Project Synopsis
Collaborators:
Dr. Sun-Lin Chung, National Taiwan University – major, trace, and isotope geochemistry
Dr. Paul Layer, University of Alaska, Fairbanks – Ar/Ar geochronology
The Reindeer Hills (RH) mélange extends along the suture zone of the Wrangellia composite terrane (WCT), which was accreted to southern Alaska by Late Cretaceous time. The mélange has an argillaceous matrix containing blocks of siltstone, lithic sandstone, and strained conglomerate that are similar to the adjacent Jurassic-Cretaceous Kahiltna assemblage. Also present are blocks of Silurian-Devonian limestone, metabasalt of unknown age, and granite that yields an Ar-Ar age of 72.7 +/- 1.3 Ma on biotite. The mélange is cut by rhyolite dikes that yield Ar-Ar ages of 70.8 +/- 0.5 on alkali feldspar and 72.1 +/- 1.3 Ma on biotite. The dikes and granite exhibit geochemical trends that, along with their ages, indicate that they were related to the Late Cretaceous-Paleogene Alaska Range-Talkeetna Mountains arc.
Deformation in the mélange shows NNW-SSE shortening and E-W right-lateral strike-slip. Above the melange is a thrust sheet of Late Paleozoic to Triassic(?) chert, limestone, shale, and pillow basalt. The entire thrust sheet is folded into a tight ENE-trending synform, consistent with NNW-SSE shortening. The thrust contact is defined by a zone of increased deformation with a pervasive E-W slaty cleavage in the mélange and an anastamosed cleavage in the chert. A tightly deformed interval of Kahiltna assemblage exposed along the Parks Highway between the RH mélange and the McKinley fault shows ESE-WNW trending fold axes and northward vergent mesoscale folds and reverse faults.
If the mélange contains blocks of Kahiltna assemblage and it is cross-cut by latest Cretaceous dikes, then the mélange was formed during mid to Late Cretaceous time. We interpret that deformation of the Kahiltna and the mélange, including emplacement of the thrust sheet, occurred in the suture zone of the northward accreting WCT. The thrust sheet is likely an imbricate of the WCT. The dikes and granite in the mélange are also deformed, indicating that deformation of the mélange unit continued after Late Cretaceous time. Late stage structures in the mélange indicate deformation in a zone of right-lateral simple shear, consistent with strike-slip along the McKinley fault. Collectively, the RH region records development and deformation of mélange that is consistent with Late Cretaceous suturing of the WCT followed by Paleogene right-lateral strike-slip along the McKinley fault.