Structural and kinematic evolution of the teslin suture zone, Yukon: record of an ancient transpressional margin

Abstract

The Teslin suture zone (TSZ), Yukon, which forms the fundamental boundary between rocks deposited along the ancient margin of North America, and allochthonous terranes to the west, preserves a complex history of pre-mid-Jurassic convergence and dextral strike-slip translation which is overprinted locally by late Cretaceous dextral strike-slip shear. The TSZ, comprised of a 15–20 km thick structural sequence of ductilely deformed sedimentary and volcanic strata, basalt, peridotite and granitoids, is divisible into distinct elongate structural domains based on the distribution of two well-defined but differently oriented stretching lineations, L ds and L ss, L ds and L ss both formed during non-coaxial ductile deformation; L ds trends E-W and plunges down-dip, whereas L ss trends NNW-SSE and plunges shallowly. Two 1–2 km wide, NNW-trending anastomosing shear zones of L ss cross-cut L ds structures, indicating L ss is younger. Other field relations indicate further that L ds began forming earlier than L ss, followed by a period of coeval L ds and L ss formation; latest motion was dominantly parallel to L ss. Synkinematic mineral assemblages associated with both L ds- and L ss-related structures record greenschist to albite-epidote amphibolite facies conditions. L ds tectonites record a complex movement history: to the west textural asymmetries indicate west-side-down, or normal shear, whereas in eastern domains top-to-the-east thrust-style asymmetries dominate. L ss tectonites consistently record dextral, or top-to-the-north, shear parallel to L ss. Tectonic motion began as chiefly penetrative dip-slip shear ( L ds) at a high angle to the present trend of the TSZ, and evolved to dominantly dextral strike-slip shear ( L ss) parallel to the trend of the zone. The geometry, kinematics and sequence of deformation in TSZ tectonites evoke a general model of terrane accretion during oblique plate convergence involving initial shortening at a high angle to the convergent margin and progression to margin-parallel translation.