Download active deformation in zagros-makran transition zone inferred from

Fourth International Conference of
Earthquake Engineering and Seismology
12-14 May 2003 Tehran, Islamic Republic of Iran
ACTIVE DEFORMATION IN ZAGROS-MAKRAN TRANSITION
ZONE INFERRED FROM GPS MEASUREMENTS
R. Bayer1, E. Shabanian 2, V. Regard3, P. Vernant4, F. Nilforoushan5,
M. Abbassi6, J. Chery7, M. Tatar8, E. Doerflinger9, M. Peyret10,
M. Daignières11, O. Bellier12, D. Hatzfeld 13, M. Mokhtari14
1
Professor, LDL/ISTEEM, Univ. Montpellier II, France. [email protected]
Engineer, IIEES, Tehran, I. R. Iran. [email protected]
3
PhD Student, CEREGE, Univ. Aix Marseille III, France. [email protected]
4
PhD Student, LDL/ISTEEM, Univ. Montpellier II, France. [email protected]
5
Head of Geodynamics Section, NCC, Tehran, I. R. Iran. f-nilf@ ncc.neda.net.ir
6
Head of Seismotectonic Department, IIEES, Tehran, I. R. Iran.
2
7
mammad @neda.iiees.ac.ir
Research CNRS, LDL/ISTEEM, Univ. Montpellier II, France. [email protected]
Head of department of Seismology, IIEES, Tehran, I. R. Iran. @neda.iiees.ac.ir
9
Research Engineer, LDL/ISTEEM, Univ. Montpellier II, France. [email protected]
10
Research Engineer, LDL/ISTEEM, Univ. Montpellier II, France.
8
11
[email protected]
Professor, LDL/ISTEEM, Univ. Montpellier II, France. [email protected]
Professor, CEREGE, Univ. Aix Marseille III, France. [email protected]
13
Reseach Director, LGIT/IRIGM, Univ. Grenoble, France.
12
14
[email protected]
Director of Seismology department, IIEES, Tehran, I. R. Iran. [email protected]
ABSTRACT
The Bandar Abbas-Strait of Hormuz zone is considered as a transition between the Zagros
collision and the Makran oceanic subduction. In this area, the transfert of the deformation is
mainly accomodated along the NNW-SSE trending reverse right lateral Zendan-Minab-Palami
faults (ZMP) system.
We used GPS network measurements collected in 2000 and 2002 to better understand the
distribution of the deformation between the collision zone and the Makran subduction.
Analysing the GPS velocities we show that 1) Arabia and Eurasia converge in this area at
velocity of 23 mm/yr, 2) rate of shortening in the Eastern Zagros is ~8mm/yr, 3) velocities on
both sides of the ZMP system are in agreement with a NNW-SSE trending reverse right lateral
faulting at rate of 10 mm/yr.
1. INTRODUCTION
The present-day N-S convergence between the Arabian and the Eurasian plates is partially
accommodated in southern Iran along the Zagros fold and thrust belt with a shortening of ~ 8
mm/yr in the central part of the belt . Along the iranian coast, the Oman oceanic lithosphere
subducts at rate of ~ 18 mm/yr near the Iran- Pakistan boundary (Vernant et al., 2002). The
Musandam peninsula-Bandar Abbas-Strait of Hormuz zone, also called the “ Oman line ”, is
considered to be the transition between the Zagros continental collision and the Makran oceanic
subduction. The Zendan-Minab-Palami fault (ZMP) system connects the Eastern Zagros and the
Western Makran domains from the Main Zagros Thrust area to the Western end of the Makran
wedge. To the East, the active Jiroft-Sabzevaran fault system is considered as the south
prolongation of the Nayband fault system. In order to better understand transfert of the
deformation, from collision to subduction, an integrated program including GPS, tectonics and
seismological studies have been conducted between 2000 and 2002. In this paper, we present
the results of 2 GPS campaigns realized in the transition zone. The estimated velocity field is
discussed in relation with geometrical and kinematics constraints deduced from tectonic
observations (Regard et al., 2003a,b).
2. TECTONIC AND GEOMORPHIC BACKGROUND
The ZMP fault system is 20-30 km wide and composed of 3 parallel faults which evidence for
a late quaternary reverse right-lateral faulting (fig. 1; Regard et al., 2003a). The ZMP system is
tied to the Main Zagros Thrust. NE of the studied zone, The NS Jiroft-Sabzevaran system is
characterized by a right lateral strike slip motion and bends southward along the southern limit
of the Jaz Murian depression. In the ZMP zone, the estimated N 45°
Figure1. Structural map of the Zagros-Makran transition zone from Regard et al (2003a).
Black filled circles indicate the location of the GPS sites.
trending compressional stress is oblique to the N-S trend of the Arabia-Iran convergence rate
(DeMets et al., 1990; Vernant et al.,2002). The right lateral strike slip component of the ZMP
system increases from the South to the North. Combining tectonic and geomorphic analysis
with cosmonucleide dating reveals a total right lateral velocity of ~10 mm/yr for Zendan-Minab
sytem and ~5mm/yr for Jiroft-Sabzevaran system (Regard et al., 2003b). For this last, the
estimated rate is close to the 2mm/yr proposed by Walker and Jackson (2002) for the Nayband
fault, northward prolongation of the Jiroft fault.
3. GPS MEASUREMENTS
It was decided in 2000 to install a GPS network covering the area extending from the eastern
Zagros to the western Makran (fig.1). The network consists of 15 benchmarks covering a
domain from Bandar Abbas to Jask, in longitude, and from Jask to Hajiabad, in latitude. The
extension of the network is ~250 km with a mean distance between sites of 60 km. The network
was measured in january 2000 and the measurements were repeated in january 2002. For both
epochs, the benchmarks were measured during 2 days, except some reference stations installed
permanently during the campaign (Minab site in 2000-2002 and Razd in 2002)
4. DATA PROCESSING
Analysis of the data is realized with GAMIT/GLOBK software packages. IGS stations are used
to locate our local network within the ITRF 2000. Solutions are computed using relaxed IGS
final orbits. Horizontal repeatability of baseline is ~1,7 mm in 2000 and ~2,6 mm in 2002 (see
fig. 1 and table 1). The accuracy of the horizontal rates deduced from the repeatabilities in 2000
and 2002 is ~1,6 mm/yr. The local GPS velocity solutions are combined with global
observations given by SOPAC from January 1995 to march 2002 (Bock et al.,1997; solutions
available at http://lox.ucsd.edu). GPS velocities and their 95% confidence ellipses are estimated
1) in ITRF 2000, 2) in an Eurasian fixed reference frame, 3) with respect to the Arabian plate.
16 European stations are chosen to fix the Eurasian plate. Pole coordinates (49,918 N, -0,856 E,
0,5241deg/yr) for Arabia given by Vernant et al, (2002) is used for computing the final
velocities with respect to Arabia. GPS velocities deduced from the GPS Global Network of Iran
(1999 and 2001 campaigns) complete our local results (Vernant et al, 2002).
Figure 2. Baseline component repeatability versus baseline length for 2000 and 2002. Strategy : relaxed
orbits, free ambiguities and fixed ambiguities. The best repeatability is obtained by fixing ambiguities
Table 1. Baseline component repeatability for different solutions
2000 relaxed IGS orbits, free ambiguity
fixed ambiguity
2002 relaxed IGS orbits, free ambiguity
fixed ambiguity
North
1,4 mm
1,4 mm
0,8 mm
1,0 mm
East
2,3 mm
1,7 mm
3,2 mm
2,6 mm
Up
3,7 mm
3,4 mm
3,5 mm
3.8 mm
5.DISTRIBUTION ANALYSIS OF THE GPS VELOCITIES
In the Eurasia fixed reference frame, velocity rates show a N 10° orientation at all sites of the
Zagros–Makran transition zone (fig.3a). Sites located in Musandam peninsula and along the
Iranian coastal sites are characterized by velocities ~23mm/yr. From the coast to the Main
Zagros thrust, the rate of shortening in the Eastern Zagros is 8mm/yr. Crossing the ZMP system
provide evidence for a decrease of velocity of 10 mm/yr in the N10° trend. East of these faults,
rates are 13 mm/yr. At the latitude of Kahnuj, rates on both sides of the Jiroft-Sabzevaran faults
system varies from 16mm/yr at west to east 13mm/yr at east.
In the Arabia fixed reference frame (fig.3b), velocities of iranian coastal sites are “ radially
oriented ” around the Musandam peninsula. The transpressional tectonic regime is clearly
confirmed by the distribution of rates. An increase of the right strike slip component along the
ZMP system is also observed from the south to the north.
Figure 3. Velocity distribution estimated from GPS data collected during the 2000 and 2002 campaigns
A) in Eurasia fixed reference frame B) in Arabia fixed reference frame.
6.CONCLUSIONS
Slow velocities are evidence for a weak strain beneath the Persian gulf . The ZMP system is a
major tectonic discontinuity zone as it is shown by teleseismic tomography (Yaminifard et
al.,2003) and Bouguer anomaly map (Ravaut et al., 1997). This deformation zone accomodates
and transfers at shallow depth the convergence from the Eastern Zagros to the Makran
subduction within a transpressional tectonic regime at velocity of 10 mm/yr. No partionning is
associated with the faults system. Focal mechanism for shallow earthquakes are in agreement
with the right strike slip fault regime of the ZMP faults (Yamini Fard et al.,2003). Absence of
strong historic earthquakes and present-day relative low seismicity in the south may be
explained by long recurrence intervals or by aseismic behaviour at all times. The JiroftSabzevaran system and oblique faults spreading toward the Makran accomodates less motion.
Focal mechanisms (reverse faults) associated with deep earthquakes (20 km and deeper)
(Yamini Fard et al. 2003) probably characterize the underthrusting of the NE wedge of the
Arabian crust beneath the Iranian crust at velocity of 8mm/yr . Rotation of residual velocities at
proximity of the Musandam peninsula may be caused by the indentation of the Musandam
promontary into the Iranian crust.
7. Acknowledgements
This study was supported by INSU-CNRS (program « Intérieur de la Terre »), by MAE
(Ministère des Affaires Etrangères) and by the International Institute of Earthquake Engineering
and Seimology (IIEES) in the frame of a co-operative research program. We thank M. G.
Ashtyani, president of IIEES who has supervised this work. We thank also M. Mokhtari for
support and administrative assistance and IIEES teams for field work assistance. The NCC have
joined this study in 2002 by participating to the 2002 survey. We thank F. Tavakoli, director of
the land-surveing and geodetic department of NCC for support. We thank also NCC team of
Bandar Abbas for field assistance.
8. REFERENCE
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