Early Tertiary exhumation of the flank of a forearc basin, southwest Talkeetna Mountains, Alaska

Abstract

New geochronologic and thermochronologic data from rocks near Hatcher Pass, southwest Talkeetna Mountains, Alaska, record earliest Paleocene erosional and structural exhumation on the flank of the active Cook Inlet forearc basin. Cretaceous plutons shed sediments to the south, forming the Paleocene Arkose Ridge Formation. A Paleocene(?)-Eocene detachment fault juxtaposed ~60 Ma metamorphic rocks with the base of the Arkose Ridge Formation. U-Pb (analyzed by Sensitive High Resolution Ion Micro Probe Reverse Geometry (SHRIMP-RG)) zircon ages of the Cretaceous plutons, more diverse than previously documented, are 90.3±0.

Integrated provenance record of a forearc basin modified by slab‐window magmatism: detrital‐ zircon geochronology and sandstone compositions of the Paleogene Arkose Ridge Formation, south‐central Alaska

“…a–c). These new ages fall within the range of 79–67 Ma U‐Pb zircon ages reported from lithologically similar intrusions that crop out 15–35 km to the east in the Hatcher Pass area (Harlan et al ., ; Bleick et al ., ). Southeast of Willow Creek, Arkose Ridge Formation strata in the Government Peak area also nonconformably overlie previously undated granitoid intrusions (Donaghy, ).…”

“…2 Ma older than the oldest reported U‐Pb zircon ages from granitoid plutons that crop out to the north in the Hatcher Pass area and ca . 10 Ma younger than spatially limited gabbroic intrusions that crop out <10 km to the east near Government Peak East (Harlan et al ., ; Bleick et al ., ).…”

“…Locally faulted against the Upper Cretaceous–lower Paleocene plutons is schist that yields 65–60 Ma 40 Ar/ 39 Ar cooling ages and slightly older maximum depositional ages based on the youngest population of detrital zircons (Kps on Fig. ; Harlan et al ., ; Bradley et al ., ; Bleick et al ., ). Along the northeastern margin of the Arkose Ridge Formation outcrop belt, basaltic and intermediate lavas, subordinate mafic and felsic pyroclastic deposits, and shallow intrusions of the Caribou Creek volcanic field yield 59–36 Ma 40 Ar/ 39 Ar ages with most ages <50 Ma (Cole et al ., ).…”

“…Integration of detrital geochronology and modal compositional data in the MVTM basin capitalizes on geochronologic and lithologic variations in adjacent source regions, including chiefly arc‐related igneous rocks to the north in the Talkeetna Mountains (e.g. Rioux et al ., ; Bleick et al ., ) and accretionary‐prism metasedimentary strata to the south (Plafker et al ., ; Amato & Pavlis, ; Kochelek et al ., ). Integration of >1600 detrital‐zircon ages with paleocurrents, compositional data, and previously published datasets provides the following results: (1) distinguishes multiple transverse paleodrainages that dispersed sediment southward from remnant arc plutons and nascent slab‐window volcanic centers into an axial fluvial system; (2) confirms cessation of regional arc magmatism coeval with the onset of localized volcanic centers attributed to slab‐window magmatism; and (3) offers robust evidence for exhumation of arc plutons and marine forearc basin strata followed by nonmarine sediment accumulation during a documented episode of ridge subduction.…”

Detrital Zircon Record of a Mesozoic Collisional Forearc Basin in South Central Alaska: The Tectonic Transition From an Oceanic to Continental Arc

“…Northwest of the Jurassic igneous rocks are Cretaceous plutons that crop out throughout much of southern Alaska in the Wrangellia and Yukon composite terranes (Figure ; Chisana arc and Kluane arc of Plafker & Berg, ) (Brennan & Ridgway, ; Moll‐Stalcup, ; Wallace & Engebretson, ). In the southern Talkeetna Mountains, Upper Cretaceous diorite and granite plutons have 79–67 Ma U‐Pb ages (Kg in Figure ) (Bleick et al, ; Davidson & McPhillips, ; Donaghy, ; Harlan & Vielreicher, ).…”