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BHeight (m) P1 VS = 5.7 ± 1.7 m
10 20 30
A N colluvial surface VS
downhill

Colluvial P1 channel P2
apron
P2 N VS = 3.2 ± 1.2 m S
-40 -20 0 20 40
Distance (m)

C View to N
Channel Channel
margin bottom

uphill

100 m Bedrock Scarp
face

Figure 3. (A) LiDAR hillshade map of Site A, showing an uphill (south) facing scarp cutting the surface of a steeply north-
sloping colluvial apron and channels. Red arrows point to steep face. Black and white arrows show apparent left and right
(respectively) lateral separations of channel margins. Example profile lines (P1 and P2) locations shown. Additional profile
lines are shown in Figure DR2 (see text footnote 1). (B) An example of LiDAR-derived elevation profiles from interfluve P1 and
channel P2. VS—vertical separation. (C) Field photo showing tectonic scarp in a channel at site C.

Massey et al., 2005). We identify several along steeply dipping foliation planes, we differential erosion across this strong litho-
strands of the Leech River fault that dis- mapped the position of lithologically dis- logic contrast.
place post-glacial sediments and record at tinct units and collected structural data on
least two MW >6 earthquakes since the the occurrence and orientation of foliation To further exclude topographic features
Cordilleran deglaciation ca. 15 ka (Clague and fault deformation fabrics. The topo- produced by glacial processes, we deter-
and James, 2002). These data provide the graphic scarps we identified are roughly mined local ice flow directions from bed-
first evidence for Quaternary surface rup- parallel to the previously mapped location rock striae and streamlined glacial deposits
ture along a crustal fault that lies within of the Leech River fault (Fairchild and and collected geomorphic data designed
close proximity of Victoria, British Cowan, 1982; Massey et al., 2005), but to confirm a tectonic origin. The roughly
Columbia, and suggest that the Leech River none of the identified fault scarps coincide east-west–oriented topographic features on
fault is only one of a network of active exactly with the fault contact between the the eastern half of the Leech River fault
faults that accommodate forearc deforma- Leech River Complex and the Metchosin are nearly perpendicular to the southerly
tion in southwestern Canada. Formation (Fig. 2). Instead, individual top- regional ice flow direction during the last
ographic features occur both north and glacial maximum. The LiDAR data delin-
OBSERVATIONS south of the lithologic fault boundary by eate large (km-long) drumlinoid ridges
as much as hundreds of meters. Where a with well-defined apices that are distinc-
We mapped >60 topographic features discrete contact between the basalt and tively streamlined with steep up-ice (north-
along the Leech River fault that together schist units is exposed at two locations ern) margins and upper surfaces
extend >60 km in length and span ~1 km in in the area, the fault strikes parallel to that gently slope in a southerly, down-ice
width. Individual features range in length regional foliation (300–310°) but dips more direction (Figs. 2C and 2D). Our field work
from hundreds of meters to >2.5 km, reach steeply (70–90° NE) than the foliation confirms that these ridges are mantled by
up to ~5 m in height, and form linear ridges, (~45° NE) (Figs. 2B and 2C, and GSA glacial sediments (Fig. DR1E
sags, and scarps with both north- and Data Repository1 Fig. DR1A). The western- [see footnote 1]). South-directed ice flow is
south-facing directions (Fig. 2). Along the most of these sites contains a 10- to further supported by glacial striae data
eastern half of the fault, where we focused >200-m-wide mylonitic shear zone within on bedrock near the drumlinoid ridges
our analysis, these topographic features both units, but exhibits no brittle deforma- (Fig. 2C). The observation that the mapped
coincide with displaced geomorphic sur- tion at the outcrop scale (Figs. 2B scarps strike perpendicular to the ice
faces, steeply dipping brittle faults, and and 2C). Because the mapped features do flow direction rules out their formation
uphill-facing bedrock scarps. not coincide with the lithologic terrane by ice flow–parallel processes, including
boundary, they cannot be explained by glacial scouring, grooving, molding,
In order to exclude topographic features and streamlining.
that were produced by differential erosion

1 GSA Data Repository Item 2017046, supplementary figures, is online at http://www.geosociety.org/datarepository/2017/. If you have questions, please email
gsatoday@geosociety.org.

6 GSA Today | March–April 2017