Geology Reference
In-Depth Information
Many paleoseismological studies rely on
trenches that are dug across faults to reveal the
slip in past earthquakes. Measurements of
displaced strata serve to define the magnitude of
displacement in an earthquake, whereas dating
of those strata can determine when the rupture
occurred. But do such measurements capture
the full slip history? Might they overestimate the
slip? Several recent discoveries complicate the
interpretation of trenches. As faults approach
the Earth's surface, slip commonly decreases, so
trench exposures at the surface may tend to
underestimate the total coseismic slip. This
underestimate is especially likely for “blind”
thrust faults that do not rupture the surface.
Even where faults do break the surface, is it
possible that significant strain occurs as diffuse
deformation between faults? Although difficult
to quantify, such broadly distributed strain may
represent a significant fraction of the total
deformation. In such conditions, displacement
in trenches would be underestimates of the
integrated deformation across an area. The
advent of extensive geodetic networks (mostly
relying on GPS) has provided new insights on
slip during and between earthquakes. Recent
recognition of “slow earthquakes” that occur
over hours to days (Heki
et al.
, 1997) indicates
that slip during an earthquake may be equaled
by slow slip of nearly equal magnitude. If such
slip were propagated to the surface, it would be
impossible to recognize as a slow slip event in
an exposure within a paleoseismic trench.
The rates of convergence between tectonic
plates, as well as measured rates of local
deformation, indicate that, in many active
mountain ranges, rocks are moving upward
with respect to sea level at rates of several
millimeters per year. Recall that a rate of
1 mm/yr is equivalent to 1 km in a million years.
Thus, in the absence of erosion, vertical rock
uplift rates of several mm/yr would build very
high mountains in only a few million years.
Clearly, mountains do not grow indefinitely:
only 14 peaks poke more than 8 km above sea
level. But what controls their ultimate height? Is
there a limit to the energy available to lift the
mass of rock and increase its potential energy?
Does rock strength set the height limits, or do
changing rates of erosion determine the
topography of ranges? Such questions lie at the
core of another current controversy in tectonic
geomorphology: Can the concept of “dynamic
equilibrium” be applied in active orogens at
the mountain range scale? Dynamic equilibrium
implies that, on average over time, the land-
scape maintains a steady-state form, whereby
the height of the summits, the steepness of the
valley walls, and the topographic relief fluctuate
around long-term mean values. If the mean
height of the mountains stays the same through
time, this persistence implies that rates of rock
uplift (vertical movement of rocks with respect
to sea level or the geoid) are balanced by rates
of erosion. How is this accomplished? Are
surface processes capable of eroding at several
mm/yr? Which processes are responsible (river
erosion, landsliding, glacial erosion, conversion
of rock to soil) and do these processes operate
in different ways in different mountain belts?
Or, is the traditional idea correct that rapid rates
of rock uplift are commonly compensated, not
by geomorphic agents at all, but by events of
tectonic denudation (extensional faulting) that
efficiently lower the regional height of the
landscape?
In order to answer such questions, we have
to be able to document rates of modern and
past erosion, to quantify rates of rock uplift
and changes in the mean elevation and topo-
graphic relief of mountain ranges (Fig. 1.8), and
to document the role of extension within
orogens. Also, because climate undergoes large
glacial-interglacial fluctuations at 100 000-year
intervals, rates of erosion that are responsive
to climate (most erosional processes are) will
also vary strongly through time. Consequently,
when thinking about the problem of dynamic
equilibrium and steady-state topography, it is
most useful to consider time spans that exceed a
full glacial-interglacial cycle, so that average
rates can be determined.
Such constraints present some great challenges
to researchers in tectonic geomorphology. It is
inadequate to document only modern rates
(which are difficult enough to measure
accurately!). Rates from intervals throughout a
climate cycle or rates that integrate an entire
