Tectonic and climatic forcing in orogenic processes [Elektronische Ressource] : the foreland basin point of view, Alborz mountains, N Iran / Paolo Ballato

Paolo Ballato

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• Publié par: universitat_potsdam
• Publié le: 01 janvier 2009
• Nombre de lectures: 33
• Langue: English
• Poids de l'ouvrage: 30 Mo

Extrait

Tectonic and climatic forcing in orogenic processes: the foreland basin point of
view, Alborz mountains, N Iran


Paolo Ballato



Dissertation zur Erlangung des akademischen Grades
Doktor der Naturwissenschaften (Dr. rer. nat.)
in der Wissenschaftsdisziplin Geologie

eingereicht an der Mathematisch-Naturwissenschaftlichen Fakultät
der Universität Potsdam


Potsdam, im August 2009


































Tag der Disputation: 26 November 2009






Gutachter

Prof. Dr. Manfred Strecker Prof. Dr. Anke Friedrich
Potsdam Universität LMU München




















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Published online at the
Institutional Repository of the University of Potsdam:
URL http://opus.kobv.de/ubp/volltexte/2010/4106/
URN urn:nbn:de:kobv:517-opus-41068
http://nbn-resolving.org/urn:nbn:de:kobv:517-opus-41068
ii
Abstract

Foreland-basin systems are excellent archives to decipher the feedbacks between
surface and tectonic processes in orogens. The sedimentary architecture of a foreland-basin
system reflects the balance between tectonic subsidence causing long-term accommodation
space and sediment influx corresponding to efficiency of erosion and mass-redistribution
processes. In order to explore the effects of climatic and tectonic forcing in such a system, I
investigated the Oligo-Miocene foreland-basin sediments of the southern Alborz mountains, an
intracontinental orogen in northern Iran, related to the Arabia-Eurasia continental collision.
40 39This work includes absolute dating methods such as Ar/ Ar and zircon (U-Th)/He
thermochronology, magnetostratigraphy, sedimentological analysis, sandstone and
conglomerate provenance study, carbon and oxygen isotope analysis, and clay mineralogy
study. Results show a systematic correlation between coarsening-upward cycles and sediment
5 6accumulation rates in the basin on 10 to 10 yr time scales. During thrust loading phases, the
coarse-grained fraction supplied by the uplifting range is stored in the proximal part of the basin
(sedimentary facies retrogradation), while fine-grained sediments are deposited in distal sectors.
Variations in sediment provenance during these phases of enhanced tectonic activity give
evidence for erosional unroofing phases and/or drainage-reorganization events. In addition,
enhanced tectonic activity promoted the growth of topography and associated orographic barrier
effects, as demonstrated by sedimentologic indicators and the analysis of stable C and O
isotopes from calcareous paleosols and lacustrine/palustrine samples.
Extensive progradation of coarse-grained deposits occurs during phases of decreased
subsidence, when the coarse-grained fraction supplied by the uplifting range cannot be
completely stored in the proximal part of the basin. In this environment, a reduction in basin
subsidence is associated with laterally stacked fluvial channel deposits, and is related to intra-
foreland uplift, as documented by growth strata, tectonic tilting, and sediment reworking.
Increase in sediment accumulation rate associated with progradation of vertically-stacked
coarse-grained fluvial channels also occurs. Paleosol O-isotope data shows that this increase is
related to wetter climatic phases, suggesting that surface processes are more efficient and
exhumation rates increase, giving rise to a positive feedback. Furthermore, isotopic and
sedimentologic data show that starting from 10-9 Ma, climate became less arid with an increase
in seasonality of precipitation. Because important changes were also recorded in the
Mediterranean Sea and Asia at that time, the evidence for climatic variability observed in the
Alborz mountains most likely reflects changes in Northern Hemisphere atmospheric circulation
patterns.
This study has additional implications for the evolution of the Alborz mountains and the
Arabia-Eurasia continental collision zone. At the orogenic scale, the locus of deformation did
not move steadily southward, but stepped forward and backward since Oligocene time. In
particular, from ~ 17.5 to 6.2 Ma the orogen grew by a combination of frontal accretion and
wedge-internal deformation on time scales of ca. 0.7 to 2 Ma. Moreover, the provenance data
suggest that prior to 10-9 Ma the shortening direction changed from NW-SE to NNE-SSW, in
agreement with structural data.
On the scale of the entire collision zone, the evolution of the studied basins and adjacent
mountain ranges suggests a new geodynamic model for the evolution of the Arabia-Eurasia
continental collision zone. Numerous sedimentary basins in the Alborz mountains and in other
locations of the Arabia-Eurasia collision zone record a change from a tensional (transtensional)
to a compressional (transpressional) tectonic setting by ~ 36 Ma. I interpret this to reflect the
onset of subduction of the stretched Arabian continental lithosphere beneath central Iran,
leading to moderate plate coupling and lower- and upper-plate deformation. The increase in
deformation rates in the southern Alborz mountains from ~ 17.5 Ma suggests that significant
upper-plate deformation must have started by early Miocene time, most likely in response to an
increase in degree of plate coupling. I suggest that this was related to the subduction of thicker
Arabian continental lithosphere and the consequent onset of hard continental collision. This
model reconciles the apparent lag time of 15-20 Ma between the late Eocene to early Oligocene
iii age for the initial Arabia-Eurasia continental collision and the onset of widespread deformation
across the collision zone to the north in early to late Miocene time.


















































iv Zusammenfassung

Systeme von Vorlandbecken repräsentieren bedeutende geologische Archive und dienen
dem Verständnis von Rückkopplungen zwischen oberflächennahen und tektonischen Prozessen.
Außerdem dokumentieren sie die Entwicklung unmittelbar angrenzender Bergketten. Die
sedimentären Abfolgen in Vorlandbecken reflektieren das Gleichgewicht zwischen tektonischer
Subsidenz, der Bildung langzeitlichen Akkommodationsraumes und des Sedimenteintrages,
welcher wiederum die Wirksamkeit von Erosions- und Massenneuverteilungsprozessen
wiederspiegelt. Um die Effekte von Klima und Tektonik in einem solchen System zu
erforschen, untersuchte ich die Oligo-Miozänen Sedimente in den Vorlandbecken der südlichen
Elburs Bergkette, einem intrakontinentalen Gebirge in Nord-Iran, das im Zuge der Arabisch-
Eurasischen Kontinent-Kollision herausgehoben wurde.
In dieser Studie der Vorlandbeckensedimente wurden Datierungstechniken angewandt
40 39( Ar/ Ar, (U-Th)/He Thermochronologie und Magnetostratigraphie), die Sedimente und deren
Herkunft analysiert und die Tonmineralogie, sowie Sauerstoff- und Kohlenstoffisotope
5 6untersucht. Die Ergebnisse zeigen, dass auf einer Zeitskala von 10 bis 10 Jahren eine
systematische Korrelation zwischen „coarsening upward“ Zyklen und den sedimentären
Akkumulationsraten besteht. Während sukzessiver Überschiebungsphasen werden die durch
Hebung der Bergkette bereitgestellten groben Kornfraktionen in proximale Bereiche des
Beckens geliefert und feinkörnige Fazies in distalen Beckenregionen abgelagert. Variationen in
der Sedimentherkunft in Phasen größerer tektonischer Aktivität zeugen von erosionaler
Abdeckung und/oder der Umorganisation natürlicher Entwässerungsstrukturen. Außerdem
zeigen die Untersuchungen an stabilen Isotopen, dass die verstärkte tektonische Aktivität das
Anwachsen der Topographie förderte und damit die Wirksamkeit einer topographischen
Barriere erhöhte.
Wenn aufgrund nachlassender Beckenabsenkung die grobe Kornfraktion nicht
vollständig im Nahbereich des Beckens aufgenommen werden kann breitet sie sich in ferne
Beckenregionen aus. Im Elburs wird die verringerte Subsidenz durch eine interne Hebung des
Vorlandes hervorgerufen und ist mit einer lateralen Stapelung von Flussbetten assoziiert.
Dokumentiert wird dies anhand konsequenten Schichtwachstums, tektonischer Schrägstellung
und sedimentärer Umlagerung. Gleichzeitig nehmen die Sedimentationsraten zu. Die
Sauerstoff-Isotope der Paläoböden zeigen, dass dieser Anstieg mit einer Phase feuchteren
Klimas einhergeht, wodurch Oberfläche