Western Sequence (Kimmerid-gian-Early Cretaceous)

Quehuita Fm. (Lower Member)(Bajocian?-Oxfordian)

Cerro Empexa Formation(Albian-Cenomanian)

20°55´

20°56´

20°57´

20°54´

69°53´ 69°52´

3896m

Quebrada CopaquiriQuebrada Guatacondo

0 1000m

thrust fault

normal fault

approximate trace of fault

trace of fault withstrike slip movements

anticline

overturned syncline

strike and dip

Pampa Pastillos Gravels(Upper Miocene)

Ujina Ignimbrite(Upper Miocene)

Middle-LateEocene Plutons

Conglomerates and Sand-stones (Eastern Sequence)(Late Cretaceous-Paleocene)

gravel Collahuasi Fm. (Upper Member)(Carboniferous-Early Permian)

Collahuasi Fm. (Middel Member)(Carboniferous-Early Permian)

Internal Structure of the Precordilleran Fault System (Chile):Insights from Structural and Geophysical ObservationsC. Janssen (1), A. Hoffmann-Rothe (1) , H. Brasse (2)

(1) GeoForschungsZentrum Potsdam, Telegrafenberg D 425, D-14473 Potsdam,(2) FU Berlin, Geophysik, Malteserstr. 74-100, D-1249 Berline-mail: jans@gfz-potsdam.de

G E O FO R S C H U N G S Z E N T R U M PO T S D A MS T I F T U N G D E S Ö F F E N T L I C H E N R E C H T S

IntroductionIn zones of oblique convergence, like between thePacific Nazca-plate and South America, the strikeslip component of relative movement is taken upby major continental faults. The kinematics andmechanics of faulting may yield information on therheological structure of the overriding plate (Molnar,1992). The Andes display some excellent exam-ples of strike-slip fault zones in the fore-arc region,e.g. the Precordilleran Fault system (Fig.1), thatallow to address important questions concerningthe geometry, kinematics and strength of thesefault zones. In particular, some uncertainties stillexist regarding the variations of fault zone char-acteristics (thickness and depth of fault core anddamaged zone, deformation mechanisms, fluidflow) both along and across the fault system. Here, we examine the Fracture Process Zone(FPZ) of the West Fissure Shear System by acombination of structural and electromagneticstudies.

20°54’S

20°58’S

Fig.3a: Satellite image of the Guataconda region with traceof the West Fissure. The red rectangular area marks theinvestigated.

Fig.3b: Aerial photograph of the investi-gated area.

Fig.3c: Distribution of fracture patternsderived from Fig.3b; (d) average fracturedensity; (e) Degree of orientation (see text).

Geological SettingThe West Fissure Zone (WFZ) as part of the PrecordilleranFault system is well exposed for about 170 km fromCalama northward to Quebrada Blanca (Dilles et al.,1997). The fault zone has been active since the LateEocene/ Early Oligocene (Reuter et al., 1996). An olderdextral strike-slip motion caused by subduction relatedmagmatic arc tectonics (Incaic tectonic phase) is followedby sinistral shear corresponding to a period of reducedconvergence rate and possibly reduced plate coupling(Reutter et al. 1996). During the youngest event dextralstrike slip was reactivated. The tectonic inversion of thefault system is also reflected in varying amount ofdisplacement (dextral displacement: 0.5 -2 km, sinistraldisplacement: 35-37 km; Reutter et al., 1991; Tomlinsonet al., 1997a,b).Our investigation concentrates on several localities alonga profile crossing the WFZ between 20°54’ – 20°58’ S(Fig.2 and Fig.3a). Deformation structures including folds,foliations, brittle faults and thrusts, trend obliquely to themain fault trace (Carrasco et al., 1999).

Fig.6: Magnetotelluric data (upper two sections) andthe electrical resistivity model (bottom). The sections inthe middle show the model response.

Satellite image and aerial photographs of the WFFZTo analyse the larger scale structural patterns we used satellite images andaerial photgraphs (Fig.3a and b). Fig. 3c shows the distribution of fracturepatterns derived from the aerial photograph of the Guataconda region.TheWFZ does not form a continuous trace. Instead, at the surface the fault ischaracterized by a number of distinct but inter-related fracture lines. We observea rapid decrease in the average fracture density with increasing distance tothe fault plane (fracture densities were measured as the total length of fracturesin km per unit area (Underwood, 1970; Fig.3d). This density decrease issymmetrically on both sides of the fault (Fig.4). The analysis of the fracturelines in terms of strike reveals a fairly complicated fracture network with twomain sets, namely NNW-SSE and NNE-SSW striking fractures (Fig.3c and e;the percentages for the degree of orientation refer to the number of tracesorientated subparallel to the fault). The major fracture directions are more orless parallel or form a low angle to the direction of the WFZ. According to ourfield work we assume that these fracture sets are mainly secondary faultsrelated to the WFZ.

Kinematic analysisKinematic analysis of fault slip data have revealed twofaulting events with constant orientation of principalstrain axes that dominate the fault patterns within theGuataconda area. (first event: NW-SE shortening andNE-SW extension, Fig.5a; second event: reverseorientation, Fig.5b). These orientations can be observedon both sides of the fault. Further away from the mainfault trace (100 m, 300 m, 800 m, up to 2000 m) thecomposite strain axes indicate once again the samedirections. The uniform orientation of principal strainaxes suggests that the whole FPZ is kinematicallycoherent.At distances of about 2500 to 3000 m from the mainfault trace the direction of principal strain axes changes(E-W shortening and N-S extension; Fig.5b).The fracture orientations measured in the selectedoutcrops vary among the different study areas asillustrated by the histograms in Fig. 5d. More than onemaximum of fracture orientation can be found for alllocations. The fractures are mostly oriented in the range0-40° (50°), 70-100° and 160-180°. In contrast to theresults from the aerial photograph analysis the fractureorientation on the outcrop scale is not affeceted by thefault.

ConclusionsGeological field work and magnetotelluric data from one profile across the WFZin northern Chile were used to describe the geometry and structure of the fractureprocess zone surrounding the strike-slip fault. The width of the FPZ based on thefracture density distribution is 4000 m (Fig.7a). The fracture density reaches amaximum in the fault core. The width of the process zone corresponds to the regionwhere the fault is kinematically uniform (Fig.7b).The Audio-Frequency Magnetotelluric (AMT) imaging shows a low electrical resis-tivity zone (apparent resistivity of 2-20 Ωm) congruent with the mapped surfacetrace of the fault. This conductivity enhancement is probably due to meteoric waterentering a zone of ruptured rocks along the fault trace. However, in contrast to thebroad process zone, this zone is very narrow (100 m) and only 100 m deep (Fig.7c).We assume that the discrepancy may be related to cyclical fault evolution. Atpresent, we have no indication for seismic slip along the investigated segment ofthe West Fissure. Fault models have shown that during aseismic periods of faultevolution fault healing (i.e. strength recovery) due to compaction and cementationis active. Hence, fluids and fluid transport, which are probably responsible forenhanced conductivity, are confined to the fault core.

Fig.7: The width of the FPZ estimated by (a) fracture density, (b) kinematicanalysis and (c) AMT.

Fig. 4: Fracture densities along the investigated profile

Audio-Frequency MagnetotelluricOnly in the high frequency range from 500 Hz-1Hz asuperficial conductivity anomaly reaching a depth of100 m at maximum can be observed and modelled inthe position of the fault (Fig.6). The width of this zonereaches approx. 110 m at the surface. A fault relatedlateral conductivity contrast is not obvious at greaterdepths (> 300m).

ReferencesCarrasco P.; Wilke H. and Scheider H. (1999): Post Eocene deformational events in the North segment of

the precordilleran Fault system, Copaquiri (21°S). Fourth ISAG, Goettingen (germany), 04-06/10/99Dilles, J.H.T., A.J.; Martin, M.W. and Blanco, N., (1997): El Abra and Fortuna Complexes: A porphyry copper

batholit sinistrally displaced by the Falla Oeste: Actas, v. III.Molnar, P. 1992): Brace-goetze Strength Profiles, the Partitioning of Strike-slip and thrust Faulting at Zones

of Oblique Convergence, and the Stress-Heat Flow Paradox of the San Andreas Fault, In: Faultmechanics and transport properties of rocks, ed. B. Evans, B. and Wong, T.F., Academic press

Reutter K.J.; Scheuber E. and Helmcke, D., (1991): structural evidence of orogen-parallel strike slipdisplacements in the Precordillera of northern Chile: Geol Rundschau, v. 80, p. 135-153.

Reutter K.J.; Scheuber E. and Chong, G., (1996): The Precordilleran fault system of Chuquicamata, NorthernChile: evidence for reversals along arc-parallel strike-slip faults: Tectonophysics, v. 259, p. 213-228.

Tomlinson, A.J.a.B., N., (1997): Structural evolution and displacement history of the west fault system,Precordillera, Chile: Part 1, synmineral history: Actas, v. III.

Tomlinson, a.a.B., N., (1997): Structural evolution and displacement history of the west fault system,Precordillera, Chile: Part 1, synmineral history: Actas, v. III.

Underwood, E.E., (1970): Quantitative Stereology: Wesly Publishing Company, 274 p.

Fig.2: Geological map of the Guatacondaarea (slightly modified after Carrasco et al.,1999)

Fig.1 : Overview sketch of Northern Chile andprofile location indicated by arrow (slightlymodified after Pelz et al., 1998).

Fig. 5: (a) Geotectonic map of the WFZ with the locations alongthe profile of detailed investigation; (b) Fault plane and fault slipdata of the older deformation event (red and black arrows representthe shortening and extension axes, respectively); (c) Fault planeand fault slip data of the younger deformation event; (d) Fractureorientation (the numbers on the x-axis denote 0-180°).

West Fissure

Coredamaged zone damaged zone

a)

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Iquique

Guatacondo

Tocopilla

Antofagasta

Taltal

Chuquicamata

Salar deAtacama

0°

20°

40°

80° 60° 40°

22°

20°

24°

26°100km

Coastal CordilleraLongitudinal ValleyPrecordilleraPre-Andean DepressionWestern Cordillera