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

Abstract

The collision of oceanic arcs with continental margins is an important mechanism for the growth of continents. Ancient forearc basin strata in collisional orogens provide a record of the upper crustal response to this tectonic process. In south central Alaska, Mesozoic forearc basin strata are exposed in a complete crustal section. U‐Pb detrital zircon geochronology from the forearc basin strata was analyzed within a ~107 Ma stratigraphic framework. The Jurassic strata contain unimodal detrital zircon populations that become progressively younger upsection and range from 175 to 151 Ma. These strata are derived from the active oceanic Talkeetna arc. The Cretaceous strata were deposited above multiple unconformities that collectively represent as much as ~30 Ma of nondeposition and/or erosion in the forearc basin. Erosion in the forearc basin and a general absence of detrital zircon ages between 140 and 120 Ma are interpreted as a hiatus of magmatism triggered by collision of the oceanic arc with the former continental margin. The Cretaceous strata have two main detrital zircon populations: a Cretaceous population ranging from 90 to 68 Ma that becomes progressively younger upsection and a Jurassic population with a broad range of peak ages from 194 to 144 Ma. The Cretaceous population marks the establishment of an active Cretaceous continental arc following the collisional event, and the older population reflects continued erosion of the remnant Jurassic oceanic arc plutons. Our results show that detrital zircon geochronology provides a powerful approach for delineating stages of forearc basin collision and the erosion of multiple magmatic arcs.

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America

“…Thick successions of Middle Jurassic to Upper Cretaceous siliciclastic strata and minor volcanic rocks lie inboard (north) of the Chugach accretionary complex and outboard (south) of volcanicplutonic belts attributed to arc magmatism in south-central Alaska. These strata reflect deposition in intra-arc and forearc depocenters with respect to the Talkeetna-Chitina-Chisana arcs to the north (Trop and Ridgway, 2007), and sediment was sourced chiefly from these arcs (Stevens Goddard et al, 2018). Locally, sediment was eroded from sources within the Chugach accretionary complex starting in early Late Cretaceous time.…”

“…The accretion record is missing between ca. 185-170 Ma, which corresponds to an inboard migration of the arc, when subduction erosion destroyed part of the forearc (Clift et al, 2005) and the forearc basin became well established (Stevens Goddard et al, 2018). This lack of preservation is cited by Sigloch and Mihalynuk (2017) as evidence that the accretionary complex is not linked with the Jurassic arc system despite the clear evidence globally that subduction erosion removes material from subduction complexes (e.g., von Huene and Scholl, 1991).…”

Cretaceous to Miocene magmatism, sedimentation, and exhumation within the Alaska Range suture zone: A polyphase reactivated terrane boundary

“…Cretaceous Kahiltna assemblage marine strata were subaerially uplifted and deformed by outboard-(south-) dipping thrust faults, folds, and synkinematic intrusions located inboard (north) of coeval arc-related igneous rocks (Csejtey et al, 1982;Davidson et al, 1992;Ridgway et al, 2002). This deformation is attributed to plate convergence outboard (south) of the suture zone, mainly north-dipping subduction (modern coordinates) of oceanic crust that is recorded by Upper Cretaceous arc plutons, forearc basin sedimentary strata, and accretionary prism metasedimentary strata preserved south of the suture zone (e.g., Plafker and Berg, 1994;Trop, 2008;Amato et al, 2013;Stevens Goddard et al, 2018). Along the southern part of the Cantwell basin, syndepositional displacement along south-dipping thrusts prompted deposition of fluvial-lacustrine strata in growth footwall synclines (Ridgway et al, 1997).…”

“…The Alexander terrane consists chiefly of metamorphosed volcanic arc igneous and sedimentary rocks (Berenak et al, 2014). Analysis of geologic data sets indicates the Wrangellia, Peninsular, and Alexander terranes were juxtaposed by late Paleozoic time (Beranek et al, 2014;Israel et al, 2014) and collided with the former continental margin during mid-Jurassic to mid-Cretaceous time (Haeussler, 1992;Kalbas et al, 2007;Manuszak et al, 2007;Gehrels et al, 2009;Hampton et al, 2010;Stevens Goddard et al, 2018).…”

Geology, U‐Pb Geochronology, and Hf Isotope Geochemistry Across the Mesozoic Alaska Range Suture Zone (South‐Central Alaska): Implications for Cordilleran Collisional Processes and Tectonic Growth of North America

“…This study was not designed to specifically address the timing of collision of the Wrangellia composite terrane with the continental margin of western North America but it does provide some new insights that may be useful to future studies addressing this question. The timing of collision is debated with interpretations ranging from Early Jurassic to Late Cretaceous (e.g., Box et al, ; Gehrels et al, ; Monger, ; Stevens Goddard et al, ; Trop et al, ; Trop & Ridgway, ). Our new data delineate two distinct melting events in the Alaska Range suture zone that may be related to collisional processes.…”