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Paleogene Extension in the Southern Basin and Range Province of Mexico:Syndepositional Tilting of Eocene Red Beds and Oligocene Volcanic Rocksin the Guanajuato Mining DistrictJosé Aranda-Gómez ;Fred Mcdowell

To cite this Article Aranda-Gómez, José andMcdowell, Fred(1998) 'Paleogene Extension in the Southern Basin and RangeProvince of Mexico: Syndepositional Tilting of Eocene Red Beds and Oligocene Volcanic Rocks in the GuanajuatoMining District', International Geology Review, 40: 2, 116 — 134To link to this Article: DOI: 10.1080/00206819809465201URL: http://dx.doi.org/10.1080/00206819809465201

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International Geology Review, Vol. 40, 1998, p. 116-134. Copyright © 1998 by V H. Winston & Son, Inc. All rights reserved.

Paleogene Extension in the Southern Basin and Range Province of Mexico: Syndepositional Tilting of Eocene Red Beds and Oligocene Volcanic Rocks in the Guanajuato Mining District

JOSE JORGE ARANDA-GÓMEZ Unidad de Ciencias de la Tierra (UNICIT), Instituto de Geología, Universidad Nacional Autónoma de México,

P.O. Box 1-742, Santiago de Querétaro, Qto. 76001 Mexico

A N D FRED W. M C D O W E L L

Department of Geological Sciences, The University of Texas at Austin, Austin, Texas 78712

Abstract

The Guanajuato region, located in the southern part of the Basin and Range tectonic province, is characterized by a rhombohedral fault pattern. Extensional faulting—as well as related uplift, erosion, and deposition of continental clastic deposits—was in progress at 49 Ma, as indicated by K-Ar dating of intercalated mafic volcanic lavas. Earliest extension is registered in the lower member of the Guanajuato conglomerate and produced NW-trend-ing, down to the southwest, normal faults and northeast tilting of the hanging-wall clastic sequence. Beds in the upper member of the Guanajuato conglomerate display lateral thick­ness variations, changes in size and composition of the clasts, and ENE and SE inclination, all of which suggest activity along a second set of NE-trending younger faults. Extension continued also after emplacement of the overlying Bufa ignimbrite (~37 Ma, K-Ar). Dip differences between the conglomerate and the Calderones Formation (<37 but >30 Ma), which is part of the volcanic sequence that conformably overlies the Guanajuato conglom­erate, indicate at least 13° and probably more than 20° of rotation prior to 37 Ma. Consis­tent northeast tilting in the Calderones Formation indicates that subsequent extension preferentially reactivated the NW-trending faults. This sequence of deformational events produced the rhombohedral fault pattern observed in the region. Later pulses of extension (late Oligocene and younger) produced simultaneous movement on both sets of faults, as suggested by others.

In addition to this strong evidence for Paleogene extension in the Guanajuato region, indication of Paleogene extension also may be present at scattered localities throughout central and southern Mexico, where continental red beds occur stratigraphically between pre-Cenozoic rocks and Tertiary volcanic rocks (Eocene-Oligocene). Detailed structural and stratigraphic data are lacking for these localities, but they may provide an important record of a regional Paleogene extension episode that has been largely overshadowed by the obvious evidence for extension during the Neogene.

Introduction

T H E SOUTHERN BASIN AND R A N G E PROVINCE

of N o r t h America stretches sou thward at least to the Guanajuato region (Fig. 1A) and p r o b a b l y t o t h e G u e r r e r o a n d O a x a c a reg ions in s o u t h e r n M e x i c o (Henry and A r a n d a - G ó m e z , 1 9 9 2 ; Jansma and Lang, 1997; Stewart et a l , 1996). In addition to the existence of the normal faults themselves, the

record of Neogene extension includes coarse, cont inenta l ly der ived clastic sed imenta ry rocks tha t were depos i ted synchronous ly with the formation of linear, fault-bounded bas ins ( M c D o w e l l e t a l . , 1 9 9 7 ; N i e t o -Samaniego , 1 9 9 0 ; N i e t o - S a m a n i e g o and A r a n d a - G ó m e z , 1 9 9 1 ) . A l t h o u g h the se deposits did not all form during the same time interval, they all post-date e rupt ion of

0020-6814/98/312/116-19 $10.00. 116

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GUANAJUATO MINING DISTRICT 1 1 7

FIG. 1. A. Late Cenozoic normal faults of the southern Basin and Range province in northern and central Mexico. The most obvious structures occur north of the Trans-Mexican Volcanic Belt (TMVB). Some faults in the Oaxaca and Guerrero region have been interpreted as part of the same province (Henry and Aranda-Gómez, 1992; Jansma and Lang, 1997). B. Outcrops of Eocene continental conglomerates and volcanic rocks in central and southern Mexico. Localities shown: 1 = Nazas (Aguirre and McDowell, 1991); 2 = Velardeña (Gilmer et al., 1988); 3 = Colorada (Albinson, 1988); 4 = Sombrerete (Albinson, 1988); 5 = Fresnillo (Lang et al., 1988); 6 = Zacatecas (Ponce and Clark, 1988); 7 = La Herradura (Labarthe-Hernández and Jimenez-Lopez, 1991); 8 = Ahualulco (Labarthe-Hernández et al., 1982, 1995); 9 = Pinos (Aguillón-Robles et al., 1994); 10 = Juchipila (Webber et al., 1994); 11 = Comanja (Martínez-Reyes, 1992); 12 = Providencia (Tristan-Gonzalez, 1986); 13 = Guanajuato (this paper); 14 = Tzitzio (Pasquaré et al., 1991); 15 = Taxco (De Cserna and Fries, 1981; Alba-Aldave et al., 1996); 16 = Tetela del Río (Pantoja-Alor, 1983); 17 = Tetelcingo (Pantoja-Alor, 1983); 18 = Tamazulapan (Ferrusquía-Villafranca, 1976); 19 = Zimapán (Simons and Mapes, 1957; Yta and Moreno-Tovar, 1997); 20 = Arcelia (Jansma and Lang, 1997).

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118 ARANDA-GÓMEZ AND MCDOWELL

voluminous felsic rocks in late Eocene and Oligocene time.

Isolated outcrops of coarse clastic red beds of Eocene age occur throughout much of central and southern Mexico (Fig. 1B; Table 1). These deposits rest unconformably upon folded Mesozoic rocks and underlie the late Eocene and Oligocene felsic volcanic rocks. Fragments derived from Mesozoic marine rocks are the dominant clasts in most locali­ties, but almost invariably a small proportion of volcanic clasts and/or intercalated lavas and tuffs is present (e.g., Schulze, 1953; Simons and Mapes, 1957; Enciso de la Vega, 1963, 1968; McLeroy and demons, 1965; Clemons and McLeroy, 1966; Ledezma-Guerrero, 1967; Mayer Pérez Rul, 1967; De Cserna and Fries, 1981; Labarthe-Hernández et al., 1982; Labarthe-Hernández and Jimenez, 1991; Aguillón-Robles et al., 1994; Yta and Moreno-Tovar, 1997; Jansma and Lang, 1997). These lower Tertiary conglom­erates have been interpreted as molasse deposits that accumulated passively within topographic lows during the last phases of or immediately after the Laramide orogeny (e.g., Tardy, 1980), and/or as fanglomerates d e p o s i t e d near active no rma l faults (Edwards, 1955; De Cserna and Fries, 1981; De Cserna, 1989; Yta and Moreno-Tovar, 1997; Jansma and Lang, 1997).

The southernmost part of the Mesa Cen­tral of Mexico (Raisz, 1964), which forms part of the southern Basin and Range prov­ince (Henry and Aranda-Gómez, 1992), is covered by a thick sequence of middle Ceno-zoic volcanic rocks (Labarthe-Hernández et al., 1982). This subaerial sequence rests on folded Mesozoic marine sediments and/or early to middle Tertiary continental clastic deposits (e.g., the Guanajuato conglomerate [Edwards, 1955] , Cenicera Formation [Labarthe-Hernández et al., 1982], Duarte conglomerate [Martinez-Reyes, 1992]). The volcanic rocks cover a large portion of the region located between Guanajuato and San Luis Potosí. They are cut by numerous Ceno-zoic faults with two prominent orienta­

tions—northwest and northeast (Fig. 2). A compilation of the attitude (strike and dip direction and amount of dip) of stratified Cenozoic rocks and their relation to known faults in the region has shown that most (over 90%) of the outcrops of Eocene continental conglomerates and Oligocene ignimbrites are tilted toward the northeast (Stewart et al., 1996). Qualitative observations made while compiling this regional tilt-domain map sug­gest that the average dip of the Eocene red beds is significantly larger than the inclina­tion of the Oligocene ignimbrites, despite the fact that in Guanajuato the contact between both sequences is concordant. These rela­tions are consistent with growth faulting and associated rollover folds that result in a sec­tion in which stratal dips progressively decrease upward.

This paper attempts to document and quantify the tilt difference between the Eocene and Oligocene strata and to interpret its significance. We have chosen to examine this relationship in the Guanajuato mining district, because Eocene and Oligocene strat­ified rocks occur together there, and because significant structural observations have been made that can be compiled and critically ana­lyzed for this effect. In our analysis we assume that: (1) both the Eocene fanglomer­ates and tabular bodies within the volcanic sequence (ignimbrites and epiclastic volcanic deposits) were nearly horizontal (≤ 10° of depositional dip) when deposited; and (2) subsequent tilting is related to normal fault­ing. Evidence of compressional deformation is absent from the Tertiary rocks of the region.

Guanajuato Mining District

The Guanajuato mining district contains the most prominent outcrop of Eocene conti­nental conglomerates within the southern part of the Mesa Central. In the eastern part of the district, the contact between the Gua­najuato conglomerate and the overlying vol-

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GUANAJUATO MINING DISTRICT 1 1 9

TABLE 1. Isotopic Ages of Some Early Tertiary Conglomerates in Central and Southern Mexico

Locality1

Guanajuato mining district (13)

Nazas, Durango (1)

Juchipila, Zacatecas (10)

Tetela del Río, Guerrero (16)

Taxco mining district (15)

Tetelcingo, Guerrero (17) Ahualulco, San Luis Potosí(8) Zacatecas mining district (6) Fresnillo mining district (5) Colorada (3) and Sombre-rete (4) min­ing districts

Velardeña mining district (2) Zimapán mining district (19) Mixteca Alta (18)

Arcelia graben (20)

Stratigraphic unit

Guanajuato conglomerate

Ahuichila Fm.

Red sand­stone and conglomerate

Balsas Fm.

Balsas/More-los Fm.

Tetelcingo Fm.

Cenicera Fm.

Zacatecas red conglomerate

Fresnillo Fm.

Ahuichila Fm.

Ahuichila Fm.

El Morro Fm.

Tamazulapan conglomerate

Balsas Fm.

Lat., °N

20.98

25.21

21.65

18.03

n.r. 3

17.91

22.27

n.r.

n.r.

n.r.

17.42

n.r.

Long.,

101.29

104.19

103.08

100.12

n.r.

99.51

101.17

n.r.

n.r.

n.r.

97.29

n.r.

Age, Ma

49

>43

48-24

42

>38

52

>44

>47

>38

>53?

>45

>38

>49

42

Method

K-Ar w.r.4

K-Ar Plag, bio

K-Ar (48 feldspar,

24 w.r.) K-Ar w.r.

K-Ar w.r.

K-Ar w.r. K-Ar w.r.

K-Ar? Bio

K-Ar

K-Ar K-spar

K-Ar

K-Ar

Pb-α

K-Ar

Dated stratigraphic unit

Age of andesitic lava flows interbedded with lower member

Average isotopic age of the overlying Boquillas Coloradas tuff Age of clastic sequence is bracketed by basaltic andesites

Age of olivine basalt interlayered with red beds

Age of vitrophyre in the lower part of the overly­ing Acamixtla formation Age of basalt interlayered with Balsas Fm. Age of overlying Casita Blanca andesite Oldest isotopic age in overlying volcanic sequence Oldest isotopic age in overlying volcanic sequence Age of overlying (?) dacite porphyry

Age of overlying andesite

Oldest isotopic age in overlying volcanic sequence Isotopic age detemined in interlayered tuffs in the overlying Yanhuitlán Fm. Interlayered basalt. Prob­ably the age reported by Pantoja-Alor, 1983 (locality 16)

Source2

a

b

c

d

h

d

i

e

f

f

g

i

k

I

1 Numbers in parentheses correspond to locality numbers in Figure 1. 2Sources: a = this paper; b = Aguirre and McDowell, 1991; c = Webber et al., 1994; d = Pantoja-Alor, 1983; e = Ponce and Clark, 1988; f = Albinson, 1988; g = Gilmer et al., 1988; h = Alba-Aldave et al., 1995; i = Labarthe et al., 1982; j = Yta and Moreno-Tovar, 1997; k = Schlaepfer et al., 1974, Ferrusquía-Villafranca, 1976; 1 = Jansma and Lang, 1997. 3n.r. = not reported. 4w.r. = whole-rock.

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1 2 0 ARANDA-GÓMEZ AND McDOWELL

FIG. 2. Regional geology of the area between Guanajuato (G) and San Luis Potosí (SLP). Other abbrevia­tions: S = Salinas de Hidalgo; SF =San Felipe; DH = Dolores Hidalgo; L = León; SM = San Miguel de Allende; VM =Veta Madre; AF = Aldana fault. Note that south of latitude 22°30' N, most of the area is covered by Cenozoic volcanic rocks. Most stratified Eocene fanglomerates and Oligocene volcanics are tilted to the northeast. Inset shows a rose diagram of orientation of the Cenozoic faults in the San Luis Potosí and Guanajuato 1:250,000 quadrangles. Sections A-A' and B-B' are diagrammatic and intended only to show the Cenozoic faulting style. Cenozoic volcanic rocks were grouped in a single unit. Modified from Nieto-Samaniego et al. (1997a), Stewart et al. (1996), and Ortega-Gutierrrez et al. (1992).

canic sequence is conspicuously marked by the water-deposited volcaniclastic sandstones of the Losero Formation (Echegoyén-Sánchez et al., 1970), called "Bufa sand­stone" by Edwards (1955). Field observa­tions indicate that there is no significant angular unconformity at this contact. The Losero Formation forms the base of the vol­canic section and is succeeded upward in order by the Bufa, Calderones, Cedro, and Chichíndaro formations (Echegoyén-Sanchez et al., 1970). Within the mining district the Calderones Formation is ~200 m thick and is composed of andesitic ignimbrites interbed-ded with volcaniclastic sediments. Isotopic

ages for the volcanic section (Table 2) are known from the Bufa ignimbrite (K-Ar = 37± 3 Ma [Gross, 1975]) and the Chichín­daro rhyolite (K-Ar = 32 ± 1 Ma [Gross, 1975]; K-Ar = 30.8 ± 0.8 Ma, 30.1 ± 0.8 Ma [Nieto-Samaniego et al., 1997c]).

The general distribution and attitude of both the Guanajuato conglomerate and the Calderones Formation, and the overall struc­tural relations of these units with syn- and post-volcanic faults, have been carefully doc­umented in several publications (Edwards, 1955; Echegoyén-Sánchez et al., 1970; Nieto-Samaniego, 1990; Martinez-Reyes, 1992; Nieto-Samaniego et al., 1997c). On

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GUANAJUATO MINING DISTRICT 1 2 1

TABLE 2. Available K-Ar Ages for the Tertiary Section in the Guanajuato Mining District

Unit

Chichíndaro rhyoiite

Same

Same

Bufa rhyoiite

Guanajuato conglomerate

Mineral

Sanidine

Sanidine

??

??

w.r.2

Lat., °N

21.08

20.89

n.r.3

n.r.

20.98

Long., °W

101.02

101.17

n.r.

n.r.

101.29

% K

8.426 8.590

8.191 8.482

-_

1.242 1.220

40*Ar, %

78 92 72

77 80

--

82 82

40*Ar4

10.26 9.77

10.08

10.17 9.94

--

2.381 2.404

Age, Ma ± 1σ

30.1 ± 0.8

30.8 ± 0.8

32.1 ± 1.0

37.0 ± 3.06

49.3 ± 1.0

Reference5

Nieto-Sarnaniego et al., 1997c

Nieto-Samaniego et al., 1997c

Gross, 1975

Gross, 1975

This work

1Samples are listed in stratigraphic order, from youngest to oldest. Decay constants: λß = 4.962 x 10-10/yr; λε+ε = 0.581 × l0-10/yr; 40K/K=1.167 × 10-4. 2w.r. = whole rock. 3n.r. = not reported. 4Units are × 10-6 cc STP for radiogenic argon (40*Ar). 5Gross (1975) included no analytical data for his results. 6Gross quotes this error as 3.0 and 0.3 Ma.

the basis of these structural data, the Guana­juato mining district is the region of the southern part of the Mesa Central where structural relations among the Eocene red beds and the Oligocene volcanics can best be analyzed and interpreted.

The Guanajuato Conglomerate

The following summary of the geology of the red-bed sequence exposed in the Guana­juato mining district is based on the work by Edwards (1955). The ~1500 m thick sequence of lower Tertiary red beds exposed near the city of Guanajuato is interpreted as coalescing alluvial fans deposited at the base of a fault-bounded mountain block. From clast size distributions, it is believed that the source area for the conglomerate was located a short distance (<10 km) northeast of Gua­najuato. A disconformity mapped in the clas­tic sequence separates two distinct members

that display different textures and clast com­positions.

The lower member is formed by red and brown sandstones, in places intercalated with intermediate to mafic lavas (i.e., "old volca­nic rocks" in Edwards's map) and conglom­erate beds. The upper member is a sequence of boulder beds that grade upward to pebble conglomerates. Volcanic fragments are the most abundant clast type (up to 76%) in both members. There is a significant increase in the proportion of granitic clasts (up to 28%) above the disconformity, indicating progres­sive unroofing of older rocks in the highland source area. According to Edwards, the dis­conformity between the upper and lower members represents a "hiatus of unknown, but probably considerable time ... [and] the absence of channeling, or observable angular discordance [between the upper and lower member]... indicates that little or no erosion occurred... .The great influx of coarse granite conglomerate immediately above the disconformity is considered a result of reju-

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1 2 2 ARANDA-GÓMEZ AND MCDOWELL

FIG. 3. Distribution of Eocene red beds in part of the Guanajuato mining district. Stereoplots (A-D) show the dip direction of the beds in the conglomerate, projected into the lower hemisphere. The data set was compiled from the l:20,000-scale geologic map of Edwards (1955).

venated normal faulting along the margin of the highland source" (Edwards, 1955, p. 160).

Abrupt changes in the thickness from 650 to 170 m of the upper member and its rela­tion to the NE-trending Amparo fault (Fig. 3) suggest that faulting was active during depo­sition of the upper member of the conglom­erate. Movement along the Amparo fault continued after deposition of the Bufa ignim-brite, as this unit is cut by the fault northeast of the town of Guanajuato. Post-Bufa activity produced less than 30 m of throw northeast of the town of Guanajuato, compared with 500 m of displacement in the area east of Marfil (Fig. 3). In the eastern part of the Guanajuato district, the upper contact

between the red beds and the overlying vol­canic sequence is conformable. The transi­tion between the poorly sorted and relatively coarse grained red beds in the upper part of the polymictic conglomerate and the green, medium-grained, finely laminated volcani-clastic sandstones of the Losero Formation is only a few centimeters thick. Therefore, at least in those places where Losero accumu­lated, the upper surface of the conglomerate must have been nearly horizontal at the time of deposition of the tuff. Nowhere in the dis­trict has a basal depositional contact of the red beds upon the underlying Mesozoic rocks been observed. The two units are everywhere in fault contact in the western part of the dis­trict (e.g., along the Aldana fault, Fig. 3).

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GUANAJUATO MINING DISTRICT 123

Age of the Guanajuato Conglomerate

There are two sources of information that provide age control on the deposition of the Guanajuato conglomerate—vertebrate fossils and isotopic ages. The vertebrate fossils of the Marfil local fauna, formed by rodents, lizards, and a tapir-like mammal found by Edwards (1955, p. 171) and Fries et al. (1955) in the lower member of the conglom­erate, are most probably of late Eocene age, but may range from late early Eocene to post-early Oligocene. The discovery of a miacid carnivore (Viverravus sp.) and a condylarth hyopsodontid (Apheliscus or Haplomylus) in the lower member of the conglomerate sug­gests a middle to late Eocene age (Ferrusquia-Villafranca, 1987). More precise control is provided by a whole-rock K-Ar age of 49.3 ± 1.0 Ma (Table 2) on a basalt from the south-westernmost exposures of the conglomerate, sampled ~2.5 km southwest of Marfil (Fig. 3). At the sampled locality, the basalts were mapped by Edwards as the "old volcanic rocks," which he describes as interlayered within the conglomerate unit (Edwards, 1955, his Fig. 18, p. 156). The K-Ar and fos­sil ages only define individual time points during deposition of the lower member of the Guanajuato conglomerate. They do not specify the full time interval of sedimenta­tion. An upper limit is provided by the 37 ± 3 Ma K-Ar age from the overlying Bufa For­mation (Gross, 1975). Unfortunately, this age has a rather large uncertainty. Clearly, addi­tional geochronology is warranted, both from within the Guanajuato conglomerate and from its bounding units.

Tilting in Cenozoic Rocks of the Guanajuato District

On their 1:20,000- and l:10,000-scale geologic maps, Edwards (1955) and Eche-goyén-Sánchez et al. (1970), respectively, plotted structural attitudes of beds in the Guanajuato conglomerate (n = 328) and the Calderones Formation (n = 43). These are

displayed separately in Figure 4 as lines of dip direction projected onto the lower hemi­sphere of equal-area plots. The data for the Guanajuato conglomerate display consider­able dispersion, but a large number of mea­surements are located in the eastern half of the diagram, along a broad belt that extends from 330° to 185° (Fig. 4A). Three clusters of points occur along this belt at 040˚/30˚, 073˚/29˚, and 155˚/24˚. A rose diagram of the azimuths of the dip directions (Fig. 4B) clearly shows that most beds in the Guana­juato conglomerate are inclined toward the NE and ENE, and the inclination diagram (Fig. 4C) depicts the range in tilt angle of the conglomerate from 75° to 4°, with most beds inclined between 35° and 20°.

In contrast, the data from the Calderones Formation are less dispersed; the dip direc­tions concentrate in the northeast quadrant of the diagram, along a belt that extends from 030° to 095° (Fig. 4D). The belt con­tains a single maximum at 053718°. The dip directions (Fig. 4E) cluster between 040° and 070°, with a concentration cluster at 055°. The dip angles range from 75° to 9°, with most beds inclined between 25˚ and 10˚ (Fig. 4F). One problem in comparing these struc­tural data is that the measurements from the two formations are predominantly from two structural blocks separated by the Veta Madre fault. To overcome this problem, a subset of 22 measurements from beds of the Guanajuato conglomerate that are from the same tectonic domain as most of the those from the Calderones Formation are plotted in Figures 4G through 41. Although these data show only a single mode of dip direction for the Guanajuato conglomerate, the mean inclination is comparable to that for the full data set and is significantly larger than that for the Calderones Formation,

In order to further evaluate the dispersion of the structural data for the Guanajuato con­glomerate, the data were plotted separately for its two members (Fig. 5). Despite the lack of an obvious angular unconformity between them, the orientation of the tilted beds varies

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1 2 4 ARANDA-GÓMEZ AND MCDOWELL

FIG. 4. Structural data compiled by Edwards (1955) and Echegoyén-Sánchez et al. (1970) in layered sequences of Cenozoic rocks in the Guanajuato mining district. A and D. Equal-area plots (lower hemisphere) of the dip direction of the beds. B and E. Rose diagrams of the dip direction of the beds. C and F. Angle of inclination of the beds. Figures G to I show the same data for tilting of the upper member of the Guanajuato conglomerate measured in the footwall block of Veta Madre.

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GUANAJUATO MINING DISTRICT 1 2 5

FIG. 5. Structural data compiled by Edwards for the lower (A) and upper (B) members of the Guanajuato conglomerate. Equal-area plots (lower hemisphere) show the dip direction of the beds.

significantly between the two members. The data for the lower member define a broad belt that occupies part of the northwestern and entire northeastern quadrant of the dia­gram (Fig. 5A), with the most significant con­centration of points around 042°/29°. In contrast, the data points for the upper mem­ber of the Guanajuato conglomerate cluster in the northeastern and southeastern quad­rants, and the distribution is bimodal, with maxima at 080˚/25˚ and 155˚/24˚ (Fig. 5B).

In Figures 3A-3D the tilt data are sepa­rated geographically to evaluate the influence of large syn- to post-depositional ENE-trend-

ing structures, such as the Amparo and Rocha faults. The data for both members are plotted separately for localities northeast and south­west of these major structures. For the lower member, the only significant difference is a small rotation toward more easterly inclina­tions to the north of the Rocha fault (Figs. 3A and 3B). In contrast, the upper member dis­plays marked differences across the Amparo fault. Dip directions for beds located north of the fault are 080° (Fig. 3C), whereas those for beds located south of the structure are bimodally distributed, with distinct clusters at 080° and 155° (Fig. 3D). Inspection of Edwards's map indicates that most SE-dip-ping beds occur near the southeastern corner of the mapped area, which also is the outcrop limit of the red beds in the area.

Subdividing the data in this manner revealed that the steepest dips, and most SW-dipping beds, occur in the lower member and near the trace of the Aldana fault, which is NE-trending, down to the southeast, and post-dates deposition of the conglomerate (Fig. 3). The largest dip angles might be related to more pronounced tilting in the older member of conglomerate and/or drag near the Aldana fault.

Discussion

Early extension in Guanajuato A prolonged period of Cenozoic extension

in the area around Guanajuato began as early as the middle Eocene (~49 Ma) and proba­bly continued until the Late Cenozoic (Nieto-Samaniego et al., 1997a, 1997b). Whether extension was continuous or occurred as discrete phases separated by long quiescent periods is unknown. The lower and upper members of the Guanajuato conglom­erate were preferentially tilted toward the NE and ENE, respectively, as they were being deposited (Fig. 5). There is a significant dif­ference (>13°) between the mean tilt angle of the Guanajuato conglomerate and that for the overlying volcanic sequence, represented

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126 ARANDA-GÓMEZ AND McDOWELL

TABLE 3. Summary of Tectonic and Depositional Events Inferred from the Study of the Guanajuato Conglomerate and Calderones Formation

Calderones Formation

Faulting and tilting

Deposition

Guanajuato conglomerate

Upper member

Faulting and tilting

Deposition

Lower member

Faulting and tilting

Deposition

PULSE 1

— N E — ______

PULSE 2

— S E — ___

— S E —

PULSE 3

?__NE__

__NE___

— N E —

by data from the Calderones Formation (Fig. 4). The lack of an angular unconformity between these two sequences suggests that faulting and tilting continued while the volcanosedimentary sequence was being deposited.

Structural data collected from the lower member of the conglomerate indicate that during the earliest pulse of Cenozoic normal faulting, extension was probably in a NE-SW direction and produced a large NW-trending fault zone (Table 3), which caused the north­east tilting in the sequence. As was pointed out by Edwards (1955), the fault-bounded mountain front, which acted as the source region for the lower member of the fanglom-erate, was located northeast of the Guana­juato mining district. Thus, the lower member is tilted toward this fault.

The disconformity between the upper and lower members of the conglomerate signals a change in the style of deformation and possi­bly in the source for sediments to the deposi­tional basin. A clear bimodal distribution of the dip direction in the upper member is seen south of the Amparo fault (Fig. 3). Although the largest concentration of data points still

lies within the northeastern quadrant, the maximum occurs around 080°, compared with 042° in the lower member (Fig. 5). Both the marked change in thickness of the upper member of the conglomerate (Edwards, 1955) and the inclination of the beds toward the ENE and SE suggest that another group of faults, oriented in a NE-SW direction, became active during a second phase of extension, possibly together with continued activity along the NW-trending set of normal faults (Table 3). The best examples of these NE-trending structures are the Amparo and Rocha faults in the Guanajuato mining dis­trict (Fig. 3). The concentration of beds dip­ping by as much as 25° to the southeast in the southern part of Figure 3 suggests that an important NE-trending structure, down to the northwest, was located to the south of Guanajuato.

Later extension occurred during (?) and after the deposition of the Calderones For­mation (Table 3). This pulse caused tilting of the sequence toward the northeast. Dip directions and angles for the Calderones For­mation cluster around 053°/18° (Fig. 4D). Additional dispersion arose from extension

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GUANAJUATO MINING DISTRICT 127

that occurred after deposition of the volcanic sequence (e.g., Nieto-Samaniego, 1990). Movement along numerous younger faults with variable orientations (see inset in Fig. 2) probably produced local rotation of the beds.

Aside from these major changes in the pat­tern of tilting, some of the structural disper­sion displayed in Figures 4 and 5 may be related to variation in primary dip directions in the fanglomerate. It is possible that the dis­continuous beds in the original alluvial fan deposit radiated away from the apex of the fan. Furthermore, it is very likely that the conglomerate was formed by coalescence of an unknown number of alluvial fans, each one with its own radial pattern.

In order to estimate the amount of rota­tion that occurred prior to deposition of the Calderones Formation, we have assumed that the maxima in the data of Figures 4 and 5 are representative of the average tilt angles and dip directions for beds of the lower and upper members of the conglomerate and the Calderones Formation. We further assume that the ignimbrites and volcaniclastic rocks of the Calderones Formation originally were horizontal. After rotating the Calderones to horizontal from its average tilt value, an incli­nation of 13° toward the northeast remains for the absolute maximum orientation of the conglomerates. We regard this figure as a minimum value for the amount of pre-Cal-derones (>37 Ma) rotation. Because it is very likely that the gravels in the active alluvial fans had a primary dip of approximately 5° to 9° away from the source area {i.e., toward the southwest), additional pre-Calderones tilting probably is required. We infer that extension-related rotation of at least 20° ± 2° occurred prior to 37 Ma in the Guana­juato mining district. This analysis does not include the possible effect of rollover folding during deposition of sediments on the hang­ing-wall blocks of the faults. Thus, pre-Cal­derones tilting could be even greater. In such a case, fanning in the deposit would produce steeper northeastern inclinations in the older

strata than in the younger beds (Figs. 3 and 5).

Synthetic model for tilting of the Guanajuato conglomerate

Our interpretation of the structural data indicates that the Guanajuato conglomerate and Caiderones Formation record a complex tectonic and depositional history, which is summarized in Table 3. We have attempted to simulate this pattern with a simple computer model. For the lower member of the Guana­juato conglomerate, we assigned an initial dip-direction data set that represents an allu­vial-fan deposit that accumulated at the base of a fault-bounded, 310°-trending mountain range (Figs. 6A and 6B). We assigned primary dips between 5° and 9° away from the moun­tain front (i.e., toward the southwest). Depo­sitional dip directions were distributed in a fan array from 130° to 310°, with a gradual concentration of data points toward the dep­ositional axis of the fan (i.e., 220°). Figures 6C, 6D, and 6E show the results of sequential tilting (20°, 10°, and 18°) of the data along 310°, 040°, 310° horizontal axes, respec­tively, which ultimately produces rotation of the maximum in the cluster to the northeast­ern quadrant while preserving a unimodal distribution. This exercise approximately reproduces the field data for the lower mem­ber of the conglomerate (Fig. 5A), except that the direction of the maximum concen­tration of points in the model is at least 20° east of that for the real data set. This could reflect either initial deposition/tilting along a more westerly fault during the first phase of deformation and/or tilting of the data set along a more northerly rotation horizontal axis during the second phase. Nevertheless, we restricted our parameters within a reason­able angular distance (± 20°) from the regional trends of the Cenozoic faults observed in the Mesa Central (see inset in Fig. 2).

For the upper member of the conglomer­ate (Fig. 5B), which has a bimodal distribu­tion of the field data, we added a second data

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FIG. 6. Results of computer simulation of simultaneous deposition and tilting of the lower member of the Guanajuato conglomerate. A. Map view of the source area, NW-trending normal fault, and alluvial fan. B. Initial conditions. Attitude of the beds in the ideal fan deposit are shown as dip directions. C. Model after 20° rotation along a 310°-trending horizontal axis (Phase 1, Table 3). Rotation was accomplished in 5° incre­ments. Formation of the rollover fold in the sedimentary sequence of the hanging wall was simulated by addition of the original data set after each rotation. D. Model after 10" rotation of (C) along a 040°-trending horizontal axis (Phase 2, Table 3). E. Final result after 18" rotation along a 310°-trending horizontal axis (Phase 3, Table 3). All diagrams show dip directions projected onto the lower hemisphere.

set to represent an alluvial fan deposited along a 040°-trending, fault-bounded moun­tain front (Figs. 7A and 7B). The initial bimo-dal d is t r ibut ion reflects s imultaneous deposition of alluvial fans in a basin bounded by two nearly orthogonal source areas, simi­lar to that observed in the field data for the upper member of the conglomerate (Fig. 5B). This enlarged data set then was sequentially rotated 10° and 18° along 040° and 310° horizontal axes, respectively. These rotations reduce the dispersion of the dip direction lines, causing the data to cluster around a sin­gle maximum (Figs. 7C and 7D). This second

model fails to produce the bimodal distribu­tion and predicts a maximum concentration (045˚/18˚) north of the highest concentra­tion of points (080˚/25˚) observed in the upper member of the conglomerate (Figs. 5B and 7D). Small variations (10°) in the orien­tation of the rotation axes produce signifi­cant shifts in the location of the maximum (Fig. 7E).

The effect of the formation of syntectonic rollover folds as the sedimentary sequence accumulated in the hanging wall of the active faults also was explored. For the lower mem­ber of the Guanajuato conglomerate, we pro-

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GUANAJUATO MINING DISTRICT 129

duced the initial 20˚ of northeast tilting in 5° steps, adding a new set of SW-dipping beds in each step. For the upper member, we likewise rotated the data set in 5° steps, adding each time the initial data set. The results of simul­taneous deposition and rotation is to main­tain the dispersion in the data (Figs. 6C and 7C). However, after simulation of the depo­sition ends, subsequent rotations of the data set tend to increase the clustering of data (Figs. 6D-6E and 7D).

Origin of the rhombohedral pattern of Cenozoic faults in the Mesa Central

This simple simulation of three pulses of alternating NE-SW, NW-SE, and NE-SW extension appears to successfully produce some of the structural and sedimentological features observed. Of course, it is the sim­plest approximation to the pattern of faulting in the district, inasmuch as it includes sequential activation of nearly orthogonal sets of dip-slip faults and rotation around horizontal axes parallel to those faults. The possibility of simultaneous movement/rota­tion and/or oblique-slip movement during the second and third pulses of deformation (Table 3) was not considered.

Nieto-Samaniego et al. (1997a, 1997b) pointed out that the regional fault pattern observed in the southern part of the Mesa Central was produced by triaxial deforma­tion during the late Oligocene. Simultaneous dip-slip movement along NE- and NW-trending fault systems requires that at least one of the fault trends already existed. Once two sets of normal faults are present, younger extension can simultaneously acti­vate both sets. Earlier tilting of both the red beds and in the Eocene-Oligocene volcanic sequence suggests that the early pulses of extensional deformation were biaxial.

Speculation about the regional extent of the early extension in Mexico

It seems clear, at least for the Guanajuato district, that deposition of Paleogene con­glomerates was related to active extension.

Jansma and Lang (1997) in a recent paper documented early Tertiary extension in southern Mexico. Edwards (1955) described in less detail occurrences of red beds in Taxco and Zacatecas and inferred for them an ori­gin similar to that of the Guanajuato con­glomerate. For many of the scattered localities in central and southern Mexico where Paleogene red beds are exposed (Fig. 1B and Table 1), a relationship to block fault­ing and extension has been suggested. How­ever, field data for most of these localities are extremely limited. Available evidence for the timing of deposition of these red beds places all of them within the Paleocene to middle Eocene time interval (Table 1).

Commonly, these strata are regarded either as molasse deposits passively derived from highlands remaining from Laramide compression (Tardy, 1980) or as fanglomer-ates associated with fault-bounded basins (after Edwards, 1955). In the Arcelia region of southern Mexico, Jansma and Lang (1997) have shown that the lower member of the Eocene red bed sequence (Balsas Formation) consists of molasse deposits that were folded during the last phases of the Laramide orog­eny, and that the middle and upper members are related to normal faulting. Based on the analysis of the sedimentary and structural data on the Guanajuato conglomerate and the Arcelia graben, we suggest that structural and stratigraphic data from other localities (Fig. 1B) should be accumulated to test the possibility that some of them record a Paleo­gene extensional event that affected much of central and southern Mexico.

Distribution of the red-bed localities and Eocene volcanic rocks (Fig. 1B) suggests that in Mexico there might be a relationship between extension and volcanism. Similar associations have been found in the North American Cordillera, north of 42° N Lat., with the formation of metamorphic core complexes as the most dramatic example of this link (Armstrong and Ward, 1991). A probable explanation for the onset of exten­sion immediately after the end of Laramide

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1 3 0 ARANDA-GÓMEZ AND McDOWELL

FIG. 7. Computer model for the simultaneous deposition and tilting of the upper member of the Guana­juato conglomerate. A. Map view of the source areas, NW- and NE-trending normal faults, and alluvial fans. B. Initial conditions. Attitude of the beds in the ideal fan deposit are shown as dip directions. It was assumed that there were two nearly orthogonal source areas. C. Model after 10° rotation along a 040°-trending hori­zontal axis. Rotation was accomplished in 5° increments. Formation of the rollover fold in the sedimentary sequence of the hanging wall was simulated by addition of the original data set after each rotation (Phase 2, Table 3). D. Final result after 18° rotation along a 310°-trending horizontal axis. E. Same tilting sequence using slightly different rotation axes (050° and 320°). All diagrams show dip directions projected onto the lower hemisphere (Phase 3, Table 3).

compression is the re-establishment of steep subduction, with the consequent westward shift of the Paleogene magmatic arc (Atwater, 1990). Reconstruction of slab geometries (Severinghaus and Atwater, 1990) suggests that this might be the case for central and southern Mexico. Lack of information on the distribution and volumes of the volcanism and nature of the clastic deposits prevents a more detailed analysis.

Summary and Conclusions

The Guanajuato mining district is the best-studied locality of a large number of isolated exposures of red beds that occur in a vast region in central and southern Mexico. Most of these localities are poorly documented, but invariably the clastic sequences are located between folded pre-Cenozoic strata and the widespread Tertiary volcanic cover.

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GUANAJUATO MINING DISTRICT 1 3 1

In several places, the age of volcanic rocks immediately above the conglomerates is mid­dle to late Eocene (Table 1). In some of the localities—such as in Guanajuato, Taxco, and other outcrops in the state of Guerrero (Table 1)—lava flows and/or tuff horizons are inter-layered with the red beds (Edwards, 1955; De Cserna and Fries, 1981; Pantoja-Alor, 1983). Furthermore, in Guanajuato, Taxco, and Zacatecas, the presence of volcanic clasts within the conglomerates suggests that Eocene (?) volcanic activity preceded the deposition of the clastic sequences. Exten­sion in the southern part of the Mesa Cen­tral, now part of the Basin and Range tectonic province (Henry and Aranda-Gómez, 1992), began during the middle Eocene.

In the Guanajuato area, the sedimentary record indicates that faulting, volcanism, uplift, and erosion were active at 49 Ma. The earliest extension produced NW-trending normal faults and tilting of the red beds toward the northeast. A minimum estimate of the amount of rotation related to this ear­liest period is ~13°. If primary dips (5° to 9° away from their source) in the fanglomerates are considered, this rotation can be as large as 20° ± 2°. If fanning in the syntectonic sequence is considered, additional tilting of the lower strata in the conglomerate is required.

The ENE and SE inclination of the beds in the upper member of the Guanajuato con­glomerate—together with its thickness varia­tions and changes in size and nature of the clasts (Edwards, 1955)—is interpreted as evi­dence of activity along a second set of NE-trending faults. The age of this event is brack­eted by the K-Ar ages (Table 2) for a lava flow interlayered with the lower member of the conglomerate (~49 Ma) and for the Bufa Formation (~37 Ma). The unimodal north­east tilting observed in the Calderones For­mation indicates that later extension occurred along the NW-trending faults.

Variations in the direction of extension, first in a northeast direction and later north­

west, produced the rhombohedral pattern of normal faults (Fig. 2) observed between Gua­najuato and San Luis Potosí (Aranda-Gómez et al., 1989; Nieto-Samaniego et al., 1997a, 1997b). Later extension, perhaps including that associated with Neogene Basin and Range tectonism, involved movement along both fault sets, perhaps synchronously.

Acknowledgments

Financial support for this investigation was provided to Jorge Aranda by CONACYT grant 3657PT and by the Insti tuto de Geología (UNAM), and to Fred McDowell by NSF Grant EAR-8616360. Luca Ferrari generously sustained long discussions with us about the interpretation of the tilt data around Guanajuato. Ismael Ferrusquía par­ticipated in collecting the sample for K-Ar dating and provided advice on the vertebrate paleontology of the Guanajuato district. John Fletcher, Max Suter, Chris Henry, and Luca Ferrari made constructive reviews of an ear­lier version of the manuscript. Gary Axen and John Stamatakos discussed some of the aspects of the project. Angel Nieto and col­laborators provided preprints of their papers on the tectonics of the southern part of the Mesa Central and Sierra Madre Occidental.

REFERENCES

Albinson, T., 1988, Geologic reconstruction of paleosurfaces in the Sombrerete, Colorada, and Fresnillo districts, Zacatecas State, Mexico: Econ. Geol., v. 83, p. 1647-1667.

Aguillon-Robles, A., Aranda-Gómez, J. J., and Marín-Solís, J. D., 1994, Noticia acerca de la presencia de sedimentos continentales y volcan-ismo asociado (¿Eoceno?) en la región de Pinos, Z a c : GEOS, v. 14, no. 5, p. 80

Aguirre-Díaz, G. J., and McDowell, F. W., 1991, The volcanic section at Nazas, Durango, México, and the possibility of widespread Eocene volcanism within the Sierra Madre Occidental: Jour. Geo-phys. Res., v. 96, p. 13, 373-13,388.

Alba-Aldave, L., Morán-Zenteno, D. J., Angeles, S., Corona, R., and Altuzar-Coello, P., 1996, Geo-

Downloaded By: [UNAM] At: 00:33 20 April 2010

1 3 2 ARANDA-GÓMEZ AND McDOWELL

química de las rocas terciarias de la región de Taxco-Huautla: Actas INAGEQ, v. 2, p. 39-44.

Alba-Aldave, L., Reyes-Salsas, M., Angeles-Garcia, S., Morán-Zenteno, D., and Corona-Esquivel, R., 1995, Estratigrafia y geoquimica de las rocas ter­ciarias de la región de Taxco-Tilzapotla: GEOS, v. 15, no. 2, p. 101-1102.

Aranda-Gómez, J. J., Aranda-Gómez, J. M., and Nieto-Samaniego, A. F., 1989, Consideraciones acerca de la evolutión tectónica durante el Ceno-zoico de la Sierra de Guanajuato y la parte merid­ional de la Meseta Central: Univ. Nac. Autón. Mexico, Inst. Geol., Revista, v. 8, no. 1, p. 33-46.

Armstrong, R. L., and Ward, P., 1991, Evolving geo­graphic patterns of Cenozoic magmatism in the North American Cordillera: The temporal and spatial association of magmatism and metamor-phic core complexes: Jour. Geophys. Res., v. 96, p. 13,201-13,224.

Atwater, T., 1990, Plate tectonic history of the north­east Pacific and western North America, in The geology of North America, vol. N : The eastern Pacific Ocean and Hawaii: Boulder, CO, Geol. Soc. Amer., p. 21-72 .

Clemons, R. E., and McLeroy, D. F., 1966, HojaTor-reón 13R-1(1) con resumen de la geología de la Hoja Torreón, Estados de Coahuila y Durango: Univ. Nac. Autón. México, Inst. Geol., Carta Geol. de México Serie de 1:100,000.

De Cserna, Z., 1989, An outline of the geology of Mexico, in The geology of North America, vol. A: The geology of Nor th America—An overview: Boulder, CO, Geol. Soc. Amer., p. 233-264.

De Cserna, Z., and Fries, C., Jr., 1981, Hoja Taxco 14Q-h(7) con resumen de la geología de la Hoja Taxco, Estados de Guerrero, México y Morelos: Univ. Nac. Autón. México, Inst. Geol., Carta Geol. de Mexico Serie de 1:100,000.

Echegoyén-Sánchez, J., Romero-Martinez, S., and Velázquez-Silva, S., 1970, Geología y yacimientos minerales de la parte central del Distrito Minero de Guanajuato: Consejo de Recursos Naturales no Renovables, Boletín 75, 35 p. , 3 maps.

Edwards, J. D., 1955, Studies of some early Tertiary red conglomerates of central Mexico: U.S. Geol. Surv., Prof. Pap. 264-H, p. 153-183.

Enciso de la Vega, S., 1963, Hoja Nazas 13R-k(6) con resumen de la geología de la Hoja Nazas, Estado de Durango: Univ. Nac. Autón. México, Inst. Geol., Carta Geol. de México Serie de 1:100,000.

, 1968, Hoja Cuencamé 13R-1(7) con resu­men de la geología de la Hoja Cuencamé, Estado de Durango: Univ. Nac. Autón. México, Inst. Geol., Carta Geol. de México Serie de 1:100,000.

Ferrusquía-Villafranca, I., 1976, Estudios geológico-paleontologicos en la Región Mixteca, pt. 1: Geología del área Tamazulapan-Teposcolula-Yan-huitlán, Mixteca Alta, Estado de Oaxaca, México: Univ. Nac. Autón. México, Inst. Geol., Boletín no. 97, 160 p.

, 1987, Reubicación geocronológica del Con-glomerado Guanajuato basada en nuevos mamíf-eros, in Programa, resúmenes y guía de excursión del Simposio sobre la geología de la región de la Sierra de Guanajuato, Guanajuato, Gto., 28-30 de abril, p. 2 1 - 2 3 : Univ. Nac. Autón. México, Inst. Geol.

Fries, Carl, Jr., Hibbard, C. W., and Dunkle, D. H., 1955, Early Cenozoic vertebrates in the red con­glomerate at Guanajuato, Mexico: Smithsonian Misc. Coll., v. 123, no. 7, 25 p.

Gilmer, A. L., Clark, K. F., Conde C., J., Hernandez C , I., Figueroa S., J. I., and Porter, E., 1988, Sierra de Santa Maria, Velardeña mining district, Durango, Mexico: Econ. Geol., v. 83 , p. 1802-1829.

Gross, W. H., 1975, New ore discovery and source of silver-gold veins, Guanajuato, Mexico: Econ. Geol., v. 70, p. 1175-1189.

Henry, C. D., and A r a n d a - ó m e z , J. J., 1992, The real southern Basin and Range: Mid- to late Cen­ozoic extension in Mexico: Geology, v. 20, p. 701-704.

Jansma, P. E., and Lang, H. R., 1997, The Arcelia graben: New evidence for Oligocene Basin and Range extension in southern Mexico: Geology, v. 25, no. 5, p. 455-458.

Labarthe-Hernández, G., and Jiménez-Lopez, L. S., 1991, Cartografía geológica de las hojas Cerritos del Bernal, Santo Domingo, El Estribo y La Her-radura, Estado de San Luis Potosí: Univ. Autón. San Luis Potosí, Inst. Geol., Folleto técnico 113, 63 p.

Labarthe-Hernández, G., Jiménez-López, L. S., and Aranda-Gómez, J. J., 1995, Reinterpretación de la geología del centro volcánico de la Sierra de Ahualulco, S.L.P: Univ. Autón. San Luis Potosí, Inst. Geol. Metalurg., Folleto táécnico 121, 30 p.

L a b a r t h e - H e r n á n d e z , G. , T r i s t á n , M . , a n d Aranda-Gómez, J. J., 1982, Revisión estratigráfica del Cenozoico de la parte central del Estado de San Luis Po to í : Univ. Nac. San Luis Potosí, Inst. Geol. Metalurg., Folleto técnico 85, 208 p.

Lang, B., Steinitz, F. J., Sawkins, F. J., and Simmons, S. F., 11988, K-Ar studies in the Fresnillo silver district, Zacatecas, Mexico: Econ Geol., v. 83, p. 1642-1646.

Ledezma-Guerrero, O., 1967, Hoja Parras 13R-1(6) con resumen de la geología de la Hoja Parras,

Downloaded By: [UNAM] At: 00:33 20 April 2010

GUANAJUATO MINING DISTRICT 1 3 3

Estados de Coahuila, Durango y Zacatecas: Univ. Nac. Autón. México, Inst. Geol., Carta Geol. de México Serie de 1:100,000.

Martínez, R. G., Morán, D. J., Martiny, B., and Macías, C., 1997, Geochemistry and geochronol-ogy of the Tertiary Volcanic Province of southern Mexico: Eur. Union Geosci., 9th Meeting, Stras­bourg, France, p. 475.

Martinez-Reyes, J., 1992, Mapa geológico de la Sierra de Guanajuato con resumen de la geología de la Sierra de Guanajuato: Univ. Nac. Autón. Mexico, Inst. Geol., Cartas geológicas y mineras 8, escala 1:100,000.

Mayer Pérez Rul, F. A., 1967, Hoja Viesca 13R-1(5) con resumen de la geología de la Hoja Viesca, Estados de Coahuila y Durango: Univ. Nac. Autón. México, Inst. Geol., Carta Geol. de Méx­ico Serie de 1:100,000.

McDowell, F. W., Roldán-Quintana, J., and Amaya-Martínez, R., 1997, The interrelationship of sedi­mentary and volcanic deposits associated with Tertiary extension in Sonora, Mexico: Geol. Soc. Amer. Bull., v. 109, p. 1349-1360.

McLeroy, D. F., and Clemons, R. E., 1965, Hoja Pedriceñas 13R-1(4) con resumen de la geología de la Hoja Pedriceñas, Estados de Coahuila y Durango: Univ. Nac. Autón. México, Inst. Geol., Carta Geol. de México Serie de 1:100,000.

Nieto-Samaniego, A. F., 1990, Fallamiento y estrati-grafía cenozoicos en la parte sudoriental de la Sierra de Guanajuato: Univ. Nac. Autón. México, Inst. Geol., Revista, v. 9, no. 2, p. 146-155.

Nieto-Samaniego, A. F., Alaniz-Alvarez, S. A., and Labarthe-Hernández, G., 1997a, La deformación cenozoica post-laramídica en la parte sur de la Mesa Central, Mexico: Rev. Mex. Cien. Geol., in press.

Nieto-Samaniego, A. F., and Aranda-Gómez, J. J., 1991, Significado geológico de los sedimentos continentales del Cenozoico Superior del centro de México y su representación en la quinta edición de la Carta Geológica de México, in M e m o r i a , Convenc ión Sobre la Evolución Geológica de México, Pachuca, Hidalgo, Junio 23-28, p. 136-137.

Nieto-Samaniego, A. F., Ferrari, L., Alaniz-Alvarez, S. A., Labar the-Hernández , G., and Rosas-Elguera, J., 1997b, Variation of Cenozoic exten­sion and volcanism across the southern Sierra Madre Occidental volcanic province, Mexico: Geol. Soc. Amer. Bull., submitted.

Nieto-Samaniego, A. F., Macías-Romo, C , and Alaniz-Alvarez, S. A., 1997c, Nuevas edades isotópicas de la cubierta volcanica cenozoica de la

parte meridional de la Mesa Central, Mexico: Rev. Mex. Cien. Geol , v. 13, no. 1, p. 117-122.

O r t e g a - G u t i e r r e z , F., Mi t r e -Sa l aza r , L. M . , Roldán-Quin tana , J., Aranda-Gómez , J. J., Morán-Zenteno, D., Alaníz-Alvarez, S. A., and Nieto-Samaniego, A. F., 1992, Carta geológica de la República Mexicana, 5th ed.: Mexico City, Univ. Nac. Aut. México, Inst. Geol. y Secretaría de Energía, Minas e Industria Paraestatal, Consejo de Recursos Minerales, map with explanatory text, 1:2,000,000 scale, 74 p.

Pantoja-Alor, J., 1983, Geocronometría del magma-tismo Cretácico-Terciario de la Sierra Madre del Sur: Bol. Soc. Geol. Mex., v. XLIV, no. 1, p. 1-20.

Pasquaré, G., Ferrari, L., Garduño, V H., Tibaldi, A. and Vezzoli, L., 1991, Geologic map of the cen­tral sector of the Mexican volcanic belt, states of Guanajuato and Michoacan, México; Geol. Soc. Amer. Map and Chart Series MCH-072.

Ponce S., B. F., and Clark, K. F., 1988, The Zacatecas mining district: A Tertiary caldera complex asso­ciated with precious and base metal mineraliza­tion: Econ. Geol., v. 83, p. 1668-1682.

Raisz, E., 1964, Landforms of Mexico, 2nd ed.: Cambridge, MA, Office of Naval Research (map).

Schlaepfer, C. J., Rincón-Orta, C , Silver, L. X, Solo-rio-Munguía, J. G., and Schmitter-Villada, E., 1974, Una edad radiométrica del Eoceno medio en sedimentos continentales de la Mixteca Alta de Oaxaca: Bol. Asoc. Mex. Geol. Petrol., v. 26, nos. 4-6, p. 237-284.

Schulze, G., 1953, Conglomerados terciarios conti­nentales en la Comarca Lagunera de Durango y Coahuila y sus relaciones con fenómenos ígneos, geomorfológicos y climatológicos: Inst. Nac. Investig. Recursos Minerales (México), Boletín 30, 52 p.

Severinghaus, J., and Atwater, T., 1990, Cenozoic geometry and thermal state of the subducting slabs beneath western North America: Geol. Soc. Amer., Memoir 176, p. 1-22.

Simons, F. S., and Mapes-V, E., 1957, Geología y yacimientos minerales del distrito minero de Zimapan, Hidalgo: Inst. Nac. Investig. Recursos Minerales (México), Boletín 40, 282 p.

Stewart, J. H., Anderson, R. E., Aranda-Gómez, J. J., et al., 1996, Map showing Cenozoic tilt domains and associated structural features, western North America, in Faulds, J. E., and Stewart, J. H., Accommodation zones and transfer zones: The regional segmentation of the Basin and Range Province: Geol. Soc. Amer. Spec. Pap., in press.

Tardy, M., 1980, Contribution a l'etude geologique de la Sierra Madre Orienta le du Mexique :

Downloaded By: [UNAM] At: 00:33 20 April 2010

1 3 4 ARANDA-GÓMEZ AND MCDOWELL

Unpubl. doctoral thesis, Univ. Pierre et Marie Curie de Paris, 459 p.

Tristan-Gonzalez, M., 1986, Estratigrafía y tectónica del graben de Villa de Reyes en los estados de San Luis Potosí y Guanajuato: Univ. Autón. San Luis Potosí, Instituto de Geología, Folleto técnico 107, 91 p.

Webber, K. L., Fernandez, L. A., and Simmons, W. B., 1994, Geochemis t ry and mineralogy of the Eocene-Oligocene volcanic sequence, southern Sierra Madre Occidental, Juchipila, Zacatecas, Mexico: Geofís. Int., v. 33 , no. 1, p. 77-89.

Yta, M. and Moreno-Tovar, R., 1997, La mineraliza­­ión en los distritos mineros Pachuca-Real del Monte y Zimapán—su papel en la evolución met-alogenica del Estado de Hidalgo, México, in II Convent ión sobre la Evolución Geológica de México y Recursos Asociados, Pachuca, Hidalgo, Libro guía de las excursiones geológicas, Excur­sion 3 : Inst. Investig. Cienc. de la Tierra de la Univ. Autón. de Hidalgo e Inst. Geol. Univ. Nac. Autón. de México, p. 73-87.

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