# Active tectonics around the Yakutat indentor: New geomorphological constraints on the eastern Denali, Totschunda and Duke River Faults

Anaïs Marechal, J.F. Ritz, Matthieu Ferry, Stephane Mazzotti, Pierre-Henri Blard, Regis Braucher, Dimitri Saint-Carlier

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## To cite this version:

Anaïs Marechal, J.F. Ritz, Matthieu Ferry, Stephane Mazzotti, Pierre-Henri Blard, et al. Active tectonics around the Yakutat indentor: New geomorphological constraints on the eastern Denali, Totschunda and Duke River Faults. Earth and Planetary Science Letters, 2018, 482, pp.71-80. 10.1016/j.epsl.2017.10.051. hal-01671315

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https://inria.hal.science/hal-01671315v1

Submitted on 13 Jun 2025

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# Active tectonics around the Yakutat indentor: New geomorphological constraints on the eastern Denali, Totschunda and Duke River Faults

Anaïs Marechal¹, Jean-François Ritz¹, Matthieu Ferry¹, Stephane Mazzotti¹,², Pierre-Henri Blard³, Régis Braucher⁴, Dimitri Saint-Carlier³

¹Geosciences Montpellier, UMR5243, Université de Montpellier - CNRS, Place E. Bataillon, 34
095 Montpellier, France

²School of Earth and Ocean Sciences, University of Victoria, Victoria BC, Canada

³Centre de Recherches Pétrographiques et Géochimiques, rue Notre Dame des Pauvres, 54 500
Vandoeuvre les Nancy, France

⁴CEREGE, UMR7330, Laboratoire des Nucléides Cosmogéniques, Plateau d'Arbois, 13 545
Aix-en-Provence, France

Corresponding author: Jean-François Ritz <jean-francois.ritz@gm.univ-montp2.fr>

## Key Points:

* New slip rates on the eastern Denali, Totschunda, and Duke River Faults

* Major role of the Totschunda Fault in the regional kinematics

* Minor role of the eastern Denali Fault in the regional kinematics

* Slip partitioning between these major faults at and near their junctions

* Strain transfer from the Yakutat collision syntaxis to the inland fault systems
20 **Keywords**

21 • Yakutat collision, eastern Denali Fault, Totschunda Fault, Duke River Fault, active

22 tectonics, slip rates

23

24
## Abstract

The Yakutat collision in SE Alaska – SW Yukon is an outstanding example of indentor tectonics. The impinging Yakutat block strongly controls the pattern of deformation inland. However, the relationship between this collision system and inherited tectonic structures such as the Denali, Totschunda, and Duke River Faults remains debated. A detailed geomorphological analysis, based on high-resolution imagery, digital elevation models, field observations, and cosmogenic nuclide dating, allow us to estimate new slip rates along these active structures. Our results show a vertical motion of 0.9 ± 0.3 mm/yr along the whole eastern Denali Fault, while the dextral component of the fault tapers to less than 1 mm/yr ~80 km south of the Denali–Totschunda junction. In contrast, the Totschunda Fault accommodates 14.6 ± 2.7 mm/yr of right-lateral strike-slip along its central section ~100 km south of the junction. Further south, preliminary observations suggest a slip rate comprised between 3.5 and 6.5 mm/yr along the westernmost part of the Duke River thrust fault. Our results highlight the complex partitioning of deformation inland of the Yakutat collision, where the role and slip rate of the main faults vary significantly over distances of ~100 km or less. We propose a schematic model of present-day tectonics that suggests ongoing partitioning and reorganization of deformation between major inherited structures, relay zones, and regions of distributed deformation, in response to the radial stress and strain pattern around the Yakutat collision eastern syntaxis.

## 1 Introduction and tectonic setting

The Yakutat collision zone is an outstanding example of continental indentor tectonics (Fig. 1). At the transition between subduction and strike-slip plate boundaries, the thickened
oceanic crust of the Yakutat block (carried by the Pacific plate) collides with the North America plate at ~50 mm/yr, at the border between Alaska and the Yukon Territory (Bruns, 1983; Pavlis et al., 2004). This collision results in a strong partitioning of the deformation, with ~13 mm/yr block-like WNW-ward motion of southern Alaska (Elliott et al., 2013; Mériaux et al., 2009) and diffuse inland deformation with shortening rates of 6 - 10 mm/yr north and east of the syntaxis (Leonard et al., 2008; Marechal et al., 2015). Inland, the Denali Fault is clearly defined as a right-lateral strike-slip structure (e.g., Plafker et al., 1994). The Mw = 7.9 earthquake, which occurred along its central part in 2002 and propagated onto the Totschunda Fault (Haeussler et al., 2004), raises the question of the slip rate evolution along the Denali Fault from its central segment to its eastern segment.

Fault slip data, regional kinematics, and strain analyses indicate a lateral eastward decrease of the Denali Fault slip rate from 12 – 14 mm/yr on its central part to 6 – 10 mm/yr at the junction with the Totschunda Fault, which accommodates the remaining 5 – 7 mm/yr (Matmon et al., 2006; Mériaux et al., 2009; Bemis et al., 2015). Further south, near the Alaska border and into Yukon Territory, Richter and Matson (1971) did not recognize any evidence of activity in the geomorphology along the eastern Denali Fault. However, Seitz et al. (2010) documented paleo-earthquakes and estimated ~3 mm/yr horizontal slip rate from offset mounds along the Kluane Lake section (Fig. 1). South of Kluane Lake, GPS data show 2.0 ± 0.9 mm/yr of dextral motion along the eastern Denali Fault, while a 5-10 mm/yr NE-SW shortening rate is estimated across the region comprising the eastern Denali and Duke River Faults (Marechal et al., 2015). GPS data also suggest that the deformation within the Yakutat syntaxis is radiating from an indentor corner with velocities rotating clockwise from NW to NE around the syntaxis of the Yakutat indentor (Leonard et al., 2007; Marechal et al., 2015) (Fig. 1).
These GPS and geomorphological data raise the question of how the regional tectonics
accommodates the distributed deformation field along the eastern Denali, Totschunda and Duke
River Faults. In this paper, we present new constraints on the kinematics (sense of motion) and
the slip rates along these three faults from a detailed geomorphological study. Our observations
bring new insight into the present-day geodynamics of the Yakutat collision.

**2 Morphotectonic analysis**

**2.1 Geormorphological setting**

The fault system studied here (Fig. 1) is comprised of i) the eastern Denali Fault that runs
along a major glacial valley (Shakwak Trench) extending between the USA/Canada border and
Kluane Lake, ii) the Totschunda Fault that marks the NE flank of the Wrangell Mountains, and
iii) the Duke River Fault that bounds the St. Elias Mountains to the north. This region is located
at high latitudes (60-63.5° N) and high elevations (700-3500 m.a.s.l.) and was repeatedly covered
with massive glaciers and icecaps from the Cordilleran Ice Sheet during the Quaternary (Duk-
Rodkin et al., 2004). Paleoclimate studies relying on radiocarbon- and cosmogenic-dated glacial
landforms and deposits establish a chronology of major and minor episodes of ice advance and
retreat. Many studies provided age constraints for the region for the past 300 ka (eg Denton,
1974; Duk-Rodkin et al., 2004; Matmon et al., 2010; Kaufman et al., 2011; Turner et al., 2016).
Overall, the maximum late Pleistocene glaciation occurred during MIS 4/3 (40-70 ka) and was
followed by an intermediate episode at 25-30 ka. At 29.6 ± 0.46 kyr BP (MIS 3), the Macauley-
Kluane glacier filled the Shakwak trench entirely and reached a maximum thickness of 300 ± 50
m; this marks the Last Glacial Maximum (LGM). Within the Shakwak trench, ice retreat is
attested by similar landforms within the Kluane Lake area at 12.5 ± 0.2 ka BP and near the
92 USA/Canada border at 11.0 ± 0.16 ka BP. Re-calibrating these two ages using OxCal 4.2 (Bronk
93 Ramsay, 2008) and IntCal13 (Reimer et al., 2013) yields 14700 ± 300 cal years BP and 12950 ±
94 100 cal years BP, respectively.

## 2.2 Methodology

96 Our geomorphological study is based on the analysis of aerial photographs, satellite
97 images, and digital elevation models at different resolutions (i.e. DEM from WorldDEM, IfSAR,
98 Pleiades/High Elevation Aerial/Low Elevation Aerial photogrammetry, and field RTK (see
99 Suppl. Material text S1) that was complemented by field observations and measurements. These
100 data allow us to determine offsets and therefore the kinematics along faults.

101 In terms of age constraints, we mainly use published data that are both the closest to our
102 studied sites and correspond to the type of markers we analyze. We recalibrated radiocarbon
103 dating of Denton (1974) using OxCal (Bronk-Ramsey, 2008) to constrain the age of the ice
104 retreat in or near the main Shakwak Trench, whereas we use cosmogenic ¹⁰Be ages obtained by
105 Matmon et al. (2006) on stabilized moraines within the northern part of the Totschunda fault to
106 constrain the age of ice retreat in this region. In summary, the age constraints for the LGM
107 retreat are 11.9 ± 1.8 kyr BP along the Totschunda Fault, 12.9 ± 0.2 kyr BP along the Duke River
108 Fault and 13.8 ± 0.6 kyr BP along the Denali Fault. The maximum extent of the late Pleistocene
109 glaciation is dated to 27.5 ± 3.3 regionally (Matmon et al., 2010). Table 1 presents a summary
110 and the locations of the ages used in this study. These age constraints combined with measured
111 fault offsets are used to determine slip rates along faults. We also performed cosmogenic
112 radionuclides analyses at two sites (Ramshole Creek and Washe Creek) along the Totschunda
113 fault immediately north of the Chisana Glacier, in order to constrain ages of offset markers
114 locally.
For the offset, age, and slip rate estimations, we calculate mean values and associated uncertainties that are propagated at all stages of the calculations through standard equations (cf. Supplementary Material text S3).

## 2.3 Eastern Denali Fault

### 2.3.1 Reverse slip rate along the eastern Denali Fault

In order to study the present-day kinematics along the eastern Denali Fault, we analyze morphological markers (glacial and alluvial) displaced by the fault along a 300-km-long section between the Totschunda junction to the north and the Duke River junction to the south (Figure 1). Numerous structures show the glacial origin of the Shakwak Trench valley all along the Denali Fault: drumlins, kame terraces, moraines and kettles. Transverse alluvial markers (rivers incisions, fan deposits) imprinted this glacial surface after the ice retreat.

From landscape analysis on satellite images and in the field, we observe surfaces perched above the Shakwak Trench valley (Figure 2). Figure 2B shows one of these surfaces (S1) that stands at ~1900 m in elevation and cuts through ancient geological structures, defining a small isolated plateau perched 800 m above the surrounding LGM abrasional surface. This feature defines an abrasive surface, most likely associated with an ancient glacier. Such kind of abrasive surfaces are also found on northern margin of the Shakwak Trench where (surface S1' in Figure 2C) but are lower in elevation. Figure 2D shows the difference of elevation between the two ancient glacial surfaces suggesting, especially if they are correlative, that the St. Elias Mountains are undergoing active uplift.

Topographic profiles across the Denali Fault, along the entire studied section, show systematic vertical displacements (SW side up) of the Shakwak Trench LGM surface (Fig. 3).
These profiles show that cumulative vertical offsets since the LGM range from 6.3 ± 0.1 m to 18.5 ± 3.6 m (see Suppl. Material text S2 and Figure S2). All along the fault, elongated ~10-m-high mounds define the fault trace across the LGM surface and separate a southwestern block uplifted with respect to a northeastern one (Fig. 3B). We interpret these large-scale moletrack features as the combination of uplift along the Denali Fault associated with a superficial pop-up structure and erosion of the uplifted LGM surface by stream incision, dissecting the scarp into aligned mounds (Fig. 3C). From the suite of topographic profiles (Fig. 3A), we calculate a mean vertical offset of 13.0 ± 3.7 m for the LGM surface across the Denali Fault.

This vertical deformation also occurs within the post-glacial features, as shown by a detailed geomorphological study at Koidern Creek where a large alluvial fan is offset by the eastern Denali Fault (Fig. 4). At this site, the Shakwak Trench LGM surface displays typical glacial landforms: striations, kame terraces, drumlins, and moraines (Fig. 4). The Koidern creek incises into the LGM surface over a distance of 5 km from the mountain foothills until the Denali fault, where it deposits a ~6 km-wide alluvial fan. This geometry shows that the LGM surface already exhibited a scarp at the time of the fan emplacement. Detailed analysis of PleDEM and RTK DEM highlights vertically offset features (Fig. 4D): profile AA' across the LGM surface shows a 6.6 ± 0.1 m vertical offset; Profile BB' across the alluvial fan shows a vertical offset of 2.3 ± 0.1 m; profile CC' across the youngest alluvial terrace T0b shows a 0.9 ± 0.1 m vertical offset. No offset of terraces T2 and T1 is apparent because of dense vegetation and anthropogenic disturbance (road), respectively. Note that we do not observe horizontal displacements within the Koidern Creek fan features, notably within T0-T1 and T1-T2 risers (Fig. 4B).
From the 13.0 ± 3.7 m mean vertical uplift affecting the Shakwak Trench LGM surface, and the timing of its abandonment (13.8 ± 0.9 kyr (Denton, 1974)), we estimate a mean vertical slip rate of 0.9 ± 0.3 mm/yr. Assuming a near-vertical (75 ± 10°) dip for the strike-slip crustal-scale eastern Denali Fault yields a horizontal shortening rate of 0.3 ± 0.1 mm/yr).

### 2.3.2 Right lateral slip rate along the eastern Denali Fault

Thirty kilometers south of the Denali-Totschunda junction, Matmon et al. (2006) estimated a right-lateral slip rate of 8.3 ± 2.3 mm/yr from clear right-lateral offset features (Fig. 1). Beyond ~80 km from the junction, there are no longer any obvious right-lateral offset features, while a vertical component is observed all along the eastern Denali Fault (Fig. 3). Eighty kilometers south of the junction, where the fault cut through the ~1 km-wide Stuiver Creek, we do not see evidence of right-lateral offset. Instead a clear vertical offset is observed (Fig. 5A). Neither the riser between the abandoned glacial surface (T2) and a first alluvial terrace (T1c) nor the riser between T1c and a second alluvial terrace (T1b) show any significant lateral displacement (Fig. 5B. We conclude that if a right-lateral offset is affecting the Shakwak Trench LGM surface, it would be smaller than ~8 m, which corresponds to the smallest object identifiable on our PleDEM dataset (4 pixels, 2 m per pixel). Considering the age of abandonment of the Shakwak Trench LGM surface (13.8 ± 0.9 kyr (Denton, 1974)), this suggests that dextral slip – if any – along the eastern Denali Fault would be slower than ~1 mm/yr.

## 2.4 Right-lateral slip rate on the Totschunda Fault

In contrast to the southern section of the eastern Denali Fault, significant right-lateral cumulative offsets mark the entire Totschunda Fault. Figure 6 shows outstanding examples
where the fault crosses the main glacial valleys of Chisana Glacier and Ramshole Creek. There, we observe both glacial and alluvial features deformed by the fault. Our analysis of the geomorphology on satellite images, DEMs, and in the field allows mapping the area between the left bank of Chisana Glacier and Ramshole Creek (Fig. 6B): A ancient glacial surface (S2) lies at the foothills of the Nutzotin Mountains and exhibits an incised stream channel (Ga) that is beheaded from its source, and demonstrating the right-lateral shift of the S2 surface. Further north, a younger glacial surface S1 is inset within S2. Within Ramshole Creek, the riser between S1 and an alluvial surface is also offset in a right-lateral sense. To the south, the surface S2 is also incised by a narrow linear U-shaped valley (Washe Creek) that we interpret as contemporaneous with the S1 surface. This valley divides upstream into two branches that are affected by the Totschunda Fault. The northern branch shows a clear right-lateral cumulative displacement. The southern branch is beheaded by the fault, and shows a rough right-lateral offset. However, the erosion of its southern extension by the Chisana Glacier makes difficult to define clear piercing points.

### 2.4.1 Ramshole Creek

Within the Ramshole Creek area (Fig. 6C), we observe a shifted riser separating the glacial surface (S1) from an abandoned alluvial terrace (T1). This terrace, the first one abandoned above the present active streambed (T0) necessarily formed after the LGM through fluvial processes. A large part of S1 is washed and eroded (S1b) by streams emanating from the Nutzotin Mountains. One of these streams deposited a small debris cone (c) along the fault scarp (Fig. 6C). The projection of the S1/T1 riser onto the fault allows defining piercing points, from which we estimate a maximum offset (A-A') of 199.9 m (assuming no erosion of the riser) and a
minimum offset (A-A') of 145.3 m (assuming the riser was eroded). This yields an average right-lateral offset for the S1/T1 riser of 172.6 ± 27.3 m.

The deposits associated with the glacial surface S1 and alluvial terrace T1 are derived from metabasalts. In order to estimate the ages of these two geomorphic markers, we dug a 2 m-deep soil pit (CROSS) in S1a to analyze the distribution with depth of cosmogenic ³He and ¹⁰Be, and we collected amalgamated surface samples on both T1 (CROSS7) and T0 (CROSS8) (Fig. 6C). Cosmogenic ³He were measured in pyroxene phenocrysts following the procedure described in Protin et al. (2016), and cosmogenic ¹⁰Be was measured in quartz (see Suppl. Material Table S1). Unfortunately, the ³He concentrations are too scattered to allow modelling a consistent attenuation profile. This scattering suggests that inheritance in radiogenic ³He is large and variable, hampering the use of ³He cosmogenic nuclide to determine the age of surface S1. The vertical ¹⁰Be attenuation profile measured -50 to -190 cm below the S1a surface allows determining a best-fit age model using the numerical approach of Saint-Carlier et al. (2016). This yields estimates for an exposure age of 6-20 ka for surface S1 (Fig. 7A), with an inheritance of 5 × 10⁴ at/g and an erosion rate ranging from 2 to 10 × 10⁻³ cm/kyr (see Supp. Material text S4).

The age of alluvial terrace T1 was computed by combining the ¹⁰Be concentration measured in amalgamated surface samples (Table S1.). Sample CROSS8 (in T0) provides an estimate of the geological inherited background concentration. The ¹⁰Be concentration measured in CROSS8 (23 ± 11 × 10⁴ at/g) was subtracted from the concentration measured in CROSS7 (39 ± 7 × 10⁴ at/g) at the surface of T1. The exposure age was then computed using a production rate of 4.24 ± 0.05 at/g/yr, with the time dependent scaling of Lal/Stone (Balco et al., 2008). This yields an age of 12 ± 6 ka for the terrace T1 (Fig. 7A).
We used the relative chronology between the glacial surface S1 and the alluvial terrace T1 (the former being older than the latter in the morphology) to refine the probability density function of the T1 and S1a ages and reduce their uncertainties. We applied a Bayesian filtering from Blard et al. (2013) and Parnell et al. (2011), enforcing the stratigraphic relationship (S1 older than T1). We obtained an age of 22 ± 10 ka (1σ) for the glacial surface S1 and 10 ± 6 ka (1σ) for the alluvial terrace T1 (Fig. 7B).

The estimated age of the T1 surface is consistent with Matmon et al. (2006) (Table 1), who constrained the ice retreat, and therefore the beginning of the subsequent fluvial incision, further north along the Totschunda Fault at 11.9 ± 1.8 kyr. The riser between T1 and T0 (the present stream bed) shows no significant offset, indicating that the displacement affecting the S1/T1 riser was accumulated after the abandonment of T1. Dividing the average right-lateral offset for the S1/T1 riser (172.6 ± 27.3 m) by our estimated T1 age (10 ± 6 ka) gives a poorly constrained maximum slip rate of 17.3 ± 10.7 mm/yr. Using the more accurate age from Matmon et al. (2006) (Table 1) gives a more precise maximum slip rate of 14.5 ± 3.2 mm/yr for this portion of the Totschunda Fault.

### 2.4.2 Washe Creek site

The U-shaped valley of Washe Creek defines a narrow glacial valley (S1) inset in the higher and larger S2 glacial surface (Fig. 6B). We found lateral moraine deposits where the valley subdivides into 2 branches, attesting to its glacial origin (Fig. 6D). The northern branch, which was fed by glaciers descending from the Nutzotin Mountains, is clearly displaced along the Totschunda Fault. Restoring the offset across the fault yields a right-lateral cumulative offset of 406 ± 30 m (piercing points B-B', based on the reconstruction of the U-shaped valley using both risers). Within the S2 surface, a distance of 950 ± 100 m (piercing points C-C') separates
the offset beheaded gully Ga from the first drainage outlet along the northeastern slope of the Nutzotin Mountains. This corresponds to the minimum right-lateral offset for the S2 glacial surface.

We collected amalgamated surface samples on surfaces S1 and S2, and dug a soil pit in S2 to analyze the distribution with depth of cosmogenic ³He and ¹⁰Be. Surface cobbles sampled in glacial surface S2 (CHISA9) and within a glacial moraine deposited on surface S1 (CHISA11) give scattered and inconsistent results, both at the surface and at depth (Fig. 6D, Table S1 in Suppl Material). This inconsistency might be due by the reworking of sediments by periglacial processes. The concentrations obtained from the vertical profile in S2 (soil pit CHISA) do not decrease with the increasing depth (Table S1). These data suggest that inheritance is large and variable, precluding the possibility of determining the age of the surface, as in Ramshole Creek. Given that S1 is the first abandoned glacial surface above the present-day Chisana Glacier, we consider that the Washe Creek valley (S1 surface) was formed during the LGM at the stage MIS 3, 27.5 ± 3.3 kyr ago, as proposed by Matmon et al. (2010) farther north. This is consistent with our age determination of the S1 surface (22 ± 10 ka) at Ramshole Creek and with other studies in the region (cf. Section 2.1). Dividing the Washe Creek cumulative offset (406.0 ± 30.3 m) by this age yields a slip rate of 14.6 ± 2.1 mm/yr.

Combining our estimates in Ramshole and Washe Creeks yields an average slip rate of 14.6 ± 2.7 mm/yr on the Totschunda Fault.

## 2.5 Reverse slip rate on the Duke River Fault

Our preliminary observations in the Duke River Fault area allows highlighting a fresh scarp affecting the youngest abandoned glacial surface in the White River valley (Fig. 8, see
location on Figure 1) at the northern range front of the St Elias mountains. The 10-km wide glacial valley was glaciated during the LGM until ice retreat at $12.9 \pm 0.1$ kyr (Denton, 1974) (Table 1). The valley is characterized by several glacial markers such as abrasion surfaces, moraines and kame terraces. The Duke River scarp is clear from the Totschunda junction to ~30 km to the east. The fault offsets glacial and alluvial markers, as highlighted in the Holmes Creek valley, from which we can measure vertical offsets ranging between ca. 20 and 80 m on postglacial terraces and glacial surfaces (Fig. 8b).

Assuming that the glacial surface Sg-f is associated with the LGM regional age of $27.5 \pm 3.3$ kyr (see Table 1), the maximum offset (profile P3, Fig. 8c) yields a maximum vertical slip rate of $3.0 \pm 0.4$ mm/yr. A similar maximum vertical slip rate ($3.0 \pm 0.1$ mm/yr) is obtained by assuming that the alluvial terrace T3 (profile P2, Fig. 8c) formed during the ice retreat ($12.9 \pm 0.2$ kyr). In contrast, the offset of younger terrace T2 (profile P1, Fig. 8c) yields a minimum vertical slip rate of $1.8 \pm 0.1$ mm/yr based on a maximum age of $12.9 \pm 0.2$ kyr. The sinuosity of the scarp suggests that the fault dips to the south with a relatively low angle typical of a thrust fault. Assuming a $30^\circ$ dip yields minimum and maximum horizontal shortening rates across the fault of $3.6 \pm 0.1$ mm/yr and $6.0 \pm 0.5$ mm/yr, respectively. To first order, this rate is in good agreement with the shortening rate estimated from the integration of the local seismicity catalog ($1-6$ mm/yr; Leonard et al., 2008).

## 3 Discussion and Conclusions

Combined with previous studies, our analysis of high-resolution topography and imagery, field observations, and cosmogenic dating along the eastern Denali, Totschunda, and Duke River Faults brings new insight into the active deformation of the Yakutat collision zone. In particular, slip rates along the eastern Denali and Totschunda Faults appear to follow opposite but
complementary evolutions before their junction with the central Denali Fault (Fig. 9): Right-lateral strike-slip along the eastern Denali Fault decreases southward over a distance of ~80 km, from 8.3 ± 2.3 mm/yr at the junction to less than 1 mm/yr at Stuiver Creek west of the Alaska-Yukon border; In contrast, right-lateral strike-slip along the Totschunda Fault increases southward from 6.0 ± 1.2 to 14.6 ± 2.7 mm/yr over a similar distance (~100 km). This pattern suggests a significant transfer of deformation between the central Denali, eastern Denali, and Totschunda Faults. Assuming that southern Alaska translates (or rotates) northwestward at about 10 – 15 mm/yr relative to the central Alaska – Yukon Plateau region (Elliott et al., 2013; Mériaux et al., 2009; Bemis et al. 2015), this motion is nearly fully accommodated along the central Denali Fault and the southern section of the Totschunda Fault, but distributed nearly equally between the northern sections of the Totschunda and eastern Denali Faults (Fig. 9 insert).

This particular pattern of slip rates and gradients implies a relay between the northern section of the eastern Denali Fault and the southern section of the Totschunda Fault, where about 6 – 9 mm/yr of NW-SE extension are predicted, assuming that the slip transfer does not occur through vertical axis rotation (Fig. 9). This observation is in agreement with the strain rates derived from regional GPS data that show a patch of NW – SE extension in this area (Marechal et al., 2015). It is unclear whether this extension is localized on a few structures or distributed across the whole region. However, micro-earthquake activity near the southern termination of the 2002 Denali earthquake suggests the presence of NE-SW crosscutting structures or deformation zones (Doser, 2014) that may accommodate part of the predicted NW-SE extension. It is also worth noting that the extension rate depends very strongly on slip rates on the main faults, currently limited to three local estimations.
On the basis on our analysis, the role of the eastern Denali Fault in the present-day regional tectonics is negligible past the Denali – Totschunda junction. Most of the fault exhibits small (less than 1 mm/yr) NW-SE shortening and no clear evidence of strike-slip motion. This pattern could be explained by the effect of the Yakutat collision and in particular the indentation corner at its eastern syntaxis. Figure 9 illustrates the pattern of strain and stress lines radiating from the syntaxis proposed by Marechal et al. (2015) on the basis of regional GPS data. Within this framework, the collision results in fault-normal compression along most of the eastern Denali Fault, thus reactivating this major structure in shortening and strongly limiting the potential strike-slip motion.

Further south, the accommodation of the SE Alaska motion remains unresolved. GPS data do not discriminate between distributed shear across the whole region or localized strike-slip along a hypothetical Connector Fault system (Marechal et al., 2015). Seismicity clusters and focal mechanisms can be interpreted as localized deformation along a Connector Fault (Doser, 2014) or as very limited distributed shortening (Leonard et al., 2008). If our hypothesis on the dip of the Duke River fault is correct, our estimated slip rate of 3.5 – 6.5 mm/yr on the westernmost part of the fault only represents about 20 – 60% of the SE Alaska motion, thus excluding a simple strain transfer model in which the Connector Fault is the only structure transferring slip from the Fairweather Fault to the Totschunda Fault.

Deformation within this region, directly north of the present-day syntaxis, is likely affected by the strong rotation (roughly NW-SE to NE-SW) of the stress and strain lines around the indentation corner and the intense deformation within the strain knot evidenced by peaks in geodetic strain rates (Marechal et al., 2015), seismicity (Doser, 2014), and exhumation rates
(Spotila and Berger, 2010; Falkowski et al., 2014). Numerical models of the Yakutat underthrusting and collision suggest a southward (outward) migration of this strain knot over the last ~1 Ma (Koons et al., 2010). Altogether, these elements may point to a very deformed, weakened region that accommodates the recent southward propagation of the central Denali – Totschunda fault system without significant strain localization onto major structures.

The fault slip pattern and tectonic model developed in this study (Fig. 9) provides significant constraints to the question of lithospheric strength contrast and strain localization near mature fault zones. Major strike-slip faults tend to develop at or near lithospheric-scale strength discontinuities (e.g., Molnar and Dayem, 2010), with a positive feedback in strain localization as mature fault zones also represent major zones of weakness in the crust and lithospheric mantle (e.g., Platt and Behr, 2011). Due to their maturity and large cumulated strain, the Denali and Totschunda Faults likely correspond to such low-strength strain-localizing structures, as shown by the block-like deformation pattern in southeastern Alaska (Fig. 9). The abrupt “shut-off” of the eastern Denali Fault near the Alaska - Yukon border marks a sharp transition from block-like tectonics to distributed deformation in a collision syntaxis setting, which should provide strong constraints on the strength reduction and dynamics of these major continental faults.

354

355

356 Acknowledgments

We acknowledge the ISIS program from the French Space Agency (CNES) for providing Pleiades images, the Tandem-X Science Program from the German Space Agency (DLR) for providing Intermediate WorldDEM (https://tandemx-science.dlr.de), and the United States

359
360 Geological Survey and the Geographic Information Network of Alaska for providing the IfSAR

361 DEM. We thank our colleagues from the Yukon Geological Survey and the US Geological

362 Survey in the Anchorage office for their assistance in the field mission preparations. We also

363 thank 2 anonymous reviewers whose thorough reviews helped improving the manuscript. This

364 work was supported by the French "Agence Nationale de la Recherche" grant ANR-12-CHEX-

365 0004-01 (DefDyCor).

366
367 **Figure captions**

368 **Figure 1.** Tectonic setting of the Yakutat collision. White arrow shows the Yakutat – North America convergence rate; black arrows show the GPS data (after Marchal et al., 2015); colored marks and rectangles identify fault slip rate estimates from previous studies (orange : Matmon et al., 2006; green: Seitz et al., 2010; blue: Marechal et al., 2015). Black frames show sites analyzed in this study.
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372

373 **Figure 2.** A: Slope map of relief north and south of Shakwak Trench valley, near Kluane Lake, pointing out abandoned glacial surfaces. B: Google Earth oblique view of glacial surfaces (S1) at ~1900 m in elevation south of the eastern Denali Fault. C: Google Earth oblique view of glacial surfaces (S1') at ~1450 m in elevation north of the eastern Denali Fault. D: Topographic profile across the Shakwak Trench valley showing the difference of elevations between perched ancient glacial surfaces on both sides of the Denali fault.
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378

379 **Figure 3.** A: Vertical offsets of the LGM abrasional surface along the Denali Fault from the Totschunda junction to Kluane Lake. Yellow rectangles represent the location of offset profiles (see Suppl. Material text S2 and Figure S2); black frames show sites analyzed in this study. B: Aerial photography of the Denali Fault at Koidern Creek (cf. Fig. 4A for orientation). C: Interpretative 3D block diagram showing the superficial pop-up structure along the eastern Denali Fault.
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384

385 **Figure 4.** Morphology and offsets on the eastern Denali Fault within Koidern Creek. A: Shaded relief map (WorldDEM) pointing out the alluvial fan (dashed line) deposited at the outlet of Koidern Creek at its intersection with the Denali fault. B: Slope map (WorldDEM) showing the alluvial terraces (T0, T1 and T2) inset in the alluvial fan (F), itself incised in the glacial surface.
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C: 3D elevation model from kinematic GPS survey along the fault in the northwestern bank of the alluvial fan (see corresponding profiles B-B' in Figure 4D). D: Topographic profiles and mean offsets along the Denali Fault in the glacial surface (AA'), the alluvial fan (BB'), and the T0 alluvial terrace (CC').

**Figure 5.** A: Alaska High Altitude Photography (AHAP) DEM oblique view showing the vertical topographic scarp observed along the eastern Denali Fault within Stuiver Creek (see Fig. 5B for orientation); red arrows point out the fault scarp. B: Slope map with alluvial terraces in colors (T0, T1a, T1b and T1c) inset within an older glacial surface affected by fluvial erosion (Sg-f).

**Figure 6.** Geomorphology and offsets on the Totschunda Fault between the valley of Chisana Glacier and Ramshole Creek. A and B: Shaded relief map (WorldDEM, see suppl. material text S1) and its geomorphological interpretation, respectively: red arrows and red line point out the Totschunda fault, yellow and green patches define S2 and S1 ancient glacial surfaces. Ga points out a beheaded gully incised in S2 surface. C: Slope map (Alaska Low Altitude Photography (ALAP) DEM) of Ramshole Creek area with geomorphic features (T0 and T1: alluvial terraces, c: debris cone, S1a: glacial surface, S1b: same surface but washed and eroded by post-glacial fluvial processes); dashed brown lines represent the risers used to define piercing points (white circles) for offset estimates; yellow squares are sampled sites. D: Shaded relief map (ALAP DEM) of Washe Creek area with geomorphic features (S1 and S2 glacial surfaces, Ga: beheaded gully on S2 surface, Ga': closest corresponding source for Ga; dashed green and blue lines and circles correspond to mean piercing lines and points, respectively, from which were estimated S1 and S2 offsets); yellow squares are sampled sites.
**Figure 7.** A: Uniform probability density functions of the cosmogenic $^{10}$Be ages of glacial surface S1A and alluvial terrace T1. B: Probability densities of S1a and T1 ages after Bayesian filtering, taking into account the stratigraphic relationships (S1a is older than T1).

**Figure 8.** A: Slope map (HAAP DEM) showing the LGM glacial surface affected by the reverse Duke River fault in the White River valley; Pink diamonds are locations of samples in Denton (1974) used in this study (see Table 1). B: Shaded relief map (WorldDEM) showing the fault scarp associated with the Duke River Fault: T0, T1, T2 and T3 correspond to alluvial surfaces inset within an older glacial surface affected by fluvial erosion (Sg-f); C1 and C2 are debris cones. C: Topographic profiles across the fault scarp.

**Figure 9.** Faults slip rates and tectonic model of the Yakutat collision area. Black lines and yellow and white rectangles show horizontal fault slip rates estimated in the present and previous studies, respectively [*] for the Duke River fault, a range of slip rates between a minimum and a maximum value is indicated, assuming a fault dip of 30°. Solid blue curves show the radiating stress and strain pattern (from GPS measurements) around the collision syntax axis and strain knot (dashed blue oval) (Marechal et al., 2015). CDF: central Denali Fault, EDF: eastern Denali Fault, TF: Totschunda Fault, DRF: Duke River Fault, CF: hypothetical Connector Fault (Spotila and Berger, 2010). Upper right inset: Schematic tectonic model of fault slip and strain partitioning in response to the SE Alaska motion. Large, medium, and thin lines show faults that accommodate nearly 100%, 50%, or 0% of the regional motion. Grey areas show regions of distributed strain.

**Table 1.** Synthesis of ages used in this study.
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<table><caption>Table_1_EPSL</caption><thead><tr><th>Area</th><th>Sample</th><th>Published age (kyr)</th><th>Reference</th><th>Calibrated age (kyr BP)</th><th>Geomorphic marker</th><th>Age used (kyr BP)</th></tr></thead><tbody><tr><td>Denali</td><td>Y-1386</td><td>12.5 ± 0.2</td><td>Denton (1974)</td><td>14.7 ± 0.3</td><td>LGM Shakwak surface</td><td></td></tr><tr><td>Denali</td><td>GSC-714</td><td>11.0 ± 0.16</td><td>Denton (1974)</td><td>12.9 ± 0.2</td><td>LGM Shakwak surface</td><td>13.8 ± 0.9</td></tr><tr><td>Regional</td><td>DFDD4</td><td>28.6 ± 3.2</td><td>Matmon et al. (2010)</td><td></td><td>Wash Creek</td><td></td></tr><tr><td>Regional</td><td>DDDL-8</td><td>28.2 ± 3.1</td><td>Matmon et al. (2010)</td><td></td><td>Wash Creek</td><td>27.5 ± 3.3</td></tr><tr><td>Regional</td><td>DR2-5</td><td>25.7 ± 2.8</td><td>Matmon et al. (2010)</td><td></td><td>Wash Creek</td><td></td></tr><tr><td>Totschunda</td><td>DFDP</td><td>11.1 ± 1.2</td><td>Matmon et al. (2006)</td><td></td><td>Ramshole Creek</td><td></td></tr><tr><td>Totschunda</td><td>DFDP</td><td>13.3 ± 1.4</td><td>Matmon et al. (2006)</td><td></td><td>Ramshole Creek</td><td></td></tr><tr><td>Totschunda</td><td>DFNM</td><td>13.3 ± 1.4</td><td>Matmon et al. (2006)</td><td></td><td>Ramshole Creek</td><td>11.9 ± 1.8</td></tr><tr><td>Totschunda</td><td>DFNM</td><td>11.8 ± 1.3</td><td>Matmon et al. (2006)</td><td></td><td>Ramshole Creek</td><td></td></tr><tr><td>Totschunda</td><td>DFNM</td><td>10.1 ± 1.1</td><td>Matmon et al. (2006)</td><td></td><td>Ramshole Creek</td><td></td></tr><tr><td>Duke River</td><td>I-6092</td><td>10.9 ± 0.2</td><td>Denton (1974)</td><td>12.9 ± 0.1</td><td>upper White River</td><td></td></tr><tr><td>Duke River</td><td>SI-1103</td><td>11.1 ± 0.1</td><td>Denton (1974)</td><td>12.9 ± 0.1</td><td>upper White River</td><td></td></tr><tr><td>Duke River</td><td>Y-2306</td><td>11.3 ± 0.2</td><td>Denton (1974)</td><td>13.1 ± 0.2</td><td>upper White River</td><td>12.9 ± 0.2</td></tr><tr><td>Duke River</td><td>Y-2301</td><td>10.9 ± 0.2</td><td>Denton (1974)</td><td>12.9 ± 0.1</td><td>upper White River</td><td></td></tr></tbody></table>
<figure><img src="image_3.png" /></figure>
<figure><img src="image_4.png" /><figcaption>Figure2_EPSL</figcaption></figure>
<figure><img src="image_5.png" /></figure>

Distance from Totschunda junction (km)

ESE

WNW

B

<figure><img src="image_6.png" /></figure>

C
<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /><figcaption>figure7_EPSL</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Figure8_EPSL</figcaption></figure>
<figure><img src="image_16.png" /></figure>