Figure 2: a) Seismicity ranging from 0 to 15 km of depth from 1960
to present-day: Seismic events denoted as blue dots (USGS Earthquake
Database). Faults are delineated with solid pink lines and labeled with
white lettering. b) Published (see Supplementary File 18) and new
apatite (U-Th)/He ages for southern Alaska, listed alphabetically:
Enkelmann et al. (2017), McAleer et al. (2009), McDermott et al (2019),
Spotila et al. (2004), Spotila and Berger (2010). Major faults shown as
semi-transparent pink lines; these are labeled in (a). Approximate slip
rates of the major faults shown in white lettering with pink arrows
denoting primary sense of slip on each structure. These modern slip
rates are from Marechal et al. (2018) and Brothers et al. (2020).
Approximate Figure 3 outline shown with yellow box.
The injection of a dike at 30 Ma into the Totschunda fault zone and
hypabyssal dikes ca. 25-23 Ma emplaced proximal to, but only on the west
side of the Totschunda fault implies fault movement and transtension
during the Oligocene (Brueseke et al., 2019). Additional dacitic and
andesitic porphyry of Oligocene age along the southern Totschunda fault
(this study) also implies Oligo-Miocene slip on the Totschunda fault.
Lastly, Milde (2014) collected and analyzed two apatite fission track
samples from Cretaceous plutons near the central Totschunda fault and
far away (>10 km) from any large Oligocene magmatic bodies
that indicate rapid cooling in the Late Oligocene. Milde interprets that
these samples were rapidly exhumed in the Late Oligocene on the basis of
the presence of Oligocene ages on the west side of the Totschunda fault
and Cretaceous ages on the east side of the Totschunda fault.
The restoration of translated Oligocene-Early Miocene boulder
conglomerate packages onto the Totschunda from the Central Denali fault
(Allen et al., 2022) and coeval focused magmatism along the Totschunda
fault (Trop et al., 2022, this study) further support Totschunda fault
motion in the Late Oligocene. 23-19 Ma intra-arc extension magmatism in
the Sonya Creek Volcanic Field has also been linked to coeval motion
along the Totschunda and Eastern Denali faults (Berkelhammer et al.,
2019). Intra-arc extensional basin formation and magmatism west of the
southern Totschunda fault at ca. 13-5 Ma has also been linked to
inferred motion on this structure (Trop et al., 2012). The ca. 10-2 Ma
Frederika Formation (White River ‘Tillites’) in a small basin abutting
the southern Totschunda fault (e.g., Eyles and Eyles, 1989) are
discussed in the next section (2.2) as the formation of that basin is
also interpreted to reflect slip on the Totschunda fault in the Late
Miocene-Pleistocene. Lastly, the Euchre Volcano erupted directly from
the Totschunda fault (Brueseke et al., 2019; Trop et al., 2022)
providing indirect evidence of slip and extension in the late Pliocene
(ca. 3 Ma).
Northwest of the Totschunda fault, the central Denali fault has acted as
a conduit for magmatism since at least the Late Cretaceous based on
generally continuous magmatism from ca. 95 to 25 Ma, both proximal to,
and directly along the main fault trace (e.g., Regan et al. 2020, 2021;
Benowitz 2022a). Likewise, the Totschunda and Duke River faults (which
intersect at the southern terminus of the Totschunda fault; Figures 1,
2) appear to have acted as conduits for Wrangell arc magmatism since ca.
30 Ma (Trop et al., 2022, this study). The Duke River and Totschunda
faults have probably been connected since Cretaceous times (Cobbett et
al., 2016; Trop et al., 2020) with slip at times apparently being
transferred from the Duke River onto the Totschunda (Marechal et al.,
2018; Choi et al., 2021). Hence, the deformation history of the Duke
River fault potentially informs on the deformation history of the
Totschunda fault. Eocene to Miocene strike-slip related basins (Ridgway
and Decelles, 1993) and Miocene leaky transform magmatism (Skulski et
al., 1992; Cole and Ridgway 1993; Trop et al., 2022) along the Duke
River fault suggest the Duke River fault was active during most of the
Cenozoic. Therefore, it is possible that slip was transferred from the
Duke River to the Totschunda from the Eocene to the present (e.g.,
Marechal et al., 2018) but direct geologic evidence for Eocene -slip
along the Totschunda fault is lacking.
2.2 White River ‘Tillites’
The White River ‘Tillites’ were deposited in a small transtensional
basin immediately east of the Totschunda fault. This basin contains
diamictites and debris flow deposits with interlayered lavas and tuffs
derived from the Wrangell volcanic arc (Figure 3). Eight K-Ar whole rock
ages from selected tuffs and volcanic flows interbedded in these
sediments range in age between 10 and 2 Ma indicating that the sediments
were deposited from Late Miocene to Pleistocene (Denton and Armstrong,
1969). Sediments in this basin are considered part of the Frederika
Formation which is also found outside the field area to the south of the
Wrangell mountains (MacKevett 1978; Trop et al., 2012; Figure 3).
Deposition of the ‘tillites’ are interpreted as being syn-tectonic with
slip along the Totschunda fault (Eyles and Eyles, 1989; Trop et al.,
2012) based on observed “proximal-sedimentation and a sustained and
plentiful supply of coarse debris in a high relief environment” and the
possibility that some large debris flows in the section were earthquake
derived. The sediments of the White River Tillites are also crosscut and
deformed by thrust splays from this fault system. Apatite fission track
ages on cobbles from this basin were collected to inform on the
provenance and/or the subsequent exhumation history of the basin.