Oligocene-Neogene lithospheric-scale reactivation of Mesozoic terrane accretionary structures in the Alaska Range suture zone, southern Alaska, USA

Abstract

Terrane accretion forms lithospheric-scale fault systems that commonly experience long and complex slip histories. Unraveling the evolution of these suture zone fault systems yields valuable information regarding the relative importance of various upper crustal structures and their linkage through the lithosphere. We present new bedrock geologic mapping and geochronology data documenting the geologic evolution of reactivated shortening structures and adjacent metamorphic rocks in the Alaska Range suture zone at the inboard margin of the Wrangellia composite terrane in the eastern Alaska Range, Alaska, USA. Detrital zircon uranium-lead (U-Pb) age spectra from metamorphic rocks in our study area reveal two distinct metasedimentary belts. The Maclaren schist occupies the inboard (northern) belt, which was derived from terranes along the western margin of North America during the mid- to Late Cretaceous. In contrast, the Clearwater metasediments occupy the outboard (southern) belt, which was derived from arcs built on the Wrangellia composite terrane during the Late Jurassic to Early Cretaceous. A newly discovered locality of Alaska-type zoned ultramafic bodies within the Clearwater metasediments provides an additional link to the Wrangellia composite terrane. The Maclaren and Clearwater metasedimentary belts are presently juxtaposed by the newly identified Valdez Creek fault, which is an upper crustal reactivation of the Valdez Creek shear zone, the Late Cretaceous plate boundary that initially brought them together. 40Ar/39Ar mica ages reveal independent post-collisional thermal histories of hanging wall and footwall rocks until reactivation localized on the Valdez Creek fault after ca. 32 Ma. Slip on the Valdez Creek fault expanded into a thrust system that progressed southward to the Broxson Gulch fault at the southern margin of the suture zone and eventually into the Wrangellia terrane. Detrital zircon U-Pb age spectra and clast assemblages from fault-bounded Cenozoic gravel deposits indicate that the thrust system was active during the Oligocene and into the Pliocene, likely as a far-field result of ongoing flat-slab subduction and accretion of the Yakutat microplate. The Valdez Creek fault was the primary reactivated structure in the suture zone, likely due to its linkage with the reactivated boundary zone between the Wrangellia composite terrane and North America in the lithospheric mantle.

Oligocene-Neogene lithospheric-scale reactivation of Mesozoic terrane accretionary structures in the Alaska Range suture zone, southern Alaska, USA: Reply

“…Dodds and Campbell (1988) synthesized data leading to the classification of the Pyroxenite Creek intrusion as a part of the Alaska-type zone ultramafic suite that is present throughout southeastern Alaska and adjacent regions of Yukon. The geochemical data from spinel group minerals presented in figure 13 of Waldien et al (2021) clearly show that the zoned hornblende-biotite-pyroxenite body in Ann Creek and other samples of mid-Cretaceous ultramafic rock within the Clearwater metasediments classify as Alaska-type ultramafic intrusions. Stout (1965) and Stout (1976) presented detailed petrographic descriptions of the hornblende-biotite-pyroxenite body in Ann Creek, and Bittenbender et al (2007) described and dated similar ca.…”

“…We have two companion publications in revision that present a Cenozoic restoration of displacement on the eastern Denali fault system, thus that topic was not a focus of Waldien et al (2021). We encourage the invested reader to access those publications when they come online later in summer of 2021.…”

“…Topics of discussion raised by Lowey (2021) include the correlation of ultramafic-intermediate intrusions hosted within the Clearwater metasediments/Dezadeash Formation and their displacement by the Denali fault. These topics were not main points of Waldien et al (2021), but instead logical extrapolations of the information presented and synthesized therein. Here we re-emphasize the key findings of Waldien et al (2021) and discuss only the relevant aspects of the ultramaficintermediate intrusions and their displacement.…”

“…These topics were not main points of Waldien et al (2021), but instead logical extrapolations of the information presented and synthesized therein. Here we re-emphasize the key findings of Waldien et al (2021) and discuss only the relevant aspects of the ultramaficintermediate intrusions and their displacement.…”

Cretaceous to Oligocene magmatic and tectonic evolution of the western Alaska Range: Insights from U-Pb and 40Ar/39Ar geochronology

“…The complex deformation zone is bounded on the south by the Talkeetna thrust and on the north by the Hines Creek fault (Ridgway et al, 2002;Brennan et al, 2011); parts of the Kahiltna assemblage were metamorphosed to upper amphibolite facies during the Late Cretaceous and exhumed along the Valdez Creek shear zone in the Paleocene (Davidson et al, 1992;Ridgway et al, 2002). The suture zone was then crosscut by the Denali fault as the tectonic regime along the margin shifted from oblique collision to strike-slip translation after the Paleocene (e.g., Waldien et al, 2020).…”