Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province, coupled with U-Pb geochronology of their zircons using sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG), yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly mafic xenoliths (constituting less than two percent of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated through magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n = 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (ca. 138–60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile addition of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached upper amphibolite-facies (sillimanite grade) to granulite-facies conditions within a series of extension-related metamorphic culminations or gneiss domes, which developed within the Cretaceous magmatic belt. Metamorphic gradients and inferred geotherms (~30–50 °C/km) from both the gneiss domes and xenoliths are too high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures. Deep-crustal seismic-reflection and refraction data reveal a 30–35-km-thick crust, a sharp Moho and reflective lower and middle crust. Velocities do not support a largely mafic (underplated) lower crust, but together with xenolith data suggest that Late Cretaceous to early Paleocene mafic intrusions are likely increasingly important with depth in the crust and that the elevated temperatures during granulite-facies metamorphism led to large-scale flow of crustal rocks to produce gneiss domes and the observed subhorizontal reflectivity of the crust. This unique combined data set for the Bering Shelf region provides compelling evidence for the complete reconstitution/re-equilibration of continental crust from the bottom up during mantle-driven magmatic events associated with crustal extension. Thus, despite Precambrian and Paleozoic rocks at the surface and Alaska’s accretionary tectonic history, it is likely that a significant portion of the Bering Sea region lower crust is much younger and related to post-accretionary tectonic and magmatic events.

The Kodiak Border Ranges ultramafic complex, Afognak batholith, and Shuyak Formation on Kodiak and Afognak Islands together form the lower, middle, and upper portions, respectively, of a Jurassic–Triassic island-arc crustal section. The Kodiak section exhibits structural and geochemical trends similar, but not identical to, the Tonsina-Nelchina segment of the Talkeetna arc, located >500 km to the northeast. Exposed at the base of the Kodiak section is cumulate clinopyroxenite with associated dunite, wehrlite, and layered gabbro. In the inferred middle to upper crust, tonalite and quartz diorite of the Afognak batholith intrude Shuyak Formation basaltic flows, basaltic pillow lavas, and volcaniclastic sedimentary rocks. Despite the fault-bounded nature of the lower crustal and mantle rocks, continuous chemical trends in elements such as MgO, Ni, Cr, Nb, Sr, Y, and rare-earth elements exist across all three units. Modeling of these data suggest that Kodiak arc evolution occurred in two main stages: (1) a gabbroic initial melt underwent fractional crystallization that produced a pyroxenitic root and a gabbroic lower crust, and (2) melt in equilibrium with the gabbroic lower crust underwent assimilation-fractional crystallization to produce mid-crustal plutonic and upper-crustal volcanic rocks. Kodiak Island exposes the oldest and thinnest portion of the Talkeetna arc, with ages from the Afognak batholith ranging from ca. 215–185 Ma. In the eastern and western Talkeetna arc, magmatism migrated northward after ca. 180 Ma in response to inferred forearc erosion. Forearc erosion coupled with differential subduction-channel movement juxtaposed blueschist-facies rocks with middle and lower crustal arc rocks. These processes occurred earlier and to a greater degree in the western Talkeetna arc, causing the arc to split in half, separating the Kodiak and Alaskan Peninsula parts of the Talkeetna arc.

Fossils within accreted terranes are typically used to describe the age or origin of the exotic geologic blocks. However, accretion may also provide new pathways for faunal exchange between previously disconnected landmasses. One such landmass, the result of accretion, is Beringia, that entity encompassing northeastern Asia and northwestern North America and the surmised land connection between the two regions. The present concept of Beringia as a Quaternary subcontinent includes a climatic component in the form of glacial advances and retreats driving changes in sea level. These changes may have facilitated exchanges of marine biota between the Pacific Ocean and Arctic Basin, or exchanges of terrestrial faunas and floras between Asia and North America. The Beringian ecosystem includes specializations of the flora and fauna, especially in the vertebrate fauna. A review of tectonic reconstructions and the striking taxon-free parallel patterns in data on the Cretaceous and Quaternary fauna and flora suggest that a generalized concept of Beringia should be formally extended back in time to the Cretaceous. A significant shift in emphasis of defining variables occurs with this extension. Climate, in the form of meteorological phenomena, and geologic history are important variables in the previously recognized definition of Beringia. The extension of Beringia into the Cretaceous implies that Beringia is rooted in its accretionary rather than its climatic history; in other words, the geographic pattern as the result of tectonics is the defining parameter for Beringia.

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.