Active Tectonics Impact on Society (1986) / Chapter Skim
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6. Coastal Tectonics
6. Coastal Tectonics
Pages 95-124
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From page 95... ...
Past sea levels derived from the geologic record comprise a composite datum for measuring long-term crustal movement. Consequently, coastal tectonics also includes mapping and dating marine strandlines and separating the vertical crustal movements from the past sea-level fluctuations they si ~d~ art__ -~ ,'-,~',~ , ~ ~ _ _= ~ FIGURE 6.1 Emergent marine strandlines form steplike terraces on the Palos Verdes Peninsula of southern California.
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From page 96... ...
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.
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From page 97... ...
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)
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From page 98... ...
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.
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From page 99... ...
,,~ l /L E V E L >~ ~ / / Kit AGE STRANDLINES i,\' it' HA _ rO ~ O EPISODI C ~UPLI FT -20 m FIGURE 6.7 Holocene sea-level changes and origin of emergent Holocene strandlines. In contrast to Pleistocene strandlines, emergent Holocene strandlines represent discrete uplift events or storm events, not sea-level fluctuations.
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From page 100... ...
Along many subsiding coastlines, strandlines formed during sea-level lowstands probably dominate the submergent geomorphic record but are difficult to distinguish from strandlines formed during highstands (Moore and Fornari, 1984~. The most detailed tectonic datum for deriving uplift from emergent Pleistocene strandlines on coastlines throughout the world is the paleosea-level curve obtained by subtracting well-documented constant tectonic uplift from the relative sea-level record of emergent coral-reef strandlines on the Huon Peninsula of Papua New Guinea (Figures 6.2B and 6.6)
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From page 101... ...
highstands appear in the emergent geologic record only where uplift rates are sufficiently high (>0.3-1.0 m/ka) to elevate them above present sea level (Figure 6.6~.
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From page 102... ...
. Along many rapidly uplifting coastlines, prehistoric Holocene strandlines are similar to strandlines produced by abrupt vertical crustal movements associated with major historical earthquakes (Sugimura and Naruse, 1954; Plafker, 1965; Wellman, 1969; Matsuda et al., 1978~.
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From page 103... ...
Consequently, marine shells deposited on the older, transgressive shoreline could be reworked into sediments deposited on the younger, regressive shoreline. On the other hand, the two-part configuration of the Holocene sea-level curve provides an independent means of approximating the ages of emergent Holocene strandlines and deriving minimum uplift rates.
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From page 104... ...
Many studies of long-term crustal movements in coastal areas focus on uplift mainly because the emergent strandline record is better exposed and easier to interpret than the submergent record. Most long-term crustal movements recorded by marine strandlines reflect sustained tectonic deformation along active plate boundaries, but the most rapid known rates of vertical crustal displacement reflect transitory volcanic tumescence and glacio-isostatic rebound.
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From page 105... ...
1538, about 40 yr after the uplift rate increased from 10 mm/yr to about 100 mm/yr. The general subsidence that followed this small eruption has now reversed.
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From page 106... ...
However, because these rapid displacements are usually produced by episodic coseismic uplift events that are sufficiently large (1-15 m) to be expressed in the geologic record as emergent Holocene strandlines, they are extremely important in studies of paleoseismicity (see Figures 6.256.28 below)
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From page 107... ...
Where data from marine strandlines and other tectonic markers are sufficiently abundant, regional compilations of vertical crustal movements provide valuable insights into both local and regional tectonic processes. Regional data are most conveniently and clearly expressed planimetrically by isobases (Figure 6.12)
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From page 108... ...
The numerical values of these two constants are unique for any pair of Pleistocene strandlines and remain fixed regardless of the differential uplift rates. Therefore, the graphically derived values for these constants can be used to correlate undated strandline pairs from widely separated areas, such as from one island to another (Figure 6.14)
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From page 109... ...
as a function of the elevation a second (Ed; the strandline pairs must be from localities with different but constant uplift rates, such as along a tilted coastline (Figure 6.13B) , or on widely separated islands.
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From page 110... ...
The strandline data indicate that the tilt rates of the two blocks have been constant but significantly different. If the fault separating the two parts of the island is vertical, the 120-ka strandline is offset about 75 m and the 6-ka strandline is offset about 4 m, which yield similar average slip rates of 0.6 m/ka and 0.7 m/ka, respectively.
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From page 111... ...
_10 m 4 _o C: ALASKA ~ km 100 11 it W SW 0 50 km 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)
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From page 112... ...
If this apparent decrease in crustal deformation is documented by successively older strandlines, it could be misinterpreted as a reduction in local or regional tectonic strain. Vertical Fault Movement Because marine platforms and strandlines are virtually horizontal structural markers, they record vertical fault movement more clearly than lateral movement.
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From page 113... ...
O I ~ ~ - - - - RA 8 1 - - - ~ NW 0 20 SE 1 1 B: VENTURA km 0 2 ~,0 6 k a _ _ NW SE FIGURE 6.20 Vertical component of fault displacement recorded by offsets in longitudinal profiles of marine strandlines. A, Progressively greater vertical offset of successively older marine strandlines documents continual fault activity over at least the past 700 ka on the Taranaki coast of North Island, New Zealand.
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From page 114... ...
If the size of an average coseismic slip event is assumed or is known from a historical seismic event, it can be divided into the slip rate to yield the average earthquake recurrence interval. However, where discrete slip events are recorded geologically, earthquake recurrence can be determined more directly.
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From page 115... ...
COSEISMIC UPLIFT AND EARTHQUAKE RECURRENCE In several seismically active coastal areas historical coseismic uplift has produced conspicuous emergent marine strandlines 1-15 m above sea level. The best doc umented examples of historical coseismic strandlines are found in Japan (Sugimura and Naruse, 1954; Nakamura et al., 1965; Ota et al., 1976, Matsuda et al., 1978; Shimazaki and Nakata, 1980)
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From page 116... ...
. A, If average uplift rate is constant, the incremental coseismic uplift events may follow either a time-predictable or a displacement-predictable pattern.
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From page 117... ...
A, Average uplift appears to be constant, and coseismic uplift events appear to follow a time-predictable pattern.
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From page 118... ...
The lowest strandline at 3.5 m above sea level was produced by uplift associated with the great 1964 earthquake in southern Alaska. Radiocarbon dates on peat and wood from the wave-cut platforms of the five highest strandlines indicate that the time interval between uplift events increased gradually from about 0.5 ka to 1.4 ka and that the uplift rate decreased from 14 m/ka to 5.6 m/ka.
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From page 119... ...
At several localities, especially along the southeast coast of North Island, large earthquakes are recorded by emergent Holocene strandlines. At least five coseismic uplift events are recorded by six
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From page 120... ...
FUTURE RESEARCH This brief review of coastal tectonics illustrates the breadth and scope of neotectonic deformation and seismic history that can be derived from the study of marine strandlines, which so conspicuously record the dynamic interaction between the fluctuating sea level and mobile tectonic plates along many of the world's active coastlines. Without sea-level changes, discrete strandlines would not be produced and the long-term record of Pleistocene crustal movements in coastal areas would be extremely difficult if not impossible to extract from a relative sea-level record.
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From page 121... ...
Late Quaternary sea levels and crustal movements, coastal British Columbia, Can.
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From page 122... ...
. Emergent Holocene marine terraces at Ventura and Cape Mendocino, California-indicators of high tectonic uplift rates, Geol.
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From page 123... ...
. Holocene marine terraces and seismic crustal movements in the southern part of Boso peninsula, Kanto, Japan, Geogr.
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From page 124... ...
. Holocene marine terraces and crustal movements of Sado Island, central Japan, Geogr.
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