Permo‐Triassic tectonism in volcanic arc sequences of the western U.S. Cordillera and implications for the Sonoma orogeny

The Permo‐Triassic Sonoma orogeny, western U.S. Cordillera, involved tight folding and imbricate thrust faulting of basinal strata of the Golconda allochthon. This event is generally interpreted in terms of accretion of upper Paleozoic volcanic arc rocks of the western U.S. Cordillera (“McCloud arc”) to the North American continent, either via back arc basin closure or arc‐continent collision. The structural history of the McCloud arc during the Sonoma orogeny is poorly understood, however, as are the implications of this history for tectonic models. This paper presents a synthesis of the Permian to Triassic stratigraphic and structural evolution of the McCloud arc, incorporating new data from the Pine Forest Range, northwest Nevada. The synthesis indicates that, on a regional scale, the arc was affected by widespread uplift but only local tilting, folding and faulting in the Late Permian to Early Triassic. The timing of Permo‐Triassic tectonic activity in the McCloud arc is thus similar to the middle Permian to Early Triassic age of deformation in the Golconda allochthon but the structures produced in the two groups of rocks differ markedly. The significance of this contrasting structural response to Sonoma age tectonism can be evaluated by comparison with other better understood examples of arc‐continent accretion (back arc basin closure in the southern Andes, “compressional” arc‐continent collision in New Guinea and “extensional” arc‐continent collision in the Mediterranean). This comparison indicates that the contrasting structural history of the McCloud arc and Golconda allochthon is entirely compatible with an arc‐continent accretion model for the Sonoma orogeny and cannot be used as a basis for concluding that the McCloud arc was not involved in the orogenic event. The comparison further indicates that either back arc basin closure or “compressional” arc‐continent collision are the most likely settings for the Sonoma orogeny. In contrast, an “extensional” Mediterranean‐style arc‐continent collision, as has been recently proposed for the earlier (middle Paleozoic) Antler orogeny, is probably not an appropriate analogue for the Sonoma orogeny. The weak structural response of the McCloud arc during Sonoman compression also provides new constraints on the specific processes involved in arc accretion, either via arc‐continent collision or back arc basin closure. These constraints suggest that there may be few fundamental differences between the two models during the final stages of arc accretion. Regardless of tectonic setting, Permo‐Triassic uplift and associated deformation in the McCloud arc should be considered as much a response to the Sonoma orogeny as is tight folding and imbricate faulting in the Golconda allochthon.