1. S0
Hangay
Dome
Western
Altai
Gurvan
Sayhan
Nemegt Uul
Tien
Shan
70km
Gobi Altai
Fig 1
Introduction
The Gobi Altai Mountains in Mongolia provide a superb natural laboratory for
studying active processes of oblique deformation, strain partitioning and restraining
bend nucleation and growth along major transpressional fault systems. Nemegt Uul
is a recently uplifted mountain range formed at a restraining bend along the
sinistral Gobi-Tien Shan fault system in southern Mongolia. In this study, structural
and lithological transects, mapping, and image analysis of Landsat images and
DEMs are used to document the 3D architecture of Nemegt Uul, highlighting the
importance of polyphase deformation in the evolution of the region.
The basement rocks in the range consist of Palaeozoic greenschist-grade
metasediments and metavolcanic successions cut by a variety of intrusive rocks
that are thrust to the north and south over range bounding Jurassic-Cretaceous
clastic deposits. Ophiolitic rocks in the west of the range are thrusted towards the
north over Palaeozoic metasediments, suggesting Nemegt Uul marks a major
Palaeozoic suture zone. The area has a complex history of deformation. At least
two phases of folding and ductile thrusting are associated with Palaeozoic terrane
accretion and ophiolite obduction in southern Mongolia (D1 and D2). Brittle normal
faults which cut Palaeozoic structures, and syn-sedimentary normal faults
bounding Jurassic-Cretaceous basins indicate a phase of Mesozoic extension
(D3). Cenozoic faults (D4) define the asymmetric bivergent flower structure seen
today.
Polyphase deformation and intraplate mountain building at the Nemegt Uul restraining bend in the Gobi Altai
Mountains, Mongolia.
Stephen Rippington (sjr32@le.ac.uk), Dickson Cunningham, Richard England
Crustal Processes Group, Department of Geology, University of Leicester, UK
Cenozoic faults
Faults of unknown age
Figure 15. Generalised cross-section for central Nemegt Uul. Major faults shown, with those identified as
Cenozoic from cross-cutting field relationships marked in bold. Fault relationships at depth are unknown, but
conservative extrapolation of faults to depth suggests that Nemegt Uul is an asymmetric positive flower structure.
D2 thrust faults
D3 normal faults
D4 oblique-slip thrust faults
Stretching lineation/slickenline
Figure 14. Stereonets showing fault planes and stretching
lineations/slickenlines for the D2 contractional event, D3 extensional
event, and D4 transpressional event. Strong parallelism between fault
planes formed in different deformation events, suggests that D1 and D2
contractional faults and fabrics have been reactivated by subsequent
deformation events
Fig 15Figure 1. Location map of Nemegt Uul, Gobi Altai Mountains, Mongolia. Nemegt Uul is forming at a restraining bend
along the left-lateral Gobi-Tien Shan strike-slip fault system (marked in yellow).
Fig 2
Figure 2. Landsat 5 image of Nemegt Uul, with major faults and transect locations marked. Nemegt Uul is bound by
outward directed thrust faults, which place Devonian-Carboniferous basement over Cretaceous clastic basin fill.
Figure 3. Map of Nemegt Uul showing principal lithologies and major faults in the range. Metamorphosed blocks are
fault bound. Volcanic rocks and large proportion of meta-volcaniclastic rocks suggest basement rocks were
deposited in arc-proximal environment. Devonian ophiolitic rocks in the west may define a Palaeozoic suture zone.
Fig 4
Figure 4. Large E-W trending F1 fold in unmetamorphosed Carboniferous sandstones and conglomerates. Bedding is folded on a 500m scale. D1 was driven by N-S
contraction, folding bedding and forming S1 cleavage.
Figure 5. Axial planar S1 cleavage in folded
quartz-mica sandstone.
Figure 6. D2 tight N-vergent F2 folds of S1 cleavage
and bedding (S0).
Figure 7. Relict F1 folds of bedding (S0) with fold limbs
parallel to S1 cleavage.
Fig 6
S1
S0
Fig 7
Fig 8
Fig 9
Figure 8. Muscovite and quartz in tight F2
folds in phyllite.
Figure 9: North vergent D2 thrust shear zone cutting N
vergent F2 folds.
Figure 10: D3 brittle normal fault, displacing phyllites
and cutting all ductile fabrics.
Figure 11. Close up of D4 fault connecting N and S fronts. Fault changes vergence along strike. Slip vectors on fault show left-lateral oblique-slip is dominant.
Fig 11
Fig 10 Conclusions
1.Palaeozoic basement rocks record two phases of contractional
ductile deformation and ophiolite obduction during arc terrane
accretion.
2.Mesozoic and Cenozoic faults reactivate Palaeozoic fabrics,
giving rise to strong parallelism between multiple generations of
faults.
3.Late Cenozoic transpression at a restraining bend along the
Gobi-Tien Shan fault system has formed a long, thin, bivergent
sigmoidal shaped mountain range.
4.Cenozoic transpressional deformation is oblique-slip dominated,
with little partitioning into thrust and strike-slip components.
Fig 12 Fig 13
Figure 12. Cenozoic (D4) left-lateral oblique slip
fault connecting north and south fronts. Granite is
being thrust north-east on a south dipping fault
plane.
Figure 13. South-vergent Cenozoic (D4) range bounding
thrust fault on south side of Nemegt Uul. Cretaceous clastic
basin sediments are tilted basinward as Silurian phyllites
are thrust south.
S0
Fig 14 D2 PZ D3 MZ D4 CZ
View E
Fig 5
S1
S0
View E View E
View W
View W
Granites
Carboniferous mafic volcanics
Carboniferous unmetamorphosed Palaeozoic sediments
Devonian Ophiolitic rocks
Silurian – Devonian greenschist grade metasediments
View W View W
Fig 3