Late Cretaceous through Cenozoic Strike‐Slip Tectonics of Southwestern Alaska

Abstract

New geologic mapping and geochronology show that marginparallel strike-slip faults on the western limb of the southern Alaska orocline have experienced multiple episodes of dextral motion since-100 Ma. These faults are on the upper plate of a subduction zone-350-450 km inboard of the paleotrench. In southwestern Alaska, dextral displacement is 134 km on the Denali fault, at least 88-94 km on the Iditarod-Nixon Fork fault, and perhaps tens of kilometers on the Dishna River fault. The strike-slip regime coincided with Late Cretaceous sedimentation and then folding in the Kuskokwim basin, and with episodes of magmatism and mineralization at ~ 70 ,-60, and-30 Ma. No single driving mechanism can explain all of the-95 millionyear history of strike-slip faulting. Since-40 Ma the observed dextral sense of strike slip has run contrary to the sense of subduction obliquity. This may be explained by northward motion of the Pacific Plate driving continental margin slivers into and/or around the oroclinal bend. From 44 to 66 Ma, oroclinal rotation, perhaps involving large-scale flexural slip, may have been accompanied by westward escape of crustal blocks along strike-slip faults. However , reconstructions of this period involve unproven assumptions about the identity of the subducting plate , the position of subducting ridges, and the exact timing of oroclinal bending, thus obscuring the driving mechanisms of strike slip. Prior to 66 Ma , oblique subduction is the most plausible driving mechanism for dextral strike slip. Cumulative displacement on all faults of the western limb of the orocline is at least 400 km, about half that on the eastern limb; this discrepancy might be explained by a combination of thrusting and unrecognized strike-slip faulting. l\t Figure 1. Map of Alaska, the northeastern Pacific, and parts of Canada and the Russian Far East showing key features mentioned in text. Geologic offsets and A' (Tintina fault), Band B' (Denali fault), and C and C' (Kaltag fault) are indicated. BRF Border Ranges fault; Castle Mountain fault. Strike-slip faults of Beringian margin after Worrall (1991).

Persistent long-term ( c. 24 Ma) exhumation in the Eastern Alaska Range constrained by stacked thermochronology

“…Intrusion of alkaline dykes occurred in the Kimball region at about 68 Ma (Foley 1985) and may have been related to a short-lived extensional event. The time of the change in cooling rate is also coincident with an interpreted period of movement along the western Denali Fault at around 66 Ma (Miller et al 2002). Thus, we interpret that the approximately 6 8C/Ma cooling rate at about 68 Ma derived from sample 01KIM may be related to the onset of movement of the Denali Fault in the region.…”

“…6). We choose a rate of about 10 8C/Ma to avoid rate changes that are simply a reflection of minor thermal perturbations along the Denali fault zone and to take into account the constrained Tertiary background exhumation rate along the long-lived (c. 85 Ma: Miller et al 2002) Denali Fault strike-slip fault system. The use of a standard rapid-rate-change limit (c. .10 8C/Ma) significantly greater than the demonstrated background cooling rate allows us to compare cooling rate trends between samples and the sample set as a whole from the high peak region of the Eastern Alaska Range.…”

The Active Yakutat (Kula?) Plate and Its Southcentral Alaska Megathrust and Intraplate Earthquakes

“…As an example, the total displacement on the SW Alaska faults, which are on the W limb of the Alaska orocline, is about 450 km [114]. This is only about half of the total fault displacements determined for the E limb of the orocline [177].…”

“…This event occurred near the Denali fault, which has had dextral strike-slip movement and was the fault involved with the generation of the Mw 7.9 earthquake of 3 November 2002, which started just to the E of this 1932 event [9]. Such documentation of sinistral strike-slip earthquake events on sinistral faults such as the BC and KAT as well as sinistral strike-slip earthquakes on a net dextral ILI and ILIS indicate some sinistral fault movements do indeed occur in S Alaska [113] with regional dextral movements obviously dominating [114,115].…”

“…Another serious problem is the large offset disparities between E and W of the curved fault systems [114,176] of the Alaska orocline. As an example, the total displacement on the SW Alaska faults, which are on the W limb of the Alaska orocline, is about 450 km [114].…”