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Alpine thermal and structural evolution of the highest external crystalline massif : the Mont Blanc
(2005)
The alpine structural evolution of the Mont Blanc, highest point of the Alps (4810 m), and of the surrounding area has been reexamined. The Mont Blanc and the Aiguilles Rouges external crystalline massifs are windows of Variscan basement within the Penninic and Helvetic nappes. New structural, Ar-40/Ar-39, and fission track data combined with a compilation of earlier P-T estimates and geochronological data give constraints on the amount and timing of the Mont Blanc and Aiguilles Rouges massifs exhumation. Alpine exhumation of the Aiguilles Rouges was limited to the thickness of the overlying nappes (similar to 10 km), while rocks now outcropping in the Mont Blanc have been exhumed from 15 to 20 km depth. Uplift of the two massifs started similar to 22 Myr ago, probably above an incipient thrust: the Alpine sole thrust. At similar to 12 Ma, the NE-SW trending Mont Blanc shear zone (MBsz) initiated. It is a major steep reverse fault with a dextral component, whose existence has been overlooked by most authors, that brings the Mont Blanc above the Aiguilles Rouges. Total vertical throw on the MBsz is estimated to be between 4 and 8 km. Fission track data suggest that relative motion between the Aiguilles Rouges and the Mont Blanc stopped similar to 4 Myr ago. Since that time, uplift of the Mont Blanc has mostly taken place along the Mont Blanc back thrust, a steep north dipping fault bounding the southern flank of the range. The "European roof'' is located where the back thrust intersects the MBsz. Uplift of the Mont Blanc and Aiguilles Rouges occurred toward the end of motion on the Helvetic basal decollement (HBD) at the base of the Helvetic nappes but is coeval with the Jura thin-skinned belt. Northwestward thrusting and uplift of the external crystalline massifs above the Alpine sole thrust deformed the overlying Helvetic nappes and formed a backstop, inducing the formation of the Jura arc. In that part of the external Alps, similar to NW-SE shortening with minor dextral NE-SW motions appears to have been continuous from similar to 22 Ma until at least similar to 4 Ma but may be still active today. A sequential history of the alpine structural evolution of the units now outcropping NW of the Pennine thrust is proposed
Cenozoic magnetostratigraphy and magnetic properties of the southern Issyk-Kul basin, Kyrgyzstan
(2014)
We present paleomagnetic data from the northern flank of the Tianshan range, southeast of Lake Issyk-Kul (Kyrgyzstan). 613 cores were collected in two parallel sections with a total thickness of 960 m (Chon Kyzylsuu, CK) and 990 m Jeti Oguz, JO), as well as 48 cores at six sites in a nearby anticline. Rock magnetic analyses identify both magnetite and hematite in the fluvial-lacustrine sediments. The concentration of both minerals, the magnetite:hematite ratio, and the average magnetite grain size increase upward in both sections. Anisotropy of anhysteretic remanent magnetization defines a tectonic fabric with sub-horizontal maximum axes that parallel the strike direction together with intermediate and minimum axes that streak out about a great circle orthogonal to the maximum axes suggestive of a tectonic fabric emplaced during folding. Stepwise thermal demagnetization isolates interpretable magnetization components in 284 samples that define 26 polarity chrons in CK and 19 in JO. A positive fold test, dual polarities and systematic changes in rock-magnetic parameters with depth suggest that the high temperature magnetization component was acquired coevally with deposition. An age model based on a visual magnetostratigraphic correlation of both sections with the geomagnetic polarity time scale defines absolute ages from 26.0 to 13.3 Ma, with a fairly constant sedimentation rate of 9-10 cm/ka. A correlation based on a numerical algorithm arrives at a slightly different conclusion, with deposition ages from 25.2 to 11.0 Ma and sedimentation rates from 5 to 8 cm/ka. In comparison with sedimentation rates found at other magnetostratigraphic sections in the Tianshan realm, we infer that the sedimentary record in this part of the Issyk-Kul Basin precedes the more rapid phase of uplift of the Kyrgyz Tianshan. The onset of deposition and concomitant erosion of the adjacent Terskey Range is in good agreement with independent assessments of the exhumation history of this mountain range, with erosion increasing at 25-20 Ma and accelerating after 11-13 Ma. (C) 2014 Elsevier B.V. All rights reserved.
We present new U-Pb LA-ICP-MS data from the Central Andean foreland basins combined with new and published stratigraphic information in order to reconstruct the Miocene fragmentation of the Andean foreland between 26 and 28 degrees S. The disruption of this foreland basin and the subsequent development of elevated intermountain basins have been the focus of several studies. However, the absence of temporal constraints in the Miocene to Pliocene sedimentary record of the low elevation Choromoro and Tucuman foreland basins has presented an obstacle for precise paleogeographic reconstructions. We describe 11 discontinuous stratigraphic sections and use the U-Pb LA-ICP-MS method to date 10 pyroclastic-bearing sediments in order to reconstruct the stratigraphic evolution of the Choromoro and Tucuman basins. We combine our results with published strati graphic and thermochronologic data from adjacent basins to present a refined Miocene paleogeographic model. In a first stage, a continuous Early Miocene foreland lacustrine basin developed, filling up the preexisting Paleogene topography. The second stage is characterized by basin unroofing around similar to 12 Ma; the easily eroded sedimentary cover was removed, leading to the uplift of the underlying basement rocks and the segmentation of the lacustrine system. In the third stage, relief increase took place after similar to 6 Ma due to the low erodibility of the basement blocks; as a result, stable fluvial systems developed. Progressive relief development caused pronounced unconformities in the basins and the development of proximal fluvial-gravitational depositional systems after 3 Ma. This model emphasizes on the relations between tectonics, climate, and erodibility, and their control on the evolution of the depositional systems and relief.
Late Miocene-Pliocene onset of fluvial incision of the Cauca River Canyon in the Northern Andes
(2022)
The incision of kilometer-scale canyons into high-standing topography is often used to constrain the surface uplift history of mountain ranges, controlled by tectonic and geodynamic processes.
However, changes in climate may also be responsible for canyon incision. This study deciphers the timing of incision of the similar to 2.5-km-deep Cauca River Canyon in the Central Cordillera of the Northern Andes using the cooling (exhumation) history of rocks from the canyon walls and a regional analysis of channel steepness in rivers.
Ten bedrock samples and one detrital sample were collected on the eastern border of the canyon between 300 m and 2300 m of elevation.
Bedrock and detrital AFT data yield ages from 50 to 38 Ma, while two bed-rock AHe ages from the valley bottom yield ages of 7-6 Ma.
The AHe ages and inverse thermal history models reveal a previously unidentified late Miocene (ca. 7-6 Ma) pulse of exhumation that we interpret as the age of a single incision event that formed the Cauca River Canyon.
We conclude that the Cauca River Canyon was carved as a response to rock uplift in the northern Central Cordillera and propagation of an erosion wave into the mountain range starting in the latest Miocene.