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Active Tectonics: Studies in Geophysics 7 Tectonic Geomorphology of Escarpments and Mountain Fronts LARRY MAYER Miami University ABSTRACT The morphology of fault-generated mountain fronts and escarpments can indicate relative tectonic activity. Fault slip rates can be estimated using erosion rates or dated landform elements. The use of landforms to solve for the timing and magnitude of tectonic perturbations is analogous to an inverse problem where given a landform morphology one must solve for the variables that caused or affected it. Models that predict the rate of landform evolution can be used to solve for the ages of uplifts when properly calibrated with independent age determinations. Basalt flows or air fall tuffs are useful for establishing a denudation chronology and permit erosion rates to be calculated. Future research should develop additional relations among tectonics and landforms rendering the solution to this inverse problem as unique as possible. INTRODUCTION Tectonic geomorphology is the study of landforms that result from tectonism and the interaction between tectonic and geomorphic processes. Tectonic geomorphologic studies constrain solutions to an inverse problem: given selected morphologic attributes of tectonic landforms, can one determine the corresponding tectonic history; or similarly, given information about the tectonic history, can one determine how landforms will evolve? Studies of tectonic geomorphology can discern the nature, timing, and distribution of faulting and seismicity that have occurred over tens of thousands of years, and in the case of mountain fronts and escarpments, an order of magnitude longer. Instrumental recordings of seismic data are available for a very short time; historical records of earthquakes are also useful. Studies of paleoseismicity, which is past seismicity based on interpretation of small-scale geomorphic or stratigraphic features, enable scientists to determine the past behavior of seismogenic structures and, hence, better evaluate the risk to society resulting from future earthquakes. The length of time a landform records depends on its survival time, or how rapidly the landform evolves. Two types of landform that have long survival times are escarpments and mountain fronts. Other tectonic landforms may have considerably shorter survival times. The relation among the different types of tectonic records is illustrated in Figure 7.1. The precision of the record is inversely proportional to the length of record so that as the time recorded increases we cannot be certain about short-term variations in uplift. In addition, each type of record measures the uplift history of a landform differently. This difference is sometimes expressed by different average uplift rates, which are calculated by
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Active Tectonics: Studies in Geophysics FIGURE 7.1 Graphs showing hypothetical uplift history by plotting cumulative uplift versus the length of record. Mountain fronts record long periods of time, whereas fault scarps and historical earthquake data record progressively shorter time periods. The circle-in-square on the mountain fronts graph outlines the time period shown on the fault scarp graph. Likewise, the circle on the fault scarps graph indicates the time period covered by the historical earthquakes graph. dividing the total (cumulative) uplift by the period of record. The variation of uplift rate estimates reflects short-term tectonic fluctuations that may be superimposed on long-term trends. Thus, long-term averages of past uplift rates serve to constrain forecasts of uplift. How mountain fronts and escarpments evolve and several methods for their use in tectonic geomorphology are discussed below. The most popular early attempt to relate landform morphology with tectonics was William Morris Davis’ cycle of erosion. Davis (1899) envisioned a closed geomorphic system where, following a pulse of rapid uplift, landforms evolved through a sequence of characteristic landforms. Each landform assemblage in the Davisian sequence of “youth,” “maturity,” and “old age” stages differed morphologically from the others. Davis’ cycle of erosion was highly intuitive and logical but suffered under the blow of equifinality. In other words it was possible to produce the landform assemblages in each stage by variables other than sequential denudation following rapid uplift. For example, rock types and geologic structure differ from area to area, and the effectiveness of erosional geomorphic processes also varies; yet these differences were not accounted for in the cycle of erosion. Despite the shortcomings of the Davisian scheme, the basic concept of landform adjustment following tectonism remains intact. The controversy surrounding the Davisian cycle of erosion may have provided the impetus for quantitative field investigations, the key to unraveling landform history. Recent studies (Bull and McFadden, 1977; Wallace, 1977, 1978; Bucknam and Anderson, 1979) have examined the interaction of tectonics and landforms using an empirical approach. In these studies dating or determining rates of change of a landform or landform assemblage is an ultimate goal on which tectonic interpretations may be based. Data that allow the rates of landform change to be estimated can be used to determine how landforms may respond to tectonic perturbations. Hillslopes can result from tectonism such as faulting, and, therefore, hillslope evolution and slope processes are fundamental to understanding how erosion and tectonics interact to produce any given landform. SLOPES Slopes are basic landform elements that are naturally combined to form landform assemblages. Hillslopes are that portion of the sloping landscape within a drainage basin that contribute sediment (by several transport processes) and runoff to streams. Slope systems denote landform assemblages that operate or evolve as an integrated package of surficial processes and are commonly delineated by their topographic expression. Equilibrium in slope systems can refer either to slope
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Active Tectonics: Studies in Geophysics forms that tend to persist over time or to conditions where the rate at which regolith or soil develops is balanced by the rate of soil removal. In the latter context, equilibrium slopes are suggested by field observations that show soil (sensu lato) thickness to be relatively constant over a slope. Constant soil thickness requires a balance between production of soil by weathering and its removal by erosion. Departures from this balance are either tipped toward erosion or weathering, and the resulting slopes are termed, respectively, weathering-limited and transport-limited slopes (Young, 1972). Weathering-limited slopes denote a condition where the rate of soil removal exceeds that of soil production. Bare rock slopes are an example of a weathering-limited slope. Transport-limited slopes refer to a condition where the rate of soil production does not limit the rate of soil removal. A valley-side slope cut by drainages on the erodible materials of “badlands” is an example of a transport-limited slope. As a slope erodes, its forms may change. Four types of slope evolution that describe some common patterns of slope change are decline, replacement, retreat, and rounding. Each type of slope evolution is the geometric result of the distribution of net erosion along a slope. During the history of some slopes, more than one type of slope evolution can take place. The relation between slope evolution and erosion can be described using a mass-flux diagram (Figure 7.2). Mass flux is a rate of material movement per unit of slope area and is analogous to the concept of erodibility. A mass-flux diagram illustrates the position on a slope where either erosion or deposition is most efficient. Slope decline is important in areas with stable base-level and mature topography. Though characterized by erosion throughout the length of the slope, decline results FIGURE 7.2 Mass-flux diagram illustrating the differences in slope models in terms of how erosion or deposition is distributed along the profile. The solid lines mark the initial condition, and the dotted lines indicate profile change after some time. On the lower portion of the figure, white areas of the profile indicate erosion while the dark areas (for rounding and replacement) indicate deposition. from more effective erosion near the upper portions adjacent to the drainage divides. The situation of parallel retreat occurs when erosion is constant along a slope and therefore vertical lowering of the slope occurs everywhere at the same rate. Parallel retreat is common, but not restricted to, areas where the upper surface is protected from erosion by a resistant cap rock. Slope replacement describes the replacement of the original slope by one controlled by deposition. Material eroded from the escarpment is deposited at the base of the slope and accumulates at a slope less steep than the original escarpment. With time, the depositional slope replaces the original slope. Slope rounding describes a symmetric erosion-deposition feature of some slopes that decreases the curvature all along a slope. Fault scarps in alluvium show this form of evolution. Other models of slope evolution can be envisioned by relating the amount of material eroded on a slope to some morphologic factor such as slope gradient or slope curvature. Slope rounding and decline are similar except for the deposition that is found in slope-rounding evolution. In addition, as a slope evolves, the dominant pattern of evolution may change. For example, a slope may initially change by slope replacement and later by some other evolutionary type. In the case of fault scarps formed in alluvium, early slope development is characterized by slope replacement and later by slope rounding (see Nash, Chapter 12, this volume). FAULT-GENERATED MOUNTAIN FRONTS The Basin and Range physiographic province of the United States is characterized by fault-bounded mountain blocks. These mountains, formed where faults displaced the topographic surface, are termed fault-generated mountains (Figure 7.3). The morphology of mountain fronts is strongly affected by the width of the range. The range width determines the maximum possible drainage basin size that can develop. As virtually all of the area within the mountain block is sloping (i.e., draining), drainage basins fill all available space. For a given mountain range, larger watersheds that tend to have a characteristic shape also have a regular spacing along the mountain front. The spacing depends on basin shape. Regular spacing of basins measured as a ratio between the distances of the basin mouths along the range front to length of the basin was noted by Wallace (1978). Drainage basin characteristics within the mountain range, primarily shape and size, affect the morphology of the mountain front and how that front may evolve. Large circular watersheds do not effectively fill space at
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Active Tectonics: Studies in Geophysics FIGURE 7.3 Idealized fault-bounded mountain front and related terminology. the mountain front and therefore, depending on other factors, may allow for extensive triangular facets (Figure 7.4). In some instances, triangular facets represent the eroded plane of faulting at the mountain front, and the existence of facets indicates that sufficient time to obliterate them has not elapsed. The overall morphology of typical Basin and Range mountain blocks reflects their integrated erosional-tectonic history. Because a lag time exists between the time of faulting at the structural front and transmission of this base-level perturbation upstream, the tectonic history may be reflected differently in different parts of the drainage basin. For this reason, morphologic studies have emphasized the geomorphic features near the mountain front. Several morphologic features of mountain blocks may reflect recent tectonism. Bull and McFadden (1977) suggested that the sinuosity of a mountain front is related to the time elapsed since active high-angle faulting ceased (see Keller, Chapter 8, this volume). Linear or curvilinear mountain fronts have low sinuosity and may represent recent fault movement, while mountain fronts FIGURE 7.4 Relation between basin shape and morphology of the mountain front. deeply embayed by pediments have high sinuosity and may represent tectonic quiescence or rock control. A maximum limit on sinuosity is achieved where the spur ridges separating adjacent drainage basins are not eroded away. This relation is shown schematically in Figure 7.5. Using simple geometric shapes for hypothetical pediment embayments one can show that the maximum possible sinuosity is dependent on drainage-basin spacing. Given a drainage-basin spacing and given the constraint that the spur ridges do not erode back, sinuosity increases as erosion progresses, and, therefore, sinuosity increases with time. Unfortunately, the constraint that spur ridges do not erode is not valid. Spur ridges do erode but at rates that are much slower than the stream erodes it valley. With decreasing stream length and, hence, decreasing mountain width, the rate of stream valley erosion may approach that of the spur ridges. Thus for narrow ranges, sinuosity may be low and practically independent of time since faulting. In general, sinuosity is dependent on basin spacing, time, and range width. Stream valley erosion, and particularly the morphology of a stream valley in cross section, has also been proposed as a tool in interpreting tectonic activity. Bull and McFadden (1977) suggested that the width of the valley floor near the mountain front is a useful index that measures effectiveness of stream downcutting (see Keller, Chapter 8, this volume). Bull and McFadden suggested that a simple ratio of valley floor width to average height of the adjacent divides be used as this index and demonstrated statistically significant variation in the ratio for different tectonic settings. Stream-valley morphology affords a fruitful ap- FIGURE 7.5 Hypothetical mountain front used to derive a relation among sinuosity, range width, and basin spacing. In this derivation it is assumed that the spur ridges do not erode back. Different geometric shapes for the embayment can be used but show a similar linear relationship between sinuosity and basin spacing.
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Active Tectonics: Studies in Geophysics FIGURE 7.6 Conceptual representation of valley erosion. The relative rates of two processes, stream downcutting and slope recession, determine the morphology of the resultant valley. proach to tectonic history based on several geomorphic postulates proposed in the context of Basin and Range physiography. First, streams downcut in response to vertical uplift, and downcutting becomes less important as the stream gradient readjusts to the new lowered base level. Second, when the mode of erosion of the valley-side slopes is dominated by slope retreat, then morphology of the stream valley is related to both the rate of stream downcutting and the rate of slope retreat. This concept is illustrated on Figure 7.6 and simply states that the volume of material removed when a stream valley is formed is related to both stream and hillslope erosional processes and that the resultant valley depends on the relative rates of the two processes. Morphologic descriptions of stream valleys, within the context of the specific postulates noted above, can then be used to make some meaningful interpretation. A FIGURE 7.7 A numerical index of valley morphology can be calculated by comparing the area of the actual valley cross section with the area of a semicircle with radius equal to the height of the adjacent drainage divide. simple scheme for describing stream-valley morphology is shown on Figure 7.7. The V ratio incorporates information about shape of the valley that can, in turn, be related to vertical uplift. V ratios near 1 indicate a U-shaped valley. Ratios greater than 1 indicate a valley much wider than deep, whereas very small ratios represent active downcutting that is common for streams flowing in V-shaped valleys. V ratios, shown in Figure 7.8, are not unique. The same V ratio may describe valleys with different morphology; however, these ratios are generally comparable when the valley depths are about equal between groups or are normalized for valley depth. Larger V ratios may indicate relatively less active vertical tectonism. FAULT-GENERATED ESCARPMENTS Fault-generated escarpments are characterized by a cliff or free face and an associated talus slope. Rock materials fall off the free face and accumulate on the talus slope. Free-face slope angles are commonly greater than 40°, whereas the angles on the talus slopes are largely controlled by the angle of repose of the materials comprising the talus. Some escarpments show a slope-replacement mode of evolution (Figures 7.9 and 7.10), while others show slope retreat. The location of drainage divides with respect to the cliff face in large part determines the morphologic evolution or escarpments. The spatial relationships be- FIGURE 7.8 Comparison between generalized valley morphologies and their associated V ratios. The V ratio is defined on Figure 7.7. Larger ratios indicate that slope recession is more important than stream downcutting and, depending on the actual rates, may indicate tectonic quiescence.
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Active Tectonics: Studies in Geophysics FIGURE 7.9 The 1887 rupture along the Pitaycachi Fault scarp in Sonora, Mexico, that shows the early stages of profile evolution to be characterized by slope replacement. tween drainage networks, discharge, and the escarpment physically limit such morphologic characteristics as linearity, hypsometry, and dissection. Where an escarpment is also a drainage divide (Figure 7.11) there is a tendency toward maintaining linearity and also for a slope-retreat mode of escarpment evolution. The Mogollon Rim in central Arizona is an example of a high escarpment that is also a drainage divide (Figure 7.11), separating flow onto the Colorado Plateau from flow into the Basin and Range province. The Hurricane Cliffs are another example where linearity is maintained to some extent by the coincidence of drainage divide and escarpment. The Grand Wash Cliffs, in contrast, are deeply embayed (Figure 7.12). This is due in part to their antiquity, but also because the main drainage divide did not coincide with the escarpment. Significant drainages cut the Grand Wash Cliffs, the largest of which is the Colorado River. Streams flowing across fault-generated escarpments erode valley embayments. Many of the embayments in their most simplified planimetric form resemble triangular cuts into the uplifted escarpment. We can consider the embayment length (the distance upstream from escarpment front to the head of the embayment) to be the height of the triangle and the width of the embayment at the escarpment front to be the base of the triangle. The geometry of these embayments is dependent on the size of the stream in the valley. The relationship between stream length and embayment length is linear. For embayments eroded into cliffs around the Grand Canyon, embayment length is proportional to the length of the stream. These relations imply that the regression line relating stream length and embayment length for an escarpment has temporal significance and can be used for relative dating of escarpments. Older escarpments should plot above younger escarpments, all else being comparable. Another factor influencing the geometry of escarpment embayments is the rate of slope retreat. The more rapid the slope retreat relative to headward advance of the embayment, the wider the embayment mouth. Because the embayment’s length and width dimensions grow at different rates, the resulting geometry can be referred to as anisometric and, where well described by
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Active Tectonics: Studies in Geophysics FIGURE 7.10 The Vermillion Cliffs in northern Arizona is at this locale characterized by slope replacement. The talus slope consists of debris that forms at the expense of the cliff face. FIGURE 7.11 Profiles of two escarpments. The one on the left has drainage over the escarpment face, a situation that promotes dissection of the escarpment. The profile on the right shows that the escarpment and the drainage divide are one and the same, a situation that promotes linearity regardless of age. a power function, as allometric. The allometric characteristics of embayment growth may permit morphology-derived dating of certain fault-generated escarpments. In any event, the width of the embayment mouth of larger streams, it seems reasonable to assume, is a function of time. RATES OF GEOMORPHIC PROCESSES Rates of stream downcutting, escarpment retreat, pedimentation, and other geomorphic processes permit estimates of the rates of tectonic processes such as vertical uplift. Pediments, for example, are bedrock surfaces formed by erosion of a mountain block that gently slope toward the basin and commonly are covered by some
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Active Tectonics: Studies in Geophysics FIGURE 7.12 Computer-enhanced topographic image of the Shivwitz Plateau in northwestern Arizona. North is at top of photo. The north-south trending upper Grand Wash Cliffs shows deep embayment by streams traversing. The canyons in the south and eastern portions of the photo are tributaries to the Colorado River in the Grand Canyon.
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Active Tectonics: Studies in Geophysics thickness of gravel (Figure 7.13). They form only under conditions where the rate of piedmont erosion is greater than the rate of uplift across a range-bounding fault. Where rock control is not a factor, pediments represent a period of tectonic inactivity and thus are binary in nature. Pediment widths are dependent on the time since active faulting along the range front and also on drainage basin slope and length. Estimates of pedimentation rates are generally in the range of 300–1000 m per million years (m.y.) (Young and Brennan, 1974; Wallace 1978). A pediment 2 km wide may therefore represent a period of tectonic quiescence that lasted greater than 2 m.y. Pediments that form in conjunction with cliff retreat from fault-generated escarpments can be used in an analogous fashion. Basalt flows and volcanic ash deposits provide an unique opportunity to determine the ages of tectonic events and the rates of geomorphic processes. They can preserve a datum that records previous river levels or a prefaulting topography. For example, if a basalt flowed down a river channel and subsequent tectonically induced entrenchment results in topographic inversion with the old channel preserved by the basalt, then the amount and geometry of downcutting can be determined (Figure 7.14). The Grand Wash in northwestern Arizona is an example of topographic inversion (Figure 7.15). Basalts flowed down the Grand Wash valley about 7 m.y. ago (Hamblin et al., 1981). Base-level fall, perhaps related to the downcutting by the Colorado River, resulted in entrenchment. When downcutting by large streams is caused by tectonic uplift, the total amount of downcutting approaches the total amount of uplift. Because a former river level is both preserved and dated by basalt flows, average downcutting rates can be calculated, which turn out to be 26 m/m.y. for the Grand Wash (Hamblin et al., 1981). Similar calculations indicate that the Hurricane fault, near the town of Hurricane, Utah, has a minimum vertical slip rate of 300 m/m.y. for the last 0.3 m.y. The amount of downcutting, however, decreases upstream from the base-level fall. Rice (1980), using four dated basalt flows, estimated a 95 m/m.y. average downcutting rate of the Little Colorado River over the past 2.5 m.y. Alternatively, if an area containing basalt flows has FIGURE 7.13 Pediment surface in the Mohave Desert of California exposing bedrock. The pediment-mountain front junction is abrupt and characterized by a sharp break in slope.
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Active Tectonics: Studies in Geophysics FIGURE 7.14 Profile of stream channel showing the relation between the present channel and portions of an older channel preserved by basalt flows. Downcutting induced by faulting has left the older channel remnants much higher than the present channel. been tectonically warped or tilted, the basalts may record irregularities, anomalous gradients, or gradient reversals. Classic examples of basalt-filled channels being used as long-term tiltmeters can be found in studies of late Cenozoic tectonism in the Sierra Nevada Mountains of California (Bateman and Wahrhaftig, 1966). Use of volcanic ashes to date erosional or depositional surfaces are also abundant (Young and Brennan, 1974). Field relations using dated basalts have supported tectonic geomorphologic inference regarding classical geomorphic problems including the development of the western margin of the Colorado Plateau and the evolution of the Grand Canyon (McKee and McKee, 1972; Hamblin et al., 1981). DISCUSSION The tectonic geomorphology of fault-generated topographic fronts can be used to describe long-term tectonic FIGURE 7.15 Aerial oblique view looking north at Grand Wash. The Virgin Mountains are located at the northernmost part of the photograph. The flat-topped mesas are capped by basalt flows that record a time when the base level in Grand Wash was much higher. The basalt flows provide a chronostratigraphic datum that can be used to calculate average rates of stream downcutting and make inferences about tectonic uplift.
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Active Tectonics: Studies in Geophysics history. Forecasting tectonic activity requires a long record, and therefore the tectonic geomorphology of landforms with long survival times will be an important ingredient in estimating tectonic hazards. Rates of geomorphic processes such as pedimentation or stream downcutting can act as a clock that starts ticking following the formation of a tectonic landform. Present limited knowledge of these rates suggests that during periods of active faulting in the Basin and Range province, fault-slip rates of about 0.1–1 m per 10,000 yr or greater are needed to generate high topographic escarpments, and these periods of active faulting may last on the order of a million years. The Hurricane escarpment, in southern Utah, for example, was formed by an average fault-slip rate of 3 m per 10,000 yr. Landforms, such as scarps, produced by slip rates less than 1 m per 100,000 yr may be sufficiently obliterated by erosion to preclude the accumulation of relief across a mountain front. Relations between the recurrence interval of ground-rupturing earthquakes and mountain front development need closer study. Many of the methods described above can be easily used and rapidly applied, and, therefore, regional studies are both possible and desirable. The need for more data on geomorphic rates is a limiting factor on the interpretation of such analyses, and therefore Quaternary dating is a corequisite for continued research in the field of tectonic geomorphology. REFERENCES Bateman, P.C., and C.Wahrhaftig (1966). Geology of the Sierra Nevada, in Geology of Northern California, California Division of Mines and Geology Bull. 190, pp. 107–172. Bucknam, R.C., and R. E.Anderson (1979). Estimation of fault scarp ages from a scarp-height—slope angle relationship, Geology 7, 11–14. Bull, W.B., and L.D.McFadden (1977). Tectonic geomorphology north and south of the Garlock Fault, California, in Geomorphology in Arid Regions: Annual Binghamton Conference, D.O. Doehring, ed., State University of New York at Binghamton, pp. 115–136. Davis, W.M. (1899). The geographical cycle, J. Geogr. 14, 481. Hamblin, W.K., P.E.Damon, and W.B.Bull (1981). Estimates of vertical crustal strain rates along the western margins of the Colorado Plateau, Geology 9, 293–298. McKee, E.D., and E.H.McKee (1972). Pliocene uplift of the Grand Canyon region—a time of drainage adjustment, Geol. Soc. Am. Bull. 83, 1923–1932. Rice, R.J. (1980). Rates of erosion in the Little Colorado valley, Arizona, in Timescales in Geomorphology, R.A.Cullingford et al., eds., John Wiley, New York, pp. 317–331. Wallace, R.E. (1977). Profiles and ages of young fault scarps in northcentral Nevada, Geol. Soc. Am. Bull. 88, 107–172. Wallace, R.E. (1978). Geometry and rates of change of fault-generated range-fronts, north-central Nevada, J. Res. U.S. Geol. Surv. 6, 637–650. Young, A. (1972). Slopes, Oliver and Boyd, Edinburgh/London, 288 pp. Young, R.A., and W.J.Brennen (1974). Peach Springs Tuff—its bearing on the structural evolution of the Colorado Plateau and development of Cenozoic drainage in Mohave county, Arizona, Geol. Soc. Am. Bull. 85, 83–90.
Representative terms from entire chapter: