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Phil. Trans. R. Soc. A (2012) 370, 2173–2192
doi:10.1098/rsta.2011.0605
River history and tectonics
BY C. VITA-FINZI*
Department of Mineralogy, Natural History Museum, London SW7 5BD, UK
The analysis of crustal deformation by tectonic processes has gained much from the clues
offered by drainage geometry and river behaviour, while the interpretation of channel
patterns and sequences benefits from information on Earth movements before or during
their development. The interplay between the two strands operates at many scales:
themes which have already benefited from it include the possible role of mantle plumes
in the breakup of Gondwana, the Cenozoic development of drainage systems in Africa
and Australia, Himalayan uplift in response to erosion, alternating episodes of uplift
and subsidence in the Mississippi delta, buckling of the Indian lithospheric plate, and
changes in stream pattern and sinuosity along individual alluvial channels subject to
localized deformation. Developments in remote sensing, isotopic dating and numerical
modelling are starting to yield quantitative analyses of such effects, to the benefit of
geodymamics as well as fluvial hydrology.
Keywords: fluvial processes; tectonics; isostasy; geochronology
1. Introduction
Active tectonism affects the characteristics of river channels, and fluvial activity
influences the geometry and tempo of fold and fault development and even the
gross behaviour of continental land masses. Both concepts have long been grasped
but they are gaining in scope and specificity from recent conceptual and technical
developments in Earth science and a variety of fields.
Some of the most thought-provoking large-scale studies of river history hinge
on simplistic views of channel morphology just as early studies of continental drift
drew inspiration from rudimentary cartography. But fluvial analysis, in the field
as well as in the laboratory and underpinned by refined dating and computing
techniques, is starting to help resolve issues—such as the consequences of the
collision between India and Asia—which have hitherto focused on kinematic
narratives. The outcome sheds light on processes in the Earth’s mantle as well as
in the crust.
The examples discussed here can be grouped into three. The first two analyse
the adaptation of rivers to existing geological structures and their response to
tectonic change: broadly equivalent to the passive and active tectonic controls of
Summerfield [1]. The categories evidently overlap, as when erosion of a growing
anticline comes to expose alternating resistant and weak strata to which the
*cvitafi[email protected]
One contribution of 10 to a Theme Issue ‘River history’.
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stream has to adapt, and they are complicated by climatic and other factors. The
third group of studies focus on how far fluvial action drives or at least modifies
tectonic processes nearby and far afield. Again the distinction is mainly one of
convenience, as river behaviour may be enhanced or damped by the deformation
it promotes.
2. The plan view
In reviewing the field of fluvial tectonics we begin by considering channels in
plan view. To be sure, there is no such thing as a two-dimensional watercourse,
but many of the pioneering studies in the 1950s and 1960s that put fluvial
geomorphology on a quantitative basis concentrated on mappable features
including stream order, sinuosity and channel width. What is more, river patterns
at regional and continental scale played a part in the elaboration of plate tectonics
and mantle plume models.
(a) Domes and drainage
Plate tectonics, like its precursor continental drift, initially involved tracing the
breakup of land masses by fitting together the resulting fragments. The centuriesold recognition of the match between the west African and South American
Atlantic coasts was eventually confirmed and improved by a computer-based fit
of the 500 fathom margins by Bullard et al. [2]. Here, and at other reconstituted
margins, river channels played a useful role in establishing or explaining the extent
of misfit. The apparent 120 km overlap between the two continents near the Niger
delta, for example, could be explained by delta growth following final separation
in Albian (Cretaceous) times [3].
The cause of continental breakup remained a secondary consideration until
its reality could finally be taken for granted, contrary to Harold Jeffreys’ [4]
standpoint that drift was untenable because it was mechanically unfeasible. One
candidate which remains in contention is the action of mantle plumes, upwellings
which nucleate at the boundary between the Earth’s core and its mantle and
which rise through the mantle and the crust to drive volcanic activity and other
symptoms of anomalous heat. Plumes are suspected of promoting the disruption
of the supercontinent of Gondwana during the last 200 Myr (figure 1). The
associated updoming appeared to account for the present drainage pattern of a
number of continental basalt provinces, although dome-flank patterns are likely
to be complicated by the effects of secondary rifting (figure 1c). In southeast
Africa the Orange River system dates in its essentials from the Jurassic, say
190 Ma when activity in the Karoo flood basalt province was at its peak [5]; the
ensuing deformation would then explain why the east-flowing rivers apart from
the Limpopo are much shorter than those flowing west [7]. On the other hand, the
plume-head model for the Deccan of peninsular India conflicts with stratigraphic
evidence, which shows that easterly drainage of the peninsula predates the
uplift [8].
The survival of radial drainage systems for 60 Myr or more seems barely
plausible even if the dome continues to rise buoyantly in response to surface
erosion or to the addition of magma (‘underplating’) to its base; and the pattern
would also be equally consistent with late deformation of ancient rock formations,
as in the Lake District of northwest England where Palaeozoic rocks were raised
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(a)
(b)
AF
a
b
AN
SA
(c)
a
b
1000 km
Figure 1. (a) Location of mantle plumes (circled) thought to have contributed to the breakup of
Gondwanaland approximately 200 Ma. Based on Cox [5]. (b) Pattern of separation between South
America (SA), Africa (AF) and Antarctica (AN) (a, proto Atlantic; b, proto Indian Ocean). After
www.wits.ac.za/. (c) Domal drainage disrupted by rifting. Adapted from Cox [5] and Moore &
Blenkinsop [6]. (Online version in colour.)
into a dome in Tertiary times. But even if South African drainage systems
since Gondwana times have been disrupted by structural effects, the exposure
of buried topographies and river capture, it may be possible to recognize the
‘first-order imprint’ imposed on the drainage by successive plumes. The Paraná
plume pattern, which was superimposed on the earlier Karoo pattern, would
thus be responsible for a dominant eastward drainage orientation since the early
Cretaceous [6].
The search for corroboration has been taken into the subsurface. Regional
rather than radial drainage was identified in space-borne imaging radar (SIR-C)
data in the Selima sand sheet of southwest Egypt [9]. The imagery revealed
large flood features, which to judge from fault alignments were to some extent
structurally controlled and provisionally dated to the Tertiary and on which were
superimposed a variety of drainage channels thought to date from the Quaternary.
A similar story has emerged in southeast Australia. Two major rivers draining
to the Murray Basin, the Lachlan and Macquarie, may represent superimposition
from sediments dating from the Mesozoic whose direction was determined by
uplift and tilting about 95 Ma [10].
Rivers also played an important part in tracing the regional deformation
resulting from convergence between India and Asia: ‘long linear valleys and
adjacent ridges characteristic of active strike-slip faulting’ were boldly displayed,
particularly in China and Mongolia, in the images used in the classic study by
Tapponnier & Molnar [11]. Total shortening was estimated at 1500 km at least and
wholly in continental lithosphere. An estimated 300–700 km of this displacement
was taken up by crustal shortening and underthrusting; the remainder was
relegated to lateral extrusion, a mechanism which had been previously proposed
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(a)
(b)
KL
RR
Figure 2. (a) Major drainage systems implicated in the indentation of Asia by India. RR, Red River
fault; KL, Kunlun fault. (b) Streams dislocated 15–72 m by left-lateral slip on portion of Kunlun
fault (ASTER image dated 20 July 2000, courtesy of NASA). The dark area shows vegetation
where groundwater has been dammed by the fault. (Online version in colour.)
for Turkey and Iran and modelled by the deformation of a plastic body subject
to indentation by a flat rigid die [12]. Comparison with imagery of the San
Andreas fault in California encouraged the estimate of 500–1000 km of eastward
displacement of southeast China on the major faults that originate in central
China (figure 2).
The work has proved to be inspirational, in the Mediterranean and in The
Philippines no less than in Tibet, but also controversial. A telling illustration
is provided by the Ailao Shan or Red River, which occupies a shear zone running from southeast Tibet to the south China Sea and which in the extrusion
model took up 500–1000 km of movement by Indo-China in response to Indian
penetration. Yet slip along the Red River fault is right-lateral, with offsets of
25–54 km at most [13]. Moreover, the faulting should have cut through the entire
lithosphere and not just the crust; the heat produced by shearing should have
resulted in partial melting and metamorphism of crustal rocks; and the shearing
itself should have affected granites along the fault. None of these criteria applies,
and any extrusion must have happened at least 29 Myr after India collided
with Asia [14].
Again, river terraces and other geomorphological indicators along the Kunlun
fault (figure 2) at millennial scale (rather than short-term seismological data)
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(a)
(b)
(c)
(d )
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Figure 3. Effect of experimental deformation on channel form [21]. The axis of uplift or subsidence
is marked by filled triangle; stream flow left to right. (a,b) Subsidence in a meandering channel
after 4 and 48 h, respectively, showing increased sinuosity upstream; (c,d) uplift on a meandering
channel after 16 and 40 h, respectively, showing increased sinuosity downstream. Photographs taken
at Engineering Research Center, Colorado State University, Fort Collins (courtesy of Prof. Ouchi).
(Online version in colour.)
show that, along the easternmost 150 km of the fault, slip rates fall from over
10 mm yr−1 to less than 2 mm yr−1 within thickened crust, suggesting that slip is
absorbed within the plateau rather than manifested as eastward extrusion [15]. In
southeast Tibet river patterns which appear to indicate extensive tectonic shear
have been otherwise explained by surface uplift accompanied or followed by river
capture and flow reversal in streams which once drained to the China Sea by way
of a single system [16].
(b) Channel patterns
Despite the inclusion of field sites in California and other tectonically lively
areas, ‘tectonism’ is sometimes seen simply as a more plausible source of
channel steepening than aggradation or degradation [17]. But the search for
laws governing channel equilibrium yielded relationships between geometry and
discharge which would allow disequilibrium (including that due to tectonics)
to be identified [18]. These inferences are increasingly complemented by
instrumentation for more direct assessment of the tectonic factor. In the Long
Valley caldera, for example, streams, springs, water wells and other components of
the hydrological system are monitored continuously so that the effect of extension
in the resurgent dome within the caldera can be evaluated.
Experiments using flumes [19,20] suggest that, in meandering channels
within alluvium, aggradation tends to follow uplift both above and below the
axis of deformation while near the axis itself degradation is accompanied by
increased sinuosity. Subsidence sees degradation upstream and downstream, with
aggradation near the axis [21] (figure 3). The relationship will be complicated
by associated changes in slope, width and other variables, and obscured once
its effects propagate upstream or downstream though perhaps not if the
deformation—as where a stream crosses a growing anticline—is persistent. And it
is increasingly clear that rooted plants (alfalfa sprouts in one flume study) favour
the development in alluvium of single-thread meandering channels.
More generally, such relationships are constantly adjusting in a state of quasiequilibrium to variations in discharge and are governed by a tendency towards
minimum work and the uniform distribution of energy utilization [22]. It therefore
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New Madrid
7.4
Reelfoot lake
100
16 000 yr BP
m
12 000 yr BP
10 000 yr BP
80
0
20
40
60
80
km
100
10 km
Figure 4. Active and fossil meanders on the Mississippi near New Madrid (Missouri) and location
of Reelfoot fault (toothed line), Reelfoot lake and one of the three main events of 1811–1812 (7.4 on
7 February 1812). Google Earth image. Inset lower left: three successive flood plains near latitude
36◦ implying that the zone of unloading shifted eastwards. Adapted from Calais et al. [23]. (Online
version in colour.)
seems mistaken to equate any one channel characteristic, such as the presence
of braiding, or the average wavelength of a train of meanders, with a single
controlling factor, such as discharge, sediment calibre or slope.
Yet that kind of oversimplification has on occasion proved fruitful. Three
intraplate earthquakes with magnitudes of 7.5 or above struck at New Madrid
(Missouri) in 1911–1912. At least one of them was produced by the Reelfoot
thrust fault (figure 4). Trenching across the fault scarp revealed three slip events
dated to AD 900, 1450 and 1812. An earlier event at about AD 300 left traces
in the region but not at the fault, and the 900 and later events account for only
8–9 m of the cumulative 15 m of displacement represented by deposits at the
scarp [24].
Intraplate earthquakes are among the most destructive and least understood,
in part because they recur at intervals that may measure centuries or millennia,
and it is therefore essential to extend the scanty instrumental and historical record
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as far back as possible in order to identify any pattern in their behaviour. Two
additional earthquakes were identified in the local valley deposits by mapping
abandoned meander loops over a 100 km reach and identifying meanderingstraightening events in pervasively meandering reaches upstream of the fault.
Dating by radiocarbon (14 C) and optically stimulated luminescence indicated ages
of approximately 2200–1600 BC and AD 900 for two such episodes [24]. Channel
straightening can result from changes other than a reduction in gradient, such as
an increase in discharge or bed load or perhaps a spontaneous meander cutoff,
but the AD 900 straightening episode is reasonably ascribed to a change in slope
caused by the faulting event of the same date, with an estimated slip of 1 m,
especially as straightening was confined to a 30 km reach immediately upstream
of the fault. This was taken as endorsement of a tectonic interpretation for the
2200–1600 BC straightening episode, which saw several centuries of low river
sinuosity with a strong similarity to the ‘New Madrid’ series of faulting events
that was launched by the AD 900 earthquakes.
If correct, this comparison suggests that the Reelfoot fault displays earthquake
clusters separated in time by prolonged quiescence. More generally, it shows that
fluvial tectonics is an effective device for investigating deforming structures which
would escape detection even by prolonged and expensive monitoring by geodetic
and levelling survey because their development may span centuries and millennia
and elude analysis in sections however neatly excavated and richly endowed with
numerical ages.
3. Sediments and gradients
Entire drainage systems may respond to structural controls such as tilting,
doming, faulting and jointing by developing appropriate dendritic, parallel,
radial, trellis, rectangular, annular and centripetal drainage geometries. The
association generally takes time to develop by successive approximations that
embody some kind of natural selection and it has been argued that river patterns,
especially if carved in rock (rather than fickle alluvium), ought therefore to be
long-lasting [25].
In his discussion of channel slope Leopold [22] emphasized that the
familiar moderately concave longitudinal profile (like other aspects of channel
morphology) represented a compromise between uniformity of energy loss and
minimum work. Yet longitudinal profiles and sharp breaks in their gradients (or
knickpoints) remain enduringly enticing to students of tectonics and not solely
because the requisite small-scale profiles can be constructed from topographic
maps or digital elevation models (DEMs). The major attractions are that the
method can be applied to bedrock as well as alluvial rivers, can integrate records
spanning millions of years, and will record major tectonic events.
Presumably with such aims in mind the profiles of the major trans-Himalayan
rivers were investigated in the context of current seismic activity [26]. All but one
(the Subansiri) were found to display low gradients within the high plateau north
of the High Himalayas, within the Sub-Himalayas and the Lesser Himalayas,
and in the alluvial plains to the south, whereas within the High Himalayas their
gradients were two (or more) times steeper than the theoretical profile of a graded
river. The poor correlation between lithology and steep river gradients suggests
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that tectonic movement rather than differential erosion plays the main role in
shaping the profiles; uplift is balanced by erosion, and topography is ‘invariant’ in
a steady-state setting. This analysis is supported by the correspondence between
the steep reaches and the basement thrust front, which is marked by thrust
earthquakes, and the presence of the highest terraces above river level in the
gorges through the high range.
Profile analysis therefore revealed large-scale tectonic activity which, as at
New Madrid, seismology had failed to detect because the instrumental record
was short and patchy. As the investigators put it, in the Himalayas topography
may be a better indicator of active sub-surface tectonics than surface structures.
The argument is especially convincing if changes in channel long profile are
associated with datable sediments. The classic study of this kind was conducted
in Pallett Creek, an ephemeral stream in California which flows from the San
Gabriel Mountains into the Mojave Desert and crosses the San Andreas fault
55 km northeast of Los Angeles. In order to uncover any evidence for earthquakes
other than the great 1906 and 1857 events the valley deposits were trenched over
a distance which ultimately amounted to 50 m and a depth of 5 m. The evidence
that was sought consisted of sandblows (or sand volcanoes), fissures and offsets
[27]. The study yielded evidence of 12 large earthquakes between AD 260 and
1857 separated by intervals of about 145–200 years [28]. The original 14 C ages
had errors of 50–100 years at the 95 per cent confidence level. Progress in dating
methodology reduced these errors to less than 23 years for 10 of the events; the
interval between events now ranged from 45 years to 330 years but the past 10
earthquakes were found to have occurred in four clusters of 2 or 3 events separated
by gaps of 200–250 years.
The question of chronology is worth pursuing for two main reasons. First,
there may be some kind of pattern in the local earthquake sequence, such as the
clustering we have already encountered at New Madrid, as it would colour (and
perhaps even discourage) any attempt to forecast earthquakes. Second, if events
of the same age have been reported elsewhere in the region it should be possible to
estimate earthquake size; event V of AD 1480 ± 15 at Pallett Creek, for instance,
appears to have ruptured the entire southern half of the San Andreas fault over
a distance of about 300 km. Yet the earthquake history of Wallace Creek, also
crossed by the San Andreas fault 300 km to the northwest, is surprisingly different.
The channel has been offset horizontally by a total of 128 m; the interval between
the last three great earthquakes was 240–450 years, twice as long as at Pallett
Creek [29].
Pallett Creek and Wallace Creek illustrate stream sequences which reveal local
seismic history and contribute to wider assessment of earthquake chronology if
only by complicating it. They also bear on regional tectonics by showing how far
movement over recent centuries compares with the expectations of plate tectonics.
At Wallace Creek the average rate of slip—both during earthquakes and by creep
between them—is about 34 mm yr−1 over the last 13 000 years, close to the rate
obtained by geodetic methods for 1977–1981 of about 30 mm yr−1 , a measure
of agreement which is sometimes taken to show that strain (i.e. the capacity
to generate an earthquake) is not building up on this stretch of the fault. Yet
the average motion between the North American and Pacific plates over the
last 3.16 Myr points to motion at about 50.2 mm yr−1 along the San Andreas
fault [30].
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The deficit is arguably being taken up by other fault systems near the
Californian coast and in the Basin Range, but there remains the interesting
alternative that plate motion is not necessarily steady; as it happens, GPS
measurements for relative movement between the Pacific and North American
plates persistently differ significantly from the geological rates. For 1996–1998,
for example, they indicate a rate of about 49.4 mm yr−1 [31]. Various explanations
for the discrepancy have been proposed, including a real change in the direction
of plate motion over the past 3.2 Myr [30]. Any information on this count is to
be treasured for what it might say about the mechanisms of plate tectonics, and
valley history is uniquely placed to bridge the large gap in the time intervals
used by geodetic and geological approaches and also to provide near-continuous
records rather than average rates.
The Pallett and Wallace fault systems added detail to the San Andreas system;
a growing number of fractures are being revealed in novel locations by surface
folds. Perhaps the first of these ‘blind’ faults was revealed in Algeria in 1980
when the El Asnam earthquake (Ms = 7.3) of 10 October nucleated on a segment
of the Oued Fodda fault (figure 5) [35]. The middle segment of the fault is aligned
transversely to Wadi Chelif. Reverse slip was marked by a number of topographic
effects indicating both elastic and brittle deformation. A temporary lake on the
Chelif was initially ascribed to blockage by the fault but was soon found to
represent downbuckling of the channel. Exposures (and, later, excavations) into
the lake bed revealed eight large events marked by temporary ponding which
antedated 1980. Organic material in the deposits yielded 14 C ages sufficient to
identify six earthquakes similar to that of 1980. Although the interval between
large earthquakes ranged between 300 and 500 years, those with magnitudes
greater than 7 clustered around 4000 years ago and also within the last 1000
years [36]. Upwarping was vividly recorded by deformation of an alluvial terrace
dating from medieval times, as shown by its burial of a Roman site and 14 C dating
of charcoal in the alluvium. The terrace could be traced into a gorge cut by the
Chelif, where it had risen by 5 m above its normal level.
Soon after the El Asnam earthquake, on 2 May 1983, there occurred the
Coalinga (Mw 6.5) earthquake in the San Joaquin valley of California [37]. Here
too a blind reverse fault had created a surface fold; levelling showed 40 cm
of uplift and 25 cm of subsidence along a survey line crossing the anticline.
The Los Gatos river also crosses the anticline and a terrace dated to 2550
years rises from an elevation of 1–3 m to 10 m above the river. The recurrence
interval for events comparable to that of 1983 was therefore put at approximately
100 years.
El Asnam and Coalinga prompted a search for active, possibly seismogenic,
structures in areas where geologists had previously focused on surface faults.
In California itself buried faults are now seen to pose as serious a threat as
the blatantly exposed San Andreas fault. The Northridge (Mw 6.7) earthquake
of 17 January 1994, in the San Fernando valley, is a case in point. With
associated ground acceleration as high as 1.7 g it proved an unexpected hazard
in an area which had been thoroughly investigated for active structures. The
year 2003 saw what was perhaps the first precautionary study of a blind
thrust, when tilted river deposits beneath what is now central Los Angeles were
interpreted as evidence of four major (Mw 7.2–7.5) events during the last 11 000
years [38].
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(a)
(c)
80°
70°
74° 55¢
74° 50¢
90°
75° 0¢
30°
13° 00¢
V
IV
1
20°
Gu
rup
2
III
ur
Nuvel 1A
12° 55¢
II
10°
I
Nethrav
athi
1
Mangalore
(b)
2
12° 50¢
36 km
39 km
41 km
5 km
Figure 5. (a) Hypothetical pattern of buckling of the Indian plate (dashed lines) in response to
collision with Asia [32] (Nuvel 1A after [33]). The model gains support from field evidence including
(b) channel pattern and (c) displacement away from the major watershed (dashed line) at lineament
III [34]. 1 and 2 show successive channels of rivers Gurupur and Nethravarthi. (Online version
in colour.)
The search for quantitative tectonic signals from sediments and landforms
continues to gain impetus from such stark findings [39]. Active folding is of
course of more general interest as a symptom of deformation in progress, and
it may lead to the regional syntheses required for refining tectonic models. In
Central Otago, New Zealand, a peneplain has been deformed by folds driven by
buried reverse faults, and drainage analysis shows (for example) how separate fold
segments can coalesce into ridges [40]. In the Manawatu region anticlinal ridges
have also developed above buried reverse faults, this time in a mid-Quaternary
marine surface. The drainage systems are consequent on the growing folds but are
no longer perpendicular to the structural contours on the flanks of the anticlines,
presumably because of regional tilting [41]. The estimated rate of tilting is similar
to that from river terraces in the Rangitikei River, an interesting endorsement
of such indirect methods for tracing crustal movements. In Epirus (northwest
Greece) a compressional component of motion has long been identified from the
topography and from limited seismological data. Field studies, including analysis
of river long profiles, revealed features which combined dip-slip with a strike-slip
component of motion [42]. The observed deformation could be reproduced by
applying a slip vector of approximately 260–280◦ across Epirus as a whole.
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Such patterns become especially interesting where they are discordant, as
their history can help to resolve the role of stream antecedence. The issue has
long occupied students of drainage in the Zagros Mountains of Iran. The debate
was shortcircuited by studies which showed how river capture too could create
transverse drainage systems [43] and more recently by evidence that fold segments
some 20–40 km long may coalesce into chains of over 100 km. This process distorts
any throughgoing drainage but, as in Otago, the pattern of tributary streams
records the progress of fold propagation [44] and it may help to distinguish
between fault-bend folds, with lengths determined by the driving reverse fault,
and detachment folds, which undergo lateral propagation at the fault tips [45].
The detachment folds exhibit serial folding, the principle that folds have grown
in sequence rather than simultaneously [46]. The idea was given prominence in
a series of simple laboratory experiments in which stratified materials over a
detachment surface were compressed by a piston [47]. The implication for the
Zagros was that folding began at the rear of the fold belt and is currently
in progress at the deformation front on the Gulf. A river section close to the
coast showed that part of the shortening was taken up by reverse faulting
(the Chah Shirin thrust); analysis of uplifted marine terraces had shown that
Holocene shortening was indeed concentrated in the frontal fold to the tune
of 19 mm yr−1 , close to the average approximately 20 mm yr−1 opening rate for
the Red Sea during the last 3–4 Myr. There are obvious implications for seismic
hazard assessment, in the sense that, as many of the Zagros folds are driven by
concealed reverse faults, the zones most prone to shallow earthquakes and ground
deformation can be provisionally identified from the topography. More important,
the force required to sustain the locked and developing folds against gravity yields
a value (less than 1 kbar) which can be compared with competing models for the
convergence between Arabia and Eurasia [48], making gravity glide resulting from
Red Sea opening a plausible mechanism for compressing the Zagros.
Inland the vertical component of valley deformation could be measured only
by laborious levelling until the development of space geodesy. On the coast sea
level provides a convenient datum, as in the Musandam peninsula of northeast
Oman, which links Arabia to the Zagros. A strongly indented coastline betrays
the submergence of a valley network. The onshore valleys are floored with a
calichefied gravel unit. It is stratigraphically and petrographically related to the
Tehran Alluvium of north and west Iran, whose accumulation had ceased by
about 6000 years ago. The Musandam valleys can be traced offshore by plotting
the cemented river gravels by geophysical means and are seen to extend to their
greatest depths towards the northeast. After allowing for sea-level rise over the
last 6000 years the age/depth data yield a maximum average subsidence rate of
8.5 mm [49]. There is good agreement with the results of satellite measurements
(by differential synthetic aperture radar interferometry), which indicate a seismic
subsidence at 10 mm yr−1 at Ras al-Khaimah, southwest of the peninsula, and
5 mm at Khor Fakkan, southeast of the peninsula, the latter possibly associated
with earthquake activity [50].
Geologists working in the region have long recognized that Musandam was
subsiding in contrast with the Oman Mountains further south, which were known
to be rising. The process could not previously be quantified even though the
tectonics of Musandam were known to be in response to extension of the Red Sea.
We now see that Arabia is transmitting this extension from the Red Sea to Iran
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without deforming significantly. In short, by allowing subsidence to be quantified
the valleys of Musandam helped to demonstrate the truth of an axiom of plate
tectonics, namely that deformation is strongly concentrated at plate margins.
The residual strain acting at plate interiors to marginal compression can
be lethal as it gives rise to large earthquakes which, as at New Madrid, are
too infrequent to stimulate adequate building codes or evacuation procedures.
Peninsular India is a case in point, as its interior has been struck by a number
of very violent earthquakes at locations which are not self-evidently seismic.
Consider, for example, the Coimbatore (M 6.0) earthquake of 28 February 1900,
the Kilari/Latur (Mw 6.1) earthquake of 24 August 1993 and the Koyna (Mw 6.5)
earthquake of 10 December 1967.
It has been suggested that collision between India and Eurasia has given rise to
a major downfold south of the Himalayas and that, as the Indian plate traverses
this topographic low, successive parts of the plate are subjected to compression
which from time to time is sufficient to trigger earthquakes [51]. An alternative
view is that the plate develops a number of buckles which undergo progressive
compression. Evidence for two such upwarps began to accumulate in the 1980s
and 1990s, the first of them running east from Cochin (I on figure 5) [52] and
the second between Mangalore and Madras (II). Along the latter there was
evidence for a thin crust and anomalously positive values for gravity, both of them
consistent with extension and the presence of mantle rocks relatively close to the
surface, microseismicity associated with pervasive rock fracturing, and above all
the displacement of river channels away from the proposed axis of uplift [34].
Four more buckles were later proposed on the basis of compressional
earthquakes, rather than surface deformation, with a spacing of approximately
400–800 km and some anticlockwise rotation in the north in response to the
irregular margin of the deforming plate [32]. The Indian plate extends south into
the Indian Ocean, and the submerged portion displays a series of undulations
with a wavelength of about 200 km which are widely seen to represent plate
compression. Modelling on the basis of an elastic thickness of 35 km—for oceanic
crust—yields buckles with a wavelength of approximately 150–200 km [53]; a
greater wavelength would be expected in the emergent part of the plate with
a thickness of 65–79 km.
The analysis will eventually shed light on the force required to drive the plate
and hence the relative importance of gravity, ridge push and basal drag in its
displacement. Buckling would require a force of about 1013 N m−1 ; sustaining the
Tibetan Plateau against gravity requires between 4 × 1012 and 1013 N m−1 [54].
In short, there is broad equivalence in the force expended by two major crustal
processes at work in the region.
4. Fluvial influence on tectonic processes
The possibility that fluvial erosion and deposition could influence tectonic
processes has long been recognized and indeed quantified. Fridtjof Nansen [55]
was among the first to postulate isostatic adjustment to loading and unloading;
he understandably focused on ice as the primary agency, and deglaciation has
remained an important topic of research into mantle properties because both the
history of ice retreat and the progress of land uplift and deformation at high
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River history and tectonics
New Orleans
6
4
2
m 0
–2
–4
–6
50 km
91°
90°
89°
88°
0
5
10 15 20
× 1000 yr BP
25
30
Figure 6. The Mississippi delta (Multi-angle Imaging SpectroRadiometer image, courtesy of
NASA); inset shows surface deflection over time at different longitudes at 30◦ N based on a threedimensional viscoelastic model. Note uplift followed by susbsidence with dissipation of the signal
with distance along the shoreline. Adapted from Blum et al. [56]. (Online version in colour.)
latitudes can be traced in sufficient detail for such matters as mantle viscosity to
be evaluated. Glacial lakes, notably Bonneville, offer similar advantages including
a built-in horizontal datum.
(a) Local adjustment
Isostatic adjustment in the world’s major deltas has also long been recognized.
Despite the difficulties that arise in evaluating the relative contribution of fault
movement, sediment compaction, fluid and gas extraction and other sources of
subsidence, isostatic modelling for the Mississippi delta (figure 6) suggests that
channel incision during glacial episodes of low sea level and channel filling during
the ensuing rises suffice to induce alternating episodes of uplift and subsidence
amounting to almost 8 m over a distance of some 150 km along the Gulf coast [56].
An elastic model was used to estimate the effects of a load 40 m thick and 80 km
wide with density 1800 kg km−3 and a lithosphere with an elastic thickness Te
of 30 km. Load removal would lead to an elastic uplift of approximately 12 m in
the valley centre and 9.7 m at the margins. The field evidence indicates six such
episodes of channel-belt deposition between 54 000 and 12 000 years ago separated
by periods of valley cutting.
The study suggests that other large deltas have probably undergone repeated
uplift and subsidence, especially where glacial meltwaters have provided enhanced
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2186
C. Vita-Finzi
erosion at times of low sea level. More important in the present context, the detail
with which delta deposits can be traced and dated offers scope for refined tests
of competing mechanisms, a kind of gravitational titration. Evidently this holds
only for deltas riddled with boreholes and for which distortion by the human
factor is well documented. In the Mississippi delta regional subsidence can be
shown to be locally compounded by fault reactivation induced by hydrocarbon
extraction [57,58].
In deciding whether erosion alone can trigger an isostatic response the key
issue is the stress level required to exceed crustal resistance. Faulting can set
in motion buoyancy forces which either reduce or accentuate the effects of
the original displacement provided the structure is substantial enough for the
resulting imbalance to exceed the elastic strength of the lithosphere. Fluvial
activity may have an analogous effect and allow crustal strain originating in
plate motions and other regional sources to be identified. At New Madrid,
GPS data show that any motions across the seismic zone measure less than
0.2 mm yr−1 , which suggests that strain is not accumulating at a steady rate
and that existing faults have been reactivated by a local process. A plausible
explanation is that upward flexure of the lithosphere consequent on river erosion
reduced normal stresses in the upper crust sufficiently to unclamp faults close to
failure [23], analogous to the mechanism that has been proposed for locations at
high latitudes where fault movement followed unloading by glacial melting and
glacial erosion.
The fluvial record for the central and lower Mississippi shows that some 6 m
of sediments were removed over a zone 60 km wide between 16 000 and 12 000
years, BP and a further 6 m over a zone 30 km wide between 12 000 and 10 000
years, BP (figure 4). Finite-element modelling, on the assumption that unloading
was linear between 16 000 and 10 000 years BP, suggests that with a strong
mantle (little viscous relaxation) the unclamping stress increases linearly with
load removal and is then constant. With a weak mantle, stresses continue to
build up long after unloading is over. In either case faults in the New Madrid
fault zone continue being unclamped as much as 10 000 years after significant
alluvial erosion came to an end. In brief, existing faults here are reactivated by
small stress changes resulting from flexure that has been induced by erosion,
so that seismicity migrates from fault to fault ‘drawing elastic energy from the
long-lived strain reservoir’ [23, p. 610].
Note that interaction between fluvial activity and deformation can be traced
away from faults and alluvium and without the assistance of isostatic rebound.
The effect on fold development lends itself well to analysis at scales smaller
than 200 km provided surface processes are rapid relative to the rate at which
deformation can operate. If the crust is already stressed to the failure limit,
erosion and deposition can accentuate buckling rate and increase buckling
wavelengths: if the topography is increased it is because the dynamic folding
instability is amplified through a change in the distribution of gravitational
loads and changes in the thickness—and hence the strength—of the deforming
elastic–plastic layer [59]. A fine illustration of this process is the Oxaya antiform
in northern Chile (figure 7), a structure which displayed its greatest rate of
growth between 6.7 and 2.7 Ma when transverse stream incision attained up
to 1500 m and coincided with plastic deformation of the crust under regional
compression [61].
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River history and tectonics
(b) 10 km
(c)
Altiplano
n
ter ra
es lle
W ordi
C
(a)
(i)
0.002
0.001
(ii)
0
0.002
0.001
0
0
0.2
0.4
0.6
normalized distance
Figure 7. (a) Deflection and topography resulting from deformation (37% shortening) in the
presence of (i) relatively efficient and (ii) relatively inefficient surface processes. Heights and
distances normalized for comparison. Dashed line, topography; full line, deflection. Based on
Simpson [59]. (b,c) Buckling amplified by river incision: the Oxaya antiform, north Chile (thick
dashed line). Adapted from Zeilinger et al. [60]. (Online version in colour.)
(b) Mountain chains
Uplift resulting from erosional unloading is evidently not confined to major
deltas at times of low sea level and enhanced stream flow. It is at its most
spectacular in some of the loftiest mountain ranges. In an analysis of stream
antecedence in the Himalayas, Wager [62] argued that a stream may maintain
its course while a mountain rises in its path, citing the Yo Ri gorge reach of
the Arun as evidence: like other streams in the region the Arun runs parallel
to the Himalayan range for hundreds of miles and suddenly turns south to
run across the High Himalaya between Mt Everest and Kanchenjunga. The
notion that isostatic rebound contributes to mountain uplift dates back at
least to Clarence Dutton, who argued in 1899 that the effect of erosion in the
mountains was abetted by the counterflow of material displaced by deposition
near the coasts.
Some have argued that transverse courses in the region often owe more to
capture and superimposition than antecedence. As often happens the battle lines
eventually become blurred. Many rivers cross the Himalayan axis at bends or
at points where headward erosion was easy and perhaps where it subsequently
led to localized isostatic uplift; at some of the crossings the stream is in effect
superimposed on a discordant structure.
For a quantitative analysis it may therefore be simpler to deal with river
systems and rock masses in bulk. The positive feedback between erosion and
uplift can be viewed as a special case of the interaction between tectonics and
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climate where removal of a thickness DT of crust of density rc compensated with
a crustal root in mantle of density rm leads to surface lowering by about DT /6: a
‘gentle highland’ of mean elevation h incised by rivers nearly to sea level would fall
in mean elevation to about 0.83 h but the peaks would rise to 1.8 h or higher [63].
Nevertheless, detailed case studies show that the influence of erosion on
mountain building is complex and often subject to feedback. An example in the
eastern Himalaya which has parallels with the Oxaya antiform is the Namche
Barwa-Gyala Peri massif, also antiformal, where high relief and rapid erosion
by what is deemed the most powerful river in the Himalaya, the Brahmaputra,
point to a balance between rock uplift and river incision at least for the last
1 Myr [64]. Such observations have led to the suggestion that river anticlines
represent the most recent phase of Himalayan deformation. The Arun river also
crosses an anticline which coincides with a local rainfall maximum, and it may be
that the coupling between tectonics and erosion is manifested to different degrees
within an active orogen in response to climatic variations and their effect on
erosion [65].
More generally, deep valleys may indicate isostatic response to incision rather
than the massive post-Pliocene net uplift that has been widely proposed for the
Alps and Rockies. Accelerated sedimentation could give an equally misleading
impression of enhanced tectonics: a fourfold increase in deposition rate over
the last 2 Myr in the Gulf of Mexico, for example, has been equated with
rapid uplift of the Rockies whereas the onset of widespread glaciations in the
Northern Hemisphere may be responsible for the necessary increase in erosion
at source [66].
Assuming periodically spaced, uniform, long, parallel, V-shaped valleys, and
crust and mantle densities, respectively, of 2800 and 3300 kg m−3 , the rate of
mountain uplift will be 42 per cent of the rate of valley incision. Measured incision
averaging 172 m in the Sierra Nevada would account for only 9 per cent of the
uplift that has been estimated for the last 10 Myr, whereas as much as 1500 m
(20–30%) of the present elevation of the Himalayan peaks may be an isostatic
effect [67]. Valley incision by trans-Himalayan rivers at the margins of the Tibetan
Plateau in the late Cenozoic, possibly accelerated by climate changes, could have
accomplished the requisite unloading.
Of course the extent to which compensation is realized depends on the flexural
rigidity of the lithosphere and the wavelength of the topography. It can be
estimated using a formulation by Turcotte & Schubert [68], which is applicable to
topographic wavelengths of 102 –103 km and flexural rigidities of 1020 –1025 N m and
encompasses many mountain ranges. An important result is that at topographic
wavelengths of less than about 100 km even low flexural rigidity will suppress
local compensation: valley incision influences mountain peak elevation where
the mountain range is large or the lithosphere is weak or thin. Flexural rigidity
of the lithosphere is in the region of 1023 –1024 N m beneath the Sierra Nevada,
which is 200–400 km wide; compensation is 1–5%; for the Himalayas (400–500 km
wide) the rigidity is estimated to be 1022 –1023 N m and the compensation could
be as much as 45–95% [67]. The answer may embody both isostasy and climate
change: in the Sierra Nevada any uplift must be primarily due to tectonic factors,
while—assuming a plateau 250 km wide and 6 km high falling to 0 km 300 km from
the watershed—stream incision at the margin of the Tibetan Plateau indirectly
accounts for the great height of Himalayan peaks.
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5. Discussion
The drainage pattern of rivers contains ‘unique information about the past
and present tectonic regime. The longitudinal profile of a river is sensitive to
the ongoing process of uplift and can be used to recognize active structures’
[26, p. 343]. This distinction is at its most valid where the network is incised in
bedrock and the river flows in alluvium; for, as we have seen, stream networks can
be disrupted by seismicity at a stroke and individual profiles may embody traits
inherited from a very different tectonic past. What is more, solitary channels as
much as river systems can trigger disproportionate tectonic reactions.
Perhaps more instructive today would be a distinction between historical and
geodynamic case studies. An important function of research on fluvial tectonics
is explanatory. Analysis of Neogene strata, for example, shows that uplift of the
eastern Cordillera of South America diverted the Orinoco drainage from Lake
Maracaibo to the Atlantic [69]. There is even dedicated computer software (e.g.
[70]) to facilitate such tasks as determining flow direction, knickpoint selection
and stream profile generation from DEMs. But the way forward surely lies in
applying the results to the dynamics of deformation and thence to the geophysical
mechanisms that are at work. Buckling of continental lithosphere, as in India,
leads to some understanding of the forces at work where plates converge. The
pattern of movement along a major strike-slip fault, such as the Red River fault,
mentioned earlier, or the Altyn Tagh fault on the Tibetan Plateau [71], may show
that a region deforms internally by a network of faults in the shallow brittle crust
rather than by slip on a transform fault cutting through the entire lithosphere
and separating two relatively rigid blocks. Isostatic processes in major mountain
rivers could conceivably promote ‘channel flow’ of low viscosity in the mid-crust
[72] whereupon ‘rivers may be the authors not only of their own valleys, but
in some circumstances the structural geology of the surrounding mountains as
well’ [65, p. 14].
This amounts to using the data of river history experimentally, much as has
long been possible with changing ice loads in glaciated areas, to assess the
properties of the crust and mantle. Although many of the processes at issue are
nonlinear (e.g. [73]), and topographic analysis remains at best a qualitative tool
for neotectonic analysis [74], advances in fluvial chronology continue to increase
its value to the experiment.
I thank Eric Force for his comments on a draft of this paper, Shunji Ouchi for discussion and
photographs of his experimental studies, Ray Weldon II for preprints, NASA for images, and Tom
Blenkinsop, Mike Blum, Eric Calais, Mustapha Meghraoui, Andy Moore, Guy Simpson, K. R.
Subrahmanya, Graham Yielding and Gerold Zeilinger for illustrations.
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