2. Geologic Background

2.1 Alaska Range Suture Zone and the Totschunda-Denali Fault system

The complex Mesozoic to Cenozoic Alaska Range suture zone lies between the peri-Laurentian Yukon-Tanana composite terrane to the north (intermontane terranes) and the more recently accreted, late Paleozoic and Mesozoic intra-oceanic plateau and volcanic arc rocks of the accreted Wrangellia composite terrane (insular terranes) to the south (Figure 1; Coney et al., 1980; Nokleberg and Richter, 2007; Dusel-Bacon et al., 2013; Jones et al., 2017). Rocks within the Alaska Range suture zone consist primarily of Late Jurassic to Late Cretaceous marine sedimentary strata and associated metamorphic equivalents (Figure 1) (Ridgway et al., 2002; Manselle et al., 2020). The section of the suture zone where parts of this study were conducted is bounded to the north by the near-vertical Denali fault and the Totschunda fault to the south (e.g., Richter, 1976; Allam et al., 2017).
The ~2000 km long, dextral strike-slip Denali fault system is the northern boundary of the Alaska Range suture zone in our region of study and has likely been active since at least ca. 70 Ma (Cole et al., 1999; Miller et al., 2002; Benowitz et al., 2014; McDermott et al., 2019) with ~480 km of documented slip occurring since ca. 52 Ma. The ~480 km of offset since 52 Ma is based inpart on the correlative offset ca. 57 Ma. Ruby Range-East Susitna Batholiths (Figure 1; Riccio et al., 2014) and offset ca. 52 Ma Shakwak-Ann Creek Plutons (e.g., Waldien et al., 2021a). These plutons are located within and immediately adjacent to the Maclaren-Kluane schist which is the principal piercing point on the Denali fault (Figure 1). The Maclaren schist and these younger 57 and 52 Ma plutons have been displaced ~480 km from their correlative piercing points (e.g., Forbes et al., 1974; Waldien et al., 2021a). Furthermore, the Mesozoic Clearwater-Nutzotin-Dezeadesh Basin complex has been dissected and slivered with the Clearwater sliver being displaced ~480 km from the Dezeadesh basin and the Nutzotin sequence being displaced ~360 km from Dezeadesh basin (Figure 1) (e.g., Eisbacher, 1976; Lowey et al., 2019; Waldien et al., 2021a).
Because the Maclaren schist and Kluane schist are correlative and have been separated by the Denali Fault, the Maclaren schist cannot be translated down the Totschunda fault and must be restored only to the junction of the two faults (so this translated crustal block can be restored on the Eastern Denali fault to the Maclaren’s paleo-connection with the Kluane schist). Currently, the Maclaren schist has been translated roughly 125 kms away from the Totschunda-Denali fault intersection. Since the separation has occurred since 52 Ma, this piercing point provides a limit of 125 kms of horizontal displacement of the Totschunda fault since 52 Ma. Additionally, the Clearwater metasediments have been displaced ~125 km from the correlative Nutzotin Basin (Figure 1) providing an additional piecing point with a nearly identical displacement (Waldien et al., 2022). We therefore infer the Totschunda fault has contributed ~125 kms of the 480 km total displacement of the Maclaren schist (e.g., Eisbacher, 1976; Waldien et al., 2021a; Allen et al., 2022; Trop et al., 2022; this study). Palinspastic restoration of offset volcanic products of the Wrangell volcanic field along the Totschunda fault provide additional markers that suggest ~85 km of the ~125 km of slip occurred since ca. 6 Ma (Berkelhammer et al., 2019; Trop et al., 2022). More speculatively, there is an apparent ~80 km offset (age unknown) of the Border Ranges fault system along the dextral Art Lewis fault (Bruhn et al., 2004; 2014) which is a southern segment of the Connector fault (Figure 1). This Connector fault offset observation is consistent with the offset constraints along the Totschunda fault (Trop et al., 2022).
Besides the dissected Wrangell arc, piercing points that inform on the long term (106 years) slip rates along the Totschunda fault are lacking. MacKevett (1978) suggested that the Station Creek Formation just north of the Duke River fault and the Slana Spur Formation/ Tetelna Volcanics near the Totschunda-Denali intersection are possibly correlative based on age (Permian), lithology (interlayered meta-volcaniclastic sediments and lava flows), and similar stratigraphy (Figure 1). These geologic units are separated by ~180 km, but modern dating techniques, detailed mapping, and compositional analysis have not yet been applied to these rocks to corroborate this interpretation.
Using surface exposure ages on offset geomorphological features, Matmon et al. (2006) and Haeussler et al. (2017) determined Pleistocene-Holocene slip rates along the Denali fault system. West of the Totschunda-Denali fault intersection average slip rates on the Denali fault were determined to be ~12.9 mm/yr, east of the intersection ~5.3 mm/yr, and ~7.4 mm/yr along the Totschunda fault. Using field observations, high-resolution imagery, digital elevation models, and cosmogenic nuclide dating, Marechal et al. (2018) document different Pleistocene-Holocene slip rates for the Totschunda and Eastern Denali faults. Marechal et al. (2018) determined rates of ~14.6 mm/yr on the Totschunda fault as compared to <1 mm/yr for the northern Eastern Denali fault (Figure 2). This discrepancy in Pleistocene slip rates for the Eastern Denali Fault and Totschunda fault reported by Matmon et al. (2006) and Marechal et al. (2018) is not easily explained, but the two research groups did not make slip observations at the same locations. Matmon et al. (2006) made slip determinations based on offset features within ~ 30 km of the Totschunda-Denali fault junction. Marechal et al. (2018) made slip determinations along the entire length of the Eastern Denali fault and note that further than 80 km from the Totschunda-Denali junction, there is no observed horizontal motion on the Eastern Denali fault. Marechal et al. (2018) sampled the Totschunda fault ~120 km south of the Totschunda-Denali fault intersection. We prefer the Marechal et al. (2018) determinations because they align with the long-term geologic record (Waldien et al., 2018, 2021a and Allen et al., 2022) and seismology modeling (Oglesby et al., 2004; Choi et al., 2021).
-Richter and Matson (1971), using offset geomorphological features of unknown age (probably Pleistocene) inferred ~10 kms of slip on the Totschunda fault since ca. 1 Ma. Although this Pleistocene slip rate on the Totschunda (~10 mm/yr) is not well constrained, this constraint is similar to the Holocene rate (~14.6 mm/yr) suggested by Marechal et al. (2018). There is also significantly less seismic activity along the Eastern Denali fault compared to along the Totschunda fault (Figure 2). A compilation of regional thermochronology data (Figure 2) show a pattern of young apatite Helium ages (AHe) along the margins of the Fairweather fault, the inferred Connector fault, and to the north (our data) along the southern Totschunda fault. This data also shows a relative absence of young AHe ages on the southern Eastern Denali fault. Additionally, the 2002 7.9M Denali fault earthquake ruptured the Totschunda fault not the Eastern Denali fault due to the fault’s preferential geometry with regards to the regional stress field (Oglesby et al., 2004) adding credence to the Totschunda being the strand of principal slip. We interpret these patterns to further support the Totschunda fault being the primary structure transferring stress inboard from the plate margin since the Late Miocene as compared to the Eastern Denali fault.
Despite most of the lateral displacement on the Totschunda fault occurring since 52 Ma, the fault is likely a Cretaceous structural feature. The Totschunda fault forms part of the inboard suture of the Wrangellia composite terrane with North America as shown by geological mapping (e.g., Richter et al., 1976; MacKevett et al., 1978) and magnetic maps of the region (Bankey et al., 2002). Receiver function analysis from Allam et al. (2017) show a 10 km moho offset across the Totschunda fault (deeper to the west). Regional Wrangell composite terrane (intermontane) suturing along the Totschunda fault by ca. 117 Ma has also been suggested by the presence of well-dated subaerial sedimentary units with dinosaur footprints that uncomformably overlap both the Wrangell composite terrane and the Cretaceous Nutzotin basin (Fiorillo et al., 2012; Trop et al., 2020). Trop et al. (2020) infer that since these overlap assemblage rocks preserve land-based fauna, and cover both Wrangellia and the inferred sutured basin, they must record a minimum suturing age. There is also thermochronologic evidence along the central Totschunda fault of a Cretaceous rapid cooling episode which has been interpreted as exhumation during Cretaceous suturing (Milde, 2014; this study) and a dikelet has been dated to ca. 114 Ma (40Ar/39Ar K-feldspar) that was injected into pre-existing fault gouge along the Totschunda fault zone (Trop et al., 2020).