Geomorphic significance of postglacial bedrock scarps on normal-fault footwalls

• Published in: Journal of Geophysical Research: Earth Surface Vol. 116; no. F1
• Main Authors: Tucker, Gregory E., McCoy, Scott W., Whittaker, Alexander C., Roberts, Gerald P., Lancaster, Stephen T., Phillips, Richard
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01.03.2011
• Subjects: Bedrock; climate; erosion; Erosion rates; faceted spurs; frost cracking; Geomorphology; Holocene; Hydrology; Limestone; Mathematical models; normal faults; Plate tectonics; Rocks; Weathering
• ISSN: 0148-0227; 2169-9003; 2156-2202; 2169-9011
• DOI: 10.1029/2010JF001861

The existence of well‐preserved Holocene bedrock fault scarps along active normal faults in the Mediterranean region and elsewhere suggests a dramatic reduction in rates of rock weathering and erosion that correlates with the transition from glacial to interglacial climate. We test and quantify this interpretation using a case study in the Italian Central Apennines. Holocene rates are derived from measurements of weathering‐pit depth along the Magnola scarp, where previous cosmogenic 36Cl analyses constrain exposure history. To estimate the average hillslope erosion rate over ∼105 years, we introduce a simple geometric model of normal‐fault footwall slope evolution. The model predicts that the gradient of a weathering‐limited footwall hillslope is set by fault dip angle and the ratio of slip rate to erosion rate; if either slip or erosion rate is known, the other can be derived. Applying this model to the Magnola fault yields an estimated average weathering rate on the order of 0.2–0.4 mm/yr, more than 10 times higher than either the Holocene scarp weathering rate or modern regional limestone weathering rates. A numerical model of footwall growth and erosion, in which erosion rate tracks the oxygen‐isotope curve, reproduces the main features of hillslope and scarp morphology and suggests that the hillslope erosion rate has varied by about a factor of 30 over the past one to two glacial cycles. We conclude that preservation of carbonate fault scarps reflects strong climatic control on rock breakdown by frost cracking.