2 Geologic Background
2.1 Jurassic to Recent Southern Alaska Convergent Margin Tectonics
Southern Alaska has been a convergent margin since at least the Triassic (Kusky et al., 2007; Trop and Ridgway, 2007), with periods of a) accretionary events (e.g., Jura-Cretaceous: Trop et al., 2020; Eocene: Garver and Davidson, 2015), b) oroclinal bending changing the orientation and shape of the plate margin (e.g. Gillis et al., 2022), c) slab-window/transform tectonics with a gap in arc magmatism (ca. 60-50 Ma; Terhune et al., 2019), d) periods of “normal” subduction (Kula slab–ca. 100-60; Pacific slab–45-30 Ma; Terhune et al., 2019; Trop et al., 2019; Jones et al., 2021; Benowitz et al., 2022) and, e) flat-slab subduction of the Yakutat oceanic plateau with initiation of the associated slab-edge Wrangell Arc (30 Ma to Recent; Ebherhart-Phillips et al., 2006; Worthington et al., 2012; Berkelhammer et al., 2019; Brueseke et al., 2019; Trop et al., 2022).
The accretion of the Wrangellia composite terrane to North American affinity crust to the north-west was the largest addition of crust to the continent in the last 200 million years (Trop and Ridgway, 2007). The generally accepted model is that the Wrangellia composite terrane, primarily oceanic crust, collided along North America’s western coast at around ca. 100 Ma and then was translated >2000 km north along margin-parallel strike-slip fault systems (e.g., Tikoff et al., 2022). The ARSZ (Trop et al., 2019) is the suture region between the Wrangellia terrane and rocks of North American affinity to the north (Figure 1). The Denali Fault system delineates the northern boundary of the ARSZ (Trop et al., 2019, 2022), and the Talkeetna Fault delineates the southern boundary of the ARSZ (Brennan et al., 2011). By ca. 50 Ma the Chugach-Prince William Terrane had been translated and accreted into place south of the Border Ranges Fault system of southern Alaska (Figure 1) (Garver and Davidson, 2015).
Crustal thickness variations exist across the Border Ranges Fault system (Mann et al., 2022), the Talkeetna Fault (Brennan et al., 2011), and the Denali Fault system (Allam et al., 2017) with clear magnetic contrasts across all three structures (Saltus and Hudson, 2022). The Moho offset across the Denali Fault system is ~10 km (Veenstra et al., 2006; Rossi et al., 2006; Allam et al., 2017; Mann et al., 2022; Yang et al., this issue ). The Denali Fault also has a well-defined across-strike lithosphere thickness variation: the lithosphere is at least 15 km thicker and is colder to the north of the Denali Fault (>65 km) compared to the south (Miller et al., 2018; Gama et al., 2022). The Arctic Plate north of the Kobuk fault has an even thicker crust (>45 km; Miller et al., 2018; Yang et al., this issue ) and an apparently cratonic (McClelland et al., 2021) thick (~200 km) and cold lithosphere (O’Driscoll and Miller, 2015; Jiang et al., 2018; Gama et al., 2022; Pavlis et al.,this issue ). These crust and lithospheric variations are primarily inherited (i.e. older than ca. 100 Ma) with some post docking shorting along the ARSZ (Ridgway et al., 2002) and Eocene extension south of the Hines Creek Fault (Gillis et al., 2022). Notwithstanding these tectonic events, the overall trend of thinner-hotter lithosphere to the south and progressively thicker-colder lithosphere inboard has been long-lived (O’Driscoll and Miller, 2015).
2.2 Mobility of the system and varying age of the subducting slab
The exact location of the paleo-southern Alaska trench is not known, but we assume the trench was near the Border Ranges Fault transform system ca. 95 Ma till ca. 50 Ma (Terhune et al., 2019) and then jumped out closer to its modern position after the accretion of the Chugach-Prince William Terrane (Garver and Davidson, 2015), with additional trench modifications with the arrival of the Yakutat Terrane (e.g., ca. 30 Ma; Brueseke et al., 2019) (Figure 1). Southern Alaska is also bisected by three major dextral strike-slip fault systems. The Tintina Fault has seen ~490 km of displacement, primarily during the Eocene (Saltus et al., 2007). The Denali Fault system has experienced ~480 km of displacement since ca. 52 Ma (Waldien et al., 2021). The Border Ranges Fault system has experienced at least ~700 km of offset between ca. 58 Ma and 50 Ma (Smart et al., 1996). More speculatively the Teslin-Tintina Fault system (not shown) of the Yukon Territory may have experienced 1900 km of displacement since ca. 70 Ma (Johnston et al., 1996). Thus, the location of the ARZS has varied with time relative to stable North America. For the Cenozoic, this is clearest for the Central Alaska Range, where this inboard region of the ARZS has been translated up to ~480 km along the Denali Fault system since 52 Ma (Waldien et al., 2021). Furthermore, North America/Alaska was ~10° degrees further to the north during the late Cretaceous as compared today (e.g., Tikoff et al., 2022) and North America continues to move southwest relative to the Pacific Plate (Anders et al., 1993; DeMets and Dixon, 1999) likely leading to trench encroachment.
Since 84 Ma there have been at least five significant proto-Pacific and Pacific Northeast plate motion changes. Based on Doubrovine and Tarduno (2008), at 84 Ma proto-Pacific and Pacific Northeast slab movement had an orientation of ~356° NE, then at 67 Ma orientation rotated 14° counterclockwise, th-en at 62 Ma rotated 11° clockwise, and then at 50 Ma rotated 16° clockwise. At 25 Ma there was a 8-15° counterclockwise slab rotation (Jicha et al., 2018) and an 18° clockwise rotation at ca. 6 Ma (Engebretson et al., 1984; Austermann et al., 2011). The rate of convergence has also varied, with rates from 50-200 km/Ma during the last 84 My (Doubrovine and Tarduno, 2008; Elliot et al., 2010). The age and therefore temperature of the subducting slab under Wrangellia has clearly varied through time, but determining the age of ancient subducted slabs is not always determinable as all previous slab material has descended into the mantle. At 50 Ma, the age of the subducting Pacific slab under Alaska was likely young given subduction had reinitiated after the break-off of the Kula plate (e.g., Terhune et al., 2019). The majority of the currently subducting Yakutat slab formed ca. 50 million years ago (Wells et al., 2014).