Contrasting deformation styles in the Domeyko Fault System, northern Chile.
Subduction of an oceanic plate under the Pacific margin of South America has heen prevalent since
Jurassic times. Magmatic and deformation centres have migrated eastward since suhduction began.
Northern Chile houses two north-south trench linked strike-slip fault systems, the Atacama Fault Zone and
the Domeyko Fault System (DFS). The DFS lies within the Chilean Precordillera from 2 10 to 28°S.
Lateral movement began on the DFS in the Eocene. The DFS can be divided into three segments which
have apparently undergone differing deformation histories. This study has focused on the central segment
of the DFS, to determine fault kinematics and to establish a relative chronology of deformation.
Observations have been made in more detail than previous investigations and have heen used to infer the
deformation history . Shallow level faulting has resulted in heavily fractured zones with occasional
slickenline surfaces. It is difficult to infer kinematics of faulting from these. Much effort has been
expended in developing techniques to analyse fracture patterns associated with brittle faulting under
conditions of plane strain, simple shear.
A novel approach of analysing the shapes of clasts of rock defined by secondary fractures within a fault
zone has been used. The clasts approximate ellipses when viewed in 2 dimensions. Combining ellipse
orientation and aspect ratio from mutually perpendicular sections through the fault zone allowed
calculation of an ellipsoid representative of the clasts of rock in 3 dimensions. Independent determination
of the fault kinematics using stratigraphic relationships across the fault, fracture distribution, incremental
strain axes and palaeomagnetic analysis has all owed evaluation of the new technique. The shapes of rock
clasts are found to be related to the kinematics of the fault system. Up to a critical stage of development of
the fault zone the axes of the rock clasts parallel the slip direction, intermediate strain axis and pole to the
boundary faults. Which rock clast axis parallels which structural feature depends upon the spacing and
curvature of fractures and stage of development of the fault zone. Analysis of the shapcs or rock clasts
defined by fractures can avoid bias of the data set towards thicker fractures or against irregular fractures,
which can occur when measuring fracture orientations directly. The degree of development of the fault
zone varies laterally along the fault over short distances. This causes the shape.: fabric of the rock clasts to
change, so predictions of connectivity within a fault zone are limited.
The history of the central segment of the DFS determined from this study is found to occur with earlier
workers. The complementary deformation histories produced from two scales of ohservation verifies the
reliability of the chronology. Lateral movements along the DFS are thought to begin in the Eocene with a
sinistral transpressive event which occurred along all three segments of the DFS. En echelon folds, east
and west verging thrusts and clockwise rotations associated with sinistral faulting along the master fault of
the segment are documented. Later, in the Oligocene, dextral faulting occurred. large clockwise
palaeomagnetic rotations, determined from Palaeozoic samples beside the master fault, indicate sinistral
displacements have been larger than dextral disp acements. It is inferred that only one episode of large
lateral transport occurred. This is the Eocene sinistral event. Normal faulting associated with sinistral
displ acements along the western side of the system are documented. This later sinistral faulting has not
been documented before in the central segment of the DFS. After Oligocene age dextral faulting, the three
segments of the DFS underwent separate deformation histories, as the main Andean deformation foci had