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been vertically displaced by ~4–6 m due to increase in surface elevation. We interpret We suggest that the identified scarps
slip on a steep, north-dipping reverse fault. this en echelon arrangement of topo- together compose an active fault system
graphic ridges and the lateral juxtaposi- that is up to ~1 km wide and 30–60 km
Site B tion of topographic highs and lows as long (Fig. 2A). Although individual linea-
pressure ridges, common in strike slip or ments can be traced for only hundreds of
Five kilometers east along strike from oblique slip systems (e.g., Sylvester, meters along strike, meter-high fault scarps
site A, a prominent south-facing bedrock 1988; Sherrod et al., 2008, 2016; Nelson are not easily preserved in this wet climate,
scarp extends for ~1.5 km and shows evi- et al., 2014). and the fault scarps are semi-continuous
dence for brittle deformation along its with one another along strike. Our recog-
length (site B, Figs. 2 and 4). Near the cen- QUATERNARY SLIP ON THE nition of topographic features along the
ter of this scarp, an abandoned rock LEECH RIVER FAULT western ~30 km of the fault similar to
quarry exposes two steeply north-dipping those on the eastern half (Fig. 2C) suggests
sub-parallel faults (dipping 85° to N40°E) The displaced geomorphic features, that the active fault zone extends the entire
cutting Metchosin Formation basalt (site faulted bedrock, and prominent scarps 60-km length of the fault onshore (Fig.
B1, Figs. 4A and 4B). Both faults have a collectively argue that several strands of 2A). Scarp morphology, fault orientations,
1–2-mm-wide gouge zone and exhibit sub- the Leech River fault have been active and fault kinematics suggest that the active
horizontal slickenlines (05° toward 129°) since the late Pleistocene. Our observa- strands of the Leech River fault accommo-
consistent with strike-slip motion (Figs. tions support a tectonic genesis for the date strike and dip slip motion within a
4B and DR1C [see footnote 1]). At the topographic features we identify for sev- steeply dipping fault zone or flower struc-
eastern end of site B, the scarp becomes eral reasons. First, several of the identified ture. Within a zone up to 1 km wide, we
~4 m high and uphill facing (Fig. 4A). topographic features show evidence for observe near vertical faults, variable scarp
Here, the northern (upthrown) side of the extensive brittle faulting. For example, the facing directions, laterally discontinuous
scarp consists of fractured and brittly fractured rock and gouge along the scarp surface scarps, and field evidence for
deformed Metchosin Formation basalt, at site B (Fig. 4B) require a tectonic origin strike-slip and reverse faulting. These
whereas the southern (downthrown) side and exclude formation by either ice pluck- characteristics are typical of strike slip
of the scarp contains fine-grained sedi- ing or the erosion of a bedrock foliation. systems and are similar to features
ment (P3, Fig. 4B). Similar to site A, the Second, the observation that paleo–ice observed along active oblique-reverse
apparent north-side-up displacement flow was directed to the south, at a high faults in the adjacent Pacific Northwest
across the scarp and the northward diver- angle to the orientation of the topographic (e.g., Johnson et al., 2001; Sherrod et al.,
gence of the scarp trace into topographic features (Fig. 2C), further rules out forma- 2008, 2016; Kelsey et al., 2012; Nelson et
lows signifies dip displacement along a tion by glacial processes. Finally, it is al., 2014; Personius et al., 2014; Blakely et
steeply north-dipping reverse fault (Figs. unlikely that the topographic scarps in al., 2014).
4B and DR1D). Overall, these observa- Quaternary deposits were produced by
tions suggest an origin for this feature as a landslide processes. Several of the scarps, These new results challenge the pre
tectonic scarp. including those at sites A and B (Figs. 2C vailing view that the Leech River fault was
and 2D), are uphill facing, nearly perfectly primarily an Eocene structure (cf.
Site C linear, and do not exhibit curvilinear head MacLeod et al., 1977). This interpretation
scarps that would be expected for landslides. was partly based on the observation that
Approximately 5 km east of site B, an relatively undeformed Oligocene sedi-
~1.5-km-long region contains >300-m-long The most compelling evidence for a tec- ments of the Carmanah Group (Sooke
ridges, linear sags, and swales up to tonic origin for these topographic features Fm.) lie unconformably above healed frac-
~2–5 m in height that cut across relatively comes from site A, where both the hillslope tures and mylonitic fabrics close to the
smooth, gently sloping till-mantled hill- surface and multiple channels are displaced trace of the Leech River fault near Sombrio
slopes (Figs. 4C and DR1E). These topo- vertically along an uphill facing scarp Point (Fig. 2A) (MacLeod et al., 1977).
graphic features display several differ- (Figs. 3A and 3B). The scarp at site A can- However, our results from the eastern half
ences from those at sites to the west. not represent the remnants of an abandoned of the Leech River fault show that active
Whereas sites A and B exhibit discrete logging road or placer mining excavation fault strands occur within a zone as much
topographic scarps, features in this region because the base of the scarp is not graded, as 1 km wide and these strands are not
are 10–15-m-wide elevated zones that sit and the upper and lower surfaces are verti- always co-located with observed fault-
more than ~5 m above the surrounding cally separated by >~4 m (Fig. 3B). Such related fabrics. Therefore, the location of
landscape. Moreover, while the scarps displacement in hillslope elevation, and in fault fabrics may not coincide with the
at sites A and B remain north-facing for particular the displaced channels, cannot surface trace of the active fault.
hundreds of meters along strike, the facing be produced by any mechanism other than
direction of the features in site C transi- fault displacement. Because the colluvial IMPLICATIONS FOR
tions southeastward from south- apron at this site remains both in situ and PALEOSEISMICITY
to north-facing over a short (~200 m) intact, the tectonic scarps crosscutting the
distance (Figs. 2D and 4C). colluvial surface and inset channels must The displaced channels and colluvial
be no older than the deglaciation following surface at site A suggest this section of the
These scarps have a nearly linear trace the last glacial maximum (ca. 15 ka) Leech River fault has experienced at least
across topography, but they do not exhibit (Clague and James, 2002). two, and possibly three or more, large,
clear upthrown fault blocks or a marked
8 GSA Today | March–April 2017
