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.
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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
33
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.
33
22
11
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).