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Formation of conjugate strike-slip faults is commonly explained by the Anderson fault theory, which predicts a X-shaped conjugate fault pattern with an intersection angle of ~30 degrees between the maximum compressive stress and the faults. However, major conjugate faults in Cenozoic collisional orogens, such as the eastern Alps, western Mongolia, eastern Turkey, northern Iran, northeastern Afghanistan, and central Tibet, contradict the theory in that the conjugate faults exhibit a V-shaped geometry with intersection angles of 6075 degrees, which is 30-45 degrees greater than that predicted by the Anderson fault theory. In Tibet and Mongolia, geologic observations can rule out bookshelf faulting, distributed deformation, and temporal changes in stress state as explanations for the abnormal fault patterns. Instead, the GPS-determined velocity field across the conjugate fault zones indicate that the fault formation may have been related to Hagen-Poiseuille flow in map view involving the upper crust and possibly the whole lithosphere based on upper mantle seismicity in southern Tibet and basaltic volcanism in Mongolia. Such flow is associated with two coeval and parallel shear zones having opposite shear sense; each shear zone produce a set of Riedel shears, respectively, and together the Riedel shears exhibit the observed non-Andersonian conjugate strike-slip fault pattern. We speculate that the Hagen-Poiseuille flow across the lithosphere that hosts the conjugate strike-slip zones was produced by basal shear traction related to asthenospheric flow, which moves parallel and away from the indented segment of the collisional fronts. The inferred asthenospheric flow pattern below the conjugate strike-slip fault zones is consistent with the magnitude and orientations of seismic anisotropy observed across the Tibetan and Mongolian conjugate fault zones, suggesting a strong coupling between lithospheric deformation and asthenospheric flow. The laterally moving asthenospheric flow may have been driven by the converging cratons with thick mantle lithosphere. This may have caused the shallow asthenosphere below a region sandwiched between the cratons to be squeezed out laterally. 1. Scope of work Our ability to interpret lithospheric deformation depends critically on the knowledge that relates the observed fault geometry to the causative stress state. The most commonly used relationship in this regard is the Coulomb fracture criterion, which forms the basis for the famed Anderson fault classification (or “theory”) of X-shaped conjugate faults at ~30° from the maximum compressive3 To whom any correspondence should be addressed. Donald D Harrington Symposium on the Geology of the Aegean IOP Publishing IOP Conf. Series: Earth and Environmental Science 2 (2008) 012026 doi:10.1088/1755-1307/2/1/012026 c © 2008 IOP Publishing Ltd 1 stress (σ1) direction. [1, 2, 3] all noted that this pattern is not sustainable under finite-strain deformation. When examining conjugate fault systems at orogenic scales, one may also find that the X-shaped systems rarely occur in nature. Faults in dip-slip systems tend to have a single dip direction [see 4 for contractional systems and 5 and 6 for extensional systems]. Similarly, strike-slip conjugate systems tend to exhibit a V-shaped rather than X-shaped pattern. Not only do the V-shaped conjugate strike-slip faults defy the predicted X-shaped geometry, their orientations are also inconsistent with that inferred from the Coulomb fracture criterion in that they typically lie at 60-75o from the σ1 direction. This type of conjugate strike-slip fault system occurs widely in the Alpine-Himalayan collisional system with prominent examples in the eastern Alps [7,8], Turkey [9, 10, 11], Afghanistan [12, 13], Tibet [14, 15, 16], Mongolia [e.g., 17, 18], Indochina [19], and Gulf of Thailand [20, 21, 22]. Similar structures also occur in subduction zones such as the Venezuela Andean conjugate faults that host large hydrocarbon traps [e.g., 23, 24]. Although V-shaped conjugate faults were long noted in the context of extrusion tectonics [25, 26], analogue and slip-line theory models all failed to reproduce the observed fault geometry [e.g., 7, 8, 26, 27, 28, 29]. Because V-shaped conjugate strike-slip faults are dominant features in collisional orogens and may have accommodated significant continental convergence [e.g., 19], it is imperative to understand the dynamic origin and kinematic evolution of these important yet poorly understood structures. In this paper we present a new hypothesis for the development of the V-shaped conjugate faults by advocating the important role of orogen-parallel asthenospheric flow. Our model has key implications for addressing two fundamental questions in the studies of continental dynamics: (1) do continents deform in a continuum or micro-plate fashion [30, 31]? and (2) is upper and lower crust coupled during continental collision [32; cf., 33]? Several hypotheses were proposed to explain the non-Andersonian conjugate strike-slip faults. First, they could have developed by faults initiated at ~30° from the maximum principal stress (σ1) direction following the Coulomb fracture criterion, reaching their current orientations by later verticalaxis rotation via bookshelf faulting or distributed deformation [2, 34, 35, 36]. Alternatively, the σ1 and σ3 directions may switch with time, causing the sense of fault slip to reverse, creating a nonAndersonian fault pattern. Finally, pre-existing anisotropy can also produce a non-Andersonian fault geometry [e.g., 37, 38, 39]. One may also consider applying the von Mises yield criterion to explain the observed conjugate faults. The criterion predicts two orthogonal sets of faults along the maximum strain-rate directions (i.e., slip lines), which are bisected by the principal stresses at 45o. Not only the predicted fault pattern still departs significantly from the observed conjugate faults oriented at 60-75o from the σ1 direction, the prefect plasticity has never been observed experimentally for rock deformation [e.g., 40]. Experimentally, different faults can be produced under coaxial or non-coaxial strain conditions. The former results in conjugate faults as described by the Coulomb fracture criterion while the latter generates Riedel (R), conjugate Riedel (R’) and primary (P) shears [41, 42]. More recently, careful experimental work shows that the formation of P and R’ shears under non-coaxial deformation only develops in wet clays with exceedingly high cohesion, inappropriate for most rocks; for dry sand with no cohesive strength only R shears develop [43]. The different fault patterns under coaxial and non-coaxial conditions suggest that kinematics of deformation (e.g., velocity field) must also play a controlling role in fault formation. For coaxial deformation under a pure-shear condition, the velocity and strain fields are