Basin Research - Volume 11, Issue 1, 1999

Marine terraces are ephemeral planar landforms. While tectonic and climatic forcings responsible for the generation of existing marine terraces have operated for at least 1 Myr, terraces have been completely removed by erosion above a given altitude (and hence above a given age). Above this altitude, the landscape has forgotten that it was once terraced. We ask what controls this characteristic time‐scale, which we term the ‘forget time’, in a landscape. We approach the problem with simple scaling arguments, and 1‐D numerical models of landscape evolution.

Using a simple cliff erosion model with a realistic sea‐level history, rock uplift and a cliff retreat rule, we find that the most important means of terrace removal is through the deeper transgression of a subsequent sea cliff into the landmass. The sequence of preserved terraces depends upon the history of sea cliff incursion into the landmass. The extent of sea cliff incursion depends on the duration of the sea‐level highstand, the far‐field wave energy input and the degree to which bathymetric drag dissipates wave energy. This portion of the marine terrace survival problem is an example of a common problem in geomorphology, in which the record of past tectonic or climatic events is rendered incomplete by the potential for younger events to wipe the topographic slate clean.

While sea cliffs decay through time, their form can still be recognized many hundreds of thousands of years after formation. This reflects the diffusive nature of their decay: early rapid evolution and lowering of maximum slopes yields to slower rates through time. Incision by streams, on the other hand, is rapid, as the streams respond to base‐level history driven by sea‐level changes. The rate of incision reflects the local climate conditions, and is limited by the rate of base‐level fall.

The principal means of vanquishing a marine terrace is by backwearing of slopes adjacent to these incising streams. The forget time should be proportional to the spacing between major incising streams and to the angle of hillslope stability, and should be inversely proportional to the rate of channel incision. This yields an overestimate of the forget time, as the terraced interfluves are reduced as well by the headward incision of tributary streams.

The resulting landscape may be viewed as a terraced fringe separating the sea from the fully channellized landscape. Over time‐scales corresponding to many glacial–interglacial sea‐level oscillations, this fringe can achieve a nearly steady width. The rate of generation of new terraced landscape, reflecting the uplift rate pattern, is then balanced by the rate at which the terraces are erased beyond recognition by channel and hillslope processes. The width of this fringe should depend upon the precipitation, and upon the distance to the nearest drainage divide, both of which limit the maximum power available to drive channel incision.

We use the landcape evolution model Zscape to explore quantitatively the development of mountainous topography in the Basin and Range province (formerly the Basin Ranges), USA, as a function of faulting, surface processes and microclimate. Many of the classic morphologies of mountains in the Basin and Range were described in the late part of the 19th century. The varied topography coupled with differing experiences led to a similarly diverse set of explanations. We are able to demonstrate through a variety of numerical experiments that a diverse landscape is easily obtained by the simple, steady combination of tectonic and surface processes. Numerical landscapes reveal the same features observed in the field, including facets, spur benches, piedmonts, and relatively linear and regularly spaced drainages. In all cases, a steady‐state landscape is generated on the order of 10⁶ years, so that there remains little information in the landscape that can tell us about processes or conditions prior to 1 million years ago. The fundamental form of the steady‐state landscape (including the facet and spur bench morphology) is governed by the spacing of rivers and bedrock strength and the resulting relief is for the most part strength‐limited. Neither hillslope nor facet relief is dependent on the rate of fault slip or channel incision. Relief is incision‐limited, however, nearer the headwaters of rivers where stream power is relatively low. Topographically asymmetric ranges may be generated over tectonically symmetric horsts by allowing precipitation to be driven by orographic processes, but the asymmetry is likely to be dependent on the ability to remove eroded material from the adjacent basins. The value of the experiments presented here is to demonstrate that the astute and impressive observations made by the likes of Gilbert, Davis and Dutton are reproducible using a relatively simple description of the relevant physics and that we can recognize and explain various landscape morphologies that have in the past been the subject of necessarily qualtitative reasoning.

A model for the change in shape of interfluves by concurrent tectonics and denudation was developed based on the morphometric attributes of landforms. The tributaries flowing down valley side slopes and dissecting low‐relief surfaces on the interfluves are one of the most important elements for relief‐forming processes. They were named as β‐tributaries. The valleyhead altitude (H ), the junction altitude (L) and the valley length (l ) of the β‐tributary were measured. The altitudinal difference (h=H−L), which indicates the local relief, and the average slope of tributary (tan θ=h/L) were calculated. The regression analyses among H, L, l, h and tan θ indicate that the valley length, relief and slope increase with an increase in valleyhead altitude.

Based on the functional relations above, the shape of interfluve is a function of uplift, altitude and erosion. This model is used to illustrate the change in cross‐section of interfluves during a period of sustained rock uplift.

Successive changes in shape of interfluve can be divided into two substages: (1) the early substage, characterized by trapezoidal cross‐sections with the original low‐relief surfaces and residual, shallow stream networks on the ridges; and (2) the later substage, characterized by a triangular cross‐section, with the original low‐relief surfaces removed and with the interfluves lowered by headward erosion of β‐tributaries. In central Japan, the transitional relief from the trapezoidal to the triangular cross‐section appears when the ridges of interfluves attain elevations about 1600–2000 m above sea level.

Topographic change in regions of active deformation is a function of rates of uplift and denudation. The rate of topographic development and change of an actively uplifting mountain range, the Santa Monica Mountains, southern California, was assessed using landscape attributes of the present topography, uplift rates and denudation rates. Landscape features were characterized through analysis of a digital elevation model (DEM). Uplift rates at time scales ranging from 10⁴ to 10⁶ years were constrained with geological cross‐sections and published estimates. Denudation rate was determined from sediment yield data from debris basins in southern California and from the relief of rivers set into geomorphic surfaces of known age. First‐order morphology of the Santa Monica Mountains is set by large‐scale along‐strike variations in structural geometry. Drainage spacing, drainage geometry and to a lesser extent relief are controlled by bedrock strength. Dissection of the range flanks and position of the principal drainage divide are modulated by structural asymmetry and differences in structural relief across the range. Topographic and catchment‐scale relief are ≈300–900 m. Mean denudation rate derived from the sediment yield data and river incision is 0.5±0.3 mm yr⁻¹. Uplift rate across the south flank of the range is ≈0.5±0.4 mm yr⁻¹ and across the north flank is 0.24±0.12 mm yr⁻¹. At least 1.6–2.7 Myr is required to create either the present topographic or the catchment‐scale relief based on either the mean rates of denudation or uplift. Although the landscape has had sufficient time to achieve a steady‐state form, comparison of the time‐scale of uplift and denudation rate variation with probable landscape response times implies the present topography does not represent the steady‐state form.

Well‐preserved, actively deforming folds in the Tien Shan of Kyrgyzstan provide a natural laboratory for the study of the evolution of thrust‐related folds. The uplifted limbs of these folds comprise weakly indurated Cenozoic strata that mantle well‐lithified Palaeozoic bedrock. Their contact is a regionally extensive unconformity that provides a persistent and readily traceable marker horizon. Based on the deformation of this marker, preserved fold geometries support simple geometric models for along‐strike gradients in fold amplitude and displacement along the underlying faults, linkage among multiple structures, transfer of displacement among folds and evolution of the folds as geomorphic entities. Subsequent to initial uplift and warping of the unconformity surface, steeply dipping reverse faults cut the forelimbs of many of these folds. Wind gaps, water gaps, recent faulting and progressive stripping of the more readily eroded Cenozoic strata indicate the ongoing lateral propagation and vertical growth of fault‐related folds. The defeat of formerly antecedent rivers coincides in several places with marked increases in erosional resistance where their incising channels first encountered Palaeozoic bedrock. Persistent dip angles on the backlimbs of folds indicate strikingly uniform geometries of the underlying faults as they propagate both laterally and vertically through the crust. Deformation switches irregularly forward and backward in both time and space among multiple active faults and folds with no systematic pattern to the migration of deformation. This distributed deformation appears characteristic of the entire Kyrgyz Tien Shan.