Geology Reference
In-Depth Information
Sea Level - Tectonic History
Coral Terrace Sequence
300
300
Vertical Motions
Required to
Account for
Observed Reefs
Much older reefs w/ totally
recrystallized coral
250
250
200
200
150
150
100
100
uplift
rate
Well-Preserved
Holocene Reef
Marine
Sediments
Underlying
Coral
Limestone
50
50
present
sea level
0
0
-50
-50
drill hole:
L
GM (21-ka) coral
at -55 m;
older coral limestone
at -55 to -70 m
-100
-100
Stage
5
350
300
250
200
Age (ka)
150
100
50
0
Fig. 9.33
Coral terrace sequence on the New Georgia group forearc and inferred tectonic history.
(Right) Uplifted Holocene reefs on Rendoza island (Fig. 9.32) are spatially bracketed by much older, recrystallized reefs.
Drill hole shows Last Glacial Maximum (LGM) corals at −55 m, indicating
∼
65 m of uplift since 20 ka. No Marine Isotope
Stage 5 terraces are recognized above sea level. (Left) Correlation (black lines) of sea-level history with observed and
dated terraces defines abrupt changes in uplift and subsidence over the past 250 kyr. Modified after Taylor
et al.
(2005).
for recent uplift at rates as high as 6 mm/yr
(Taylor
et al.
, 2005). Similarly, drill-hole data
reveal that Marine Isotope Stage 2 terrace deposits
that were formed at a depth of
∼
120 m are
currently only at −55 m (Fig. 9.33) and define an
average uplift rate of
∼
3 mm/yr since 20 ka. But
these rates have not been sustained for long: no
Marine Isotope Stage 5 terraces are currently
exposed above sea level, and terraces immediately
above the Holocene terraces are completely
recrystallized, indicating their overall antiquity
(Fig. 9.33). These observations provide persuasive
evidence for multiple and abrupt changes in
forearc uplift and subsidence over the past
20-300 kyr. Taylor
et al.
(2005) argue that these
changes could not be induced by a displacement
(underthrusting) mechanism (Fig. 9.3). For
example, over the past 40 kyr, as rapid subsidence
was replaced by rapid uplift, an impinging
seamount would have advanced only a few
kilometers, not enough to cause such an abrupt
transition. Instead, Taylor
et al.
(2005) suggest
that subsidence-uplift history is consistent with
impinging seamounts that jam up the subduction
process, cause temporary locking of the upper
parts of the subduction zone, and drive elastic-
like compression of the nearby forearc. Seismic
rupturing of the seamount is speculated to cause
collapse of the forearc and rapid subsidence, as is
seen in the New Georgia islands. This striking
example of abruptly changing patterns of uplift
highlights the potent insights that can be gleaned
at intermediate time scales when well-dated
geomorphic markers with known initial positions
can be tracked as they deform.
Summary
Tectonic deformation and interactions with
surface processes over intervals of hundreds of
thousands of years produce the landscapes that
we see today in many tectonically active areas.
At vertical uplift rates of about 1 mm/yr and
horizontal displacement rates of about 1 cm/yr,
hundreds of meters of uplift and several kilometers
of lateral displacement occur over these “interme-
diate” time spans. In combination with climatic
