Arizona Geological Surveyazgs.arizona.edu | repository.azgs.az.gov

August 2020

CONTRIBUTED REPORT CR-20-C

Reconnaissance study of mylonitic fabrics in the Bellota Ranch area,

eastern Santa Catalina Mountains, ArizonaJon E. Spencer and Kurt N. ConsteniusDept of Geosciences, University of Arizona

Map of the eastern Santa Catalina Mountains showing mylonitic lineation trend andplunge direction derived from averages of previous measurements

Arizona Geological Survey

Phil A. Pearthree, Director

Manuscript approved for publication in August 2020Printed by the Arizona Geological Survey

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Recommended Citation: Spencer, J.E. and Constenius, K.N., 2020, Reconnaissance study of mylonitic fabrics in the Bellota Ranch area, eastern Santa Catalina Mountains, Arizona. Arizona Geological Survey Contributed Report CR-20-C, 12 p, one appendix.

Reconnaissance study of mylonitic fabrics in the Bellota Ranch area, eastern Santa Catalina Mountains, Arizona

Jon E. Spencer and Kurt N. ConsteniusDepartment of Geosciences

University of ArizonaTucson, Arizona, 85721

Arizona Geological Survey Contributed Report CR-20-C, version 1.0(includes PowerPoint file as separate Appendix A)

The Santa Catalina Mountains in southeastern Arizona include extensive mylonitic fabrics developed within granitic and gneissic rocks that make up most of the range. These fabrics are strongest at the southern foot of the range where they dip south and project beneath the rangebounding Catalina – San Pedro detachment fault. Shear sense in the mylonitic rocks is primarily top-southwest, consistent with shearing down-dip from the detachment fault during early normal faulting and exhumation to form the metamorphic core complex. Two to three kilometers north of the foot of the Santa Catalina Mountains the mylonitic foliation is horizontal and farther north it dips to the north. The mylonitic fabrics thus dip outward from the axis of the Forerange arch with primarily top-northeast shear-sense indicators on the north side of the arch. This dominantly top-northeast fabric forms the Molino Basin shear zone, which continues eastward toward the Bellota Ranch area that is the subject of this study. We found that mylonitic fabrics in the Bellota Ranch area are generally subho-rizontal, weak, and without clear shear-sense indicators in by far ost outcrops. Four days of field study yielded clear shear-sense indicators at only eight outcrops, with six of them indicating top-southwest shear sense. We were not able to divide mylonitic rocks into top-southwest and top-northeast zones as in a previous study by Bykerk-Kauffman (2008). Lineation trend gradually changes laterally from more northeasterly in the Molino Basin area to more northerly in the Bellota Ranch area. As with the Molino Basin area, lineations associated with top-northeast shearing generally trend more northerly than lineations associated with top-southwest shearing. We conclude that mylonitic lineation in the Bellota anch area is not a simple continuation of the Molino Basin shear zone as it is dominated by top-southwest shear-sense indicators rather than top-northeast as with the Molino Basin shear zone.

ABSTRACT

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Introduction

The Santa Catalina and Rincon Mountains make up one of the largest metamorphic core

complexes in the North American Cordillera (Fig. 1; Keith et al., 1980; Davis, 1980; Force,

1997; Spencer et al., 2019). Strongly lineated mylonitic fabrics that developed during tectonic

uplift and exhumation of the ranges characterize the south flank of the Santa Catalina Mountains

and extend over much of the western and central Rincon Mountains. Shear sense in mylonites is

dominantly top-southwest (Spencer, 2006; Davis et al., 2019). This mylonitic fabric projects

down-dip directly beneath the partly buried trace of the Catalina – San Pedro detachment fault,

which was active at ~15-30 Ma (Dickinson, 1991).

Mylonitic foliation in the southern Santa Catalina Mountains forms an east-west trending

antiform known as the Forerange arch (Fig. 2). Shear sense indicators are generally top-

southwest on the south side of the arch and top-northeast on the north side (Spencer, 2006; Davis

et al., 2019). Farther north at higher elevations, a generally weakly developed mylonitic fabric

with top-southwest shear-sense indicators forms the Windy Point shear zone (Force, 1997; Davis

et al., 2019). This shear zone projects southward toward the mylonitic fabrics on the south side

of the Forerange arch, and mylonitic fabrics in the two areas may be part of a single shear zone

that is genetically related to the Catalina – San Pedro detachment fault and formed during

exhumation and uplift of the Santa Catalina Mountains (see for example Davis et al., 1986;

Spencer and Reynolds, 1989).

Unlike the Windy Point shear zone and mylonitic fabrics on the south flank of the Forerange

arch, mylonitic foliation on the north side of the arch dips northward below largely non-

mylonitic Eocene leucogranite sills with sparse screens of Proterozoic granite (Reynolds and

Lister, 1990; Naruk and Bykerk-Kauffman, 1990; Force, 1997; Fornash et al., 2013). This north-

dipping fabric forms the Molino Basin shear zone. It’s origin and significance are not well

understood but are likely related to deep crustal processes associated with core-complex genesis

(Reynolds and Lister, 1990; Wernicke, 1992). This shear zone continues eastward toward the

Bellota Ranch area northwest of Redington Pass between the Santa Catalina and Rincon

Mountains (Fig. 2). The Bellota Ranch klippe, a small erosional remnant of the upper plate of the

Catalina – San Pedro detachment fault, rests on brecciated lower plate leucogranites in this area.

East of Bellota Ranch, the mylonitic fabrics are poorly developed but form a zone that appears to

be truncated by the Catalina – San Pedro detachment fault (Bykerk-Kauffman, 2008; Spencer et

al., 2009). This report is a study of mylonitic fabrics in the Bellota Ranch area that includes a

reconnaissance geologic map of the klippe.

Geology of the Bellota Ranch area

Mylonitic fabrics. Bykerk-Kauffman (2008) mapped separate zones of mylonitic fabrics with

top-north and top-south shear sense in the Bellota Ranch area, with a high density of foliation,

lineation, and shear-sense measurements (Fig. 3A). The study described here was an attempt to

confirm the geometry of these zones and to determine their relative chronology.

In four days of field work we measured 38 mylonitic lineation orientations and determined shear

sense at eight locations (Fig. 3B; Appendix A). Mylonitic fabrics are generally weak in the study

area and, in many areas, not sufficiently developed to allow accurate measurement of orientation.

Measured mylonitic lineations are clustered around an eigenpole plunging 3° toward azimuth

020° (Fig. 4). We found only eight outcrops with clear shear-sense indicators, generally in areas

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with stronger fabric development than typical, and were not able to find indicators in sufficient

density to identify zones with uniform shear sense. The locations of the six top-south and two

top-north indicators we identified are all consistent, however, with the shear-sense zones mapped

by Bykerk-Kauffman (2008).

Lineations in general, as well as those in outcrops where shear sense was determined, are

essentially subhorizontal, indicating that mylonitic fabrics in the Bellota Ranch area are not a

simple eastward continuation of the north-dipping Molino Basin shear zone. The dominance of

top-south shearing further suggests that the zone is related to the top-southwest shear zone on the

south flank of the Forerange arch and in the Windy Point area, and that it is related to

exhumation of the core complex. Lineation orientations are more northerly at progressively more

eastward locations within the Molino Basin shear zone (Fig. 2). This trend continues in the

Bellota Ranch area (Fig. 5). The significance of this geometry is uncertain but is possibly related

to changing extension direction during mylonitic shearing or to vertical axis rotation of hanging-

wall or footwall rocks during mylonitic shearing.

Measurements of mylonitic foliations and lineations from Bykerk-Kauffman (2008) were

provided in the form of an Excel spreadsheet by Steve Richard (written communication, 2020).

Measurements in the Bellota Ranch area were separated from other measurements by separating

those to the west of 544,000 UTM easting and to the south of 3,580,000 UTM northing (NAD83,

zone 12). Poles to foliations from this subset are plotted in Figure 6, which shows a well defined

average with a principal eigenvalue defining a mean foliation plane dipping 14° to the north-

northeast and no indication of an arch as with the Forerange arch farther west. Lineations cluster

around a north-northeast trending pole (Fig. 7). Lineations with top-south-southwest shear sense

trend slightly more easterly while those with top-north-northeast shear trend slightly more

northerly (Fig. 7). This discrepancy is in the same sense as with shear-sense and trend

differences in the Bellota Ranch area as reported here (Fig. 4) and in the Forerange arch (Fig.

16B in Davis et al., 2019).

Detachment fault. The Bellota Ranch klippe is an erosional remnant of a once more extensive

assemblage of displaced fault blocks above the Catalina – San Pedro extensional detachment

fault. Previous maps of the klippe (Creasey and Theodore, 1975; Thorman and Drewes, 1981)

and the trace of the underlying detachment fault (Bykerk-Kauffman, 2008), differ significantly

(Fig. 3C) from the reconnaissance geologic map shown here (Fig. 3B). Both Thorman and

Drewes (1981) and Bykerk-Kauffman (2008) mapped the detachment fault as if it were overlain

by the peraluminous granitoids that are so prevalent in the area. We did not attempt to map faults

in the leucogranites, partly because the low relief and Quaternary alluvial cover that obscured the

leucogranites in the immediate area. The contact we mapped as the Catalina – San Pedro

detachment fault marks the boundary between highly contrasting lithologies, supracrustal

sedimentary and metasedimentary rocks displaced over heterogeneous pegmatitic leucogranite

(Fig. 3B).

Conclusion

Field study of mylonitic fabrics in the Bellota Ranch area indicates that foliations are

subhorizontal and lineations have an average trend of 020°. We conclude, tentatively because of

our small data set, that top-south shearing was dominant in the genesis of these lineated

mylonitic rocks. We did not observe boundaries of shear zones with opposite shearing directions,

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as shown by Bykerk-Kauffman (2008), but confirm that shearing occurred in both directions.

Mylonitic fabrics in the Bellota Ranch area are not a simple eastward continuation of the Molino

Basin shear zone and do not include a moderately north-dipping shear zone as at Molino Basin.

As mapped by us, the upper plate of the Catalina – San Pedro detachment fault in the Bellota

Ranch area consists only of sedimentary and metasedimentary rocks, not leucogranites as

mapped in previous studies.

Acknowledgments

We thank Ann Bykerk-Kauffman for a review that improved clarity and focus.

References cited

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the Catalina metamorphic core complex, southeastern Arizona: U-Pb, Nd, and Hf isotopic

constraints: Contributions to Mineralogy and Petrology, v. 165, p. 1295-1310;

doi:10.1007/s00410-013-0859-4

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and Davis, G.H., eds., Cordilleran metamorphic core complexes: Geological Society of

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Naruk, S.J., and Bykerk-Kauffman, A., 1990, Late Cretaceous and Tertiary deformation of the

Santa Catalina metamorphic core complex, Arizona, in Gehrels, G.E., and Spencer, J.E.,

eds., Geologic excursions through the Sonoran Desert region, Arizona and Sonora:

Arizona Geological Survey Special Paper 7, p. 41-50.

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core complexes: Evidence from near the mylonitic front: Geology, v. 18, no. 3, p. 216-

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County, Arizona: U.S. Geological Survey Bulletin 1500, p. 5-37.

2028m

RattlesnakePeak

2409mGreen Mtn.

Mylonitic lineation

Catalina Highway

Bellota Ranchklippe

Redington Road

Figure 3

Forerange arch

Figure 2. Map of the eastern Santa Catalina Mountains showing mylonitic lineation trend andplunge direction derived from averages of previous measurements (Spencer, 2006; Davis et al.,2019). Mylonitic foliation and lineation dip and plunge, respectively, outward from the axis of the Forerange arch. The Molino Basin shear zone is outlined in blue and is north of the Forerange arch. Map units: blue = dominantly metasedimentary rocks; orange = granitic andgneissic rocks; green - Cenozoic clastic deposits.

0 1 2

0 1 2 3

Mi.

km

NN

Windy Pointshear zone

Molino Basin shear zone

110° 37.5' W110° 37.5' W

32° 22.5' N32° 22.5' N

110° 45' W110° 45' W

520000 5300003580000

Top-SW shear zone

Top-SW shear zone

Top-NE shear zone

top-SW shearindicators

top-SW shearindicators

top-NE shearindicators

Bykerk-Kau�man (2008)

Thorman and Drewes (1981)

Bykerk-Kau�man (2008)

Spencer and Constenius (this report)

Alluvium (Quaternary)Clastic sedimentary rocks (Oligocene-Miocene)

Carbonate (Paleozoic)Quartzite (Paleozoic to Mesoproterozoic)

1719

10

15

182

7

99

25

6

5

5

12

1

11 4

4

11

122

6 3

111025

11

11 6

14

11

17

5

45

18

6572

534000 538000536000

3578

000

UTM NAD83, ZONE 12

54

10 mylonitic foliationbedding

mylonitic lineation withup-plunge shear sense

4 mylonitic lineation withdown-plunge shear sense

mylonitic lineation

100 10 1kmkm

NN

110° 37.5' W

32° 2

0' N

CC

BB

AA

Figure 3. Geologic maps of the Bellota Ranch area. (A) Bykerk-Kau�man (2008). (B) This report. (C) Thorman and Drewes 1981) with superposed detachment-fault traces from Bykerk-Kau�man (2008) and from this report.

Axis Eigenvalue Trend Plunge1. 0.9461 020.4 03.02. 0.0304 290.0 06.33. 0.0236 135.3 83.0

N

Top-southwest shearTop-northeast shear

11

22

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Figure 4. Lower hemisphere stereonet of lineation data from the Bellota Ranch area (see Figure 3B). Stereonet and data analysis software (Stereonet v. 10) as described by Allmendinger et al. (2012) and Cardozo and Allmendinger (2013).

0

10

20

30

40

50

60

70

520,000 525,000 530,000 535,000 540,000

Bellota Ranch area

Molino basin shear zone

UTM Easting (in meters; NAD83, zone12)

azim

uth

Figure 5. Mylonitic lineation trend vs. UTM easting (geographic coordinate) derived from data reported in Spencer (2006) and Davis et al. (2019) for the Molino basin shear zone, and from thisstudy for the Bellota Ranch area.

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Principal eigenvalue, all data (n=240)Data set: poles to ABK - tectonic foliation.txtAxis Eigenvalue Trend Plunge 1. 0.8430 216.7 76.02. 0.0902 011.1 12.73. 0.0668 102.4 05.9

Fig. 6. Bingham analysis of foliation data from Bykerk-Kau�man (2008), Bellota Ranch area, eastern Santa Catalina Mountains.Contours from all data (n=240). Average foliation plane strike = 306.8°, dip = 14°. Plot and analysis with Steronet v. 10.0 (Allmendinger et al., 2012; Cardozo and Allmendinger, 2013).

1

2

3

Principal eigenvalue, undirected (n=72)Data set: ABK - tectonic lineation.txtAxis Eigenvalue Trend Plunge 1. 0.7955 026.7 11.92. 0.1283 295.4 06.23. 0.0762 178.2 76.6

Principal eigenvalue, top NE lineation (n=117)Data set: ABK - tectonic lineation Top-N.txtAxis Eigenvalue Trend Plunge 1. 0.8533 006.3 19.42. 0.0858 193.1 70.53. 0.0609 097.0 02.1

Principal eigenvalue, top SW lineation (n=47)Data set: ABK - tectonic lineation Top-S.txtAxis Eigenvalue Trend Plunge 1. 0.8973 020.7 01.72. 0.0642 271.6 85.03. 0.0385 110.8 04.8

Principal eigenvalue, all data (n=238)Data set: ABK - tectonic lineation all types.txtAxis Eigenvalue Trend Plunge 1. 0.8128 015.1 13.62. 0.1055 276.0 32.93. 0.0817 124.3 53.7

Fig. 7. Bingham analysis of lineation data from Bykerk-Kau�man (2008), Bellota Ranch area, eastern Santa Catalina Mountains.Contours from all data (n=238). Second and third eigenpoles are not plotted because eigenvalues for these poles are so low. Plot and analysis with Steronet v. 10.0 (Allmendinger et al., 2012; Cardozo and Allmendinger, 2013).