ALBERT

Description: The interpretation of low-temperature thermochronology (LTT) data in magmatic and metallogenic provinces requires a knowledge of the geothermal field through time. There, the challenge is differentiating rapid cooling following transient perturbations of the geotherms (reheating) from exhumational cooling induced by erosion during tectonic uplift or normal faulting. The Takab Range Complex (NW Iran) is a basement-cored range of the Arabia-Eurasia collision zone that experienced voluminous Eocene to Miocene magmatism and mineralization. Our new apatite and zircon (U-Th-Sm)/He and apatite fission track data, together with field observations, a dedicated numerical thermal model, and a re-evaluation of available geochronology data document the occurrence of a complex geological and thermal history including: (a) late Cretaceous-Paleocene exhumation possibly controlled by regional contractional deformation followed by Eocene deposition; (b) Oligocene to possibly early Miocene (29 to 22–20 Ma) exhumation of basement rocks from 13 to 8 km of depth, most likely through normal faulting during a thermal anomaly that led to migmatization and partial melting; (c) early to late Miocene (∼22–20 or earlier to 11–10 Ma) regional subsidence with deposition of an up to ∼2- to 3-km-thick Oligo-Miocene sedimentary sequence in association with the emplacement of shallow intrusions, which led to a partial to total reset of our LTT systems sometime between 18 and 13 Ma; and (e) erosional exhumation after 11–10 Ma with the development of a transpressional system and a master, right-lateral, strike slip fault (Chahartagh Fault). Our data highlights the impact of magmatic reheating on LTT ages in areas affected by intense magmatism.

Description: The evolution of orogenic wedges can be determined through stratigraphic and thermochronological analysis. We used apatite fission-track (AFT) and apatite and zircon (U–Th–Sm)  He (AHe and ZHe) low-temperature thermochronology to assess the thermal evolution of the Ukrainian Carpathians, a prime example of an orogenic wedge forming in a retreating subduction zone setting. Whereas most of our AHe ages are reset by burial heating, 8 out of 10 of our AFT ages are partially reset, and none of the ZHe ages are reset. We inverse-modeled our thermochronology data to determine the time–temperature paths of six of the eight nappes composing the wedge. The models were integrated with burial diagrams derived from the stratigraphy of the individual nappes, which allowed us to distinguish sedimentary from tectonic burial. This analysis reveals that accretion of successive nappes and their subsequent exhumation mostly occurred sequentially, with an apparent increase in exhumation rate towards the external nappes. Following a phase of tectonic burial, the nappes were generally exhumed when a new nappe was accreted, whereas, in one case, duplexing resulted in prolonged burial. An early orogenic wedge formed with the accretion of the innermost nappe at 34 Ma, leading to an increase in sediment supply to the remnant basin. Most of the other nappes were accreted between 28 and 18 Ma. Modeled exhumation of the outermost nappe started at 12 Ma and was accompanied by out-of-sequence thrusting. The latter was linked to emplacement of the wedge onto the European platform and consequent slab detachment. The distribution of thermochronological ages across the wedge, showing non-reset ages in both the inner and outer part of the belt, suggests that the wedge was unable to reach dynamic equilibrium for a period long enough to fully reset all thermochronometers. Non-reset ZHe ages indicate that sediments in the inner part of the Carpathian embayment were mostly supplied by the Inner Carpathians, while sediments in the outer part of the basin were derived mostly from the Teisseyre–Tornquist Zone (TTZ) or the southwestern margin of the East European Platform. Our results suggest that during the accretionary phase, few sediments were recycled from the wedge to the foredeep. Most of the sediments derived from the Ukrainian Carpathian wedge were likely transported directly to the present pro- and retro-foreland basins.

Description: Miocene strike-slip tectonics was responsible for creating and closing short-lived (ca. 6 Ma) passages and the emergence of isolated topography in the Northern Andes. These geological events likely influenced the migration and/or isolation of biological populations. To better understand the paleogeography of the Miocene hinterland and foreland regions in the Northern Andes, we conducted a source-to-sink approach in the Magdalena Basin. This basin is located between the Central and Eastern Cordilleras of Colombia and contains an ample Miocene record, which includes Lower Miocene fine-grained strata and Middle Miocene to Pliocene coarsening-up strata. Our study presents a new data set that includes detrital U–Pb zircon ages (15 samples), sandstone petrography (45 samples) and low-temperature thermochronology from the Southern Central Cordillera (19 dates); which together with previously published data were used to construct a paleogeographical model of the Miocene hinterland and foreland regions in the Northern Andes. The evolution of the Magdalena Basin during the Miocene was characterized by playa and permanent lake systems at ca. 17.5 Ma, which may be related to a marine incursion into NW South America and western Amazonia. The appearance of Eocene to Miocene volcanic sources in the Honda Group after ca. 16 Ma suggests the development of fluvial passages, which connected the Pacific with the western Amazonia and Caribbean regions. These passages were synchronous with a time of Miocene exhumation and topographic growth (ca. 16 to 10 Ma) in the Central Cordillera and the transition from lacustrine to fluvial deposition in the Magdalena Basin. Middle to Late Miocene strike-slip deformation promoted by oblique plate convergence and the oblique collision of the Panamá-Chocó Block likely explains the synchronous along-strike fragmentation and exhumation in the Central Cordillera.

Description: We explore the spatial and temporal variations in denudation rates in the northern Pamir—Tian Shan region using 10Be-derived denudation rates from modern (n = 110) and buried sediment (2.0–2.7 Ma; n = 3), and long-term exhumation rates from published apatite fission track (AFT; n = 705) and apatite (U-Th-Sm)/He (AHe; n = 211) thermochronology. We found moderate correlations between denudation rates and topographic metrics and weak correlations between denudation rates and annual rainfall, highlighting complex linkages among tectonics, climate, and surface processes that vary locally. The 10Be data show a spatial trend of decreasing modern denudation rates from west to east, suggesting that deformation and precipitation control denudation in the northern Pamir and western Tian Shan. Farther east, the denudational response of the landscape to Quaternary glaciations is more pronounced and reflected in our data. Modern 10Be denudation rates are generally higher than the long-term AFT and AHe exhumation rates across the studied area. In the Kyrgyz Tian Shan, on average, the highest 10Be denudation rates are recorded in the Terskey range, south of Lake Issyk-Kul. Here, modern denudation rates are higher than 10Be-derived paleo-denudation rates, which are comparable in magnitude with the long-term exhumation rates inferred from AFT and AHe. We propose that denudation in the region, particularly in the Terskey range, remained relatively steady during the Neogene and early Pleistocene. Denudation increased due to glacial-interglacial cycles in the Quaternary, but this occurred after the onset and intensification of the Northern Hemisphere glaciations at 2.7 Ma.

Description: The spatio-temporal evolution for the frontal part of the Kirthar fold and thrust belt in Pakistan is constrained for the first-time using apatite (U–Th-Sm)/He (AHe) and apatite fission track (AFT) dating of samples from the Oligocene Nari Formation in combination with a published balanced cross-section. The AHe ages appear to be fully reset, allowing us to date the timing of exhumation above ramps. Comparison of partially reset AFT ages (∼22–24 Ma) with previously published zircon fission track (ZFT) ages of the Nari Formation from nearby areas suggest that rocks in the frontal zone were not buried deeper than ∼4 km after deposition. The close range of AFT ages have implications for the timing of hinterland exhumation and Oligocene age of the Nari Formation. The AHe ages suggest that deformation along the major basement ramp was active since ∼7 Ma, forming a major topographic step (∼1.5 km) in the frontal part of the Kirthar fold and thrust belt. Combined analysis of the structural cross-section and thermal modeling of the samples suggest that major cooling occurred between ∼7 Ma and ∼5 Ma. This cooling was due to the erosion of Oligocene to Miocene strata from above the samples when Precambrian to Miocene strata was thrust above the pre-existing normal fault that acted as a basement ramp. The temporal structural evolution suggests that deformation in the frontal part of the Kirthar fold and thrust belt was characterized by faster rates (∼2.5 km/Ma) of orogenic growth and exhumation between ∼7 and 5 Ma, followed by slower shortening rates (〈1 km/Ma) since ∼5 Ma.

Description: The dynamic Neogene evolution of the Western European Alps included exhumation of the external crystalline massifs, thrust propagation to the foreland, drainage network reorganization, and major climatic variations. To constrain possible interactions between those factors, accurate geomorphological and sedimentological archives are required. However, intra-orogenic areas are subject to erosion, and extensive glacial cover during the Quaternary erased most of the geomorphic markers in the Alps. For these reasons, the genesis of the main features of the modern landscape, such as the major valleys and the drainage network, remains poorly understood. This study highlights how recently discovered karstic archives from the perched paleo-karst of the Obiou peak (Dévoluy massif, SE France) record the tectonic and drainage-network evolution of this part of the Alps during the Neogene. The Obiou caves are located at 2250-2380 m elevation, ∼1600 m above the modern Drac valley; they contain fluvial deposits including sand-clay units and rounded crystalline cobbles derived from the adjacent Ecrins-Pelvoux massif. As the Dévoluy and Ecrins-Pelvoux massifs are currently separated by the axial Drac valley (a major tributary of the Isère River), these cave sediments must have been deposited by a radial drainage system before incision of the modern Drac. We report new multi-method results from these sediments, including cosmogenic-nuclide burial dating (21Ne, 10Be, 26Al in quartz), provenance analysis (clast petrography and heavy-mineral analysis), and detrital thermochronology (apatite fission-track and (U-Th)/He) combined with a paleo-environmental reconstruction from palynology. 21Ne/10Be dating of cobbles and sand constrains the burial age to 11.5 ± 1.5 Ma, providing a maximum age for the modern axial drainage system and a minimum long-term incision rate of ∼140 m/Myr for the Drac valley. Comparison of the combined data to both modern rivers and nearby Oligocene foreland-basin deposits provides evidence for two successive drainage reorganizations. Early Miocene exhumation and development of high topography in the Ecrins-Pelvoux massif, linked to localized thrusting on a crustal-scale ramp, led to initial deflection of the antecedent radial drainage network, beheading its headwaters by establishment of the axial upper Durance valley. Subsequent propagation of thrusting into the subalpine Dévoluy massif and associated uplift during the mid to late Miocene led to establishment of the modern drainage system.