McDannell, and three co-authors, reported in 2014 that in the Bendeleben Mountains, Seward Peninsula of Alaska mid-Cretaceous granites are exposed in an uplifted block bounded on its south side by an E-W striking normal fault. The Bendeleben fault has well-preserved scarps 4–7 m in height that offset Holocene moraines. Seismic activity, young normal faulting, and Quaternary basaltic volcanism are all evidence of active extension. South of the Bendeleben fault, there is a 3–4 km deep basin. Fifteen apatite (U-Th)/He ages from granitic samples of the footwall yield an Eocene-weighted mean age of 41.364.8 Ma. Biotite 40Ar/39Ar ages from the country rock of the Bendeleben pluton are 81–83 Ma.
In spite of the young fault scarps, HeFTy and Pecube thermal modeling results illustrate that rapid exhumation of the Bendeleben Mountains occurred in the Late Cretaceous-Eocene and slowed since the Oligocene. A weak age-elevation relationship of apatite helium ages and a lack of correlation between age and distance from the fault indicate that exhumation was accomplished with minimal block rotation on a steeply dipping, long-lived normal fault.
Timing of extension in the Seward Peninsula can be correlated with deformation in the offshore Hope Basin where seismic reflection lines document Early Tertiary large-magnitude normal faulting followed by minor post-Miocene reactivation. The faulting observed in the Bendeleben Mountains is part of an extensional system that spans a large portion of the Bering Strait region. The tectonic model proposed in previous studies suggests that clockwise rotation of the Bering block relative to North America is the cause of extensional deformation in western Alaska.
The authors concluded that Apatite fission-track and 40Ar/39Ar thermochronology of the exposed deep crustal rocks of the Kigluaik Mountains suggest that the existing thin crust of the Bering Region was established in the mid-Cretaceous, but during the Early Tertiary a renewed period of faulting, offshore basin formation, and erosion occurred throughout the Seward Peninsula [see Dumitru et al., 1995;Tolson, 1987]. This renewed extensional signal can be seen in thr authors' measured A-He ages from the Bendeleben Mountains. The Tertiary tectonic regime can be modeled as episodic extension related to block rotations induced by tectonic extrusion in response to Pacific margin convergence and terrane accretion. Extension in the Seward Peninsula also corresponds to normal faulting in the Hope Basin, NW of the Seward Peninsula. The authors proposed that these events are linked and are part of the same system. These results are in good agreement with regional tectonic and geochronologic information.
The weighted mean measured A-He age is 41.364.8 Ma for the Bendeleben footwall block. Apatite (U-Th)/He, biotite 40Ar/39Ar thermochronology and thermal modeling has constrained the cooling and exhumation history of the central Seward Peninsula. Inverse HeFTy t-T modeling show that rapid cooling occurred in the mid-Late Cretaceous, after batholith emplacement, and has progressively slowed into recent time. HeFTy output t-T paths provide normal fault displacement inputs in Pecube forward models. Exhumation of the Bendeleben footwall block is estimated to be approximately 0.2–0.25 mm/yr from the late Cretaceous to Eocene and approximately 0.04–0.06 mm/yr from the Eocene to present.
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