5th Swiss Geoscience Meeting, Geneva 2007
Fold Interference Patterns at the Precambrian
basement of Hassan-Robat Region, North Isfahan, Iran
Nadimi Alireza
Geology Department, Payame Noor University of Isfahan, Kohandej, Isfahan, Iran
(geotecton@yahoo.com).
Fold interference patterns are a useful tool when attempting to determine the
structural geometry of complexly deformed terranes. Folds are common
structures in many ductile shear zones. The origin of these folds is variable, as
their development may predate, be contemporaneous with, or postdate the
shearing event. Folds and other common meso- and microstructures in shear
zones (e.g. crenulations and boudins) are often asymmetric. This asymmetry is
regarded as a kinematic indicator (Ramsay 1979; Bjornerud 1989).
The Precambrian basement complex of Hassan-Robat Region, north Isfahan, in
the Sanandaj-Sirjan metamorphic belt of Iran, preserves evidence of multiple
orogenic events (Fig. 1). The Sanandaj-Sirjan Zone is a narrow band lies to the
northeast of the Zagros Fold-Thrust Belt. The SSZ is the inner crystalline zone
of the Zagros orogen, which has been highly deformed and exhumed during
Late Cretaceous-Eocene continental collision between Arabian plate and
central Iran.
The Hassan-Robat Region in the zone is an example of the exhumed
Precambrian basement complexes (Nadimi & Nadimi, 2006). In the region, the
Precambrian deformed marbles contains metamorphosed limestone and chert
has faulty contact with dolomite marble and granite with Precambrian age.
The region is a polydeformed region that records a complex shortening and
shearing history involving the development of crescent-shaped interference
pattern (Fig. 2a), disharmonic ptygmatic vertical fold (Fig. 2b), non-cylindrical
oblique fold with curved axis (Fig. 2c), disharmonic ptygmatic fold (Fig. 2d), noncylindrical recumbent with vertical axis fold (Fig. 2e and f) during the Pan
African orogeny and later phases. Measured strain (e=8.35) on the deformed
rocks show intensity of deformation.
Base on structural evidences and studied fold patterns, tectonic evolution of the
Hassan-Robat Region is interpreted as the product of three major sequential
tectonic events: 1) formation of beds of limestone and sandstones in the initial
basin. 2) Folding the strata during first orogenic phase that must has occurred in
Upper Precambrian (F1). 3) Refolding the deformed rocks during a high degree
of deformation and forming the different pattern of folds (F2). 4) Covering of the
old basements with younger sediments. 5) Exhumation of the old deformed
basement during Zagros orogeny and in a lateral extensional at the SSZ. In this
episode the old basements and strata located adjacent younger strata with
normal fault contact.
5th Swiss Geoscience Meeting, Geneva 2007
Figure 1. Topography map of Iran and its active belts. Thick words are: P-G=
Precambrian granite; P-D= Precambrian dolomitic marble; P-M= Precambrian
metamorphic and deformed marble and J-S= Jurassic slates.
Figure 2. a-f) Photos of the Hassan-Robat deformed marbles.
REFERENCES
Bjornerud, M., 1989: Toward a unified conceptual framework for shear sense
indicators. Journal of Structural Geology 11, 1045–1050.
Nadimi, A., & Nadimi, H., 2006: The Zagros Collision: Exhumation of Old Rocks
in the Northwestern Part of Zagros Mountain, Iran. Inter. Conf. Continental
Dynamics and Environmental Change of the Tibetan Plateau. PR China, 3.
Ramsay, D.M., 1979: Analysis of rotation of folds during progressive
deformation. Geological Society of America Bulletin 90, 732–738.