Exhumation and uplift of the western main cordillera between 33

Exhumation and uplift of the western main cordillera between 33

6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 273-276
Exhumation and uplift of the western Main Cordillera between 33° and
34°5
Andrés Fock" Reynaldo Charrier
Pamela Alvarez 6
2,
Marcelo Fadas
3,
Victor Maksaev
4,
Mark Fanning
5,
&
1 Departamento de Geologia, Universidad de Chile, Santiago, Chile ([email protected]); 2 Departamento de
Geologia, Universidad de Chile, Santiago, Chile ([email protected]); 3 Departamento de Geologia,
Universidad de Chile, Santiago, Chile, and LMTG-IRD, Université Paul Sabatier, Toulouse, France
([email protected]); 4 Departamento de Geologia, Universidad de Chile, Santiago, Chile
([email protected]); 5 Research School of Earth Sciences, Australian National University, Camberra,
Australia ([email protected]); 6 SIPETROL, Santiago, Chile ([email protected])
KEYWORDS: Central Chile, Andes, Apatite fission-track ages, Neogene mountain building
INTRODUCTION
ln this paper we discuss the control of major structures in the exhumation of Cenozoic rocks and surface
uplift in the Andean Main Cordillera based on new apatite fission tracks age analysis and the study of geological
cross -sections.
Fig. 1: Principal
morphostructural Units. The
box shows de Study Region
The study area is segmented in the following N-S oriented morphostructural units (Fig. 1), from west to
east: Coastal Cordillera, Central Depression, Western and Eastern Main Cordil1era (WMC and EMC,
respectively), and Frontal Cordillera. The Coastal Cordillera consists mainly of volcanic and sedirnentary
Mesozoic rocks, the Aptian - Albian Las Chilcas and Maastrichtian La Valle formations and Late Mesozoic
intrusive bodies (Sellés and Gana, 2001), which are in contact with Cenozoic rocks through west vergent thrust
faults located in the Central Depression and the western border of the WMC. The Central Depression
corresponds to a basin rooted by Eocene to Early Miocene rocks: the Cordon de los Ratones Beds (Sellés and
Gana, 2001) and the Abanico Formation (Sellés and Gana, 2001), filled by Pleistocene (?) - Holocene alluvial,
fluvial and pyroc1astic deposits. The WMC consists mainly of extensional basin-related deposits of the Abanico
Formation (Late Eocene to Early Miocene) overlain by the Miocene volcanic FareJIones Formation (Thiele,
1980; Charrier et al, 2002). In the eastern boundary of the WMC, the Abanico Formation thrusts westwardly
Mesozoic marine and continental series that form the bulk of the EMC (Fig. 2).
273
6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 273-276
o
Recentaluvlal and fluvial deposijs
MesozoicSe<llmentary Rocks
~ Mesozoic Inlrusives
_
Hipoabisal Basic Eocene - Origocene Intrusives
_
Abanlco F",marlOn
~ FarnllonesFormation
Ô
Earty Mooœne Intrusives
1::::::1 Late Mlocene Intru.ive.
~ RecentVolcanlsm
Fig. 2 (Left) Geologie Map
(Modified from
SERNAGEOMIN,2002)
34"00'5
STRUCTURE
The boundary between the Coastal Cord illera and the Central Depression in the northern part of the
Santiago basin is controlled by a west-vergent thrust-fault (e.g. Infiernillo fault). Similarly, the boundary
between the Central Depression and the WMC is controlled by a west-vergent thrust (San Ramon Fault System)
forming a 2500 m escarpment (see fig . 3, cross section A-A '), that could be responsible for the deformation of
the Abanico Formation along this boundary (Rauld, 2002).
In the central part of the WMC, the deformation consists mainly of open folds showing eastward growth
strata. Close to the eastern boundary of the WMC, the deformation increases, exhibiting tight, overturned fold s
and west-vergent faults (e.g . Chacayes - Yesillo Fault). The boundary between the WMC and EMC is
characterized by a major east-vergent fault (El Diablo Fault) that thrusts the Abanico Formation over the
Mesozoic succession (cross section B-B'). The origin of the west-vergent thrust system has lengthy been
discussed (Palma, 1991; Baeza, 1999; Bustamante, 2001 ; Charrier et al., 2002, Charrier et al., 2005). We present
here a discussion about the possible mechanism that caused this deformation and provide additional apatite
fission tracks age determinations that shed new light on this problem.
From the analysis of Fig. 3 (cross section BB') it is possible to deduce the following hypotheses for the
development of this fault system: (1) The major east-vergent El Diablo fault would corresponds to an inverted
normal fault that induced the eastward development of the Aconcagua Fold-and-Thrust belt (ATFB). Thus, the
west-vergent system would correspond to a back-thrust on the rear part of the east-vergent ATFB; (2) The El
Diablo fault on the back of the ATFB is an (east-vergent) out-of-sequence thrust of the Aconcagua Fold-Thrust
274
6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 273-276
belt developed further east and the west-vergent faults corresponds alike as in the hipothesis to a back-thrust.
A
A'
'" Vergars et al, 1999
o
If
~
lf
-
Hoioc:lWw DeQoMa
LM Miocroe ~
_-...F......-.
E"~à F.-..onn FCIfmiIIllOn
_
2;':~i c~ ~
~ loV~FormIIbOn
.......
Ria Oanas Fmnabon
_
2,46tO,92
_
Rio eow. Form.m
-
• • 'Volca'1Riv$1AItweg
o
_FIa5oonT_Ago
•
MAI Pt8çIlodaMPI8I..... Age
o
UIPbSH_PboonAgo
Fig. 3 : Geologie cross sections. For location see Fig. 2
DEFORMATIONAL HISTORY
Between 33° Sand 34° S, apparently two main deformation stages can be recognized during an
apparently countinous process of tectonic inversion. The first one took place in Late üligocene and Early
Miocene times (Kurtz et al., 1997; Godoy et al., 1999; Charrier et al ., 2002) , Most of the deformation occurred
on the basins borders, enclosing a new basin where the Farellones Formation was deposited, forming a continuos
sucession with Abanico Formation and exhibiting locally evidence for syn-tectonic depositation . The second
stage took place in Late Miocene - Pliocene times, coeval to the main stage of surface uplift of the Andean Main
and Frontal Cordillera between 33° - 34°S (Skewes y Holmgren , 1993 ; Giambiagi et al., 2003; Maksaev et al.,
2003).
EXHUMATION AND UPLIFT OF THE WESTERN MAIN CORDILLERA
14 new apatite fission tracks (AFf) ages were obtained from samples collected in the Abanico
Formation, Late Miocene Plutons (WMC) and Mesozoic Rocks (EMC), showing complex a process of
deformation, exhumation and mountain building across the Main Cordillera at this latitude during Late Cenozoic
(Fig. 1).
ln the western flank of the WMC, the AFT ages agree with the depositional ages of the Abanico
sucession according to new U/Pb SHRIMP and 4°Ar/39Ar ages. Therefore, the burial of these rocks was not
enough to warm and reset the AFT, confirming a similar conclusion of Maksaev et al. (2003) for the La übra
Pluton (cross section A-A '). In the central part of the WMC, near to San José de Maipo , the AFT ages obtained
in the stratigraphicaJly lowest levels of Abanico Formation are ca. 10 Ma, but 500 m above, the AFT ages
coincide with the depositional age (corroborated by new LI/Pb SHR1MP age and the available 40ArrAr ages in
the literature). The boundary between WMC and EMC exhibits a more cornplex situation . According to three
AFT ages obtained in the Abanico Formation in different locations along an east-west oriented section (see cross
275
6th Int ernat ional Symposium on Andean Geodynamics (ISAG 2005, Barcelona ), Extended Abstracts: 273-276
sec tion B-B') , the lowest and eas ternmos t stra tigr aph ie levels of the Aba nico Fo rma tion were coo led before the
upper and western stratigraphie levels, sugges ting that the west- vergent thrus t system might have dragged
up ward the Abanico Deposits we st of El Diabl o Fault. This situ ation woul d support hypothesis l , that E l Diabl o
Thrust is an inve rted normal faul t. O n the o ther hand, AFT exhumat io n ages o n Mesozoic rock s are cons trained
to L ate Pliocen e and Earl y Pleistocene times.
CONCLUSIONS
AFT ages sugg est that bu rial in the basin was grea ter to the eas t of the W MC than to the wes t,
coi nc iding with the similar co nclusion of Zurita (1999) for the Las Lefias Ri ver at 34° 15 ' S . This prob ably
ind icates that El Diablo Fault corr esponds to the eastern basin boundi ng fa ult. Inversion of the E l Diablo Fault
induced the development o f the Aconcagua FTB as well as the west vergent deformation . A prob abl e
react ivation of this structural sys te m oc urred in Late Pliocene based on very yo ung AFT age s from the Mesozo ic
un its, ex p laning present d ay shal low seis mici ty and high rates va lley incision (Fa das et al., this sy mposi um) .
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