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| Titel |
\textbf{Shift in Temporal Pattern of Dome Formation and Extension of the NE-Pamir Plateau} |
| VerfasserIn |
Rasmus Thiede, Edward Sobel, Lindsay Schoenbohm, Jie Chen, Daniel Stockli |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250049985
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| Zusammenfassung |
| Over decades there have been speculations about the geodynamic implications of the shift
from a long phase of Cenozoic orogen-perpendicular crustal shortening, growth and uplift, to
orogen-parallel extrusion that accommodates further shortening within the Tibetan plateau.
The Pamir mountains, the NW-prolongation of Tibet, appears to have undergone a similar
tectonic and morphologic history, however it’s evolution is not well constrained. To address
this problem we documented the crustal behavior through geologic time by analyzing
the orientation of the formation of metamorphic dome complexes through the late
Cenozoic.
The geologically young NE-Pamir is an excellent study location, as two generations of
dome complexes are exposed. During the Oligocene-Miocene, E-W-striking Muskul and
Sares domes exhumed mid-crustal rocks to the surface, followed by the late Miocene
N-S-trending extension driving the rise of two giant metamorphic dome complexes, the
Kongur Shan (7719 m) and Muztagh Ata (7546 m). Both domes are situated in the footwall
of the west dipping Kongur detachment system. Previous studies documented two
detachment generations along the southwestern termination of Muztagh Ata dome and late
Miocene onset of rapid exhumation of Kongur Shan-rocks from mid crustal depth.
Our goal is to better constrain the temporal and spatial variation of deformation
and exhumation in order to track changes in the upper crustal stress state of the
orogen.
Twenty new apatite fission-track (AFT) as well as zircon U-Th/He (ZrHe) cooling ages
from a series of vertical profiles in the footwall and spot hanging-wall-samples of the Kongur
detachment have been analyzed. This data is combined with published 40Ar-39Ar mica data.
In general, our new AFT and ZrHe cooling ages show significantly younger cooling ages in
the vicinity of the domes compared to rock samples analyzed further along strike and from
the hanging wall rocks. The youngest ages, between 3 mm/a), however, since then
exhumation rates have slowed down and not achieved rates >0.5 mm/a. Structural
observation and pervasive Ar-cooling ages of 10-8 Ma suggest that the rocks forming the
southern termination were exhumed along a N-S-deforming detachment (Shenti Fault) during
the middle Miocene before deformation shifted to E-W-oriented deformation along the
Kongur detachment. Thus this forms an important constrain for the transition from
N-S-to-E-W-oriented fault displacement. Preliminary 1D thermo-kinematic and erosion
modeling suggests that dome exhumation rates between the time of peak metamorphism and
the present has been as high as >3 mm/a, consistent with previous studies. Along
strike towards the NW rates decreased to ~0.7 mm/a over the last 3 Myr. Rocks
forming the hanging wall of the Kongur Detachment have been affected by relatively
low exhumation, with rates between 0.3 and 0.5 mm/a over the last ~8 Ma, if not
longer.
These results, in combination with structural observations, suggest a shift from
N-S-to-E-W directed dome exhumation occurred ~8 Ma. Since then, focused erosion
and exhumation has been limited mainly to the vicinity of the domes, allowing
them to exhume with up to threefold higher rates than the surrounding Pamir; the
Kongur-detachment accommodates the majority of fault displacement. One can speculate
whether the observed change in stress field of the NE-Pamir reflects a change in either
rheology, boundary conditions or gravitational potential energy, as discussed for Tibet. |
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