5 Discussion
5.1 Arc localization in the Alaska Range Suture Zone since ca. 100 Ma
Based on the mapped compilation (N= 6485 ages total) of detrital single grain and bedrock sample ages, arc magmatism has been localized along the ARZS since ca. 100 Ma (Figures 1, 4, 5, 6, and 7). This arc localization 1) occurred as far as ~300 and 550 km from a subduction interface that generally moved oceanward with time through accretion (Garver and Davidson et al., 2015; Trop et al., 2019), 2) occurred in segments of the upper-plate that were mobile along regional lithospheric and crustal-scale strike-slip faults (Allam et al., 2017; Waldien et al., 2021; Gama et al., 2022), 3) involved slabs of different ages (Engebretson et al., 1984; Wells et al., 2014), variable convergence angles and velocities (Sharp and Clague, 2006; Doubrovine and Tarduno, 2008; Jicha et al., 2018), and involved slabs of varying thicknesses (Worthington et al., 2012; Mann et al., 2022), 4) was maintained after oroclinal bending which greatly modified the margin geometry (e.g., Gillis et al., 2022) and slab break off resulting in a slab window event (Terhune et al., 2019), and 5) mostly occurred south of the Denali Fault system (including the Hines Creek segment) along theARZS from ca. 100 Ma to Recent.
By deduction, pre-existing features of the Alaska upper-plate must be playing a first-order role in localizing magmatism. Sutures can act as passive conduits for magma (Richard, 2003) and play an active role in magmatism localization (Gómez-Vasconcelos et al., 2022). However, melt generally rises vertically (Hall and Kincaid, 2001) and slabs subducting under the ARSZ must have maintained a depth roughly around 100 ± 40 km (Isacks and Barazangi et al., 1977; England et al., 2004; Syracuse & Abers, 2006) since 100 Ma to generate melt in the general vicinity of the ARSZ . Furthermore, there is no dominant structure along the spine of the Western Alaska Range Arc to facilitate arc magmatism localization (Figure 1).
5.2 Mechanisms for Arc localization in the ARSZ since ca. 100 Ma
The overriding plate in a subduction zone influences the geometry of the subducting slab through its effect on the mantle flow field: flow around the subducting slab can cause both low dynamic pressures above the slab and high dynamic pressures below, and lead to forces that counteract negative slab buoyancy and decrease subducting dip angle (Stevenson & Turner, 1977; Tovish et al., 1978; Rodríguez‐González et al., 2012; Liu, 2022). It has also been demonstrated through geodynamic modeling and natural examples that the presence of a region of thicker-colder upper-plate moving toward the trench can lead to increased mantle-flow-related hydrodynamic “suction” in the mantle wedge (van Hunen et al., 2004; Manea and Gurnis, 2007; Rodríguez‐González et al., 2012). Thus, slab dips beneath continents are generally shallower by ~20° than under oceanic lithosphere and flat-slab subduction under oceanic lithosphere has not been documented (Jarrard, 1986; England et al., 2004; Syracuse and Abers, 2006).
At ca. 80 Ma the the Wrangellia composite terrane of Alaska was at a paleo-latitude of 53° ± 8°N (Stamatakos et al., 2001) and by ca. 50 Ma the terrane was at or near modern latitudes (Panuska et al., 1990). More speculatively, at ca. 100 Ma the Wrangellia composite terrane of Alaska may have been located off the coast of the western United States (e.g., Tikoff et al., 2022). The incoming slab as well as the upper-plate characteristics varied along the >2000 km northward translation of the Wrangellia composite terrane. Regardless of the exact paleo-latitude position, the Wrangellia composite terrane would have been outboard of a plate with a thicker (>100 km) lithosphere (Porter and Reid, 2021) when translated north along the modern day Western United States (Figure 7a). At ca, 80, when the Wrangellia composite terrane was being translated along the coast of modern-day British Columbia the inboard lithosphere would have been relatively thin (~80-100 km thick) (Clowes et al., 1995). Interestingly, this is the time when arc magmatism ceased in the Central Alaska Range and shifted outboard to the Western Alaska Range and Talkeetna Mountains, which suggests slab steepening or rollback during this time period or possibly the translation of the Wrangellia composite terrane over a steeper dipping slab segment. The trench distance to thick-cold craton for the Wrangellia composite terrane would have been ~350 km from the >150 km thick Mackenzie craton when passing by the modern-day Yukon Territory around ca. 75 Ma when arc magmatism reinitiated in the Central Alaska Range (Schaeffer and Lebedev, 2014; Estève, 2020 Estève et al., 2020).
The period of ca. 60 Ma to 50 Ma was a dynamic period in Alaska history with the Kula plate breaking off, the creation of a slab window under southern Alaska, the final translation of the Chugach-Prince William Terrane into place, and syn-tectonic oroclinal bending (Figure 7b) (Garver and Davidson 2015; Terhune et al., 2019; Gillis et al., 2022). The shape of trench was modified at this time, as was any pre-existing subduction channel. However, when subduction reinitiated at ca. 48 Ma (Bezard et al., 2021; Jones et al., 2021; Benowitz et al., 2022) the continental arc once again returned to the ARSZ .
The lithosphere north of the Denali Fault system is of Laurentian (Dusel-Bacon et al., 2013, 2017; Jones et al., 2017) and Caledonian affinity (McClelland et al., 2021), where the terranes of southern Alaska are primarily of oceanic affinity (e.g., Trop and Ridgway et al., 2007). This contrast is clear in both the lithosphere being thicker and colder north and west of the ARSZ (Gama et al., 2022). Additionally, the lithosphere beneath northern Alaska is even thicker (150-200 km) and colder (Figure 7c) (O’Driscoll and Miller, 2015; Jiang et al., 2018; Gama et al., 2022). Alaska/North America has been moving to the southwest since the late Cretaceous (e.g. Tikoff et al., 2022) leading to an overall trenchward motion of interior Alaska which continues today (McConeghy et al., 2022). Hence, we prefer a geodynamic model where the upper-plate lithospheric shape and dynamics induce hydrodynamic “suction” forces that have controlled the geometry of the underlying Alaska slab since ca. 100 Ma. The ca. 30 Ma to Recent “flat” slab subduction of the Yakutat oceanic plateau beneath Alaska also likely reflects a component of upper plate hydrodynamic “suction” (Figure 7d). Many studies have demonstrated that anomalously thick/buoyant oceanic slabs alone do not lead to shallow subduction, but also need a component of mantle-wedge suction (O’Driscoll et al., 2009; Manea et al., 2012; Skinner and Clayton, 2013).
Additional features of the upper plate are also likely contributing to arc localization in the ARSZ. Sutures zones, which are regions of crustal weakness (Sykes, 1978) can act as pathways that focus rising melt (Dahm et al., 2020). It has also been suggested that upper-plate faults which penetrate all the way through the crust can funnel slab related melts into the upper crust (Marot et al., 2014), and it has been documented that extensional kinematic environs along strike-slip faults can lead to magmatism localization (Tibaldi et al., 2009; Mathieu et al., 2011; Gómez-Vasconcelos et al., 2020; Webb et al., 2020). The Hines Creek, Denali and Totschunda Faults bounding the northern ARSZare Cretaceous and still active (Miller et al., 2002; Benowitz et al., 2014, 2022; Trop et al., 2020), lithospheric-scale (Eberhart-Phillips et al., 2003; Allam et al., 2017; Gama et al., 2022) strike-slip faults (Figure 7). Eocene-dissected plutons (Regan et al., 2021), fault zone Cretaceous-Oligocene-Miocene dike swarms (Brueseke et al., 2019; Trop et al., 2019, 2020) and Cretaceous-to-Recent magmatism focused along the Denali and Totschunda Faults (Benowitz et al., 2022; Trop et al., 2022) support these structures having an active role in arc magmatism localization.
Another scenario leading to arc localization along the ARSZ since ca. 100 Ma is possible melt ponding along Totschunda and Denali Fault Moho depth offsets (Figure 7). Melt ponding has been inferred below upper-plate structures from seismic imaging (e.g., MacKenzie et al., 2010; Rondenay et al., 2010). We speculate that Moho depth offsets, such as those across the Totschunda and Denali Fault systems (e.g., Gama et al., 2022), may be acting as catchments for melt rising through the convecting mantle wedge and hence contribute to focusing magmatism along crustal-scale structures.