The Border Ranges fault system is the arc-forearc boundary of the Alaskan-Aleutian arc and separates a Mesozoic subduction accretionary complex (Chugach terrane) from Paleozoic to middle Mesozoic arc basement that together comprise an oceanic arc system accreted to North America during the Mesozoic. Research during the past 20 years has revealed a history of repeated reactivation of the fault system, such that only scattered vestiges remain of the original subduction-related processes that led to formation of the boundary. Throughout most of the fault trace, reactivations have produced a broad band of deformation from 5 to 30 km in width, involving both the arc basement and the accretionary complex, but the distribution of this deformation varies across the Alaskan orocline, implying much of the reactivation developed after or during the development of the orocline. Along the eastern limb of the orocline the Hanagita fault system typifies the Late Cretaceous to Cenozoic dextral strike slip reactivation of the fault system with two early episodes of strike slip separated by a contractional event, and a third, Neogene strike-slip system locally offsetting the boundary. Through all of these rejuvenations strike slip and contraction were slip partitioned, and all occurred during active subduction along the southern Alaska margin. The resultant deformation was decidedly one-sided with contraction focused on the outboard side of the boundary and strike slip focused along the boundary between crystalline arc basement and accreted sediment. Analogies with the modern Fairweather–St. Elias orogenic system in northern southeast Alaska indicate this one-sided deformation may originate from erosion on the oceanic side of the deformed belt. However, because the strike-slip Hanagita system faithfully follows the arc-forearc contact this characteristic could be a result of rheological contrasts across the rejuvenated boundary. In the hinge-zone of the Alaskan orocline the smooth fault trace of the Hanagita system is disrupted by cross-cutting faults, and Paleogene dextral slip of the Hanagita system is transferred into a complex cataclastic fault network in the crystalline assemblage that comprises the hanging wall of the fault system. Some of these faults record contraction superimposed on earlier strike-slip systems with a subsequent final strike-slip overprint, a history analogous to the Hanagita system, but with a more significant contractional component. One manifestation of this contraction is the Klanelneechena klippe, a large outlier of a low-angle brittle thrust system in the central Chugach Mountains that places Jurassic lower-crustal gabbros on the Chugach mélange. Recognition of unmetamorphosed sedimentary rocks caught up along the earlier strike-slip systems, but beneath the Klanelneechena klippe, provides an important piercing point for this strike-slip system because these sedimentary rocks contain marble clasts with a closest across-strike source more than 120 km to the north and east. Published thermochronology and structural data suggest this dextral slip does not carry through to the western limb of the orocline. Thus, we suggest that the Paleogene strike slip along the Border Ranges fault was transferred to dextral slip on the Castle Mountain fault through a complex fault array in the Matanuska Valley and strike-slip duplex systems in the northern Chugach Mountains. Restoration of this fault system using a strike-slip duplex model together with new piercing lines is consistent with the proposed Paleogene linkage of the Border Ranges and Castle Mountains systems with total dextral offset of ∼130 km, which we infer is the Paleogene offset on the paired fault system. Pre-Tertiary deformation along the Border Ranges fault remains poorly resolved along most of its trace. Because Early Jurassic blueschists occur locally along the Border Ranges fault system in close structural juxtaposition with Early Jurassic plutonic assemblages, the earliest phase of motion on the Border Ranges fault has been widely assumed to be Early Jurassic. Nonetheless, nowhere, to our knowledge, have structures within the fault zone produced dates from that period. This absence of older fabrics within the fault zone probably is due to a major period of subduction erosion, strike-slip truncation, or both, sometime between Middle Jurassic and mid-Early Cretaceous when most, or all, of the Chugach mélange was emplaced beneath the Border Ranges fault. In mid-Early Cretaceous time at least part of the boundary was a high-temperature thrust system with sinistral-oblique thrusting syntectonic to emplacement of near-trench plutons, a relationship best documented in the western Chugach Mountains. Similar left-oblique thrusting is observed along the Kenney Lake fault system, the structural contact beneath the Tonsina ultramafic assemblage in the eastern Chugach Mountains, although the footwall assemblage at Tonsina is a lower-T blueschist-greenschist assemblage with an uncertain metamorphic age. We tentatively correlate the Kenney Lake fault with the Early Cretaceous structures of the western Chugach Mountains as part of a regional Early Cretaceous thrusting event along the boundary. This event could record either reestablishment of convergence after a lull in subduction or a ridge-trench encounter followed by subduction accretion during continuous subduction. By Late Cretaceous time the dextral strike-slip initiated in what is now the eastern Chugach Mountains, but there is no clear evidence for this event in the western limb of the orocline. This observation suggests strike slip in the east may have been transferred westward into the accretionary complex prior to emplacement of the latest Cretaceous Chugach flysch.

Six samples collected from pre-, syn-, and post-Talkeetna arc units in south-central Alaska were dated using single-grain zircon LA-MC-ICP-MS geochronology to assess the age of arc volcanism and the presence and age of any inherited components in the arc. The oldest dated sample comes from a volcanic breccia at the base of the Talkeetna Formation on the Alaska Peninsula and indicates that initial arc volcanism began by 207 ± 5 Ma. A sedimentary rock overlying the volcanic section in the Talkeetna Mountains has a maximum depositional age of <167 Ma. This is in agreement with biochronologic ages for the top of the Talkeetna Formation, suggesting that the Talkeetna arc was active for ca. 40 m.y. Three samples from interplutonic screens and roof pendants in the Jurassic batholith on the Alaska Peninsula provide information about the tectonic setting of Talkeetna arc magmatism. All three samples contain Paleozoic to Proterozoic zircons and require that arc magmas on the Alaska Peninsula intruded into detritus that contained older continental zircons. This finding is distinct from observations from eastern exposures of the arc in the Chugach and Talkeetna Mountains, where there is only limited evidence for pre-Paleozoic zircons, and it suggests that there were along-strike variations in the tectonic setting of the arc.