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.

A distinctive yet enigmatic suite of fault-bounded ultramafic massifs occurs within accretionary complex mélange of the McHugh Complex on the Kenai Peninsula of southern Alaska. The largest and most significant of these include Red Mountain and the Halibut Cove Complex, consisting of dunite and pyroxenite with chromite seams and lesser quantities of garnet pyroxenite and gabbro. Several different hypotheses have been advanced to explain their origin. Burns (1985) correlated these fault-bounded ultramafic massifs with others known as the Border Ranges Ultramafic-Mafic Complex. Other parts of the Border Ranges Ultramafic-Mafic Complex are located several hundred kilometers away along the Border Ranges fault, marking the boundary between the Chugach terrane and the Wrangellian composite terrane in the northern and eastern Chugach Mountains. Burns (1985) suggested that this entire group of ultramafic bodies represents the deep roots of the Talkeetna arc developed on the southern margin of Wrangellia during Early Jurassic–Cretaceous subduction. In this model, bodies such as Red Mountain represent klippen thrust hundreds of kilometers southward over the McHugh Complex and now preserved as erosional remnants. Bradley and Kusky (1992) suggested alternatively that the Kenai ultramafic massifs may represent segments of a thick oceanic plate offscraped during subduction, and therefore might represent ophiolitic, oceanic plateau, or immature island arc crust as opposed to the roots of the mature Talkeetna arc. In this scenario, the Kenai ultramafic massifs would be correlative with the McHugh Complex, not the Talkeetna arc. A third hypothesis is that the Border Ranges Ultramafic-Mafic Complex may represent forearc or suprasubduction zone ophiolites formed seaward of the Talkeetna arc during early stages of its evolution and incorporated into the accretionary wedge during subsequent accretion tectonics. The implications of which of these models is correct are large because the Talkeetna arc section is the world's premiere example of a complete exposed arc sequence, including the volcanic carapace through deep crustal levels. Many models for the composition and evolution of the crust rely on the interpretation that this is a coherent and cogenetic section of arc crust. We report six new U/Pb zircon ages that show that at least some of the deep ultra-mafic and mafic complexes of the Border Ranges Ultramafic-Mafic Complex are Triassic (227.7 ± 0.6 Ma; Norian) and significantly older than structurally overlying Jurassic rocks of the Talkeetna arc (201–181 Ma, continuing plutonism until 163 Ma) but the same age as the surrounding Triassic-Jurassic-Cretaceous McHugh Complex. New geochemical data that show that rocks of the Border Ranges Ultramafic-Mafic Complex have ophiolitic affinities, with Cr-chemistry further indicating that the complex's rocks formed in a suprasubduction zone ophiolite. Regional and detailed and field observations show that rocks of the complex are similar to and can be structurally restored with other fault-bounded units in the McHugh Complex mélange, and that a crude ophiolitic stratigraphy can be reconstructed through the Border Ranges Ultramafic-Mafic Complex and McHugh Complex. We suggest that the Border Ranges Ultramafic-Mafic Complex represents the forearc oceanic basement upon which the Talkeetna arc was subsequently built. The conclusion that the Border Ranges Ultramafic-Mafic Complex does not represent the base of the Talkeetna arc but instead contains remnants of a dismembered ophiolitic complex raises questions about the validity of mass balance calculations and bulk crustal compositions, as well as models of arc development used to understand the growth of continental crust.