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An emerging paradigm that equates glaciers to ''buzz saws'' of exceptional erosional efficiency has been strengthened by short- term (,10 2 yr) sediment yields from southern Alaska. New low- temperature cooling ages from this area, the glaciated Chugach- St. Elias Mountains, constrain long-term (10 6 yr) exhumation rates. Vertically averaged exhumation rates reach ;3 mm/yr, but are an order of magnitude lower than rates based on short-term sediment yields. Whereas these exhumation rates are not exceptional for oro- genic belts, denudation patterns are strongly correlated with the distribution of glaciers, and erosion appears to keep pace with con- vergence and uplift. These findings imply a coupling between gla- cially dominated erosion and tectonics.

There is no question that extensional tectonics and related processes have played a major role in the geologic evolution of the British Isles, and in the formation and trapping of immense “super-giant” fields of hydrocarbon resources in the North Sea. The latter have been of considerable importance to the economy of the United Kingdom. This two-part volume is the second in a new series of publications by The Geological Society, “Key Issues in the Earth Sciences.” It is a compilation of 35 well-known papers on extensional tectonics published over the past 2 decades.

The geomorphic character of major river drainages in the Himalayan foothills of central Nepal suggests the existence of a discrete, west-northwest‐trending break in rock uplift rates that does not correspond to previously mapped faults. The 40 Ar/ 39 Ar thermochronologic data from detrital muscovites with provenance from both sides of the discontinuity indicate that this geomorphic break also corresponds to a major discontinuity in cooling ages: samples to the south are Proterozoic to Paleozoic, whereas those to the north are Miocene and younger. Combined, these observations virtually require recent (Pliocene‐ Holocene) motion on a thrust-sense shear zone in the central Nepal Himalaya, ;20‐30 km south of the Main Central thrust. Field observations are consistent with motion on a broad shear zone subparallel to the fabric of the Lesser Himalayan lithotectonic sequence. The results suggest that motion on thrusts in the toe of the Himalayan wedge may be synchronous with deeper exhumation on more hinterland structures in central Nepal. We speculate that this continued exhumation in the hinterland may be related to intense, sustained erosion driven by focused orographic precipitation at the foot of the High Himalaya.

The dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state. This concept is often invoked, but the nature of the steady state is commonly not specified. We identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical-model results and natural examples. These types are (1) flux steady state, (2) topographic steady state, (3) thermal steady state, and (4) exhumational steady state: they refer to the erosional flux, the topography, the subsurface temperature field, and the spatial pattern of cooling ages, respectively. Models suggest that the topography will reach a steady mean form at the scale of an orogenic belt, but perfect topographic steady state is unlikely to be achieved at shorter length scales. Thermal steady state is a precondition for exhumational steady state and in the case of temperature-dependent deformation, topographic steady state. Exhumational steady state is characterized by reset age zones spatially nested according to closure temperature, as illustrated in natural systems from New Zealand, the Cascadia accretionary margin, and Taiwan.

Late Cenozoic warping of the Colorado piedmont involved interplay of tectonic forcing, river erosion, and isostatic response to erosion. Modeled erosional isostasy closely replicates the observed pattern of deformation, but accounts for only about half its magnitude. The remainder reflects tectonic rock uplift that increases southward across the piedmont, likely reflecting proximity to the northward-propagating Rio Grande Rift. This differential uplift triggered differential erosion, concentrated on southern piedmont river systems, particularly the Arkansas River, which led in turn to differential isostatic rock uplift focused on the Arkansas drainage. Covariation of tectonic uplift, erosion, and isostatic compensation across the piedmont reflects a positive feedback between uplift-induced erosion and erosion-induced isostasy, which has progressed to the point that isostatic uplift is approximately equal to the initial tectonic forcing.

Large-scale topographic analyses show that hemisphere-scale climate variations are a first-order control on the morphology of the Andes. Zonal atmospheric circulation in the Southern Hemisphere creates strong latitudinal precipitation gradients that, when incorporated in a generalized index of erosion intensity, predict strong gradients in erosion rates both along and across the Andes. Cross-range asymmetry, width, hypsometry, and maximum elevation reflect gradients in both the erosion index and the relative dominance of fluvial, glacial, and tectonic processes, and show that major morphologic features correlate with climatic regimes. Latitudinal gradients in inferred crustal thickening and structural shortening correspond to variations in predicted erosion potential, indicating that, like tectonics, nonuniform erosion due to large-scale climate patterns is a first-order control on the topographic evolution of the Andes.

Gently sloped summits and ridges (collectively referred to as summit flats) are abundant in many Laramide ranges in the western United States. The erosion rate of summit flats is ~10 m/m.y., on the basis of the concentrations of cosmogenic radionuclides. Because erosion rates in valleys between summit flats are an order of magnitude faster, relief within these ranges is currently increasing by about 100 m/m.y. If summit-flat erosion is slower than rock uplift driven by the isostatic response to valley erosion, then this relief production could result in increased summit elevations. The mean depth of material eroded from a smooth surface fit to existing summit flats varies from 280 to 340 m in four Laramide ranges, based on geographic information system (GIS) analyses of digital elevation models. This erosion would result in a maximum of 250‐300 m of rock uplift, assuming Airy isostasy. However, because the Laramide ranges examined here are narrow relative to the flexural wavelength of the lithosphere, erosionally driven rock uplift is limited to ~ 50‐100 m. Over the past several million years, summit erosion would approximately offset this rock uplift. Therefore, we conclude that summit elevations have remained essentially constant even though several hundred meters of relief has been produced. On the basis of valley and summit erosion rates and the average depth of erosion, we estimate that relief production in Laramide ranges began at ca. 3 Ma. We hypothesize that this relief production was climatically driven and was associated with the onset or enhancement of alpine glaciation in these ranges.

A mechanical model of crustal shortening and deformation driven by the relative convergence of rigid, underlying mantle plates explains many features of convergent orogens. Results based on numerical models and supported by sandbox models show that a Coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth: (1) block uplift bounded by step-up shear zones; (2) development of a low-taper wedge over the underthrusting mantle plate; and (3) development of a low-taperwedge overlying the overthrusting mantle plate and verging in the opposite direction. When modified by isostasy, basal viscous flow, surface erosion and denudation, and sedimentation, the resultant model orogens exhibit a variety of styles with characteristics in common with small, rapidly denuded orogens, large orogens with plateaus and extensional characteristics, and active subduction margins with doubly vergent accretionary wedges and deformed fore-arc basins.

Measuring tectonic uplift of the mean surface of Earth using thermochronological data to document "uplift" histories is often plagued by confusion arising from the independent nature of the thermal and sea-level frames of reference. The technique of backstacking apatite fission-track "stratigraphy" differentiates between vertical movement of rocks toward Earth9s surface due to erosional or tectonic denudation and tectonic uplift of Earth9s surface. Apatite fission-track analysis can also be used to identify episodes of erosion where no other evidence for the erosional event exists, in addition to providing information about the timing, magnitude, and geographic distribution of erosion.

Summary. A mountain’s history includes two distinctive phases. one of active tectonism and construction followed by one of erosion and passive isostatic rebound. In the first phase uplift is driven by tectonic mechanisms while in the second phase base levels of the terrain are regionally uplifted. It is this latter phase that is modelled here. The starting model is a mountain range, initially in isostatic equilibrium, on a viscoelastic plate defined by the effective flexural rigidity D and relaxation time 7,. The rate of erosion at any time t is assumed to be proportional to the elevation at that time, with an erosional time constant 7,. For a given present-day topography the uplift, erosion, gravity, and stress can be computed through time as functions of D, 7,. 7,, and the time to at which the erosional-rebound mechanism became the dominant landscaping process. The model has been applied to the highlands of south-eastern Australia which we assume to be an erosional residue of the Palaeozoic Lachlan Fold Belt. Observations of rivers cutting through Cainozoic basalts and other geomorphological indicators of uplift can then be interpreted in terms of the isostatic rebound and there is no need to invoke active tectonic uplift mechanisms. The model parameters that fit the observations are T,= 150-3-50Myr, DT~- 2.5 x IOz4NmMyr and to- 180-200Myr. The predicted Late Palaeozoic fold-belt topography is about 75 per cent greater than the present-day values. Topographic depressions are predicted to have occurred along the margins of the formerly elevated areas and the model is relevant to understanding the evolution of basins flanking the highlands. Within the highland area erosion generally exceeds uplift but in some regions the two are approximately equal, in keeping with recent geomorphological observations. The flexural stresses associated with the erosion initially increase with time and maximum values in south-eastern Australia are predicted to occur in Late Cainozoic time. with the stress state near the surface being mainly tensional.

The Wenatchee Formation of Oligocene age (34 m.y.) rests on a profound unconformity that developed during an extended period of erosion and tectonic quiescence from perhaps 40–34 m.y. B.P. Its occurrence strongly suggests that a stripped erosion surface in the northern Cascade Range, previously considered of Miocene age, is of Late Eocene—early Oligocene age and that it is part of a more widespread surface in the Pacific North-west. The Late Eocene—early Oligocene unconformity/erosion surface is apparently of continental extent, from British Columbia to Mexico. Where it is incised on crystalline basement rocks, it now may be exhumed and exposed as a plateau or level-crested divides of concordant elevation. Where it is developed on sedimentary rocks, it may be found as a profound unconformity. Widespread eruptions of pyroclastic volcanic rocks covered much of the surface in Oligocene time and tended to protect and preserve it. The surface is particularly well documented in the San Juan volcanic field of Colorado. There it was given the name “Telluride peneplain”, which is here broadened to the Telluride erosion surface of continental extent. A magmatic gap indicating a period of magmatic quiescence is generally associated with tectonic quiescence implied by the erosion surface, but the timing varies regionally. Development of the Telluride surface and associated magmatic gaps correlates with general plate reorganization during the Eocene. An erosion surface correlates with a global tectonic transition because erosion rates immediately dominate uplift rates when compressive deformation is slowed or shut off. An analysis of the dynamic balance between erosion rates and plate convergence rates suggests that erosion may maintain a continental surface of low relief if the relative convergence rate falls to about 0.5 cm/yr or less. Gaps in magmatic activity do not correlate as precisely with the 40 m.y. tectonic transition, in part because of the possibility of time lags. A great deal of scattered geologic evidence suggests a major global event at about 40 m.y. BP. The timing does not always precisely correlate with the timing of events in western North America, which suggests that the 40 m.y. tectonic transition was spread over several millions of years: A most impressive global impact of the 40 m.y. transition is the largest drop in eustatic sealevel at about 30 m.y. BP. The 40 m.y. tectonic transition profoundly altered geologic history, including the development of continental landscapes, global sea-level, and global climate. Classical stratigraphic studies remain the best way of documenting its effect on the continental surface. The small remnant of Oligocene rocks in central Washington, overlooked in early reconnaissance studies, is of great geologic significance. Other undiscovered remnants probably exist in western North America.

This symposium concerns The Present State of Plate Tectonics . By any measure, plate tectonics can be described as “alive and well” and, in fact, a “healthy, burgeoning concept”, although only eight years have passed since (in 1967) plate tectonics suddenly won general acceptance, at least among western scientists, as the proper model for global tectonics. This concept is, of course, an integration of, and a follow-on to, sea-floor spreading, transform faulting, trench subduction and continental drift. Although some criticism has arisen, this revolution in geology and geophysics has provoked remarkably little cogent dissent. Of late, plate tectonics has bridged the gap from basic to applied science in the search for natural resources; e.g. petroleum accumulations on continental margins, porphyry copper deposits associated with subduction zones, base metal sulfides associated with pillow lavas at fossil rifts, etc. Perhaps the largest gap that remains is in a definitive understanding of plate driving forces. The viability of plate tectonics is emphasized by the several symposia at this IUGG meeting which concern, either directly or peripherally, plate tectonics. In this symposium, we have asked the organizers of such symposia, or their nominees, to present a synthesis concerning the implications of their particular specialty for plate tectonics. These include plate tectonics aspects of seismology, heat flow, plate accretion processes at ridges, paleo-oceanography, etc.

The mean denudation rate in mid-latitude river basins is directly proportional to mean basin relief. Mean annual precipitation has no noticeable effect upon the denudation rate. Without any uplift, the relief is reduced to 10 percent of its initial value in 11 m.y.; with isostatic compensation this time increases to at least 18.5 m.y. Establishment of a steady state relief (Hack, 1960) would require a constant rate of uplift for more than 20 m.y. and thus is unlikely to occur. Oscillating rates of uplift cause corresponding oscillations of relief. The quantitative relationship between uplift, relief, and denudation may permit order-of-magnitude estimates of current rates of uplift.

The study of tectonic landforms is one of the main themes of geomophological research in Japan, characterized as a tectonically active area along the subducting oceanic plates. The recent trends in the studies of tectonic landforms which include an arrangement or distribution of major ranges and plains, vertical displacement of low-relief erosion surfaces and of marine terraces, and various kinds of deformed features due to active faulting or folding, are introduced here with special reference to the Quaternary geomorphic development. A regionality of each tectonic landforms is also summarized. Further, some fundamental concept of the Quaternary tectonics, for instance, an accumulative character of each type of deformation and sequence of rate of tectonic movement, reconstruction of former stress field as well as a relation between the Quaternary tectonic movement and seismic deformation in present and historic time are discussed on the basis of analysis of the tectonic landforms. The Quaternary tectonics of the Japanese Islands are essentially characterized by the compressional stress, originated from the subduction of oceanic plates, and regionality can be interpreted as the result of the different response of each tectonic region to island arc tectonics.

The pattern of fluvial incision across the Himalayas of central Nepal is estimated from the distribution of Holocene and Pleistocene terraces and from the geometry of modern channels along major rivers draining across the range. The terraces provide good constraints on incision rates across the Himalayan frontal folds (Sub-Himalaya or Siwaliks Hills) where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces. Farther north and upstream, in the Lesser Himalaya, prominent fill terraces were deposited, probably during the late Pleistocene, and were subsequently incised. The amount of bedrock incision beneath the fill deposits is generally small, suggesting a slow rate of fluvial incision in the Lesser Himalaya. The terrace record is lost in the high range where the rivers are cutting steep gorges. To complement the terrace study, fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate. This approach allows quantification of the effect of variations in channel slope, width, and discharge on the incision rate of a river; the determination of incision rates requires an additional lithological calibration. The two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared. In the Sub-Himalaya, river incision is rapid, with values up to 10–15 mm/yr. It does not exceed a few millimeters per year in the Lesser Himalaya, and rises abruptly at the front of the high range to reach values of ∼4–8 mm/yr within a 50-km-wide zone that coincides with the position of the highest Himalayan peaks. Sediment yield derived from the measurement of suspended load in Himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range. The observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat-ramp-flat geometry of the Main Himalayan Thrust fault. The morphology of the range reflects a dynamic equilibrium between present-day tectonics and surface processes. The sharp relief together with the high uplift rates in the Higher Himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the Tibetan Plateau driven by late Cenozoic climate change, or late Miocene reactivation of the Main Central Thrust.

The high altitudes of most mountain ranges have commonly been ascribed to late Cenozoic uplift, without reference to when crustal thickening and other tectonic processes occurred. Deep incision and recent denudation of these mountain ranges, abundant late Cenozoic coarse sediment near them, and palaeobotanical evidence for warmer climates, where high mountain climates today are relatively cold, have traditionally been interpreted as evidence for recent uplift. An alternative cause of these phenomena is late Cenozoic global climate change: towards lower temperatures, increased alpine glaciation, a stormier climate, and perturbations to humidity, vegetative cover and precipitation.

From Mr. J. Hospers :— In Professor Shackleton9s paper, as well as in Dr. Dixey9s paper (1946) on “ Erosion and Tectonics in the East African rift system,” mention is made of Dr. Bullard9s gravity measurements (Bullard 1946). Bullard found that the plateaus surrounding the rift valleys are very nearly isostatically compensated, but that over the rift valleys large negative isostatic anomalies occur. Not only are these rift valleys uncompensated, but there is even more light matter under them than if they were simply cut out of the plateau, leaving the compensation unchanged. Under the rift valley there is therefore an excess of light matter and the upward hydrostatic forces will, according to Bullard, exceed the downward pull of gravity; the floors of the rift valleys will therefore rise unless held under by some other force. Consequently, this settles the argument in favour of the compression theory, a wedge-shaped block (widening downwards) being forced down between reversed faults. The crucial point in this reasoning is that if a part of the crust is not in isostatic equilibrium (that is, if the isostatic anomalies are not zero or very small) it cannot be in hydrostatic equilibrium. This is true only for rectangular and not for wedge-shaped blocks. When wedge-shaped crustal blocks float on a denser substratum there is no isostatic equilibrium in the sense that the isostatic anomalies are zero, even though the block may be in hydrostatic equilibrium. In a recent paper F. A. Vening Meinesz (“ Les ‘ graben

Summary From consideration of the Karroo and Jurassic structures within the Rift Zone it is suggested that major folding that took place in later Jurassic time raised locally to great height the resistant pre-Karroo complex, out of which, in successive cycles, the high-level residual plateaux have been carved. In general, throughout the Rift Zone the surviving larger tracts of Karroo and Jurassic sediments still occupy low-lying areas representing the original major synclines, but in some cases trough-faulting that succeeded the folding carried down into the elevated areas strips of sediments which were subsequently eroded to form ancestral " rifts "; within these " rifts " much of the Pleistocene rifting took place and thus formed narrow lake basins within an ancient topography. The pattern of the ancient residuals on the main watersheds of the Rift Zone indicates that these watersheds were original features on the Miocene peneplain, inherited from the great framework of the Jurassic folds. Until the rifting of about the Middle Pleistocene carried down the floor of the Nyasa and Tanganyika troughs to form the existing lakes, the history and form of these troughs differed in no essential respect from that of the Karroo troughs of the Luangwa Valley and the middle and lower Zambezi. These troughs had been developed in their present form by a prolonged sequence of planation, continental uplift, and erosion that intervened between the fracturing of the post-Karroo period and that of the Pleistocene. The Pleistocene rifting accordingly took place along existing troughs of ancient origin of which the form was accentuated by the erosion of the Miocene and Pliocene cycles. Similarly, many of the Pleistocene rift fractures of Tanganyika Territory followed ancient fault-line scarps and troughs. The major rifts of the northern part of the Rift System, including the Abyssinia Rift, also show a pronounced pre-Pleistocene phase of ancient origin preceding the Pleistocene rifting, and in two localities there is evidence, too, of movements of Lower Miocene age

The plate tectonics hypothesis provides new insight into interpretation of the geomorphic history of the Atlantic Provinces. The landscape of the continental margin was probably rejuvenated in Late Triassic and Early Jurassic times as a result of regional tectonic uplift and ramping associated with continental breakup. The development of some aspects of the present geomorphic expression appears to have begun as early as Late Jurassic time. Uplift was followed by broad regional subsidence along the continental margin and adjoining ocean basin and appears to have persisted to at least late Tertiary time. Unconformities across the formations of the submerged Atlantic Coastal Plain were formed during the time of broad subsidence, and these geomorphic surfaces were probably developed by subaerial processes. The lower sea levels possibly resulted from eustatic change rather than from tectonic uplift, and were possibly controlled by tectonic events along the mid-oceanic ridges.