Geodetic Insights into the Growth and Evolution of

Thrust Belts: Examples from East Iran

Alex Copley, Kirsty Reynolds, Romain JolivetCOMET, Bullard Labs, Department of Earth Sciences, University of Cambridge, UK

Key Questions:

• Which parts of thrust belts slip in earthquakes, and which

aseismically?

• How do each of these processes contribute to the growth of

topography and geological structures?

• What is the rheology of the faults?

2. The Shahdad thrust belt

1. The 1978 Mw 7.3 Tabas-e-

Golshan earthquake

1978 Mw 7.3 Tabas earthquakeBerberian (1978,1982)

Walker et al (2003,2013)

11,000 dead

13,000 population

Manuel Berberian

1978 Mw 7.3 Tabas earthquake

Blind thrusting. Minor and discontinuous surface slip on Neogene anticlines

Coseismic fault plane is low-angle (~15 degree dip), with 9km centroid depth

Berberian (1978,1982)

Walker et al (2003,2013)

Postseismic InSAR results

Stacks of 7 ERS and 11 Envisat

interferograms (cumulative

observation times of 21.7 and

46.2 years). Time span 1996 to

2009.

Signals visible in the individual

interferograms, and not

correlated with topography

Signal width ~ 5 km

Model of fault slip

• Creep on high-angle thrust

ramp

• Significantly faster than

time-averaged rate (~1—2

mm/yr; Walker et al 2013,

Walker et al 2009)

Approximate location of

low-angle fault plane at

depth (from aftershocks

and body-waveform

modelling; Berberian 1982,

Walker et al 2003)

Cumulative Mw from 1996 to

2009 = 5.7

Conceptual model of the seismic cycle at Tabas

This structural style is common in the geological record; here produced by afterslip

Walker et al 2013

The Shahdad

thrust belt and

Gowk fault

1998 Mw 6.6

Fandoqa

earthquake

Coseismic and first 6

months of postseismic

Right-lateral strike-slip with normal

component (displacements

saturated on this scale)

~8cm slip on low-angle thrust

(probably postseismic)

(Berberian et al 2001; Fielding

et al 2004)

5 to 11 years post-

earthquake

Ele

va

tio

n (

m)

LOS

disp

lace

me

nt (m

m)

Black: topography

Green: co- and early post-seismic

Red: 5 to 11 yrs postseismic

9 descending-track interferograms, 30 yrs cumulative time

Descending-track

interferogram stack

(ascending track unusable due to atmospheric effects)

Model of fault slip

Field photos (in 1998) from James Jackson

Transient or steady-state motion?Earthquake stress-changes would lead to

NORMAL-faulting

Pre-earthquake interferogram

Variation of slip rate through time

Driving forces for the steady-state creep

1.5 MPa

Lith

ost

ati

cp

ress

ure

Slip-rate vs shear stress

Inconsistent with: Diffusion or pressure-solution creep

Dislocation creep

Consistent with: rate and/or state dependent friction, e.g.

(if so, ‘a’ ~ 10-3)

> 40 mm/yr

1.6 to 1.9 MPa

3 to 8 mm/yr

1.5 MPa

Conclusions

• Creep on two ramp-and-flat fault systems

• Postseismic (both examples) and steady-state creep (Shahdad)

• Fault rheological law at Shahdad (rate-dependent friction)

• Contrasts: - postseismic timescales (< 6 months to > 30 yrs)

- ‘Flat’ behaviour (Mw 7.3 earthquake vs creeping)

-> possible effects of depth and stress pertubation?

• Construction of geomorphology and shallow geological structures

by aseismic creep

• Implications for hazard assessment (stress-shadowing and creep)

Ascending-track stack: 6 interferograms, 17.1 years