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).