Abstract The San Juan–southern Gulf Islands Archipelago of Washington State, USA and western Canada is located on the upper plate of the Cascadia subduction zone, in the forearc between the trench and volcanic arc. Onland and island investigations show many faults within the region that primarily represent old, inactive faults associated with transport, subduction and accretion of tectonostratigraphic terranes. However, until recently little geologic investigation and mapping have been done in the offshore. From these narrow straits, channels and sounds we have collected and interpreted high-resolution multibeam echosounder bathymetric data, 3.5 kHz sub-bottom and Huntec seismic-reflection profiles, and piston-cores to identify and date recently active faults. Previous studies by us focused on the earlier recognized active Devils Mountain fault zone that bounds the southern part of the Archipelago and the recently reported newly mapped active Skipjack Island fault zone that bounds the northern part. These transcurrent fault zones appear to be deforming and rotating the Archipelago. We concentrate on the unique deformation occurring within the seaways to determine the relationship and styles of faulting associated with these active bounding fault zones and relate the fault geometry and kinematics to one other subduction complex, the New Hebrides island arc of Vanuatu.

Abstract The Queen Charlotte Fault defines the Pacific–North America transform plate boundary in western Canada and southeastern Alaska for c. 900 km. The entire length of the fault is submerged along a continental margin dominated by Quaternary glacial processes, yet the geomorphology along the margin has never been systematically examined due to the absence of high-resolution seafloor mapping data. Hence the geological processes that influence the distribution, character and timing of mass transport events and their associated hazards remain poorly understood. Here we develop a classification of the first-order shape of the continental shelf, slope and rise to examine potential relationships between form and process dominance. We found that the margin can be split into six geomorphic groups that vary smoothly from north to south between two basic end-members. The northernmost group (west of Chichagof Island, Alaska) is characterized by concave-upwards slope profiles, gentle slope gradients (<6°) and relatively low along-strike variance, all features characteristic of sediment-dominated siliciclastic margins. Dendritic submarine canyon/channel networks and retrogressive failure complexes along relatively gentle slope gradients are observed throughout the region, suggesting that high rates of Quaternary sediment delivery and accumulation played a fundamental part in mass transport processes. Individual failures range in area from 0.02 to 70 km 2 and display scarp heights between 10 and 250 m. Transpression along the Queen Charlotte Fault increases southwards and the slope physiography is thus progressively more influenced by regional-scale tectonic deformation. The southernmost group (west of Haida Gwaii, British Columbia) defines the tectonically dominated end-member: the continental slope is characterized by steep gradients (>20°) along the flanks of broad, margin-parallel ridges and valleys. Mass transport features in the tectonically dominated areas are mostly observed along steep escarpments and the larger slides (up to 10 km 2 ) appear to be failures of consolidated material along the flanks of tectonic features. Overall, these observations highlight the role of first-order margin physiography on the distribution and type of submarine landslides expected to occur in particular morphological settings. The sediment-dominated end-member allows for the accumulation of under-consolidated Quaternary sediments and shows larger, more frequent slides; the rugged physiography of the tectonically dominated end-member leads to sediment bypass and the collapse of uplifted tectonic features. The maximum and average dimensions of slides are an order of magnitude smaller than those of slides observed along other (passive) glaciated margins. We propose that the general patterns observed in slide distribution are caused by the interplay between tectonic activity (long- and short-term) and sediment delivery. The recurrence (<100 years) of M > 7 earthquakes along the Queen Charlotte Fault may generate small, but frequent, failures of under-consolidated Quaternary sediments within the sediment-dominated regions. By contrast, the tectonically dominated regions are characterized by the bypass of Quaternary sediments to the continental rise and the less frequent collapse of steep, uplifted and consolidated sediments.

Abstract Multibeam echosounder (MBES) images, 3.5 kHz seismic-reflection profiles and piston cores obtained along the southern Queen Charlotte Fault Zone are used to map and date mass-wasting events at this transform margin – a seismically active boundary that separates the Pacific Plate from the North American Plate. Whereas the upper continental slope adjacent to and east (upslope) of the fault zone offshore of the Haida Gwaii is heavily gullied, few large-sized submarine landslides in this area are observed in the MBES images. However, smaller submarine seafloor slides exist locally in areas where fluid flow appears to be occurring and large seafloor slides have recently been detected at the base of the steep continental slope just above its contact with the abyssal plain on the Queen Charlotte Terrace. In addition, along the subtle slope re-entrant area offshore of the Dixon Entrance shelf bathymetric data suggest that extensive mass wasting has occurred in the vicinity of an active mud volcano venting gas. We surmise that the relative lack of submarine slides along the upper slope in close proximity to the Queen Charlotte Fault Zone may be the result of seismic strengthening (compaction and cohesion) of a sediment-starved shelf and slope through multiple seismic events.

Abstract Canada's western continental shelf can be broadly divided into three geographical regions: Salish Sea, Pacific North Coast and Vancouver Island Shelf. Each region's physiography has been uniquely impacted by a history of glaciation, tectonism, oceanography and sea-level change. Retreat of the Fraser Glaciation started from the Pacific towards the mainland of British Columbia first in the north on Haida Gwaii (16 14 C ka BP) then outer Vancouver Island (14 14 C ka BP), and generally progressed east with ice persisting on the Fraser lowland and northern British Columbia mainland, and possibly the central interbasin coast, until after 10 14 C ka BP. Retreat on the Pacific North Coast was rapid, while downwasting and stagnation occurred in the Salish Sea. Sea levels responded to this deglacial history, with submergence of most of the Salish Sea by over 250 m before 12 14 C ka BP then falling to approximately 100 m where it remained until approximately 10 14 C ka BP, and emergence of the Vancouver Island Shelf with the development of a forebulge. In contrast, the Pacific North Coast crustal loading of the British Columbia mainland and development of a peripheral forebulge towards Haida Gwaii resulted in submergence of the mainland by 120 m and emergence of the western basin by more than 120 m, with regional variation throughout. Holocene sedimentation is primarily restricted to the coastal inlets and fjords, except for the southern Salish Sea off the Fraser River.

Abstract During the late Wisconsin Fraser Glaciation on the Pacific Margin of Canada, ice moved offshore from the Coast Mountains of the Canadian Cordillera and south into the Strait of Georgia, reaching a maximum extent at about 14 000 14 C bp . Most of the strait was ice-free by 11 300 14 C bp . Deglaciation was very rapid with regional downwasting and widespread stagnation. This resulted in a stratigraphy of thick till (30–60 m), overlain by ice-proximal glacimarine sediments and a thin and discontinuous ice-distal glacimarine unit. Glaciation of Queen Charlotte Basin reached a maximum sometime after 21 000 14 C bp . Deglaciation in this region began sometime after 16 000 to 15 000 14 C bp and ice had retreated fully onto mainland British Columbia by 13 500 14 C bp . Deglaciation was rapid, with the eastward retreat of an ice shelf. This resulted in a stratigraphy of a till up to 50 m in thickness, usually turbated by iceberg scour and overlain in some areas by thin, ice-proximal glacimarine sediments and much thicker (20 m) widespread ice-distal glacimarine sediments. A significant difference between these two regions is the deglacial relative sea-level history. Rapid regression of the outer Queen Charlotte Islands shelf occurred between approximately 14 600 and 12 500 14 C bp , primarily due to rapid isostatic rebound and contemporaneous with deglaciation of the continental shelf. Sea-level reached a maximum lowstand of greater than 150 m and remained low until approximately 12 400 14 C bp . In Georgia Basin, sea-level was at a relative high stand of 50 to 200 m during initial deglaciation, falling to between 0 to 50 m below present sometime after 10 000 14 C bp . We suggest that rapid emergence on the northern margin of the outer shelf was due to forebulge effects. Further, the very limited extent of glacial ice on the Queen Charlotte Islands and the exposure to the open Pacific forced the retreat of the Cordilleran ice-sheet margin eastwards thereby resulting in dominantly ice-distal glacimarine sedimentation. In contrast, the initial relative sea-level highstand during deglaciation between the Vancouver Island and Cordilleran glaciers in the Strait of Georgia resulted in significant ice-proximal deposition and limited ice-distal glacimarine deposition.

Abstract Throughout the Holocene, the river dominated Fraser Delta on the Pacific coast of Canada has prograded by continuous channel switching and avulsion into a deep (>300m) basin. However, at the beginning of the 20th century the delta was modified to provide a navigable channel and port facilities for the city of Vancouver. Now most of the sand brought down by the river (35% of the sediment load) is removed from the system by dredging. The remaining fine-grained sediment is transported in a plume past the intertidal estuary within the distributary channels then deflected northwards by the dominant flood tidal flow into the basin. Two causeways to the south of the main channel and one to the north that cross the intertidal zone to the delta foreslope act as barriers to the dominant northward sediment transport causing estuarine and localized seabed erosion. An eroded distributary channel failure complex has been exposed on the delta foreslope, off the southern causeways, by flood tidal flows that scour the seabed and form northward migrating subaqueous dunes, further increasing the delta slope. This, combined with slow sea-level rise and seismicity, intensifies the risk of further erosion and instability of the delta, particularly along the subaqueous delta front and the intertidal estuaries.