The influence of weathering and soils on the geomorphic expression of tectonic landforms: an exception to a rule of tectonic geomorphology

Background – Blind Thrust Faults and Seismic Hazard
In southern California, one of the most tectonically active regions of the United States, it is critical to understand the potential earthquake hazard that faults pose. Along the north flank of the San Bernardino Mountains runs a series of faults collectively known as the North Frontal Thrust System (Figure 1). Many of these faults have not breeched the ground surface, but instead have pushed up anticlines in linear ridges along the mountain front (Figure 2). Such faults are known as 'blind thrusts'. The magnitude 6.7 Northridge earthquake that caused over $20 million in damage to the LA area occurred along just such a blind thrust in the southern San Bernardino Mountains in 1994.

Background – Tectonic Geomorphology and Seismic Hazard Assessment
One way to judge the past tectonic activity of such landforms, and hence the probability for earthquakes, is to use their geomorphic characteristics. Typically, the more topographically pronounced that the landform is (ex: sharp ridges with little evidence of erosion), the less time that there has been for erosion to modify it, and the higher the potential for continued activity on the fault producing it. This assumption is made because it is has been shown that faults that have been the most active in the past will continue to be in the future. It has also been shown that the size of earthquake that a fault can produce is well-correlated with the overall plan-view length of the fault. Therefore in assessing potential earthquake risk of faults, it is critical to document a fault's overall past activity as well as its geographic extent.

Case Study – San Bernardino Mountains – An exception to the rule
The blind thrust ridges along the north flank of the San Bernardino Mountains vary considerably in their morphology from subdued convexities in the west to sharp linear ridges that significantly modify the alluvial fan topography in the east (Figure 3). Using conventional geomorphic analysis, it would be assumed that the faults that produce these different landforms have varied considerably in their past tectonic activity, and also that the active faults are relatively short. A closer look at the soils and geomorphology of this mountain front, however, tells a different story. Below is a description of data that suggest that the landform contrasts along the north flank can be accounted for by differences in the soils and weathering of the deposits that are being deformed and not necessarily differences in rates or styles of deformation of the underlying faults (See Eppes et al., 2002 for the complete story).

Progressive weathering of sediment through soil development can significantly alter the sediment's physical and hydrological characteristics. The geological composition of the sediment strongly influences how soil development proceeds. Such a contrast is highly evident in the alluvial fans of the North Flank of the San Bernardino Mountains where the drainage basins on the western half of the range are composed primarily of Mesozoic granites and the basins in the east are composed of Paleozoic limestones and marbles (Figure 3). Consequently the composition of the alluvial fans issuing from these basins also varies from east to west.

Examination of 40 soil pits in these alluvial fan deposits indicates that soil development and weathering characteristics of the granite- and limestone-derived deposits diverge with time in important ways:

Soils in Granite Deposits:

• exhibit thick (up to 5 m) clay-rich (argillic) horizons
• granite clasts are commonly weathered into coarse sand/small pebble sized grus sediment
• minimal calcium carbonate accumulation is evident
• the upper 10–20 cm is characterized by grus-rich sand with minimal soil and stone pavement development

Soils in Limestone Deposits:

• exhibit thick (up to 5 m), rock-like petrocalcic (calcium carbonate cemented) K horizons.
• limestone clasts weather primarily by dissolution of calcium carbonate that is then reprecipitated in the K horizon.
• the upper 20-30 cm is younger, reworked alluvium that has been transported across the surface of the K horizon at some time in the past.
• exhibit moderately well developed desert pavements

These different soils strongly influence the geomorphic processes that are acting on them. Abundant grus and clay-rich argillic horizons facilitate erosion of granite fan surfaces, while rock-like petrocalcic soils and desert pavements inhibit erosion of limestone fan surfaces. Abundant active and inactive backfilled small drainages developing on clay horizons in granite soils (Figure 4a) provide evidence that these soils produce significant runoff when exposed. In contrast, channels in the limestone alluvial fans are widely spaced and typically deeply incised into petrocalcic horizons (Figure 4b). These channels do not migrate laterally once incised.

A Conceptual Model
Based on the above observations, a conceptual model can be derived that explains the variability in the geomorphic expression of blind thrust faults along the mountain front without calling on differences in tectonics (Figure 5) as would traditionally be the geomorphic interpretation in such a situation:

• As a blind thrust begins to grow in either granite-or limestone-derived deposits, the surface soils are eroded (Figure 5-2).
• Exposure of the underlying clay rich horizons on granite fans, however, will result in beveling of the growing fold (Figure 5-2a).
• These folds will thus retain a subdued morphology (Figure 5-3).
• In contrast, once a petrocalcic horizon is exposed at the surface, it is extremely resistant to erosion, and fractures create a macro-porosity that reduces runoff on the surface. Consequently, these folds attain a steep surface morphology that is not easily eroded away (Figure 5-3b).
• When bedrock is exhumed at the core of the fold in either setting, surface processes become similar, and fault-cored folds assume similar morphologies (Figure Figure 5-4).

Thus, the topographic expression of blind thrusts in the San Bernardino Mountains is possibly not associated with the relative activity of the underlying faults, but instead with the geomorphic properties of the soils developing in the deposits that are being deformed. This conclusion has important implications for seismic hazard and tectonic geomorphology studies of landforms associated with active faults. It is critical to understand the weathering and erosion characteristics of such landforms before drawing conclusions about their tectonic activity.

• Eppes, M.C., McFadden, L., 2008. Drainage Basin Response to Holocene Climate Changes: Importance of Rock Type and Soil Development. The Holocene, v. 18, p. 895-905
• Eppes, M.C., McFadden, L., Matti, J., Powell, R. 2002, Influence of soil development on the geomorphic evolution of landscapes: an example from the Transverse Ranges of California. Geology, v.30,n.3 p. 195-198.
• McAuliffe, J., Hamerlynck, E., Eppes., M. 2007. Landscape Dynamics Fostering the Development and Persistence of Long-Lived Cresotebush (Larrea tridentata) Clones in the Mojave Desert. Journal of Arid Environments (0.878), 69. Pg. 96-126.
• Pearce, S., Pazzaglia, F., Eppes, M.C., 2005. Ephemeral channel response to growing folds, Geological Society of America Bulletin, V. 116, p. 1223-1239.