Decoding temporal and spatial patterns of fault uplift using transient river long profiles

Abstract

We present detailed observations of rivers crossing active normal faults in the Central Apennines, Italy, where excellent constraints exist on the temporal and spatial history of fault movement. We demonstrate that rivers with drainage areas N 10 km 2 and crossing faults that have undergone an increase in throw rate within the last 1 My, have significant long-profile convexities. In contrast, channels that cross faults that have had a constant-slip rate for 3 My have concave-up profiles and have similar concavities and steepness indices to rivers that do not cross any active fault structures. This trend is consistent across the Central Apennines and cannot be explained by appeal to lithology or regional base level change. The data challenge the belief that active faulting must always be reflected in river profiles; instead, the long-profile convexities are best explained as a transient response of the river system to a change in tectonic uplift rate. Moreover, for these rivers we demonstrate that the height of the profile convexity, as measured from the fault, scales with the magnitude of the uplift rate increase on the fault; and we establish that this relationship holds for throw rate variation along strike for the same fault segment, as well as between faults. These findings are shown to be consistent with predictions of channel response to changing uplift rate rates using a detachment-limited fluvial erosion model, and they illustrate that analysis of the magnitude of profile convexities has considerable predictive potential for extracting tectonic information. We also demonstrate that the migration rate of the profile convexities varies from 1.5-10 mm/y, and is a function of the slip rate increase as well as the drainage area. This is consistent with n N 1 for the slope exponent in a classical detachment-limited stream-power erosion law, but could potentially be explained by incorporating an erosion threshold or an explicit role for sediment in enhancing erosion rates. Finally, we show that for rivers in extensional settings, where the response times to tectonic perturbation are long (in this case N 1 My), attempts to extract tectonic uplift rates from normalised steepness indices are likely to be flawed because topographic steady state has not yet been achieved.

Numerical modelling of landscape and sediment flux response to precipitation rate change

“…The existence of knickpoints within river-long profiles, assumed to be produced by a system perturbation such as a base level, has been used to provide evidence in support of the stream power law in upland areas (e.g. Whipple and Tucker, 1999;Snyder et al, 2000;Whittaker et al, 2008). Knickpoints, however, can likewise be a result of changes in lithology (Grimaud et al, 2014;Roy et al, 2015) and are certainly not a unique indicator of erosion dynamics (e.g.…”

Geomorphic analysis of transient landscapes in the Sierra Madre de Chiapas and Maya Mountains (northern Central America): implications for the North American–Caribbean–Cocos plate boundary

“…The normalized steepness index (k sn ) is widely used to investigate tectonically induced perturbations in river longitudinal profiles and has been used to propose patterns of uplift (Kirby and Whipple, 2001;Wobus et al, 2006;Whittaker et al, 2008). The relationships between slope and catchment area which define the equilibrium state channel gradient are given by Eqs.…”

“…Graded profiles show a single k sn value, while non-equilibrated profiles are characterized by changing k sn values associated with several segments. In this latter case we considered the k sn value of the lowermost segment as it is the one along which rivers adjust to new base-level conditions (e.g., Kirby and Whipple, 2001;Wobus et al, 2006;Whittaker et al, 2008). The second parameter is the estimated base-level change.…”

Natural experiments in landscape evolution

“…Thus, the central Italy normal-fault array provides natural experiments in drainage basin evolution under both steady and accelerated fault-block motion, similar to Figure 1. Whittaker et al (2007bWhittaker et al ( , 2008 compiled a database on channel forms in the region, and compared stream profiles developed on steady and accelerated footwall blocks. The steady cases show smooth, concave-upward long profiles with roughly uniform patterns of stream power and bed shear stress.…”

“…Another approach is to focus on landscape evolution on time scales much longer than a glacial-interglacial cycle. For example, the central Apennines rivers analyzed by Whittaker et al (2007aWhittaker et al ( , 2007bWhittaker et al ( , 2008 and modeled by Attal et al (2008) and Cowie et al (2006Cowie et al ( , 2008 involve transient response extending over about 18 marine oxygen isotope stages. A model calibrated to such a case might therefore be said to represent a weighted average of glacial and interglacial conditions, though understanding the nature of such averaging, and the relative importance of cold versus warm phases, deserves further research.…”