Late Mesozoic strike slip movement on the Border Ranges Fault System in the Eastern Chugach Mountains, southern Alaska

Abstract

Preliminary studies of mylonitic rocks from the Haley Creek metamorphic assemblage in south‐central Alaska suggest that the Border Ranges fault system was a strike slip, ductile‐shear zone during part of its late Mesozoic history. Evidence for this strike slip event is limited to the hanging wall of the Border Ranges fault system, where deformed crystalline rocks were apparently subjected to a retrograde, mylonitic deformation prior to subduction‐related underthrusting of the latest Cretaceous Valdez Group, but subsequent to a Late Jurassic plutonic event. The strike slip interpretation contrasts with previous interpretations, which related mylonitic fabric development to multiple deformational events during underthrusting of the Chugach terrane, a conflict resulting from the interpretation of lineation as an intersection lineation parallel to the Y axis of strain versus an extension lineation. Evidence for horizontal stretching on a vertical flattening plane includes: (1) a parallelism between all lineations, including intersection lineations, and unequivocal extension lineations developed within systems of strike slip, ductile‐shear zones, (2) extensional fragmentation of feldspars parallel to lineation, (3) local rodding and development of L‐tectonites, (4) boudin axes at a high angle to lineation, (5) crystallographic preferred orientations of quartz, and (6) syn‐kinematic crack‐seal veins with fibers parallel to lineation. The local parallelism between intersection lineations and extension lineation is interpreted as a consequence of sheath folding in a high‐strain zone, and we suggest that it is this complication that is largely responsible for confusion on the kinematic history. Despite evidence for a strain pattern consistent with a strike slip shear zone, evidence for noncoaxial strain and a clear sense of shear is elusive. Nonetheless, coaxial deformation seems unlikely given the structural position of the mylonitic rocks along an ancient plate boundary. Regional correlation of rock types is suggestive of a dextral offset of at least 50 km, although this offset could be due to younger faulting. Thus further work will be needed to clearly distinguish a dextral versus sinistral sense of shear. Important absolute age problems still remain, and thus the tectonic significance of the strike slip event is uncertain. However, the general late Mesozoic age places the event within an important time period in the history of the northern Cordillera. Thus the strike slip event may have (1) played a role in a transform history preceding Early Cretaceous thrusting along the Border Ranges fault system, (2) been a complication from collision of Wrangellia with North America, and (3) been important in the northward transport of outer Cordilleran terranes that is implied by paleomagnetism.