520 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Fig. 32.5. Submarine contour map of west end of Aleutian Ridge. Reproduced from Gates 

 and Gibson, 1956. 



The continental slope comprising the inner north side of the foredeep is 

 considered approximately as the area between the 100-fathom contour (50- 

 fathom contour frequently applicable) and the floor of the trench. It ranges 

 from 20 to 70 miles in width, is narrower near Cape St. Elias, and widest off 

 Umnak and Unalaska islands. South of Unimak Island, a pronounced widening, 

 herein termed the "Aleutian Bench," exists between the 2000- and 2500-fathom 

 contours and extends westward to Umnak Island. This bench is approximately 

 20 miles wide and 170 miles long. The bench lies several hundred fathoms 

 higher than the top of the outer seaward side of the trench. 



The average slope of the north face or continental slope is 3°-4° and 

 terminates in depths ranging from around 2,000 to 4,000 fathoms. Steeper 

 slopes, however, are found in limited areas or between successive soundings. 

 When the slope exceeds 30°, it usually occurs near the bottom of the trench 

 where the profiles show an abrupt slope or escarpment as, for instance, the 

 apparent escarpment off Cape St. Elias. 



The surveyed slopes on the north and south sides of the Umnak Island 

 locality differ materially with respect to relief and rate of descent. The north 

 side of the island is characterized by long valleys and ridges in the deeper 

 area. For instance, the maximum seaward distance of the 1000-fathom curve on 

 the north side of Umnak Island is 45 miles, whereas that on the south is barely 

 5 miles. 



The floor of the trench, 20 to 70 miles off the edge of the continental shelf, 

 undulates, but steadily descends in the 1000-mile stretch from Cape St. Elias 

 to Umnak Island. In many profiles, the converging side slopes of the trench 



meet in a narrow area defined by one or two soundings at, or close to, the base 

 of the continental slope. 



The gentle incline of the trench terminates at about 2,000 fathoms, off 

 Cape St. Elias. The trench, however, continues eastward across the continental 

 slope and then, apparently, is continuous with a depression extending across 

 the continental shelf toward Yakutat Bay. The delineation of the 100-fathom 

 curve on the shelf here is inconclusive, as it is controlled by only a few widely 

 spaced soundings. A bar with depths of 8 to 16 fathoms extends entirely across 

 the entrance to Yakutat Bay. Depths as great as 167 fathoms, however, are 

 found about 4% miles inside the bay. 



Detailed contouring of the west end of the Aleutian Ridge has led 

 Gates and Gibson ( 1956 ) to postulate that the submarine topography re- 

 flects the structure. The Aleutian Ridge with its islands is shown in Fig. 

 32.5, and the suggested structure in Fig. 32.6. The geology of the Aleutian 

 Islands will be discussed in Chapter 39, but suffice it to say here that 

 Attu, Agattu, the Semichi Islands and the southern part of Kiska lack 

 young stratovolcanoes and are composed of pre-middle Tertiary rocks 

 and subordinate amounts of upper Tertiary coarse clastic sediments and 

 subaerial lava flows. They owe their height to faulting and alpine char- 

 acter to vigorous erosion. The fault pattern of Attu and Agattu, par- 

 ticularly, is obvious and intricate. It has led to the interpretation of 

 submarine features as fault reflections. 



Four principal topographic provinces are recognized: (1) The Crest of the 

 Aleutian Ridge contains the Aleutian Islands, the Insular Shelf at depth ranging 

 from present shore lines to 70 fathoms, and the Ridge Shelf at a depth of 100 

 to 500 fathoms, all apparentiy the result of subaerial and marine erosion since 

 the middle Tertiary and of glaciation in the late Pleistocene. (2) The Insular 

 Slopes form the sides of the Aleutian Ridge. The North Insular Slope is a long, 

 steep, linear scarp that probably marks a major fracture in the earth's crust. 

 The South Insular Slope appears to be a broad, faulted and warped arch con- 

 taining numerous steep-sided linear sea valleys and canyons. Many of these 

 traverse the south slope at an angle to the maximum regional gradient, and 

 several line up with observed faults on the island. These linear topographic 

 features probably mark fault zones. (3) The Aleutian Bench is a prominent 

 step in the general slope from the islands to the Aleutian Trench, and its inside 

 edge may be the trace of a thrust fault. (4) The arcuate Aleutian Trench has a 

 steep north side, a flat floor at a depth of about 4000 fathoms, and a south side 

 containing an en echelon topographic pattern. The Trench perhaps marks a 

 major thrust zone dipping north beneath the Aleutian Ridge. 



A structural interpretation of the submarine topography suggests that the 



