Geological and Geophysical Studies of the Atacama Fault Zone in Northern Chile

Abstract

The Atacama fault, a suggested regional strike-slip fracture parallel to the coastline of northern Chile, is a major structural break that has undergone a complicated history of movement since Mesozoic time. Abundant Quaternary scarps attest to continuing activity; however, the Atacama fault does not appear to be an active strike-slip fault -- at least not along its central segment between 24°5. and 26°S. In the map area, the Coast Ranges are underlain predominantly by crystalline rocks that have been intensely broken by high-angle faults of the Atacama fault system. A eugeosynclinal section, chiefly comprising Jurassic-Cretaceous andesites, overlies Paleozoic metamorphic and plutonic rocks and has been intimately intruded by batholithic rocks of Late Jurassic to Tertiary (?) age. An older more voluminous suite of rocks ranging from gabbro to adamellite, with abundant diorite and pyroxene granodiorite, is discordantly cut by stocks of more homogeneous hornblende-and-biotite granodiorite, adamellite, and tonalite. In the Taltal area, the Atacama fault zone was obliquely cut and left-laterally off set 10 km by the northwest-trending Taltal fault during mid- to late Tertiary time. Lateral motion on the Taltal fault appears to have ceased before the Pliocene epoch. A structural knot presently constrains lateral motion along truncated branches of the Atacama fault. A northwest-curving branch that escapes truncation by the Taltal fault has been more recently active both onshore and offshore, with recent motion predominantly vertical. Strike-slip displacement occurred during the early history of the Atacama fault. Subsequent to its segmentation, individual sectors have been reactivated independently and have accommodated vertical block motions in their recent history. No convincing evidence was found for any recent lateral displacement. Aspects of the origin of alluvial scarps displaying an anomalous ridge-trench-ridge morphology remain enigmatic, but the features are not necessarily indicative of strike-slip movement as had earlier been considered. Offshore studies near the map area indicate major faulting on the upper continental slope, beginning within 10 to 20 km of the coastline, that has resulted in the tilting and down-dropping of blocks toward the trench axis. Late Cenozoic tectonics in the Coast Ranges appear to be directly related to the development of the Peru-Chile trench. A reconnaissance micro-earthquake survey along the Atacama fault shows virtually no activity, or at most very little, in the immediate vicinity of the fault. Abundant micro-after shock activity from an offshore after shock zone related to the magnitude 7.5 Taltal earthquake (Dec. 28, 1966) was detected. Gravity and magnetic profiles across the Atacama fault zone reveal no distinctive anomalies that would allow correlation of major branches over large distances. No large-scale gravity anomalies occur across the fault zone. Schemes of consistent lateral offset along the Atacama fault and its branches have not emerged from systematic regional mapping. Large vertical separation along the faults is a chief control of the regional distribution of rocks. No need is seen to hypothesize strike­-slip displacements of hundreds of kilometers along the Atacama fault zone, but suggestive lines of evidence call for a component of lateral displacement measured at least in tens of kilometers. The vertical component of total displacement probably amounts to several kilometers of uplift of the east side. Taking into account map relations which appear to constrain the amount of allowable lateral displacement, it is judged that total strike-slip displacement may be moderate, amounting only to a few tens of kilometers of right-lateral slip. Photographic materials on pp. 11, 13, 15, 72, 73, 78, 79, 129, 144, 162, 177 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered.