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Active Tectonics: Studies in Geophysics
FIGURE 6.3 Cross-sectional profiles of Holocene strandlines.
A, Erosional terraces on the Boso Peninsula of Honshu, Japan. These strandlines, unlike their Pleistocene counterparts, represent periodic coseismic uplift events, not sea-level fluctuations (see Figures 6.7, 6.25, 6.26, 6.27, and 6.28). The lowest platform (IV) records uplift that accompanied two major earthquakes in 1703 and 1923, and the three highest strandlines were dated by radiocarbon analysis of fossil shells. The average uplift rate is 3.9 m/ka (see Figure 6.25). Modified from Matsuda et al. (1978).
B, Depositional strandlines (beach ridges) at Te Araroa on North Island, New Zealand. Each ridge most likely represents a storm event not a coseismic uplift event. If uplift events are recorded in this sequence of strandlines, they are indistinguishable from storm events. The average uplift rate derived from the highest beach ridge (6 ka) is 1 m/ka. Modified from Garrick (1979).
margins consist of broad coastal plains bordered offshore by wide continental shelves and gentle continental slopes. These relatively stable coastlines are characterized by depositional landforms such as broad sandy beaches and offshore barrier bars and at low latitudes by broad coral reefs. The southeast coast of North America and the northeast coast of Australia are typical passive-margin coastlines with low-to-moderate topographic relief and subdued depositional landforms. Much of the former is bordered offshore by barrier bars (Oaks and Du Bar, 1974), and most of the latter by extensive coral reefs (Hopley, 1983).
Long-term tectonic stability along most passive-margin coastlines is expressed stratigraphically by undeformed continental and marine sediments that underlie flat coastal plains and continental shelves and geomorphically by broad accretionary strandline terraces that consist of subdued beach ridges separated by abandoned tidal flats (Oaks and Du Bar, 1974). However, rapid sediment accumulation, such as at the mouth of a large river, may isostatically depress an otherwise stable passive-margin coastline (Figure 6.4C) (Fisk and McFarlan, 1955; Hicks and Crosby, 1974). Also, occasional large earthquakes, such as the 1886 Charleston, South Carolina, seismic event on the Atlantic coast of North America (Hays and Gori, 1983), indicate that passive continental margins are not completely aseismic, even though there is little stratigraphic or geomorphic evidence in these areas of recent, near-surface crustal deformation.
In contrast to the subdued coastlines along passive plate boundaries, most coastlines along or near active continental margins consist of coastal hills or mountains bordered offshore by narrow continental shelves and
FIGURE 6.4 Tide-gauge records.
A, San Francisco, California—the secular drift of this record (~2 mm/yr) is similar to that found in other parts of the world and, therefore, probably represents eustatic rise in sea level.
B, Juneau, Alaska—the extreme apparent drop in sea-level represents crustal uplift due to either tectonic uplift or residual glacio-eustatic rebound.
C, Mississippi delta—the apparent rise in sea-level represents subsidence due to sediment compaction and isostatic adjustments of the crust to the sediment load of the Mississippi delta. The eustatic rise (2 mm/yr) was subtracted from the relative rates to obtain the uplift and subsidence rates. Modified from Hicks and Crosby (1974).