deep submarine trenches, particularly in the circum-Pacific region. These tectonically active coastlines are characterized by rugged erosional landforms such as steep sea cliffs and rocky headlands, islands and sea stacks. However, at low latitudes coral reefs commonly form narrow depositional platforms even along rugged coastlines (Figure 6.2B)(Chappell, 1974a). The mountainous west coast of South America is an active-tectonic coastline that geomorphically reflects regional crustal uplift and subsidence related to rapid plate convergence and resultant subduction along the offshore Peru-Chile Trench (Plafker, 1972). The hilly-to-mountainous coast of California in western North America is an active-tectonic coastline that geomorphically reflects slower regional uplift and local basin subsidence related to oblique plate convergence and resultant large-scale, right-lateral displacement across the San Andreas Fault system.

Long-term crustal instability along most active-tectonic coastlines is expressed stratigraphically by folded and faulted marine sediments that fill youthful structural basins and geomorphically by narrow uplifted and deformed Pleistocene strandline terraces that notch steep coastal slopes (Figures 6.1 and 6.2). Short-term instability along extremely active coastlines is expressed geomorphically by emergent Holocene strandline terraces (Figure 6.3) and by dramatic changes in the location and configuration of modern shorelines that result from rapid vertical crustal movements (Figure 6.4). Most of the Earth’s seismicity occurs along or near active-tectonic coastlines (Tarr, 1974), and many large earthquakes in these areas are recorded geomorphically by emergent Holocene strandline terraces (Figure 6.3A; also see Figures 6.256.28 below).

The subdued coastlines bordering the shallow epicontinental seas in North America (Hudson Bay) and Fennoscandia (Bay of Bothnia) are exceptions to the general rule that coastal regions undergoing rapid crustal deformation are characterized by marked topographic relief. However, the rapid crustal uplift recorded so dramatically by sequences of highly emergent Holocene strandlines in these recently deglaciated areas reflects transitory isostatic rebound (see Figure 6.10 below) not sustained tectonic deformation.

MARINE STRANDLINES

Marine strandlines are the geological and historical records of former sea levels. In the geologic record marine strandlines are the depositional and erosional remains of abandoned marine shorelines (Figures 6.1, 6.2 and 6.3), and in the historical record they are most commonly tide-gauge measurements (Figure 6.4) or high-water marks on man-made coastal structures.

A strandline, or a sequence of strandlines, is a relative sea-level record that potentially represents both real and apparent sea-level changes (Figure 6.5):

(6.1)

Real sea-level changes are absolute vertical movements of the ocean’s surface and may be local to worldwide in extent; if worldwide, they are called eustatic changes.

FIGURE 6.5 Relative, apparent, and real sea-level changes.

A, Late Pleistocene.

B, Holocene.

All sea-level records (strandlines or tide-gauge measurements) are relative, which means they potentially represent both apparent and real sea-level changes (relative=real +apparent). Apparent sea-level changes are the inverse of the vertical crustal (or ground) movements that produce them and, therefore, are the focus of all coastal tectonic studies. The apparent sea-level history is obtained by algebraically or graphically subtracting the real sea-level history from the relative record of marine strandlines. In effect, the real sea-level history is a composite tectonic datum. The real sea-level history is obtained by subtracting apparent sea level changes from a relative sea-level record. In this case, the uplifting coastline is a moving sea-level datum. E is the present elevation of a strandline; e is the original elevation of a strandline; D is the vertical displacement (D=E−e); A is the age of a strandline; R is the crustal displacement rate (R=D/A).



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