high-pressure conditions are preserved in mafic boudins and in garnet-kyanite-mica schists. Garnet in garnet-kyanite-mica schists displays inclusions of microdiamonds and swarms of non-oriented rods of rutile and quartz. The lithological and structural appearance of the melange zone resembles the exposure of ultrahigh-pressure metamorphic rocks further east at Xanthi, whereas the location at Kimi may occupy a higher structural level.

Exhumation of high- and ultrahigh-pressure metamorphic rocks by slab extraction

Exhumation of high- and ultrahigh-pressure metamorphic rocks in collisional orogens may be explained by upward extrusion of these rocks, erosion of their overburden, or extensional thinning of the overburden. Some high-pressure terranes, such as the Adula nappe in the Central Alps, fit none of these scenarios. We propose an additional way in which part of the overburden may be removed: it may sink off into the deeper mantle (slab extraction). Structural and metamorphic relationships in and around the Adula nappe indicate that the emplacement of this Alpine high- to ultrahigh-pressure nappe (to 3.2 GPa) in a pile of lower-pressure nappes resulted from the interaction of two subduction zones that accommodated the closure of two ocean basins, ultimately leading to the extraction of the intervening slab. In terms of mechanics, the cause of the exhumation is, in this case, not the buoyancy of the high-pressure rocks, but the negative buoyancy of the extracted slab.

Tertiary subduction, collision and exhumation recorded in the Adula nappe, central Alps

Abstract The Adula nappe in the Central Alps represents a lithospheric mélange assembled in a south-dipping subduction zone during the Tertiary orogenic cycle. It consists of several heterogeneous lobes which are stacked in a forward-dipping duplex geometry. Eclogites, garnet peridotites and garnet-white-mica schists record southward-increasing peak pressure conditions which culminate at 12–17 kbar/500–600 °C in the north and 30 kbar/800–850 °C in the south. Some studies infer even higher peak pressures for the garnet peridotite body of Alpe Arami. The present-day metamorphic field gradient for peak pressures exceeds the lithostatic pressure gradient. So far, only eclogites and garnet peridotites from the Cima Lunga complex in the south and the adjacent Southern Steep Belt have yielded Tertiary metamorphic ages for the peak-pressure stage. Some recent studies propose that the Adula nappe got assembled after the formation of high-pressure assemblages in eclogites and garnet peridotites and reject regional high-pressure conditions in Tertiary times. This scenario, however, is in conflict with the observed continuity of metamorphic field gradients and post-peak-pressure structures. Amphibolite facies conditions post-date formation of the Central Alpine nappe stack. In this paper, the associated field gradient is explained through southward-increasing temperatures during near-isothermal decompression. The main mylonitic foliation in the Adula nappe post-dates peak-pressure conditions. It is associated with top-to-the-north shearing and southward-increasing amounts of decompression from eclogite facies to amphibolite facies conditions. Also, the present-day supra-lithostatic field gradient for peak pressures probably results from this deformation phase and is here related to substantial vertical flattening during northward shearing. All subsequent structures affect established nappe boundaries. Pervasive Oligocene deformation events in the Adula nappe are coeval with intense shearing along the so-called Insubric mylonites and occur during ongoing isothermal decompression to around 5 kbar. They are associated with orogen-oblique to orogen-parallel stretching of unspecified amount which may considerably contribute to the exhumation of the Lepontine dome already before the onset of the well-known Miocene extension.

Age and composition of meta‐ophiolite from the Rhodope Middle Allochthon (Satovcha, Bulgaria): A test for the maximum‐allochthony hypothesis of the Hellenides

The metamorphosed thrust stack of the Rhodopes comprises a level with ophiolites (Middle Allochthon) underlain and overlain by continent-derived allochthons. The Upper Allochthon represents the European margin, but the origin of the Lower Allochthon remains controversial, with suggestions that it may be derived from an inferred microcontinent (Drama) or from the margin of Adria. Trace element compositions and Sr and Nd isotope ratios of metagabbroic amphibolites and enclosed meta-plagiogranites from the Satovcha Ophiolite, Middle Allochthon, show that they are cogenetic and represent suprasubduction zone ophiolites. U-Pb dating using laser ablation sector field inductively coupled plasma mass spectrometry of zircons from two meta-plagiogranites and a metagabbro yielded identical Jurassic ages (160 ± 1 Ma, 160.6 ± 1.8 Ma, and 160 ± 1 Ma, respectively), similar to ophiolites in the eastern Vardar Zone bordering the Rhodopes to the SW. The trace element patterns also closely resemble those of the Vardar ophiolites. The association with Late Jurassic arc-type granitoids is another feature that applies both to eastern Vardar and Satovcha. This strongly suggests that the Middle Allochthon comprises the metamorphosed northeastward continuation of the Vardar Zone. The Jurassic age of the Satovcha Ophiolite contradicts the hypothesis of Early Jurassic suturing between Europe (Upper Allochthon) and the assumed Drama microcontinent (Lower Allochthon) but is in line with the “maximum allochthony hypothesis,” i.e., the assumption that the Lower Allochthon represents Adria and that the “root” of the Vardar-derived thrust sheets is at the NE boundary of the Rhodopes.

Splitting a continent : insights from submarine high resolution mapping of the Moresby Seamount detachment, offshore Papua New Guinea

The Moresby Seamount detachment in the Woodlark Basin (east of Papua New Guinea) is arguably the best exposed active detachment fault in the world. We present the results of a high-resolution autonomous underwater vehicle survey of bathymetry, bottom water temperature, and turbidity. In combination with dredging and existing drillhole data, a synthesis of the tectonic geomorphology, kinematics, and mechanics of the detachment is provided. The detachment surface, which has a 30° northward dip and ∼8 km post-Pliocene displacement, is well preserved. Two major smooth areas are tectonically created, and megascopic (kilometer scale) slickensides indicate downdip direction of movement. The detachment is transected by a major sinistral strike-slip fault, suggesting deformation partitioning in the detachment zone in response to the 500 k.y. change in plate kinematics. The mainly gabbroic protoliths and cataclasites from the fault show pervasive syntectonic alteration, leading to large increases in abundance of quartz and, more important, calcite. Resulting quartz-rich and calcite-rich mylonites play a crucial role, as weak fault rocks and ductile microstructures point to detachment operation at low differential stress. A kilometer-sized anomaly in bottom water temperature and turbidity is found at the downdip end of the detachment zone, indicating that it hosts an active hydrothermal system, probably fed by overpressured fluids from a deep crustal source.

On the role and importance of orogen-parallel and -perpendicular extension, transcurrent shearing, and backthrusting in the Monte Rosa nappe and the Southern Steep Belt of the Alps (Penninic zone, Switzerland and Italy)

Abstract During Europe–Adria collision in Tertiary times, the Monte Rosa nappe was penetratively deformed in several stages after an eclogite-facies pressure peak: (1) top-to-the-NW thrust shearing (Mattmark phase, after 40 Ma); (2) orogen-parallel, top-to-the-SW extensional shearing and folding (Malfatta phase); (3) orogen-perpendicular, top-to-the-SE extensional shearing and folding (Mischabel phase, before 30 Ma); and (4) large-scale, upright, SE-vergent folding (Vanzone phase, c. 29–28 Ma). Structural analysis and neutron texture goniometry of quartz mylonites show that the Stellihorn shear zone in the Monte Rosa nappe accommodated a complex and multidirectional sequence of shearing movements during the Mattmark, Malfatta and Mischabel phases, and was folded in the Vanzone phase. In the tail-shaped eastward prolongation of the Monte Rosa nappe in the Southern Steep Belt of the Alps, both dextral and sinistral mylonites (Olino phase) were formed during and after the formation of the Vanzone fold, reflecting renewed orogen-parallel (SW–NE) extension contemporaneous with NW–SE shortening from c. 29 Ma onward. A similar sequence of deformation stages was identified in the Adula nappe at the eastern border of the Lepontine metamorphic dome. Important consequences arise for the Insubric fault at the southern border of the Lepontine dome: (1) the NW- to N-dipping orientation of the Insubric fault is not a primary feature but resulted from rotation of an originally SE-dipping shear zone after c. 30 Ma; and (2), the strong contrast in metamorphic grade across this fault (upper amphibolite facies to the north versus anchizone to the south) results from north-side-up faulting coupled with orogen-parallel extension of the northern block (Lepontine dome), while no such extension occurred in the southern block (Southern Alps). Extension in the northern block started in the Malfatta phase and continued in the Mischabel phase when the foliation in the area which later became the Southern Steep Belt still dipped towards south. During Vanzone/Olino deformation, further unroofing and uplift of the Lepontine dome relative to the South Alpine block took place while the Southern Steep Belt was progressively rotated into its present, overturned position, changing its character from a normal fault into a backthrust. Complex deformation paths in the Southern Steep Belt resulted from the combination of extension of the northern block with strike-slip motion along the Insubric fault.

Tectonic evolution of the southeastern part of the Pohorje Mountains Eastern Alps, Slovenia

Tectonic evolution of the southeastern part of the Pohorje Mountains (Eastern Alps, Slovenia) Field relations and deformation structures in the southeastern part of the Pohorje Mountains constrain the tectonic evolution of the Austroalpine high-pressure/ultrahigh pressure (HP/UHP) terrane. The Slovenska Bistrica Ultramafic Complex (SBUC) forms a large (ca. 8 × 1 km size) body of serpentinized harzburgite and dunite including minor garnet peridotite and is associated with partly amphibolitized eclogite bodies. The SBUC occurs in the core of an isoclinal, recumbent, northward closing antiform and is mantled by metasedimentary rocks, mostly gneisses and a few marbles, including isolated eclogite/amphibolite lenses. Before this folding, the SBUC formed the deepest part of the exposed terrane. We interpret the SBUC to be derived from near-MOHO, uppermost mantle which was intruded by gabbros in the subsurface of a Permian rift zone. During Cretaceous intracontinental subduction, the SBUC was most likely part of the footwall plate which experienced HP to UHP metamorphism and was folded during exhumation. In the Miocene, the Pohorje Pluton intruded and, subsequently, the metamorphic rocks together with the pluton were deformed probably due to east-west extension and contemporaneous north-south shortening, thus forming an antiformal metamorphic core complex.