QUATERNARY NEOTECTONIC LINEAMENTS IN THE HYPERARID ATACAMA DESERT, NORTHERN CHILE: LONG-TERM RECORDS OF FOREARC STRAIN

Located in the upper plate of the modern Peru-Chile Trench subduction zone, the hyperarid Atacama Desert is an ideal place to study forearc deformation through surface geomorphology. We studied neotectonic lineaments between ~25.5° and 26° S in the Coastal Cordillera preserved in the region’s alluvium with the goal of understanding modern forearc strain. Visible in satellite imagery, these lineaments are defined by linear to curvilinear structures of subparallel ridges that range from tens of meters to kilometers in length. Field observations and 10-cm digital surface models derived from drone imagery record a consistent, asymmetrical “ridge-trough-ridge” morphology that commonly traces into bedrock fissures (some along known bedrock faults) ~1-2 m wide often containing gypsum and calcite. Most lineaments trend ~N-S, parallel to the area’s dominant Mesozoic structural grain. Faults found in the lineament ridges have cm- to mm-scale normal-sense displacement and consistently dip moderately to steeply inwards towards the troughs, defining a graben-like structure. In outcrop these normal fault zones are enhanced by differential erosion, and in thin section faulted material is distinguishable by increased cementation between grains and within fractures penetrating grain boundaries. A tuff deposit within an alluvial fan containing several lineaments yields a zircon U-Pb age of 2.2 ± 0.1 Ma, indicating that lineaments in this fan are Quaternary in age and related to modern subduction along the Peru-Chile Trench. Evidence for thrust faulting and folding along the flanks of a few lineaments suggests the ridges may have formed in response to contractional deformation. We propose the lineaments record alternating forearc shortening and extension that may be related to interseismic and coseismic phases of the earthquake cycle, with the development of ridges and thrust faults recording interseismic shortening and normal faults and fissures which form central troughs recording coseismic extension. We expect to observe systematic changes in lineament morphology across fan surfaces of different ages if they are indeed responding to transient changes in the upper crustal stress field.