ABSTRACT The Matanuska lowland north of Anchorage, Alaska, was episodically glaciated during the Pleistocene by the merged westward flow of the Matanuska and Knik glaciers. During the late Wisconsin glaciation, glacial Lake Atna filled the Copper River Basin, impounded by an ice dam blocking the Matanuska drainage divide at Tahneta Pass and the adjacent Squaw Creek headwaters and ice dams at other basin outlets, including the Susitna and Copper rivers. On the Matanuska lowland floor upvalley from the coalesced glacier’s late-Wisconsin terminus, a series of regularly spaced, symmetrical ridges with 0.9-km wavelengths and heights to 36 m are oriented normal to oblique to the valley and covered by smaller subparallel ridges with wavelengths typically ~80 m and amplitudes to 3 m. These and nearby drumlins, eskers, and moraines were previously interpreted to be glacial in origin. Borrow-pit exposures in the large ridges, however, show sorting and stratification, locally with foreset bedding. A decade ago we reinterpreted such observations as evidence of outburst flooding during glacial retreat, driven by water flushing from Lake Atna through breaches in the Tahneta Pass and Squaw Creek ice dam. In this view, the ridges once labeled Rogen and De Geer moraines were reinterpreted as two scales of fluvial dunes. New observations in the field and from meter-scale light detection and ranging (LiDAR) and interferometric synthetic aperture radar (IfSAR) digital elevation models, together with grain-size analyses and ground-penetrating radar profiles, provide further evidence that portions of the glacial landscape of the Matanuska lowlands were modified by megaflooding after the Last Glacial Maximum, and support the conclusion that the Knik Glacier was the last active glacier in the lowland.

ABSTRACT Upper Jurassic to Upper Cretaceous forearc basin strata exposed in south-central Alaska record variations in sedimentary basin development associated with late Mesozoic terrane accretion and translation of the Insular terranes (Alexander-Wrangellia-Peninsular) along the western North American margin. Upper Jurassic strata contain a dominant population of Late Jurassic (ca. 150 Ma) detrital zircons with juvenile to intermediate isotopic compositions (ε Hf[ t ] >5) that reflect sediment input from the adjacent oceanic Talkeetna arc, located to the north in a marine forearc basin. By Early Cretaceous time, Early–Late Jurassic (ca. 160 and 198 Ma) and minor Late Devonian–Permian detrital zircon grains that have juvenile to intermediate hafnium compositions (ε Hf[ t ] >5) indicate continued exhumation of the Talkeetna arc and a new sediment contribution from erosion of Insular terrane basement. Upper Cretaceous strata record final suturing of the terrane, resulting in an influx of sediment from the inboard Intermontane terrane, represented by Ordovician–Permian detrital zircon grains with a wide range of hafnium compositions (ε Hf[ t ] >–25). Late Cretaceous detrital zircon ages (ca. 90 and 70 Ma) in these strata reflect establishment of a coeval volcanic arc. Trends toward more-evolved hafnium isotopic data within our data record ~40 m.y. orogenic cycles of crustal thickening during the Late Triassic–Early Jurassic and Late Jurassic that can be linked to Cordilleran-scale deformational events. Overall, we prefer a model where the forearc basin in south-central Alaska, the part of the Intermontane terranes that is today located in northern British Columbia, Yukon, and eastern Alaska, and the part of the Chugach–Prince William terrane found on Kodiak Island were all juxtaposed by Late Cretaceous (Turonian) time.

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.