The Tally Ho shear zone (THSZ) is a 40-kilometre long, northwest-striking assemblage of highly-deformed rocks outcropping along the western margin of Stikinia in southern Yukon. The location of the THSZ adjacent to the boundary between the oceanic Stikine Terrane and pericratonic Nisling Assemblage of the Yukon-Tanana Terrane provides an opportunity to examine the tectonic significance of a terrane-marginal high-strain structure. Detailed geological mapping along the THSZ and adjacent rocks indicates that the shear zone is a crustal-scale thrust fault that places deep-seated arc rocks of Stikinia up-section and to the east onto a package of upper crustal volcanic and volcaniclastic rocks of the Upper Triassic Lewes River Group. Timing of movement along the THSZ is constrained by U-Pb zircon age determinations of a deformed section of cumulate leucogabbro in the hangingwall of the shear zone (208 ± 4.3 Ma) and by a post-kinematic cross-cutting megacrystic granite intrusion (~173 Ma). Positive Nd values of Stikine magmatic rocks (+4.5 to +6.7) indicate magmas forming the oceanic terrane were formed with little to no input of continental crust until around the time of intrusion of a megacrystic granite unit in the mid-Jurassic (Nd -4.0). The THSZ is therefore interpreted to have formed in response to the thick-skinned collision and underpinning of the Nisling Assemblage beneath western Stikinia in the Early Jurassic resulting in the imbrication of the Stikine magmatic arc along the THSZ.

The Tally Ho shear zone (THSZ) is a 40-kilometre long, northwest-striking assemblage of highly-deformed rocks outcropping along the western margin of Stikinia in southern Yukon. The location of the THSZ adjacent to the boundary between the oceanic Stikine Terrane and pericratonic Nisling Assemblage of the Yukon-Tanana Terrane provides an opportunity to examine the tectonic significance of a terrane-marginal high-strain structure. Detailed geological mapping along the THSZ and adjacent rocks indicates that the shear zone is a crustal-scale thrust fault that places deep-seated arc rocks of Stikinia up-section and to the east onto a package of upper crustal volcanic and volcaniclastic rocks of the Upper Triassic Lewes River Group. Timing of movement along the THSZ is constrained by U-Pb zircon age determinations of a deformed section of cumulate leucogabbro in the hangingwall of the shear zone (208 ± 4.3 Ma) and by a post-kinematic cross-cutting megacrystic granite intrusion (~173 Ma). Positive Nd values of Stikine magmatic rocks (+4.5 to +6.7) indicate magmas forming the oceanic terrane were formed with little to no input of continental crust until around the time of intrusion of a megacrystic granite unit in the mid-Jurassic (Nd -4.0). The THSZ is therefore interpreted to have formed in response to the thick-skinned collision and underpinning of the Nisling Assemblage beneath western Stikinia in the Early Jurassic resulting in the imbrication of the Stikine magmatic arc along the THSZ.