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Active Tectonics: Studies in Geophysics
FIGURE 6.17 Crustal folds recorded by strandlines and illustrated by profiles.
A, Longitudinal profiles of Holocene strandlines on Wairarapa coast of North Island, New Zealand, reveal tight folds clearly expressed in underlying Tertiary rocks. Each strandline is probably of coseismic origin, which indicates that folding was incremental not continuous. Highest (6 ka) strandline dated by graphical techniques. Modified from Wellman (1971b).
B, Longitudinal profiles of Pleistocene strandlines along the Arauco coast of Chile reveal a broad warp not expressed in bedrock units but generally expressed in the generalized skyline profile of the local mountain range. Age of lowest strandline estimated to be 120 ka. Modified from Kaizuka et al. (1973).
C, Profiles of historical coseismic strandlines reveal broad crustal warps in the Gulf of Alaska (1964) and on the Chilean coast (1960). These profiles are not continuous but are defined by numerous points on highly irregular coastlines. See Figure 6.18A for isobase map of warp in Gulf of Alaska. Modified from Plafker (1972).
some highly active coastlines coseismic strandlines are warped into progressively tighter folds (Figure 6.17A) (Wellman, 1971a,b; Kaizuka et al., 1973; Gahni, 1978), which suggests that tectonic folding, like uplift and tilt, is incremental, not continuous.
On local to regional scales, vertical displacement data from deformed marine strandlines are conveniently summarized as structural contours and isobases that express folds planimetrically (Figure 6.18; also see Figure 6.21 below). Structural contours and standard isobases depict deformation of a single tectonic marker (Figure 6.18A; also see Figure 6.21A below), whereas integrated isobases depict deformation normalized from two or more tectonic markers of different ages (Figure 6.18B) (Gahni, 1978).
Isobases on the surface defined by the 1964 strandline in the Gulf of Alaska (Figure 6.18A) reveal broad, gentle warps produced by coseismic crustal deformation over an area of 200,000 km2 (Plafker, 1965). These isobases indicate that during the great 1964 earthquake an area of 60,000 km2 was uplifted an average of 1–2 m and was tilted northward; maximum uplift was 3–10 m above local, north-dipping secondary faults (Figure 6.17C) (Plafker and Rubin, 1978). The isobases also show that an area of 110,000 km2 north of the uplifted zone subsided a maximum of 2 m during the earthquake. The pattern of coseismic crustal warping associated with the earthquake is similar to the general pattern of long-term deformation documented by older Holocene strandlines. However, drowned vegetation in the uplifted area south of the epicenteral region documents subsidence during the 1.4 ka prior to the earthquake (Plafker and Rubin, 1978). This subsidence was probably pre-earthquake strain accumulation above the interplate megathrust on which the slip occurred.
The general pattern of coseismic uplift and subsidence that accompanied the 1964 earthquake in the Gulf of Alaska is similar to the pattern of crustal defor-