38 NORTH AMERICA/Northern Cordillera at 6050 m are the highest peaks on the continent The Pacific Mountains System contains a longitudinal but discontinuous coastal depression, oceanward of which are many islands and the Olympic Mountains in the south and Saint Elias Mountains in the north The continental shelf and slope along the Pacific Ocean are about 200 km wide; the base of the continental slope is about 2000 m deep south of latitude 60 N, but westwards across the northern Gulf of Alaska its depth increases to 7000 m in the Aleutian Trench off the western Aleutian Islands The Bering and Chukchi Seas are mostly less than 200 m deep, except for a 3000 m deep basin north of the western Aleutians The Cordilleran ice-sheet covered most of the region several times between Ma and 12 Ka, was up to 2.5 km thick in places, and shaped local landforms Parts of central Alaska, the eastward extension of this region into Yukon, and the Arctic coastal plain remained largely ice-free Today, ice cover is restricted to mostly retreating glacier complexes mainly at higher elevations, but which descend to sea-level in parts of southern and south-eastern Alaska Neotectonics The northern Cordillera occupies a ‘soft’ plate margin up to 1200 km wide landward of an offshore convergent–transform–convergent plate boundary (Figure 1) Earthquakes are concentrated near most plate boundaries, but are scattered across the entire region South of latitude 51 N, the small, young ( Ma) Juan de Fuca and Explorer oceanic plates converge with the North American Plate at rates of 2–4.6 cm year along the currently aseismic, locked Cascadia subduction zone, 200–300 km landward of which is the active Cascade magmatic arc Between latitudes 51 and 60 N, the plate boundary is the dextral transform Queen Charlotte–Fairweather Fault, west of which the enormous Pacific Plate moves northwards to converge with the North American Plate at rates ranging from 5.7 cm year near the transform to cm year south of the western Aleutian Islands The Pacific Plate descends beneath Alaska along the Aleutian subduction zone, and above it the Aleutian–Wrangell magmatic arc extends eastwards some 3800 km from the Komandorsky Islands near Kamchatka, along the Aleutian Island chain, and into the mountains of southern Alaska and south-westernmost Yukon The North American Plate continues for some 2000 km west of the Bering Strait to terminate against the Eurasian Plate near longitude 140 E There is widespread evidence for neotectonic activity in the northern Cordillera in addition to earthquakes and volcanoes Seismically imaged folds and thrust faults in Late Tertiary and Quaternary strata are in the accretionary wedges above the Cascadia and Aleutian subduction zones, and faults and folds occur in the Arctic shelf-slope region off northern Alaska and Canada On-land surface evidence for latest Tertiary to Holocene deformation is sparse, possibly because erosion by ice eliminated the surface expression of many small-scale tectonic features Segments of the Denali Fault in southern Alaska and south-western Yukon are active dextral strike-slip faults, as are other fault segments in central Alaska Glaciated valleys crossing the Fairweather Fault in south-eastern Alaska are dextrally offset, and neotectonic faults occur on Vancouver Island in south-western British Columbia In western Washington, east–west-trending Holocene folds and reverse faults in the coastal depression are related to north–south compression, as are structures involving Late Miocene lava flows in the Columbia Plateau, east of the Cascade arc On a larger scale, regional differential uplift of up to several kilometres in the last 5–10 million years in the Alaska Range, Saint Elias Mountains, Coast Mountains, and Olympic Mountains is documented by elevated Late Tertiary erosion surfaces, changes from wet to dry floras in regions now in rain-shadows inland of the coastal mountains, palaeodrainage patterns, and fission track studies Differential regional uplift is not well documented elsewhere, but major reversals of drainage in upper reaches of the Fraser and Columbia Rivers in central and south-eastern British Columbia, respectively, possibly reflect Neogene differential uplift of parts of the Rocky Mountains System, in addition to documented blockage by temporary ice dams and landslides during waning stages of Cordilleran glaciation The late differential uplift means that physiographical system boundaries only approximately coincide with major bedrock boundaries, many of which formed between the Early Jurassic and the Neogene Crustal Thickness Seismic refraction and reflection profiles across the northern Cordillera show the crust to be nearly 50 km thick under the south-eastern Canadian Cordillera and parts of northern Alaska (Figure 2) Below the Intermontane Plateau System in Alaska and northern Canada, it is about 35 km thick, and under the southern Canadian Cordillera only 30 km thick Above subducting lithosphere in southern Alaska and south-western Canada, the crust is apparently underplated by material detached from the lower plate and is over 50 km thick, whereas landward of the Queen Charlotte– Fairweather transform fault it is only about 25 km thick