COESEISMIC CRACK PRODUCTION AND REACTIVATION IN THE NORTHERN CHILE FOREARC

Coseismic cracks preserved in the hyper-arid Atacama Desert of Northern Chile provide a unique record of the seismic history of the modern Andean forearc, which has generated the largest earthquakes on earth. Loveless et al. (2009) mapped more than 60,000 cracks on satellite imagery and, based on boundary element modeling, suggested that they indicate repeated rupture of approximately the same segment. But, what is the time frame of the repeated rupture and what is the long-term strain rate of the forearc due to this process? West of Salar Grande, four overlapping fan surfaces contain different densities of coseismic cracks, allowing for such an assessment. The fan surfaces are highly indurated with gypsum and salt, resulting in excellent preservation of these brittle features. All surfaces are underlain by a tuff ~4 m.y. old. Older surfaces consistently have more and larger cracks than younger surfaces. Locally, cracks cutting across multiple surfaces have distinctly different morphologies and widths on each. Because cracks degrade over time, true width was approximated as the width at half of the vertical relief on each side of the crack. Density is the number of cracks present per given length and total opening is the sum of the widths along that same length. On these surfaces, widths reach >2 m and densities range from <1 to 13 cracks per 100 m. Both crack density and total opening on each surface show a positive relationship to relative age, supporting the previously anecdotal evidence for reactivation of cracks through time with multiple events. Fresh, centimeter-scale cracks are also present on all surfaces, further supporting that these features record a history rather than a singular event. Preliminary cosmogenic dating results show a complex history for the surfaces involving burial and exhumation of surface clasts, but that the ages will help to define the timing of the surfaces and thus rate of crack production and related strain rate in the forearc.