9. Bird, P. (1978b) Finite-element modeling of lithosphere deformation: The
Zagros collision orogeny, __Tectonophysics__, **50**, 307-336.

**Abstract**. In the Zagros Mountains a formerly stable continental
margin is being suddenly deformed. Finite element models of this tectonic flow
are used to relate known surface deformations, earthquake locations, and fault
plane solutions to unknown rock flow parameters and driving forces, in order to
determine the latter. These two-dimensional plane-strain flow models
incorporate the effects of nonlinear dislocation creep, frictional faulting,
geological inhomogeneity, density anomalies, and varying temperature. All
models driven by a subducting slab are unsuccessful because they require
subduction of continental crust, which does not match present seismicity.
Therefore the former oceanic slab must be detached, and the orogeny must be
driven by horizontal compression in the lithosphere. Models also show that the
subcrustal lithosphere is not shortened but acts as a stabilizing foundation.
These results imply a simple geometry of crustal deformation which can be
analytically modeled. The creep strength of the lower crust (75-100 bars)
determines the topographic slope of the Zagros. The fact that subduction is not
occurring on the old plate boundary places a limit on the shear stress
deforming the cold upper crust. This limit is 300 bars if there is limestone at
depth in the Crush Zone; otherwise 800 bars. These results are confirmed by a
final finite element model. The total driving force of the orogeny associated
with these limits is 2.8-5.5 ´ 10^{11}
dyne/cm, and the smaller amount could be provided by the gravitational
spreading of the Red Sea rift. Shear-strain heating caused by the orogeny to
date is less than 20° C. These results
imply that even unheated continental crust is considerably weaker than
laboratory friction measurements imply, and that it is mechanically decoupled
at the Moho from the stronger mantle lithosphere.

**P.S.** Units are not SI. Convert 75-100
bars = 7.5-10 MPa; 300 bars = 30 MPa; 800 bars = 80 MPa; 2.8-5.5 ´ 10^{11} dyne/cm = 2.8-5.5 ´ 10^{12} N/m. *P. Bird, 2000.08.31*