McInnes et al. (1999) applied a new thermochronometer, (U-Th)/He dating of apatite, in the district of Chuquicamata, northern Chile, attempting to place constraints on the sense and magnitude of the vertical component of on the West fault, a major regional fault which offsets the Chuquicamata porphyry copper deposit. They concluded based on the offset of apatite cooling profiles across the fault that the fault had ca. 600 m of up -on-the-west and further concluded the displaced missing piece of the Chuquicamata deposit should be located at an elevation of ca. 3600 m. We are excited about their application of the new low-temperature (~45° to 90°C) thermochronometer to a long-standing problem in Chilean porphyry copper geology, but we disagree with their geologic interpretation of the analytical data. Their interpretations are in conflict with structural data which indicate ca. 35 to 37 km left-lateral and 500 to 600 m down -on-the-west along the West Fault (Maksaev et al., 1994; Tomlinson and Blanco, 1997; Dilles et al., 1997). The source of the discrepancy arises from the misuse by McInnes et al. (1999) of the term displacement and concepts related to its determination, and their implicit assumption of perfectly horizontal regional isotherms. McInnes et al. (1999) claim that the vertical offset of the apatite cooling profiles is a measure of the vertical component of displacement on the fault. The apatite cooling profiles record the position of paleoisotherms which were planar at their time of preservation via uplift and cooling of the rocks. Determination of fault from the offset of a single planar marker or set of parallel planar markers is nonunique (Fig. 1). Consequently, in structural geology the concept of separation, which is the apparent offset of a planar marker as measured in a specified direction, is distinguished …