The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
Active Tectonics: Studies in Geophysics
multaneously record. Because marine strandlines are the physical records of past sea levels, the study of sea-level history is an integral part of coastal tectonics.
Time-transgressive sequences of displaced and deformed Pleistocene [2 m.y. to 10,000 yr (10 ka) ago] marine strandlines document patterns of and, where dated, yield rates of continual, long-term crustal deformation (Figures 6.1 and 6.2). Sequences of emergent Holocene (past 10 ka) strandlines (Figure 6.3), which occur only along the most rapidly uplifting coastlines, commonly record abrupt coseismic uplift events of 1–15 m (see Figures 6.25–6.28 below) that cumulatively comprise the long-term deformation recorded by Pleistocene strandlines. Consequently, sequences of Holocene strandlines often record past earthquakes and, where dated, yield earthquake periodicity and provide a means of forecasting future seismic events. In many areas, apparent sea-level changes documented by tide-gauge records (Figure 6.4) or subtle shifts in the location of the modern shoreline reflect ongoing vertical crustal movement. Frequently, this movement is opposite in sense to long-term trends and, therefore, may represent postearthquake crustal relaxation or pre-earthquake strain accumulation.
Coastal tectonics includes the study of both onshore (emergent) and offshore (submergent) marine strandlines and structural features. However, offshore coastal tectonics is a highly specialized field and is beyond the scope of this brief review, which focuses mainly on the formation and deformation of emergent marine strandlines and stresses their importance in determining the style and measuring the rates of recent crustal deformation, especially in highly active coastal regions. Most examples of strandline displacement and deformation cited in this review reflect sustained tectonic processes, but a few, included mainly for comparative purposes, reflect transitory volcanic and glacio-isostatic crustal deformation.
COASTAL MORPHOLOGY AND TECTONIC SETTING
On global and regional scales coastal morphology correlates closely with tectonic setting (Inman and Nordstrom, 1971). The greatest geomorphic contrast is between the subdued coastlines along passive continental margins and the rugged coastlines along convergent plate boundaries. Along most coastlines modern (active) coastal landforms are similar to their Pleistocene counterparts, which suggests that current tectonic and coastal processes have been fairly uniform over considerable periods of time.
Most exposed coastlines along passive continental
FIGURE 6.2 Cross-sectional profiles of emergent Pleistocene strandline terraces.
A, Erosional strandlines near Santa Cruz, California. Solid line is existing topographic profile. Dashed line represents original profile of each strandline terrace, which consists of a relict wave-cut platform backed by a relict sea cliff. The intersection of the platform and the sea cliff is the shoreline angle, which closely approximates the paleoshoreline. Seaward thinning wedges of alluvial sediment derived from the degrading sea cliffs overlie the wave-cut platforms. A 120-ka strandline was removed from this section when the next lower (104 ka) platform was cut; the 120-ka strandline occurs a few kilometers north of this site and is projected onto this cross section. The three lowest strandlines were dated by paleontological, amino acid, and geomorphic techniques. The average uplift rate (0.35 m/ka) was derived from the vertical displacement (28 m) of the 82-ka strandline. The three highest strandlines were dated by extrapolation of this uplift rate and by numerical analysis of the progressively gentler slopes of successively higher (older) relict sea cliffs. Modified from Hanks et al. (1984).
B, Depositional strandline terraces on the Huon Peninsula, Papua New Guinea. Each platform is a relict coral reef. U-series dates on fossil corals from these strandlines yield a history of glacio-eustatic fluctuations that serves as a tectonic datum for measuring vertical tectonic movements on other coastlines throughout the world (see Figure 6.6). The maximum average uplift rate of 4 m/ka on the Huon Peninsula was derived from the maximum vertical displacement (500 m) of the 120-ka strandline. Note that the three lowest strandlines on the Santa Cruz coastline correlate with the three highest strandlines in this sequence. Modified from Chappell (1974a).