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Deformation and low-grade metamorphism in the Triassic rocks
of the Zacatecas area (Zacatecas, Mexico)
Beckmann, A. , Caballero, J.A. , Fischer, R. , Gonzales Arroyo, A.
and Michalzik, D.
104°
1. Overview
This study is based on about 3390 measurements of textural elements and detailed mapping at a
Rio Grande
24°
N
1:10.000 scale. The study area is situated NW of Zacatecas City (Fig. 1, 2). In the study area 10
102°
Fresnillo
lithostratigraphic units have been identified belonging to the Triassic Zacatecas Group and the
Cretaceous Chilitos Formation. Cenozoic volcanic rocks and alluvial accumulations form the younger
units (Fig. 2).
Pazific
ocean
Stratigraphy
Lithology
Lithostratigrafic units
QUARTERNARY
fault
CRETACEOUS
extrusive
Rhyolite
Eocene
743
744
22°47 to Fresnillo
Pillows
clChp
150-200m
Zacatecas Group
Carnian
N
Fig. 1: Map showing location of the study area
(clChs)
70-100m
clCha
200-250m
Nevadan
Orogeny
Pimienta
Arro
0
102°37’30’’
1 km
Extrusive
Rhyolite
Limestone
lens
Fluvial deposits
Pillows
Mudstone
747
748
N
2523
B
yo E
l Bo
San Miguel
El Bote Formation
2522
te
Ar
(truBo)
>100m
Cerro El Bote
ro
yo
Cerro El Gato
La Pimienta
Formation
Ta
la
m
an
2521
te
s
49
49
(truPm)
>300m
Zacatecas
La Escondida
A San Miguel Unit
B Talamantes Unit
Legend
Talus deposits
746
745
A
Andesite
El Ahogado(truAh)
Formation >80m
TRIASSIC
study area
100 km
500 km
Laramide
Orogeny
Sediments
thrust
pilings
22°
(teRe)
50m
(teRg)
5-30m
Chilitos Formation
geologic
contours
TERTIARY
basin and
range
tectonic
(qLs)
>1 m
? Valanginian Hauterivian ?
Legend
Mexico D.F.
Tectonic events
Alluvium
Rhyolitic
dyke
Zacatecas
Gulf
of
Mexico
Conglomeratic
lens
Andesite
Fossils
Sand-,
siltstone
Phyllite
2520
1 km
0,5
0
102°37
boundary line
Rhyolitic
dyke
102°35’00’’
Fig. 3: Location of two structural units of
the Zacatacas Group
Fig. 2: Detailed geological map of the area NW of Zacatecas City
2. Structural analysis
Based on our data four tectonic deformation events (D1 to D4)
Quartz
are distinguished:
Mica
B1
-sf 3
-
The D1 deformation produced isoclinal folding (Fig. 4) and a
syngenetic sf1 axial plane foliation (Fig. 5). A very low grade
-sf 2-
metamorphism (prehnite-pumpellyit) of the Zacatecas Group is
B2
related to this tectonic event.
Subsequently,
a
second
compressive
deformation
-s f
-s
0-
(D2)
12 mm
produced open to tight folds (Fig. 6) and a related sf2 foliation
Fig. 4: Isoclinal folded bedding planes (D1)
Fig. 5: General textural structures in thin
(Fig. 7).
in rocks of the La Pimienta Formation
section
Fig. 6: Open folds (D2) in the El Bote Formation
Fig. 7: Crenulation cleavage in clay schists
Both the D1 and the D2 deformation event are restricted to the
Zacatecas Group and do not affect the Chilitos Formation.
The D3 event is documented by open folds and a related sf3
foliation (Fig. 5) in the Zacatecas Group as well as in the Chilitos
Formation. This compressional regime can be subdivided into
an early and a late subphase.
The youngest D4 deformation is related to the Cenozoic basinand-range extension.
of the El Bote Formation (thin section)
3. Interpretation
D1 and D2 deformations do not affect the Chilitos Formation and must
deformation
event
D4
tectonic
event
basin and range
tectonic
(late Tertiary)
D3
early D3
D2
D1
therefore be pre-Cretaceous (probably Nevadan) in time (Fig. 8).
Presumably related to early Laramide compression the Chilitos
Formation was thrusted upon the rocks of the Zacatecas Group. During
Laramide orogeny
(late Cretaceous
to early Tertiary)
this event the Triassic rock assemblage was also affected and split into
Laramidan
Nevadan orogeny
thrusting
(late Jurassic to
and spliting
early Cretaceous)
of the Triassic
rock assemblage
rotated and overthrusted individual structural units (e.g. Talamantes
and San Miguel Units, Fig. 3). The Laramide D3 folds and related
foliations are found in the Zacatecas Group as well as in the Chilitos
Formation. The present morphology of the Sierra de Zacatecas is
related to the Cenozoic D4 extension. The mineralized structures of the
textural
elements
Legend
N
E
W
S
thrust
NE
NE
SE
vergence of folds
W
?
axis and plunge of folds
normal fault
Zacatecas mining district also belong to this stage.
Fig. 8: Compilation of the tectonic events and the resulting textural elements in the study
area