Structural characterization of the porcupine shear zone in northeastern Alaska

Tectonic models for large-scale terrane translation along the northern Laurentian margin during the evolution of the circum-Arctic region require the presence of large-scale shear zones. The Porcupine shear zone (PSZ) in northeastern Alaska and northwestern Yukon juxtaposes the Arctic Alaska terrane with autochthonous rocks of Laurentia and is a prime candidate for such a structure. However, beyond a recent study of relationships at several sites across the 10-km-wide deformation zone in Yukon, little is known of the style of deformation and kinematic history of the PSZ. Here, structural relationships within the PSZ were examined along strike for an ~80 km stretch of the Porcupine River in Alaska. Lithologic units along the river are divided into discrete blocks of Neoproterozoic carbonate, quartzite and shale, Silurian limestone and shale, Devonian limestone and Mississippian limestone and shale, Permian limestone, conglomerate and shale, and Triassic siliciclastic and mafic rocks. Individual blocks are separated by NNE-striking, NW-dipping thrust faults or poorly exposed NE-striking zones characterized by strong brecciation and veining.

Internal deformation in the blocks varies by lithology but is dominantly brittle. Neoproterozoic rocks immediately west of the Canadian border are broken into several panels separated by NW-dipping hanging wall up oblique faults and strike-slip faults striking 030° marked by 0.5-m-thick gouge zones. Smaller scale faults define a complex array dominated by conjugate, E-W-striking reverse faults, NNW-striking strike-slip faults, and lesser NNE-striking normal faults. Neoproterozoic shale units commonly define large (tens of m amplitude) overturned folds. Further west and south, Paleozoic strata are characterized by a similarly complex array of NNW-striking normal faults, E-W-striking normal and reverse faults, and NE-striking strike-slip faults. Paleozoic shale units locally display abundant isoclinal folds with subhorizontal hinge planes. Where Devonian limestone is thrust on Mississippian shale, the younger units define SE-vergent isoclinal folds that are gently refolded about NW-plunging hinge lines. In contrast, deformation in Triassic rocks along the PSZ appears less pronounced, with Triassic strata occurring as a gently folded homoclinal panel with a relatively simple array of N-S-striking normal faults, E-W-striking reverse faults, and NE-striking strike-slip faults.

Although the kinematics are uncertain, a strike-slip displacement history for the PSZ is consistent with the abundance of steep, NE-striking faults with subhorizontal striae in most lithologic blocks, coupled with the presence of NE-trending zones of brecciation and veining at their boundaries. The fault arrays within individual stratigraphically defined blocks are consistent with secondary structures associated with strike-slip faults, but their geometry and observed kinematics do not define a simple or consistent displacement history. Moreover, the differences in deformation observed in Mesozoic versus older units suggests a complex, protracted history of displacement on the PSZ. Finally, the presence of a significant NE-striking strike-slip deformation zone along the ancestral Laurentian margin in Yukon and Alaska supports tectonic models for large-scale terrane translation. Additional field and analytical work are required to refine the timing and detailed kinematic history of the PSZ.