Secoud ISAG, Oxford (UK), 21-231911993
The LiquiAe Ofqui Fault Zone : a major Cenozoic strike slip duplex
in the Ssuthern Andes
José CEMBRANO and Francisco HERVE
Departamento de Geologia - Universidad de Chile
Casilla 13518 Correo 21, Santiago-CHILE
La zona de falla Liquifie-Ofqui (ZFLO) se extiende por casi 1000 kms en los Andes del
sur. Rocas deformadas ocurren a Io largo de la ZFLQ, que coincide con el eje Mioceno del
Batolito Norpatagonico y con cuencas extensionales terciarias. Un movimiento dextral en el
rumbo y luego normal, es consistente con la convergencia entre las placas de Nazca y
Sudamérica que ha variado de oblicua a casi ortogonal.
KEY WQRDS: Southern Andes, strike-slip duplex, Cenozoic.
The overall deformation resulting from converging plates is partitioned into strain,
displacement along discrete faults and block rotation (Lamb and Bibby, 1989;
Molnar, 1990). Attempts to assess the contribution of these three components of deformation
along different mountain belts (Dewey and Lamb, 1992) face the problem that short and long-
term slip rates, distributed strain, and the amount and sense of block rotation are generally
poorly constrained. This is the case in the Chilean Andes, where hundred-kilometer long crustal
lineaments, regarded as major long-lived strike-slip fault systems, have been causally linked to
the margin parallel component of oblique subduction of the Nazca (Farallon) plate beneath
South America (Hervé, M., 1976; Beck, 1988; Pardo Casas and Molnar, 1988).
The most relevant tectonic feature of the southern Chilean Andes, the 1000 km long
Liquifie-Ofqui fault zone (LOFZ) , has been lately the subject of geologic, geochronologic and
paleomagnetic research. An updated synthesis of a l 1 available relevant information about the
LOFZ is given below.
The Liquifie-Qfqui fault zone is represented by a series of NNE-trending crustal
lineaments, mainly corresponding to aligned glacial valleys and fjords.
Their spatial arrangement as observed in satellite images, aerial photographs and
geologic maps (Fuenzalida & Etchart, 1975; Hervé et al., 1979; Thiele et al., 1986) allows the
distinction of three categories of lineaments based on length and shape : straight long
segments; en échelon features; and curved splays.
The main trace o f the LOFZ is represented by two NNE-trending straight segments with
a left step in between. These two main segments are linked by a series of NE-trending en
échelon lineaments resulting in an extensional strike-slip duplex geometry (as described in
Second ISAG, O-tford (UK), 21 -231911 993
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Woodcock and Fisher, 1986). The third important element is a series of NW-trending, concave
to the southwest, oceanward splays off the two main segments (Fig. 1).
The LOFZ, along most of its main trace, runs through plutonic rocks of the Meso-
Cenozoic North Patagonian batholith, metamorphic rocks of the Paleozoic Accretionary
Complex and Cenozoic volcanosedimentary units (Hervé, 1976; Parada et
1992). The Miocene belt of the North Patagonian batholith has a close spatial relationship with
the main trace of the LOFZ, as does the holocene southern Andes volcanic Chain. It has been
suggested that the LOFZ is a deep-seated structure cutting through the lithosphere, favouring
magma rising and emplacement. (Parada et al., 1987; Pankhurst et al., 1992).
NS-trending strips of fault rocks, crop out in discrete areas along the LOFZ, between
extensive exposures of non-deformed rocks. A steeply-dipping foliation striking NNW
characterizes the mylonites. Limited available data on mineral stretching lineations and fault
striae are consistent with lateral displacement.
Mid-Tertiary extensional basins (Bartholomew and Tarney, 1986) and holocene
volcanoes are spatially compatible with the extensional duplex geometry described.
Paleomagnetic data on rock units adjacent to the LOFZ (Garcia et al., 1988; Cembrano
et al., 1992; Rojas et al. in press) have shown a pattern of crustal block rotation consistent with
the LOFZ geometry and motion, characterized by small counterclockwise rotations West of the
LOFZ and small to moderate clockwise rotations within and to the east of the LOFZ. Beck et al.
(in press) propose that the observed pattern is the result of small north-south displacement of
blocks along curved splays West of the LOFZ, giving rise to counterclockwise rotation in contrast
with distributed dextral shear producing clockwise rotations within and to the east of the LOFZ.
Large-scale north-south transport -as found in the US western Cordillera- has not been
documented, but available data do not rule out a few hundred km of lateral offset.
Nature and timing o f motion
Hervé (1976) proposed that it was a Cenozoic
dextral and Late Tertiary normal up-on-the-east motions. The main evidence for the early strike-
slip component was a steeply-dipping mylonitic foliation along with a conjugate set of NNE-
trending dextral and ENE-trending sinistral mesoscopic faults found in a 3km wide NNE -
trending belt of fault rocks. The normal, up-on-the-east motion, was interpreted from the fact
that deeper crustal levels crop out east of the LOFZ main trace. At 41's the LOFZ juxtaposes
weakly deformed Miocene and Cretaceous plutonic rocks (Drake et al. 1992). At 42OS, NNW
striking foliations, subhorizontal mineral lineations and structural asymetries found in both Mio-
Pliocene granitoid and their Paleozoic metamorphic wallrocks document a right lateral ductile
shear zone of pre-Pliocene age (Cembrano, 1992).
High uplift rates have been calculated for the LOFZ by Thiele et
Ma and, up to 9 mmlyear, by Hervé and Ota (in press) for the late Holocene, respectively.
al., 1987; Cembrano,
fault zone with both pre-Oligocene
al. (1986) for the last 10
The LOFZ is represented by two major NNE-trending crustal linearnents along which
isolated outcrops of ductilely-brittlely deformed rocks occur. Purely geometric considerations of
spatially related en échelon and curved oceanward features can be assimilated to a strike-slip
extensional duplex and associated splays.
Field evidence from within the LOFZ is consistent with a right lateral shear zone with
sharp variations in the nature of rock deformation along strike.
The fact that both the Miocene plutonic rock belt and Recent volcanoes spatially
coincide with the LOFZ suggests that the structure has exerted control on magma rise and
Paleomagnetic data show a pattern of block rotation consistent with the geometry and
sense of motion assigned to the LOFZ.
Right lateral followed by normal motion on the LOFZ reflects the Nazca-South America
convergence which varied from highly oblique to nearly orthogonal during the Cenozoic.
Projects FONDECYT 1931096 (J.C.) and 92/0914 (F.H.) are financing current research
on the subject.
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