Crustal balance and crustal flux from shortening estimates in the Central Andes

AbstractThe Central Andes of South America form the second largest high elevation plateau on earth. Extreme elevations have formed on a noncollisional margin with abundant associated arc magmatism. It has long been thought that the crustal thickness necessary to support Andean topography was not accounted for by known crustal shortening alone. We show that this may in part be due to a two-dimensional treatment of the problem. A three-dimensional analysis of crustal shortening and crustal thickness shows that displacement of material towards the axis of the bend in the Central Andes has added a significant volume of crust not accounted for in previous comparisons. We find that present-day crustal thickness between 12°S and 25°S is accounted for (∼−10% to ∼+3%)with the same shortening estimates, and the same assumed initial crustal thickness as had previously led to the conclusion of a ∼25–35% deficit in shortening relative to volume of crustal material. We suggest that the present-day measured crustal thickness distribution may not match that predicted due to shortening, and substantial redistribution of crust may have occurred by both erosion and deposition at the surface and lower crustal flow in regions of the thermally weakened middle and lower crust

Microfabrics and deformation processes in magmatic veins of the Thuringian Forest, Germany

The research area is located in the Ruhla-Brotterode crystalline complex in the western part of the Thuringian Forest (Germany), about 20km southsouthwest of Eisenach. The investigated outcrops occur at the eastern and western flanks of the valleys north of the villages Trusetal and Hohleborn. Deformed magmatic veins only occur in the Hohleborn area. Both areas have relative fresh outcropping rocks, due to the steep relief, former quarries and fresh road cuts. According to Obst & Katzung (2000) several periods with the formation of magmatic veins with different chemical composition occur in the Ruhla-Brotterode crystalline complex. Presumably older lamprophyric veins and younger doleritic, syenitporphyric and granitporphyric veins have been identified (Obst & Katzung 2000). Benek & Schust (1988) already pointed out that some of these magmatic veins have experienced ductile deformation. The subject of this work is the occurrence of deformed magmatic veins in the Hohleborn area. The contact to their host rocks, their petrography and their microfabrics have been investigated and related to deformation processes, which led to a better understanding of their deformation conditions within the late-to post-variscan development of the area.conferenc

Constraining the near-surface response to lithospheric reorientation: Structural thermochronology along AlpArray geophysical transects

The shape of the present-day European Alps results from a complex tectonic and climatic history since the onset of convergence between the African and Eurasian plates. Low-temperature thermochronology data are a unique archive that can trace the cooling history of rocks back in time during exhumation from upper to middle crustal levels to Earth's surface. However, the precise mechanisms that led to cooling and exhumation are still debated. In this study, we investigated the potential for mantle processes, such as potential subducting slab break-off or slab reversal, to leave a fingerprint in the rock cooling record of the present-day surface along three key, north-south oriented geophysical transects: NFP-20E, TRANSALP and EASI. Along all transects, our zircon and apatite (U-Th)/He data reveal reset Neogene (and younger) cooling ages centred around core complexes such as the Lepontine Dome and the Tauern Window indicative of late exhumation during the Cenozoic Alpine orogeny. North and south of these complexes, the cooling ages become older, forming U-shaped age distributions around the reset centres. Thermal history reconstructions along TRANSALP confirm a conspicuous southward shift of cooling towards the Southern Alps approximately at the time of deep-seated exhumation of the Tauern Window driven by motion along the mid-crustal Tauern Ramp in the Mid-Miocene. Thermo-kinematic models along the transect confirm this southward shift of deformation and (i) reproduce the distribution of cooling ages and thermal history reconstructions, (ii) are consistent with the present-day structural geometry along the transect, (iii) and the observed surface heat flux. It is possible that rock cooling is primarily driven by rock displacement along active faults and less by climatic and/or mantle buoyancy forces, which are both not included in the applied modelling approach. Our comprehensive thermochronological analyses allow two interpretations concerning mantle processes: (i) Assuming a strong coupling between the subducting and overriding plate, hence, the applicability of doubly-vergent orogen kinematics, then the thermochronological data are most consistent with an ongoing reversal in continental subduction polarity. (ii) A high degree of decoupling would negate the possibility that mantle processes are archived in the thermochronological record

Die neogene Entwicklung des zentralen Tien Schan, Kasachstan. Erste Ergebnisse von Apatit-Spaltspurdatierungen und morphotektonischer Analyse von Satellitendaten

Der Tien Schan ist ein etwa E–W erstrecktes, rund 2500km langes und bis 250km breites Gebirge in Zentralasien. Einzelne Gipfel sind über 7000m hoch. Obwohl durch die Kollision Indiens mit Asien entstanden, ist der Tien Schan ein Intraplatten-Orogen, dessen Hebung lange nach dem Beginn der Kollision vor 50Ma und weit nördlich der Sutur einsetzte (Sobel & Dumitru 1997). Von Tibet ist der Tien Schan durch das kaum deformierte Tarim-Becken getrennt. Hohe und schroffe Topographie, starke Seismizität (Molnar & Ghose 2000) und GPS-Daten zeigen, dass das Orogen auch heute sehr aktiv ist (Abdrakhmatov et al. 1996, Reigber et al. 2001). Der Tien Schan nimmt gegenwärtig etwa 40% der Gesamtkonvergenz Indiens mit Asien auf. Die Struktur des Tien Schan wird dominiert von E–Wstreichenden, nach N und S gerichteten Überschiebungen (Avouac et al. 1993, Yin et al. 1998), die sich meist deutlich in der Morphologie äußern. Auffallend ist die großräumige Gliederung des Orogens durch NW–SE-streichende dextrale Blattverschiebungen, die auch in das nördliche Vorland reichen (Tapponnier & Molnar 1979). Den Unterbau des Tien Schan bildet ein paläozoisches Akkretionsorogen (Zonenshajn et al. 1990). Im Mesozoikum entstand eine ausgedehnte Fastebene. In der späten Kreide oder dem frühen Tertiär setzte die Ablagerung kontinentaler Serien ein, die im jüngeren Känozoikum sehr mächtig werden. Die synorogenen Sedimente liegen manchmal konkordant, oft aber auch deutlich winkeldiskordant auf dem paläozoischen Sockel. In beiden Fällen bilden sie häufig asymmetrische Falten, die oft mit Störungen verknüpft sind. Geländestufen und ein starker Einfluss auf die Entwicklung des Entwässerungsnetzes weisen viele Störungen als gegenwärtig aktiv aus. Unser Untersuchungsgebiet liegt im Südosten Kasachstans. Es umfasst die Nordflanke des Tien Schan und seinen zentralen Teil mit den höchsten Erhebungen. Im Untersuchungsgebiet liegt das nach E propagierende Ende eines seismisch aktiven Störungssystems, das weiter westlich die nördliche Randstörung des Gebirges bildet, wo es unter der Millionenstadt Almaty (Alma- Ata) verläuft und eine ernste Bedrohung darstellt. Die Entwicklung dieses Störungssystems soll über verschiedene Zeitskalen mit verschiedenen Methoden untersucht werden...conferenc

Thermo-Kinematic Evolution of the Eastern Alps along TRANSALP: Exploring the Transient Tectonic State towards Slab Reversal

The Eastern Alps are shaped by the indentation of Adria into Europe and exhibit a doubly-vergent lithospheric wedge geometry. Immediately after the subduction of the Penninic ocean, pro- and retro-wedges have been established in the European and Adriatic plates, respectively. Recent tomographic studies, depicting several detached slab fragments beneath the Alps, have been interpreted as evidence of continuous southward subduction, contrary to an often-invoked subduction polarity reversal. Systematic changes in orogen-scale exhumation, driven by rock displacement along active faults, should reflect such change in subduction polarity. Low temperature thermochronology can evaluate upper lithospheric cooling as a response to changes in tectonic and/or erosional boundary conditions. This study investigates whether a potential change in locations of the pro- and retro-wedges is reconcilable with observed crustal re-organisations, exhumation patterns and mantle tomography. A suite of thermo-kinematic forward models driven by a new 2D structural-kinematic reconstruction of continental collision along the TRANSALP profile in the Eastern Alps has been subject to systematic sensitivity analyses encompassing variations in shortening rates, thermophysical parameters and topographic evolution, supplemented by new apatite and zircon fission-track data. Results from the thermo-kinematic modelling reproduce: (i) the orogen-scale structural geometry, (ii) the distribution of low-temperature thermochronometer ages, (iii) independently determined time-temperature paths, and (vi) the present-day surface heat flux. We suggest that the observed thermochronologic record along the TRANSALP profile is primarily driven by cooling through rock displacement along active faults. Our thermo-kinematic reconstruction emphasises a systematic southward shift of deformation, in particular in the Southern Alps, since onset of motion along the Tauern Ramp. Interpreting both, the Tauern Ramp as a mega retro-thrust and the southward shift of deformation in the Southern Alps, as a response to new Coulomb-wedge criterions, then our results are consistent with a Mid-Miocene reversal of continental subduction polarity. This time frame is compatible with a detachment of the European slab and a tectonic re-organisation of the Eastern Alps since ~10-25 Ma

Structural Thermochronology along Geophysical Transects through the Alps

Changes in the deep lithosphere (e.g., slab break-off or a switch in subduction polarity) potentially result in orogen-wide structural reorientations and changes in the pace and location of exhumation and Earth surface processes. In this project we combine bedrock thermochronology and balanced cross sections with thermo-kinematic modelling to reconstruct the cooling and exhumation history along geophysical profiles crossing the Central and Eastern Alps. Available thermochronological data together with new apatite and zircon (U-Th)/He ages taken along the NFP-20E, TRANSALP and EASI profile is used to test and improve existing across-strike, orogen-wide balanced cross sections. This ‘structural thermochronology’ method yields reliable information about the structural and kinematic evolution of the Alps since continental collision. As an example, thermochronological data along TRANSALP can be fitted with a kinematic model suggested by balanced cross sections and both datasets suggest a general shift from pro- to retro-wedge deformation, potentially related to a switch in subduction polarity

Thermo‐kinematic evolution of the Eastern European Alps along the TRANSALP transect

The eastern European Alps are shaped by the indentation of Adria into Europe. Recent tomography, depicting detached slab fragments, has been interpreted as evidence of continuous southward subduction of European lithosphere, contrary to an often-invoked subduction polarity reversal. Orogen-scale exhumation, driven by rock displacement along active faults, may reflect subduction polarity within the framework of doubly-vergent Coulomb wedge theory, provided the absence of rheological contrasts across the colliding plates. Low-temperature thermochronology can evaluate crustal cooling in response to changes in tectonic and erosional boundary conditions. This study investigates the consistency of observed crustal re-organization, exhumation, and mantle processes in the Eastern Alps. Thermo-kinematic forward models driven by reconstructions of crustal shortening along TRANSALP were subjected to variations in shortening rates, thermophysical parameters, and topographic evolution, supplemented by new fission-track data. The thermo-kinematic models reproduce: (i) the orogen-scale structural geometry, (ii) the distribution of thermochronometer ages, (iii) observed time-temperature paths, and (vi) the present-day surface heat flux. Results suggest that exhumation is driven by rock displacement along active faults without the need to involve mantle-driven buoyancy forces. Taken together, results identify two possible scenarios: if the Tauern Ramp is a retro-thrust and the southward shift of deformation in the Southern Alps is a response to new Coulomb-wedge conditions, then our results suggest a Mid-Miocene reversal of the subduction polarity. Alternatively, crustal deformation does not reflect mantle processes due to a high degree of inter-plate decoupling

Neogene Exhumation History along TRANSALP: Insights from Low Temperature Thermochronology and Thermo-Kinematic Models

Many convergent orogens such as the eastern European Alps display an asymmetric doubly-vergent wedge geometry. Loci of deepest exhumation are located above the overriding retro-wedge, whereas increased fault activity occurs in the pro-wedge on the subducting plate. The main drainage divide separates steeper from more gently sloping topography on the two wedges of different critical taper. We performed apatite and zircon (U-Th)/He analyses densely spaced along the TRANSALP geophysical transect in combination with thermo-kinematic models based on cross-section balancing. Our new low temperature thermochronology data and thermo-kinematic model results underline (i) deepest levels of exhumation across the Tauern Window until the Pliocene and (ii) higher Late Neogene exhumation rates south of the Periadriatic Fault relative to the north, while seismic activity is focussed across the Southern Alps. Our proposed mantle-to-surface link positions the retro-wedge north of the Periadriatic Fault subsequent to subduction polarity reversal during continental collision. Present-day drainage divide migration trends and imaged locations of mantle-lithospheric slabs beneath TRANSALP suggest ongoing, slow slab reversal since Adriatic indentation in the Eastern Alps

Geochronology, Geochemistry, and Geodynamic Implications of Permo-Triassic Back-Arc Basin Successions in the North Pamir, Central Asia

AbstractThe Permo-Triassic period marks the time interval between Hercynian (Variscan) orogenic events in the Tien Shan and the North Pamir, and the Cimmerian accretion of the Gondwana-derived Central and South Pamir to the southern margin of the Paleo-Asian continent. A well-preserved Permo-Triassic volcano-sedimentary sequence from the Chinese North Pamir yields important information on the geodynamic evolution of Asia’s pre-Cimmerian southern margin. The oldest volcanic rocks from that section are dated to the late Guadalupian epoch by a rhyolite and a dacitic dike that gave zircon U-Pb ages of ~260 Ma. Permian volcanism was largely pyroclastic and mafic to intermediate. Upsection, a massive ignimbritic crystal tuff in the Chinese Qimgan valley was dated to 244.1±1.1 Ma, a similar unit in the nearby Gez valley to 245±11 Ma, and an associated rhyolite to 233.4±1.1 Ma. Deposition of the locally ~200 m thick crystal tuff unit follows an unconformity and marks the onset of intense, mainly mafic to intermediate, calc-alkaline magmatic activity. Triassic volcanic activity in the North Pamir was coeval with the major phase of Cimmerian intrusive activity in the Karakul-Mazar arc-accretionary complex to the south, caused by northward subduction of the Paleo-Tethys. It also coincided with the emplacement of basanitic and carbonatitic dikes and a thermal event in the South Tien Shan, to the north of our study area. Evidence for arc-related magmatic activity in a back-arc position provides strong arguments for back-arc extension or transtension and basin formation. This puts the Qimgan succession in line with a more than 1000 km long realm of extensional Triassic back-arc basins known from the North Pamir in the Kyrgyz Altyn Darya valley (Myntekin formation), the North Pamir of Tajikistan and Afghanistan, and the Afghan Hindukush (Doab formation) and further west from the Paropamisus and Kopet Dag (Aghdarband, NE Iran)