Large-Magnitude Extensional Deformation in the South Mountains Metamorphic Core Complex, Arizona: Evaluation with Paleomagnetism

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Large-magnitude extensional deformation in the South Mountains metamorphic core complex, Arizona: Evaluation with paleomagnetism

Richard F. Livaccari Department of Earth and Planetary Sciences, University of New Mexico, John W. Geissman } Albuquerque, New Mexico 87131 Stephen J. Reynolds Department of Geology, Arizona State University, Tempe, Arizona 85287

ABSTRACT data from these two sides suggests that the INTRODUCTION back-dipping mylonitic front was synkine- Paleomagnetic data are used to test con- matically tilted about 10 down-to-the- Cordilleran metamorphic core complexes troversial aspects of Cordilleran metamor- southwest. The data support a folded shear (MCCs) and their associated structures (de- phic core complexes, including the original zone hypothesis for origin of the mylonitic tachment faults and mylonites) are the in- dip of extensional structures, origin of the front and the interpretation that footwall ferred products of large-magnitude Cenozo- mylonitic front, and applicability of rolling- rocks possess primary, Miocene-age TRMs ic extension (tens of kilometers of normal hinge models. We obtained paleomagnetic or TCRMs. A second regional fold test in- slip; e.g., Crittenden et al., 1980; Reynolds data (115 sites, 82 accepted for analysis) volved data from sites on both ﬂanks of and Spencer, 1985; Howard and John, 1987; from the weakly deformed interior of a syn- the topographically prominent northeast- Coney, 1987). Aspects of metamorphic core kinematic, footwall intrusive suite and Prot- trending mountain range–scale antiform. complex evolution not fully understood in- erozoic footwall rocks of the South Moun- The negative result from this fold test dem- clude the origin of the mountain range–scale tains metamorphic core complex, central onstrates that this structure formed early in antiforms that strike parallel with the exten- Arizona. These rocks yield dual polarity, the extensional history and prior to magne- sion direction and the origin of the mylonitic high unblocking temperature, and high to tization acquisition by the plutons. We ob- front (e.g., Yin, 1991). The most controver- moderate coercivity magnetizations. Posi- tained a well-grouped footwall grand mean sial aspect of metamorphic core complexes tive baked contact tests indicate that from 62 front-side and 20 back-dipping concerns the original dip of detachment -faults and mylonites during extension (mod ؍ k , 51.7 ؍ I , 1.0 ؍ D ,82 ؍ footwall rocks possess primary thermorem- site means (N

We calculated this grand erate-angle versus low-angle). Some models .( 2.5 ؍ anent magnetizations (TRMs) or high-tem- 41.8, ␣95 perature thermochemical remanent magne- mean with the assumption that front-side and field observations suggest that metamor- tizations (TCRMs) acquired early in their sites have remained structurally untilted, phic core complexes represent crustal-scale cooling history and during ductile and brit- whereas back-dipping side sites require re- blocks originally bounded by moderate-an- tle extensional deformation of structurally moval of 10 of southwest dip. This grand gle normal faults that have isostatically tilted higher rocks. This is consistent with ther- mean is statistically indistinguishable (95% to subhorizontal attitudes (Miller et al., mochronologic data indicating rapid syn- confidence level) from time-averaged Mio- 1983; Davis, 1983; King and Ellis, 1990; kinematic cooling from crystallization cene expected directions. We thus conclude Miller, 1991; Brun et al., 1994). Rolling- through the range of laboratory unblocking that the current gentle dip of front-side my- hinge models advocate isostatically induced temperatures for the magnetic mineralogy lonites and detachment faults is original. flexural tilting of an active moderate-angle of these rocks (between about 22 and 17.5 Therefore, both ductile and brittle exten- fault (30Њ to 60Њ dip), to final abandonment Ma). sional deformations of the South Moun- as a subhorizontal structure (Buck, 1988, Paleomagnetic data are considered as two tains metamorphic core complex were ac- 1993; Wernicke and Axen, 1988; Hamilton, populations based on the structural asym- commodated along low-angle structures 1988; Manning and Bartley, 1994). As ap- metry of the South Mountains metamorphic (dip of <15 ). Our interpretation refutes plied to an elastic crust, these models re- core complex: (1) a front side characterized the widespread applicability of models that quire detachment faults and their footwalls by northeast-dipping (ϳ10 ) mylonitic fab- predict metamorphic core complexes to rep- to have been tilted 30Њ to 60Њ. This is sup- rics and brittle extensional structures, and resent tilted crustal blocks originally ported by seismic data indicating that most (2) a back-dipping side characterized by bounded by moderate-angle normal faults present-day extension is along moderately rollover of the mylonitic zone to form a and does not support rolling-hinge models dipping (30Њ to 60Њ) normal faults (Jackson southwest- or back-dipping (ϳ15 ) mylo- of metamorphic core complex evolution that and McKenzie, 1983; Jackson, 1987; Jack- nitic front. Comparison of paleomagnetic require a moderate-angle ramp. son and White, 1989; Thatcher and Hill,

Data Repository item 9520 contains additional material related to this article.

GSA Bulletin; August 1995; v. 107; no. 8; p. 877–894; 15 ﬁgures; 1 table.

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applied to this problem (Coleman, 1991; Metcalf and Smith, 1991) but has large uncertainties. Under favorable conditions, paleomagnetic methods may provide more accurate and precise data to assess footwall tilting and other controversial aspects of metamorphic core complexes (Livaccari et al., 1993; Livaccari, 1994). This study reports on paleomagnetic data from weakly deformed footwall rocks of the Miocene South Moun- tains metamorphic core complex (Fig. 11). Paleomagnetic data provide passive linear markers to compare with expected, time-averaged directions derived from cratonic paleomagnetic poles of similar age. The South Mountains metamorphic core complex is ideally suited for study because of its relatively simple structural history and because it contains footwall intrusions that are synkinematic with respect to both ductile and brittle phases of extensional deformation (Reynolds et al., 1986). The thermochronology of South Mountains intrusions is relatively well understood and indicates rapid synkinematic cooling through the range of laboratory unblocking temperatures of these rocks, and the absence of postkinematic Figure 1. Location map, generalized geologic map and interpretive cross section of the thermal events that may have remagnetized South Mountains metamorphic core complex (MCC), south-central Arizona. Also illus- footwall rocks (Fitzgerald et al., 1994). trated are paleomagnetic sampling locations that represent multiple site locations (filled circles). The synkinematic Miocene footwall intrusive suite consists of the following: Tsm, GEOLOGIC SETTING AND South Mountains Granodiorite; Ttp, Telegraph Pass Granite; Tfd, felsic dikes; and Tmd, PALEOMAGNETISM mafic dikes. These synkinematic magmas intrude the host Proterozoic Estrella gneiss (p–Ce) and Komatke granite (p–Ck). South Mountains metamorphic core complex is divided The principal structure of the South into the following structural subdivisions: (1) a front side (northeast part) characterized Mountains metamorphic core complex is a by a northeast-dipping (ϳ10 ) zone of mylonitic and brittle structures, and (2) a back- subhorizontal, ductile to brittle, high-strain dipping side (southwest part) characterized by rollover of the mylonitic zone to form a zone that accommodated large-magnitude southwest-dipping (ϳ15 ) or back-dipping mylonitic zone. extension (Fig. 1; Reynolds and Rehrig, 1980; Reynolds, 1982, 1985; Reynolds et al., 1986, 1988; Davis et al., 1986). Initial mylo- 1991; cf., Abers, 1991). Alternative models, age data from footwall rocks of the Cheme- nitic ductile deformation and later brittle supported by field observations and theoret- huevi and Bullard–Eagle Eye detachment detachment faulting were partitioned into a ical studies, suggest that detachment faults faults (southeast California and western Ar- relatively thin zone (about 100 m thick). originated and were maintained as low-an- izona) suggests that these structures origi- Both phases of deformation are kinemati- gle structures (dip of Ͻ30Њ; e.g., Davis and nated with a dip of somewhere between 15Њ cally coordinated and indicate top-to-the- Lister, 1988; Miller and John, 1988; Spencer and 50Њ (Bullard–Eagle Eye) and between northeast shear (azimuth of 60Њ). Ductile and Chase, 1989; Lister and Davis, 1989; 12Њ and 32Њ (Chemehuevi; Richard et al., and brittle extension occurred at relatively Yin, 1989; Melosh, 1990; Abers, 1991; For- 1990; Foster et al., 1990; John and Foster, shallow crustal levels (within 5 to 10 km of syth, 1992; Wernicke, 1992; Yin and Dunn, 1993). A similar approach, using apatite and surface, as indicated by the presence of mi- 1992; Scott and Lister, 1992; Parsons and zircon fission-track age determinations, con- arolitic cavities in synkinematic granitoids; Thompson, 1993a, 1993b). Thus, knowledge cluded that the Newberry Mountains de- Reynolds, 1985) and was promoted by mag- of the history of footwall tilting has become tachment fault originated with a moderate matic heat and expulsion of high-tempera- the critical link in understanding the me- angle dip (Ն45Њ) and was subsequently tilted chanics and dynamics of large-magnitude to a low angle (Dokka, 1993). These inter- extension in metamorphic core complexes. pretations support rolling-hinge models but 1GSA Data Repository item 9520 (a more-de- Recently, the history of footwall tilting suffer from uncertainties in the assumed tailed version of Figure 1, and tables of paleomagnetic statistical data and fold test and discrim- along metamorphic core complexes has geothermal gradient, which is the main var- ination test data) is available on request from been studied by several methods. For exam- iable in calculating time-depth history. Documents Secretary, GSA, P.O. Box 9140, Boul- ple, spatial distribution of 40Ar/39Ar isotopic Hornblende geobarometry has also been der, CO 80301.

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granodioritic phase. The granite is the second major phase and is exposed as a tabular body along the southwest margin of the granodiorite. The granite forms a gradational to sharp, crosscutting contact with the granodiorite. These granitoids are cut by two, compositionally distinct dike sets: felsic (Tfd) and microdioritic (Tmd or ‘‘mafic dikes’’; Reynolds and Rehrig, 1980; Rey- nolds, 1985). Dikes in both sets are subvertical, relatively thin (Ͻ2 m thick), and strike north-northwest (orthogonal to stretching lineations in mylonitically deformed rocks). The older felsic dikes are porphyritic to aphanitic and are compositionally similar to the composite pluton. The lithologically distinct mafic dikes are the youngest dike set. These synkinematic magmas intrude the host Proterozoic Estrella gneiss (p–Ce), which is exposed along the southwest part of the South Mountains (Reynolds, 1985; Reynolds and DeWitt, 1991). We also sampled mylonitically deformed parts of the granodiorite (Tm). Demagnetization behavior indicates that sites with well-grouped magnetizations with laboratory unblocking temperatures up to Figure 2. Equal area projections of South Mountains front-side and back-dipping-side 580 ЊC and/or high to moderate coercivities, site means with projected confidence cones. Solid symbols represent lower hemisphere ␣95 which are characteristic of the granodiorite, projections and open symbols represent upper hemisphere projections. Abbreviations same mafic dikes, and mylonite, reside chiefly in as for Figure 1. N number of site means. magnetite (Fig. 3). Characteristic remanent ؍ magnetizations (ChRMs) of the granite, felsic dikes, and Proterozoic rocks are more ture magmatic fluids from a suite of synkin- hypothesis of Reynolds and Lister, 1990). complex in that they are dominated either ematic intrusive rocks (Reynolds et al., S-C relations in both front-side and back- by magnetite, with minor hematite, or by 1988; Smith et al., 1991). Stable isotope data dipping-side mylonitic fabrics indicate top- hematite, with minor magnetite (Fig. 3; de- suggest temperatures of deformation along to-the-northeast shear. The South Moun- tails of rock magnetism in Appendix). Nat- the high-strain zone to be about 600 ЊC for tains detachment fault is considered to have ural remanent magnetization (NRM) inten- ductile phases and 400 ЊC for brittle phases closely followed the current topographic sities range from ϳ0.001 A/m for the (Smith et al., 1991). Both ductile and brittle crest line along the front-side part of the granitoids, felsic dikes, and Proterozoic phases of extensional deformation occurred range and to have projected over the back- rocks, to ϳ1.0 A/m for the mafic dikes. The between about 22 and 17.5 Ma (Fitzgerald et dipping part of the range (Fig. 1). granodiorite is of normal polarity, except al., 1994). This paleomagnetic study is based on de- the early mafic phase of this intrusion that is The South Mountains metamorphic core magnetization results from 957 samples col- of reverse polarity. The granite, mylonites, complex consists of the following two struc- lected from 115 sites located within the and Proterozoic rocks, as well as the felsic tural subdivisions (Fig. 1): (1) a front side South Mountains metamorphic core com- and mafic dikes, are both normal and re- (northeast part) characterized by a north- plex footwall (Fig. 1; details of paleomag- verse polarity. Some dikes, one granite site, east-dipping (ϳ10Њ) zone of mylonitic and netic methods in Appendix). Of the 115 sites and one mylonite site possess dual polarity brittle structures and (2) a back-dipping side demagnetized, we accepted 82 for inclusion magnetizations. In all cases, including dual (southwest part) characterized by rollover of in group and grand means (Fig. 2; Table 1). polarity sites, we cannot attribute the pres- the mylonitic zone to form a southwest-dip- A synkinematic, footwall intrusive suite con- ence of normal or reverse polarity magne- ping (ϳ15Њ) or back-dipping mylonitic zone stitutes the northeast part of South Moun- tizations to differences in the principal mag- (Reynolds and Lister, 1990). The upper tains and consists of two main intrusions netic phase responsible for carrying the form surface of the back-dipping mylonite (Fig. 1): the South Mountains Granodiorite ChRM. zone is referred to as the mylonitic front (Tsm or ‘‘granodiorite’’) and Telegraph Pass (Davis and Lister, 1988). Field observations Granite (Ttp or ‘‘granite’’; Reynolds, 1985). FIELD TESTS OF PALEOMAGNETIC suggest that the back-dipping side of South The granodiorite is the oldest and volu- STABILITY Mountains was tilted down-to-the-south- metrically largest part of the composite plu- west by about 15Њ during the final phases of ton and was intruded in at least two pulses; Field tests of paleomagnetic stability as- mylonitic deformation (folded shear zone an early mafic phase followed by the main sess the geologic antiquity of a magnetiza-

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tion. Here, field stability tests are used to coercivity normal-polarity magnetizations unblocking temperatures with increasing determine whether South Mountains rocks (component A, isolated below 450 ЊCorbe- distance from the dike contact (1.0 to 2.5 cm possess primary thermoremanent or high- low 10–14 mT). We interpret component A from the mafic dike; Livaccari, 1994). We temperature thermochemical magnetiza- to be a TVRM because it was isolated at interpret component B magnetizations to be tions (TRMs or TCRMs) or secondary, unblocking temperatures higher than ex- partial thermoremanent magnetizations viscous or thermoviscous remanent magne- pected for simple ambient temperature (PTRMs) acquired from partial remagneti- tizations (VRMs or TVRMs). These stabil- VRM acquisition (e.g., Walton, 1980; Kent, zation of granodiorite during intrusion of ity tests include four baked contact tests, 1985). Component A is found in the gran- mafic dikes. Some granodiorite samples and a conglomerate test of sample magne- odiorite host as either a north-northwest or within 5 cm of the mafic dikes that possess tizations from a brittlely deformed mafic north-northeast declination remanence iso- component B PTRMs also retain what we dike. The baked contact tests differentiate lated below unblocking temperatures of interpret to be a normal polarity, compo- three components of magnetization (A, B, 350 ЊC (specimens Tsm25-SSa and Tsm86- nent C magnetization, isolated at tempera- and C), which represent the first, second, Za, respectively, of Fig. 4). Reverse polarity tures above 570–620 ЊC and carried by hem- and third magnetizations unblocked in ther- magnetizations isolated in the mafic dikes at atite. Granodiorite specimens, ranging from mal demagnetization experiments. These unblocking temperatures above 450 ЊCor 5 to 10 cm from the contact do not contain magnetization components exhibit relatively peak fields above 10 mT are considered to a reverse polarity magnetization isolated at discrete and separate unblocking tempera- represent the characteristic primary TRM temperatures Ͻ400 ЊC. This is interpreted ture spectra (within a range of about (component C). This interpretation is sup- to be a result of subsequent overprinting of Ϯ50 ЊC). ported by the fact that all granodiorite spec- both dike and host by normal polarity com- Baked contact tests numbers 1 and 2 in- imens found within 5 cm of the mafic dikes ponent A TVRMs. Normal polarity rema- volve granodiorite host rock and 40- to 60- possess high unblocking temperature, re- nent magnetizations found in granodiorite cm-wide mafic dikes (Fig. 4). Magnetization verse polarity magnetizations isolated be- specimens Ͼ1 m from the dike and isolated components A, B, and C revealed in this tween 400 and 570–620 ЊC (component B in at unblocking temperatures above 450 ЊC contact test are hosted chiefly by magnetite granodiorite specimens). In contact test 2, and peak fields above 10 mT are considered and minor hematite. The mafic dikes con- component B PTRMs in granodiorite host the primary TRMs (component C). We in- tain a low to moderate temperature and low- rocks are isolated over decreasing maximum terpret contact tests 1 and 2 to be positive

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Figure 3. Representative modiﬁed orthogonal progressive demagnetization diagrams for samples of South Mountains footwall rocks. Filled circles are vector end points projected to the horizontal plane (declination), open squares represent vector end points projected to the true vertical plane (inclination). Samples include: Proterozoic Estrella gneiss (p–Ce63 Da; NRM to 660 C); South Mountains Granodio- rite (Tsm34 Ea; NRM to 660 C); Telegraph Pass Granite (Ttp45 Aa; NRM to 670 C); felsic dike (Tfd13 Da; NRM to 125 mT); maﬁc dike (Tmd11 Ha; NRM to 100 mT); and a mylonite (Tm46 Ca; NRM to 600 C).

contact tests (Everitt and Clegg, 1962), in- polarity, component B magnetizations. For sidered to contain pregranite magnetiza- dicating that component C magnetizations specimens 2–4 cm from the contact, demag- tions. The origin of this normal polarity carried by magnetite and hematite in gran- netization trajectories are commonly curvi- magnetization is unclear, but it may be a odiorite and mafic dikes and isolated at un- linear, suggesting the presence of two mag- burial-related viscous PTRM acquired dur- blocking temperatures above 450 ЊCor netizations (components C and B) with ing a prekinematic crustal thickening event. above peak fields of 10 mT are primary overlapping blocking temperature spectra. Therefore, baked contact test 4 is consid- TRMs. Plots of directional data (Fig. 5B) more ered to be positive. Baked contact test 3 involves a narrow clearly demonstrate a decrease in the amount The results of baked contact tests 1 and 2 (Յ30 cm wide) mafic dike intruding a gran- of thermal resetting of host granite away indicate that component C magnetizations ite host (Fig. 5). The mafic dike possesses a from the dike. We interpret these observa- isolated in the granodiorite and mafic dikes normal polarity component C magnetizations to indicate a positive contact test, dem- are chiefly carried by magnetite and repre- tion carried chiefly by magnetite. Character- onstrating that magnetizations residing in sent primary TRMs, acquired early in the istic magnetization of the granite host is of hematite were acquired early in the cooling cooling history of these rocks. Data from reversepolarityandcarriedprimarilybyhem- history of the granite host. contact tests 3 and 4 suggest that magneti- atite. Granite host rocks Ͼ1 m from the dike A more regional scale contact test be- zations carried by hematite in the granite possess a normal polarity magnetization iso- tween the granite and host Proterozoic are of high-temperature TCRM origin, with lated below 10 mT or below 450 ЊC, which is rocks involved results from eight Proterozo- remanence acquisition occurring early in the similar to the component A TVRM of baked ic sites at distances of 25 m to 1.5 km from cooling history of these rocks. Baked contact contact tests 1 and 2 (e.g., specimen the contact with the granite (baked contact tests 3 and 4 are further interpreted to in- Ttp81-Fa of Fig. 5). We interpret magneti- test 4; Fig. 6). Five sites in Proterozoic rocks dicate early acquisition of magnetizations zations isolated at high unblocking temper- Ͻ550 m from the contact with the granite carried by hematite in the granite, a result of atures in granite specimens to represent pri- yield well-grouped, reverse polarity magne- synkinematic circulation of high-tempera- mary, reverse polarity TRMs or TCRMs tizations similar to those of the granite. Sites ture (600 ЊC) magmatic fluids during ductile (component C). Granite specimens within 4 in Proterozoic rocks 650 m or more from the deformation (Smith et al., 1991). A pattern cm of the normal polarity mafic dike possess granite contact yield normal polarity mag- of heterogeneous circulation of high-tem- normal polarity PTRMs (component B) un- netizations that vary from poorly to well perature fluids (see Smith et al., 1991) would blocked at temperatures between 300 and grouped on the site level and yield poor sta- explain why some granite sites possess re- 575 ЊC (Fig. 5). Granite specimens within 1 tistical results when compiled into a group verse polarity magnetizations carried chiefly cm of the contact yield well-defined, normal mean. These normal polarity sites are con- by magnetite rather than hematite. This

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Figure 4. Baked contact tests 1 and 2. Modiﬁed orthogonal progressive demagnetization diagrams and equal area projections of site

means with ␣95 conﬁdence cones. Symbols for equal area projections and orthogonal demagnetization plots same as for Figures 2 and .number of site means ؍ partial thermoremanent magnetization; N ؍ respectively. PTRM ,3

suggests that some granite sites were un- Mountains footwall rocks. Consequently, all including mafic dike sites with high ␣95’s re- affected by fluid circulation and acquired a normal polarity magnetizations defined in jected from inclusion in the group mean. primary reverse polarity magnetization in principal component analysis and used in This dispersion is argued to be caused by magnetite, whereas other sites acquired a calculation of site means were isolated at brittle deformation (brecciation) that oc- reverse polarity remanence in hematite. unblocking temperatures above 450 ЊC and curred after remanent magnetization acqui- Several granite sites have normal polarity above peak fields of 10 to 14 mT. sition. Our inherent assumption is that this magnetizations carried primarily by hem- We applied a paleomagnetic ‘‘conglomer- dike initially possessed a single polarity atite; we interpret these sites to reflect ate’’ test to a mafic dike (site Tmd89) that magnetization (either normal or reverse) ac- spatially limited, late-stage fluid circula- intrudes detachment-fault breccia and is quired before deformation. Alternatively, if tion continuing after a reverse to normal brecciated by closely spaced, roughly or- this dike originally possessed a dual polarity field reversal. Because hematite is of high thogonal, brittle fractures (fracture spacing remanence, then our test for randomness temperature, chemical origin, we cannot de- of 1–5 cm). This dike has been slightly af- becomes less straightforward. Inverting all fine the temperature range over which mag- fected by hydrothermal alteration character- samples with a negative inclination that may netizations carried by hematite were istic of brittlely deformed rocks (green brec- have originally been of reverse polarity al- blocked (e.g., Geissman and Van der Voo, cia of Smith et al., 1991). Component C lows calculation of a revised site mean and 1980). Reverse polarity component C mag- magnetizations in dike specimens are car- statistics. This site mean also yields low pre-

netizations of magnetite-dominated granite ried chiefly by magnetite with minor hema- cision results (n ϭ 11, k ϭ 3.7, ␣95 ϭ 27.8Њ). specimens, however, were unblocked be- tite and, for this site, are isolated at unblock- Thus random (single polarity) to highly dis- tween about 400 and 580 ЊC, consistent with ing temperatures above 400 ЊC and peak persed (if dual polarity) magnetizations are a TRM residing in magnetite. It is possible fields above 20 mT. In situ sample directions interpreted to be the product of deforma- that the low/moderate temperature and are random at the 95% probability level tion of the mafic dike by brittle microfaults

low-coercivity, normal-polarity secondary (N ϭ 11, k ϭ 1.3, ␣95 ϭ 77Њ; each sample was related to detachment faulting. This ran- TVRM (component A) identiﬁed at all drawn from a separate and coherent struc- domness test further suggests that compo- three baked contact tests and in back-dip- tural block). No other maﬁc dike sampled nent C remanent magnetizations are pri- ping sites may be common to all South yielded similar, highly dispersed directions, mary TRMs acquired early in the cooling

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Figure 5. Baked contact test number 3. A. Modiﬁed orthogonal progressive demagnetization diagrams and equal area projection of site

means, with ␣95 conﬁdence cones. B. Equal area projections of thermal demagnetization directional data. Symbols for equal area -partial thermoremanent magne ؍ projections and orthogonal demagnetization plots same as for Figures 2 and 3, respectively. PTRM .number of specimens ؍ tization; n

history of these rocks and before brittle de- expected directions, whereas the in situ that front and back-dipping side data rep- formation of maﬁc dikes. back-dipping group mean is displaced, in an resent two different data populations. In a eastward sense, from the expected direc- qualitative assessment, we observe that the

STRUCTURAL CORRECTIONS, FOLD tions (Fig. 7B). Field observations suggest ␣95 circles of front-side and back-dipping TESTS, REVERSAL TESTS, AND that the back-dipping side of South Moun- side group means overlap, but neither esti- STATISTICAL SIGNIFICANCE TESTS tains was tilted down-to-the-southwest mate of the mean direction is contained

OF PALEOMAGNETIC DATA about 15Њ during the ﬁnal phases of mylo- within the ␣95 circle of the other mean nitic deformation (folded shear zone hy- (Figs. 7B and 8). This suggests, but does not Based on the structural asymmetry of the pothesis of Reynolds and Lister, 1990). The prove, that at a 95% probability level both South Mountains metamorphic core com- location of the in situ back-dipping group data sets are two different samplings of the plex, paleomagnetic data are considered as mean along the down-to-the-southwest tilt same data population (i.e., at a 95% prob- two populations: a front-side population and a path is consistent with between 10Њ and 15Њ ability level the fold hypothesis is not re- back-dipping side population (Fig. 1; Ta- of tilting from folding of the mylonitic front quired). The fold test of McElhinny (1964) ble 1). We have calculated in situ group about a horizontal 330Њ axis (Fig. 8). was applied by untilting a single back-dip- means for all back-dipping sites (6 p–Ce, 2 If, however, front-side and back-dipping ping limb by either 10Њ or 15Њ. This test was Ttp, 1 Tmd, and 11 Tfd sites; N ϭ 20, D ϭ side group means represent two independ- inconclusive at a probability level of 97% for

13.2Њ, I ϭ 47.5Њ, k ϭ 21.2, ␣95 ϭ 7.3Њ) and all ent samplings of a single data set (rather 10Њ of dip correction and a probability level front-side sites (2 p–Ce, 17 Tsm, 13 Ttp, 8 than two different data sets), then separa- of 99% for 15Њ of dip correction. That is, Tfd, 17 Tmd, and 5 Tm sites; N ϭ 62, D ϭ tion of these group means by 10Њ to 15Њ need at a very high probability level, acceptance

0.2Њ, I ϭ 51.0Њ, k ϭ 60.1, ␣95 ϭ 2.4Њ; Figs. 7B not be attributed to folding. Therefore, ac- of the fold hypothesis is statistically in- and 8). The in situ front-side group mean is ceptance of the folded shear zone hypothe- conclusive. Speciﬁcally, there is no statis- statistically indistinguishable from Miocene sis for origin of the mylonitic front requires tically signiﬁcant increase in the concen-

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Although not strictly appropriate, application of the McFadden and McElhinny (1990) reversal test also supports the assumption that the back-dipping side of South Mountains has been tilted to the southwest by about 10Њ. The reversal test is designed to test whether populations of normal and reverse polarity data differ by 180Њ. This should be the case if populations have adequately averaged paleosecular variation and are not ‘‘contaminated’’ by an unrecog- nized magnetization. The reversal test was applied by artificially inverting all back-dipping side site means to reverse polarity and all front-side site means to normal polarity. Comparison of in situ directions resulted in a failed reversal test. When data for the back-dipping sites were corrected by removing either 10Њ or 15Њ of southwest dip along a 330Њ strike, the reversal test passed with a classification of ‘‘B’’. Results of both the reversal and mean discrimination tests are consistent with the field observations of Rey- nolds and Lister (1990) and corroborate the folded shear zone origin for the mylonitic front at South Mountains (Figs. 7 and 8). These results further imply that component C remanent magnetizations isolated in these rocks are primary TRMs or high-temperature TCRMs acquired before folding of the mylonite zone. MCCs are characterized by topographically prominent mountain range–scale antiforms and intervening basin-scale synforms trending parallel to the extension direction. The result is a series of mountain range– scale corrugations in metamorphic core Figure 6. Regional baked contact test number 4. Simplified geologic map of the central complex mylonites and detachment faults. part of South Mountains illustrating location of rock types and paleomagnetic sampling Mechanisms for the origin of these struc- sites in host Proterozoic Estrella gneiss (p–Ce). Symbols for equal area projections are the tures include the following: (1) upward iso- same as for Figure 2. static pushing of either undulatory crustal roots or buoyant synkinematic plutons (Yin, 1991; Lister and Baldwin, 1993); (2) buck- tration of site mean directions after sites significantly decreases this probability ling due to compression perpendicular to ex- performing the structural correction (as level to 48.0% (i.e., the fold hypothesis is tension (synkinematic constrictional folds, indicated by no significant change in the acceptable because after 10Њ restoration Yin, 1991); and (3) extension-parallel folds concentration parameter k). there is a 52.0% probability that front-side imposed on a compliant lower plate by The McFadden and Lowes (1981) mean and back-dipping side data become a single grooves on the underside of the upper plate discrimination test is a more precise way to population). Removing 15Њ of southwest dip (Spencer and Reynolds, 1991). determine at what probability level a ‘‘null from back-dipping sites decreases this prob- A fold test of front-side site mean data hypothesis’’ of a common true mean (i.e., all ability level to only 81.7% (i.e., the fold hy- from the two flanks (‘‘limbs’’) of the topo- data drawn from a single population) is re- pothesis is acceptable because after 15Њ res- graphically prominent northeast-trending jected. This test applied to in situ front-side toration there is a 28.3% probability that mountain range–scale antiform provides a and back-dipping side means indicates that front-side and back-dipping side data be- means of evaluating these hypotheses. The the null hypothesis of a common true mean come a single population, but this is less ac- fold test of McElhinny (1964) gave a nega- is rejected at the 98.8% probability level ceptable as the case of removing 10Њ of dip). tive result (Ͼ99% probability level); unfold- (i.e., the fold hypothesis is acceptable be- The mean discrimination test then supports ing each ‘‘limb’’ by 15Њ (approximate dip of cause at a 98.8% probability level the means the assumption that the back-dipping part of mylonite fabrics) results in much greater dis- were drawn from two populations). Remov- South Mountains has been tilted about 10Њ persion of component C magnetizations ing 10Њ of southwest dip from back-dipping down-to-the-southwest (along a 330Њ strike). (Fig. 1). We offer two possible interpreta-

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Figure 7. Equal area projections of group means and grand means, with ␣95 conﬁdence cones, for South Mountains paleomagnetic data. Also illustrated are lower hemisphere projections of expected, time-averaged Miocene paleomagnetic directions for South Mountains (gray squares). Expected directions calculated from paleomagnetic pole data of Diehl et al., 1988 (38–22 Ma); Irving and Irving, 1982 (20 Ma); and Mankinen et al., 1987 (17–14 Ma). Solid symbols represent lower hemisphere projections (all data inverted to normal polarity). A. .mylonite; other abbreviations same as for Figure 1 ؍ In situ group means of front-side synkinematic intrusions and the mylonites. Tm B. In situ group means of all front-side and back-dipping sites. Front-side group mean is calculated from the Tsm, Ttp, Tfd, Tmd, and Tm group mean directions illustrated in A. Back-dipping sites group mean is calculated from the 20 site means illustrated in Figure 2. The ‘‘tilt corrected’’ back-dipping sites group mean is corrected for 10 of down-to-the-southwest tilt. C. Group means of front-side reverse and normal polarity group means used in the reversal test. D. Grand mean number 2 is calculated from 82 front-side and back-dipping site means accepted for analysis (possessing what we interpret to be primary magnetizations). Grand mean number 2 is calculated with the assumption that front-side sites have remained structurally untilted, whereas sites obtained from the back-dipping side of South Mountains require removal of 10 of dip.

tions of this negative fold test. Acquisition of ation of the Miocene ﬁeld. A paleomagnetic been adequately averaged involves applica- primary component C remanent magnetiza- pole for all South Mountains data was cal- tion of a reversal test. The reversal test (Mc- tions in synkinematic footwall intrusions was culated using site-mean virtual geomagnetic Fadden and McElhinny, 1990) was applied postmylonitic and followed folding of the poles (VGPs) of in situ front-side sites and to all normal and reverse polarity front-side mylonitic zone along the northeast-trending tilt-corrected (ϳ10Њ) back-dipping sites sites (normal polarity group mean, N ϭ 42,

mountain range–scale antiform. Alterna- (pole lat. 89.7ЊN, pole long. 109.5ЊE, N ϭ 82, D ϭ 359.4Њ, I ϭ 53.4Њ, k ϭ 62.9, ␣95 ϭ 2.8Њ; tively, antiforms represent the oldest struc- R ϭ 79.148, K ϭ 28.4, A95 ϭ 3.0Њ). The dis- reverse polarity group mean, N ϭ 20, D ϭ tures of this metamorphic core complex persion (circular standard deviation, ␦63 181.4Њ, I ϭϪ45.7Њ, k ϭ 75.9, ␣95 ϭ 3.8Њ; see (along with earliest formed mylonites), or S) is 15.2Њ, with the 95% confidence in- Fig. 7C). The result was a negative reversal and the antiforms were enhanced by up- terval between 17.0Њ and 13.7Њ for the VGP test. The reversal test passes if the observed ward pushing of a buoyant pluton during distribution (Cox, 1969). This observed an- angle between these two group means is intrusion, but before acquisition of pri- gular dispersion is less than the expected Ͻ4.6Њ. The data set fails the reversal test mary magnetizations (see Yin, 1991; value predicted by paleosecular variation because the two group means are 7.8Њ apart. Lister and Baldwin, 1993). We favor the sec- model G of McFadden et al. (1991). Using The low dispersion associated with both po- ond interpretation because it is corroborated their paleolatitude range of 20Њ to 30Њ, for larity groups likely reflects the ability of by positive baked contact tests that indicate the interval of 5.0 to 22.5 Ma, model G these rocks to partially average paleosecular the presence of primary TRMs and high- yields a predicted value of 19.0Њ, with the variation at the site level and, by implication, temperature TCRMs (component C) ac- 95% confidence interval between 20.5Њ and the intrusion level by cooling relatively quired early in the cooling history of these 17.7Њ. We interpret the overall consistency slowly through their blocking temperatures. rocks and synchronous with extensional de- to suggest that South Mountains rocks have This is consistent with the time period formation (as discussed above). approximately averaged paleosecular varia- spanned by the different intrusions (ϳ5 We have applied several tests to deter- tion, with the caveat that some averaging m.y.), the medium-grained texture of all mine whether the South Mountains rocks may occur at the site level. Another assess- granitoids sampled, the horizontal distribu- have adequately averaged paleosecular vari- ment of whether paleosecular variation has tion of sampling locations (sites are up to 10

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TIMING OF ACQUISITION OF REMANENT MAGNETIZATIONS

Information on the thermal and structural history of the South Mountains metamorphic core complex is critical to linking timing of acquisition of remanent magnetizations in undeformed parts of synkinematic intrusions to periods of extension. The thermochronology of undeformed footwall granodiorite has been studied with isotopic (U-Pb zircon, 40Ar/39Ar hornblende and biotite, and K-Ar biotite) and fission-track apatite age determination methods (Fig. 9; Rey- nolds et al., 1986; Fitzgerald et al., 1994). Unfortunately, there are large uncertainties in the cooling rates derived from these data due to a relatively large (Ϯ4.0 m.y.) error in the U-Pb zircon data. 40Ar/39Ar and K-Ar age determinations have relatively low associated errors of Ϯ0.5 m.y., whereas the fission track age determinations have errors of Ϯ1.0 m.y. (Fitzgerald et al., 1994). The overall cooling rate indicated by these data is 192 Ϯ 74 ЊC/m.y., between 22 and 17.5 Ma (from an assumed 750–800 ЊC for zircon to 110 ЊC). Fitzgerald et al. (1994) suggest a likely decrease in cooling rates from 334 Ϯ 276 ЊC/m.y. between 22 and 20 Ma (800 to 350 ЊC) to 86 Ϯ 34 ЊC/m.y. from 20 to 17.5 Ma (350 to 110 ЊC). The period of inferred rapid cooling between 22 and 20 Ma is considered to reflect both extensional denudation and ambient cooling of the granitic intrusions; whereas the decreased rate of cooling between 20 and 17.5 Ma is a result of Figure 8. Equal area projection with superimposed geologic map and interpretive cross extension only. Cooling rate estimates for section of the South Mountains metamorphic core complex. In situ group means (squares), lower-plate rocks at South Mountains are with ␣95 confidence cone, of all front-side and back-dipping sites are also illustrated (same similar to other metamorphic core com- as Fig. 7B). All symbols represent lower hemisphere projections (data inverted from reverse plexes from western Arizona where thermo- polarity). The open circles near zero represent the Miocene expected directions. The down- chronologic data indicate rapid synkine- to-the-southwest tilt path (wide gray arrow) is based on tilting of these expected directions, matic cooling rates Ͼ40 ЊC/m.y. (Foster et in 10 increments (open circles at 10 , 20 , and 30 ), about a 330 horizontal axis perpen- al., 1993). dicular with the lineation in the mylonites (‘‘tilt axis’’). Location of the in situ group mean Field relations indicate that mylonitiza- of back-dipping sites is interpreted to indicate that the southwest-dipping mylonitic front tion of footwall granitoids began shortly was synkinematically tilted down-to-the-southwest by about 10 . This supports the folded after crystallization at 22 Ma (ϳ750 ЊC; Rey- shear zone hypothesis for origin of the mylonitic front. Location of the in situ group mean nolds, 1985). Therefore, we interpret acqui- of front-side sites is interpreted to indicate that the front side of South Mountains has sition of primary, component C magnetiza- remained structurally untilted and unfolded. tions in footwall granitoids and secondary magnetizations in Proterozoic host rocks to have occurred after warping of the high- km apart), and the number of dikes sampled clinations of reverse polarity magnetiza- strain zone along northeast-trending, moun- (24 separate felsic and mafic dikes). A rea- tions may be artificially low due to an un- tain range–scale arches and synchronous sonable explanation for low between-site removed normal polarity, component A with later stages of mylonitization of struc- dispersion is that paleosecular variation has magnetization. Therefore, a failed rever- turally higher rocks (between 22 and 20 been at least partially averaged on a within- sal test is considered an unavoidable con- Ma). Felsic dikes that cut strongly my- site level. Artificially increasing the inclina- sequence of low dispersion at the be- lonitized parts of the granodiorite and gran- tion value of the reverse polarity group tween-site level for both polarities and ite intrusions are only weakly mylonitized. mean by 3.2Њ (Ϫ45.7Њ to Ϫ48.9Њ), results in a incomplete removal of a contaminating Therefore, component C magnetizations positive, classification ‘‘A’’ reversal test. In- normal polarity magnetization. found in undeformed parts of felsic dikes

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41.8, ␣95 ϭ 2.5Њ; Fig. 7; Table 1). These two grand means are statistically indistinguishable and reflect a consistency of data sets on all levels. Grand mean no. 2 is considered more appropriate (Fig. 7D), because it in- cludes two front-side site means from Prot- erozoic rocks that we could not include in grand mean no. 1. We have calculated the grand means with the assumption that footwall rocks of the front-side part of South Mountains have remained structurally untilted and unfolded, whereas sites obtained from the back-dipping side of South Moun- tains require correction for 10Њ of southwest dip (along a 330Њ strike, orthogonal with the Figure 9. Cooling curve, with error bars, for the South Mountains Granodiorite (Tsm; lineation in mylonites). The grand means data from Fitzgerald et al., 1994). The overall cooling rate indicated by these data is are statistically indistinguishable (95% con- C/m.y., between 22 and 17.5 Ma (from ϳ750 to 110 C). The wide gray arrow fidence level) from time-averaged expected 74 ؎ 192 illustrates the interpretation of a decrease in cooling rate from 334 ؎ 276 C/m.y. between directions for Miocene time (Fig. 7D). We and 20 Ma (ϳ750 to 350 C) to 86 ؎ 34 C/m.y. for the period from 20 to 17.5 Ma (350 interpret the agreement to indicate that the 22 to 110 C). current dip of front-side mylonites and detachment faults (dip of 5Њ–15Њ northeast), combined with the restored attitude of back- are interpreted to have been acquired dur- site level (e.g., site Tm56), suggesting a late- dipping mylonites (roughly horizontal), is ing final phases of ductile deformation of kinematic acquisition of magnetization in original. structurally higher rocks. Mafic dikes cross- these rocks. We interpret acquisition of We interpret these data to demonstrate cut the Proterozoic rocks, granodiorite and well-grouped, magnetite-carried magnetiza- that large-magnitude extension was accom- granite intrusions, felsic dikes, and mylo- tions of both polarities in mylonites to be modated along low-angle structures (dip of nites (Tm) and are not mylonitically de- related to synkinematic circulation of mag- 0Њ–15Њ northeast) during both ductile and formed. Mafic dikes that intersect the South matic fluids along the high-strain zone. Sta- brittle phases of deformation. Progressive, Mountain detachment fault are highly ble isotope data interpreted to reflect rock isostatically induced unroofing and asym- brecciated and thus are postkinematic with equilibration with magmatic fluids suggest metric arching accompanied this deforma- respect to ductile deformation and pre- to temperatures of deformation along the high tion (along a broad arch striking 330Њ, per- synkinematic with respect to brittle defor- strain zone to be about 600 ЊC for ductile pendicular to lineation in mylonites; see mation (Reynolds and Rehrig, 1980; Rey- phases and 400 ЊC for brittle phases (Smith map view inset of Fig. 10), resulting in 10Њ of nolds, 1985). Primary component C magne- et al., 1991). These rocks are interpreted paleomagnetically resolvable back-tilting of tizations found in mafic dikes and the to have been above maximum magnetite the back-dipping side mylonitic zone. The mylonites are interpreted to have been ac- unblocking temperatures during active rollover of the mylonitic zone at South quired during the transition from ductile to mylonitization and fluid circulation. We Mountains is more pronounced than the brittle deformation. The slower, yet still interpret rapid late-kinematic cooling, ac- broad, open rollovers found in other meta- rapid, cooling of footwall rocks below companied by acquisition of remanent morphic core complexes where there are no 350 ЊC, between 20 and 17.5 Ma, is tenta- magnetization in the mylonites, to have synkinematic plutons (Reynolds and Lister, tively correlated with the transition from occurred just before the onset of brittle 1990). This suggests the influence of local ductile to brittle deformation associated detachment faulting as the mylonites isostatic adjustments related to buoyant syn- with intrusion of mafic dikes and cooling of cooled through their range of blocking kinematic intrusions, in addition to regional the mylonite. Brittle faulting as young as 17 temperatures (below 580 ЊC). isostatic adjustments, as a cause of back-tilt- Ma is indicated by tilted upper-plate rocks ing of the mylonite zone (Reynolds and of the Tempe Buttes andesite (dipping 30Њ DISCUSSION AND CONCLUSIONS Lister, 1990). Because back-tilting of the to 45Њ southwest; 17.8 Ma K-Ar biotite), mylonite zone was facilitated by southwest- cropping out about 5 km northeast of South We obtained two well-grouped grand side-down, high-angle shear zones, it is con- Mountains (Pewe et al., 1987). Fission-track means from all accepted site and group ceivable that some back-tilting may not be age determinations on apatites from unde- mean directions. We calculated grand mean paleomagnetically detectable. That is, down- formed granodiorite indicate that the South no. 1 from the front-side group means (Tsm, dropping of individual blocks by subparallel, Mountains metamorphic core complex has Ttp, Tfd, Tmd, and Tm) and the tilt-cor- subvertical shear zones would result in no resided at temperatures of Ͻ110 ЊC since 17 rected group mean of all back-dipping sites net tilting of the blocks, but an overall warp-

Ma (end of major deformation; long track (N ϭ 6, D ϭ 1.5Њ, I ϭ 51.4Њ, k ϭ 510.2, ␣95 ing of the mylonitic zone. If this were the lengths with a mean length ϭ 14 ␮m; ϭ 3.0Њ). We calculated grand mean no. 2 case, then an additional restoration (along Fitzgerald et al., 1994). from site mean data (62 front-side site with the 10Њ required by the paleomagnetic Mylonitic rocks (Tm) possess well- means and 20 tilt-corrected, back-dipping data) of 10Њ–20Њ may be necessary to return grouped, dual polarity magnetizations on a site means; N ϭ 82, D ϭ 1.0Њ, I ϭ 51.7Њ, k ϭ back-dipping mylonite to the same plane as

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ping structures (Spencer, 1984). Use of paleomagnetic data to assess the validity of rolling-hinge models that require a moderate-angle ramp involves knowing at what stage in the deformation history footwall rocks acquired remanent magnetizations. As maintained by Holm et al. (1993), an apparent lack of tilt of footwall rocks does not preclude large amounts of tilting during unroofing related to migration of the footwall through a moderate-angle rolling-hinge ramp. This is because in the rolling-hinge model, footwall rocks that tilt at the onset of unroofing (corresponding to the onset of movement of footwall through a rolling- hinge ramp) will effectively untilt as the rolling hinge migrates through the upper crust (Fig. 11). Certainly, if all phases of the South Mountains synkinematic magmatic suite were emplaced and acquired remanent magnetizations while the footwall was in its lower subhorizontal attitude (Fig. 11), then complete migration of the footwall through a moderate-angle rolling-hinge ramp would not be paleomagnetically detected. The South Mountains granodiorite and granite are midcrustal plutons, intruded during duc- Figure 10. Interpretive structural cross sections and map of the South Mountains met- tile phases of extension. We argue that these amorphic core complex (MCC) illustrating the structural evolution of this metamorphic intrusions acquired primary magnetizations core complex based on paleomagnetic data. These data demonstrate that large-magnitude before unroofing by a rolling-hinge ramp. extension occurred along low-angle structures during both ductile and brittle phases of The timing of remanent magnetization ac- deformation. Extensional deformation was accompanied by progressive, isostatically in- quisition in the dikes (Tfd and Tmd) and the duced unroofing and arching along a 330 striking arch (perpendicular to lineation in mylonites (Tm), relative to the deformation mylonites as depicted in the map view inset). Arching resulted in 10 of paleomagnetically history, then becomes critical, because data detectable back-tilting of the mylonitic zone along the back-dipping side of South Moun- for these rocks also do not suggest signifi- tains, and insignificant warping of the front-side structures. We interpret the paleomag- cant tilt. These rocks were intruded and ac- netic data to indicate that the northeast-trending folds formed early in the extensional quired their remanent magnetizations dur- history (northwest-southeast cross section of upper right corner). These folds constitute the ing the latest phases of ductile deformation prominent northeast-trending mountain-range-scale antiform of the South Mountains met- and the transition from ductile to brittle amorphic core complex. Abbreviations are same as in Figure 1. deformation (discussed above). Especially important in this regard are mafic dikes intruded during brittle deformation. A transi- front-side structures. The front side of biting (15–6 km) normal faults that have tion from ductile to brittle behavior is a South Mountains, however, lacks similar tilted to subhorizontal attitudes (Miller et likely response to unroofing as the footwall high-angle shear zones that could have al., 1983; Davis, 1983; Miller, 1991; King and moves up a rolling-hinge ramp (Wernicke caused undetectable tilting of front-side Ellis, 1990; Brun et al., 1994). Furthermore, and Axen, 1988). Acquisition of remanent structures. Our interpretation of the paleo- South Mountains paleomagnetic data are magnetizations in dikes and mylonites is magnetic data suggests that only minor not considered to support rolling-hinge therefore considered to have occurred dur- warping of front-side structures could have models of metamorphic core complex evo- ing footwall unroofing. For South Moun- resulted from the isostatic adjustments that lution that require unroofing of metamor- tains this would result in the net lack of tilt- caused back tilting of the mylonites (see phic core complexes to have occurred along ing of older granitoids and, for younger map view inset of Fig. 10). a moderate-angle ramp (dip of 30Њ–60Њ; dikes and mylonites, tilting of some 30Њ–60Њ Our interpretation of the data from South Fig. 11; Buck, 1988, 1993; Wernicke and down-to-the-southwest (assuming vertical Mountains also calls into question the wide- Axen, 1988; Hamilton, 1988; Holm et al., dike injection during footwall motion up the spread applicability of models that predict 1993; Manning and Bartley, 1994). Rather, rolling-hinge ramp). A rolling-hinge ramp all metamorphic core complexes to repre- our data favor models of extensional un- mechanism is thus inconsistent with South sent crustal-size blocks, originally bounded roofing and isostatic rebound of metamor- Mountains paleomagnetic data and the ob- by moderately dipping (60Њ–30Њ) and deeply phic core complexes along more gently dip- served subvertical attitude of the dikes.

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ing. In summary, South Mountains paleomagnetic data cannot be used to directly support rolling-hinge models of metamorphic core complex evolution (Fig. 11). Rather, paleomagnetic data support a less- complicated model of a gently inclined high- strain zone during all phases of deformation (mylonites and detachment faults) with coeval pluton cooling and tectonic unroofing (Fig. 11). Holm et al. (1993) have interpreted paleomagnetic data from footwall rocks of the Black Mountains of the Death Valley region to support rolling-hinge models of metamorphic core complex evolution. Structural in- terpretations of Black Mountains based on paleomagnetic data, however, are complex and require an episode of regional tilting (20Њ–40Њ) and local folding, followed by a substantial amount (50Њ–80Њ) of clockwise, vertical axis rotation. If the Holm et al. (1993) interpretation of vertical axis rotation is correct, then the early episode of regional tilting may reflect unroofing associated with movement of the footwall up a moderate-angle rolling-hinge ramp. This interpretation, however, requires additional evidence to substantiate claims of 50Њ–80Њof clockwise, vertical axis rotation. The paleomagnetically detected tilt in the Black Mountains block footwall rocks should only be provisionally interpreted as supporting the rolling-hinge model. Coleman (1991) and Coleman and Walker (1994) have interpreted structural and preliminary paleomagnetic data from footwall rocks of the Miocene Mineral Moun- tains metamorphic core complex, west- Figure 11. Interpretive structural cross sections illustrating application of the rolling- central Utah, to indicate large-magnitude, hinge model (Wernicke and Axen, 1988; Hamilton, 1988) to the structural evolution of the yet local, footwall tilting. This tilting is con- South Mountains metamorphic core complex (MCC). Application of this model would sidered an isostatic response of the footwall result in the net lack of tilting for the older synkinematic granitoids (Tsm and Ttp), and to extension along the initially moderate-an- tilting of some 30 to 60 down-to-the-southwest for younger dikes and mylonites (assuming gle Cave Canyon detachment fault. Footwall subvertical intrusion of dikes as the footwall moved up the rolling-hinge ramp between 22 tilting of the Mineral Mountains metamor- and 17 Ma). This rolling-hinge model is not supported by our interpretation of South phic core complex varies from nonexistent Mountains paleomagnetic data. Abbreviations are same as in Figure 1. along its western front side, to up to 85Њ (east-down) along its eastern back-dipping side. The untilted front side and steeply Alternatively, all phases of the synkine- rapid cooling rates of footwall granitoids in- tilted back-dipping side of Mineral Moun- matic magmatic suite may have acquired dicated by thermochronologic data (Fig. 9; tains are interpreted to be separated by a characteristic magnetizations while the de- Fitzgerald et al., 1994) and the paleomag- broad zone of gradually increasing tilt (ϳ10 tachment fault and its footwall were in the netic baked contact tests. Baked contact km wide; Coleman, 1991; Coleman and lower subhorizontal attitude, with the tran- tests indicate reheating of granodiorite and Walker, 1994). This style of tilting at Min- sition from ductile to brittle deformation re- granite host rocks to laboratory unblocking eral Mountains is not dissimilar to South lated to ambient cooling of all intrusions. temperatures between 400 and 625 ЊCby Mountains, where the front side remains un- The granitoids and dikes may then have mafic dikes (Figs. 4 and 5). This suggests tilted and the back-dipping side is tilted moved through a rolling-hinge ramp after that these intrusions cooled to at least about 10Њ to 20Њ. What differs considerably is remanence acquisition, resulting in no pa- 400 ЊC before the onset of brittle deforma- the amount of tilting of the back-dipping leomagnetically detectable tilting. We do tion. Rapid cooling requires the need for side, and the width of the zone between an not favor this possibility because of the very coeval pluton cooling and tectonic unroof- untilted front side and a tilted back-dipping

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side (Ͻ1 km at South Mountains versus 10 for their hospitality and gratefully acknowl- treated as individual sites (e.g., Tmd25 and km or more at Mineral Mountains). The edge M. L. Alter (Arizona State University), Tsm25). contrast may be attributed to crustal density R. F. Butler (University of Arizona), D. Ba- We cut samples into one or two, standard-size, 2.5 by 2.2 cm specimens. Remanent magnetiza- differences resulting from a volumetrically har, M. J. Grubensky, B. K. Horton, and tions were measured using a 2G Enterprises larger synkinematic pluton in the Mineral H. D. Rowe (University of New Mexico Model 760R three-axis superconducting rock Mountains, which enhanced isostatic uplift [UNM]) for field assistance. G. D. Acton magnetometer in a magnetically shielded room. over a broader area in the Mineral Moun- (UNM) wrote the outlier rejection program We typically demagnetized samples in 10–20 pro- tains than in the South Mountains. The fun- for estimation of site means. We also thank gressive steps to peak alternating fields up to 135 mT with a 2G Enterprises instrument, or to tem- damental interpretation of detachment faults the City of Phoenix Parks and Recreation peratures up to 680 ЊC with a Schonstedt demag- associated with Mineral Mountains meta- Department for permission to collect core netizer (TSD-1 furnace). In cases where samples morphic core complex as initially having a samples. This manuscript was improved by possessed high-coercivity magnetizations, alter- broad listric geometry (moderate-angle comments and reviews from R. F. Butler, nating field demagnetization was followed by breakaway flattening with depth; Coleman G. H. Davis, J. F. Diehl, L. B. Goodwin, thermal demagnetization. Demagnetization results were examined using and Walker, 1994) is similar to our inter- K. E. Karlstrom, R. S. Molina-Garza, and modified orthogonal demagnetization diagrams pretation of detachment faults at South G. A. Smith. (Zijderveld, 1967). Least-squares analysis of Mountains (Fig. 10). Kirschvink (1980) was used to calculate the best- Development of low-angle detachment APPENDIX: PALEOMAGNETIC METHODS fit vectors to the demagnetization data for each structures is favored in a rheologically lay- specimen, usually involving 4 to 12 (unanchored) demagnetization steps applied to isolate magne- ered continental crust (Davis and Lister, All samples were collected as independently oriented cores using a portable, gas-powered cor- tizations. Samples with magnetization vectors iso- 1988; Lister and Davis, 1989; Yin, 1989; ing drill. We usually obtained sample azimuths lated by principal component analysis with max- Reynolds and Lister, 1990; Melosh, 1990; with both magnetic and sun compasses. We de- imum angular deviation (MAD) values of Ͼ15Њ Forsyth, 1992). Detachment structures may tected no significant local magnetic anomalies af- were rejected from further analysis. Because of initiate at moderate dips in brittle upper fecting magnetic compass readings. The number the common presence of north-declination, moderate positive inclination secondary remanence crust and flatten at midcrustal levels (about of samples collected at each site ranged from as few as 5, to as many as 82. For baked contact tests (component A, discussed below), demagnetiza- 10–15 km depth) due to the low strength of (e.g., a mafic dike intruding a granodiorite), 5–15 tion steps used to isolate primary TRMs and faults or the rotation of stress axes because samples of each rock type, plus 10–50 samples TCRMs (component C magnetizations, discussed of ductile, perhaps fluid-like (Wernicke, across the contact zone, were taken. In these below) usually ranged from unblocking tempera- 1992), flow of a weak middle crust beneath cases, the samples of each rock type collected at tures of 400 ЊCto600ЊC (or 680 ЊC) and peak a distance well removed from the contact were alternating fields of 14–135 mT. In some speci- the brittle to ductile transition zone (Spen- cer and Reynolds, 1989). For South Moun- tains, synkinematic intrusions and associated high magmatic fluid pressures (Smith et al., 1991) probably raised the brittle to ductile transition zone to between 5 and 10 km depth. This facilitated nucleation of a low-angle detachment structure at shallow crustal levels that, after magmatic activity, cooled relatively quickly. Elsewhere, the dip of this detachment structure may have flat- tened at deeper crustal levels, resulting in an along-strike warped geometry of the detachment structure, with only the structurally highest level (i.e., South Mountains) currently exposed at the surface. Variations in dip of master detachment fault structures, ranging from low-angle to moderate-angle, may then be controlled by lateral variations in thermal and mechanical properties of a layered crust (Davis and Lister, 1988; Lister and Davis, 1989; Yin, 1989; Reynolds and Lister, 1990; Melosh, 1990; Forsyth, 1992).

ACKNOWLEDGMENTS

This work was funded by Sigma Xi, the Student Research Allocation Committee at the University of New Mexico, and National Science Foundation award EAR-92-06524. Figure A1. Curves showing the stepwise acquisition of a direct-current isothermal remanent mag- We thank S. B. Keith and S. Ruff of Mag- netization (IRM) and back-ﬁeld demagnetization of saturation IRM (SIRM) obtained from eight maChem Exploration in Phoenix, Arizona, representative granodiorite (Tsm), granite (Ttp), felsic (Tfd), and maﬁc dike (Tmd) specimens.

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Figure A2. Curves illustrating the response in alternating field demagnetization of the natural remanent magnetization (NRM), saturation isothermal remanent magnetization (SIRM), and anhysteretic remanent magnetization (ARM) of four representative granodiorite (Tsm), granite (Ttp), and mafic dike (Tmd) specimens. Tmd50 represents a dual polarity mafic dike.

mens considered contaminated by component A, only the highest unblocking temperature or coercivity steps were used, and we usually anchored these to the origin in principal component analysis. Calculations of site mean directions interpreted to be primary, early-acquired magnetizations, were based on the methods of Fisher (1953). Cal- culations of the group and grand mean directions were based on the methods of both Fisher (1953) and Bingham (Onstott, 1980). Demagnetization data acquired from both alternating field and thermal demagnetization were used for site mean determinations. We gave each specimen unit weight when results from only one specimen of a given sample were used in the analysis. In cases Figure A3. Curves illustrating the where we analyzed multiple specimens of a single response in thermal demagnetization sample, we applied unit weight to each specimen of the natural remanent magnetiza- direction to determine the average sample directions (NRM) and saturation isother- tion. At sampling locations where a single rock mal remanent magnetizations (SIRM) type possessed a remanence of dual polarity, sam- of three representative granodiorite ples defining each polarity subset were treated as (Tsm), granite (Ttp), and mafic dike two separate populations and we determined two (Tmd) specimens. means. Specimens were considered outliers and rejected from the estimate of the site mean direction if (1) the angular distance of the sample

direction was both Ͼ2␪63 (␪63 ϭ angular standard deviation of population deﬁning the site mean) and Ͼ10Њ from the site mean or (2) the angular distance of the sample direction was Ͼ30ЊC from

the site mean (regardless of the value of ␪63). We performed this outlier rejection method in two iterative steps. Site means of secondary, partial TRMs were also calculated for samples involved in baked contact tests. Because of the small scale of some contact tests (secondary, partial TRMs isolated in host rock samples Ͻ5 cm from a contact) and potential inhomogeneous reheating of host rocks, all individual specimen PTRMs were considered

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/8/877/4649975/i0016-7606-107-8-877.pdf by guest on 28 September 2021 Figure A4. Photomicrographs of ferromagnetic minerals taken with reflected light in air of representative samples of South Mountains intrusive suite and Proterozoic rocks. The scale bar in the lower left of each micrograph represents 10 ␮m. A. Proterozoic Estrella gneiss (p–Ce6), euhedral, unoxidized magnetite grains in groundmass adjacent to a biotite phenocryst. B. Telegraph Pass Granite (Ttp7), cracks in quartz partially filled with hematite. C. Mafic dike (Tmd5), euhedral magnetite grains in groundmass with hematite along cleavage planes (martite). D. Felsic dike (Tfd8), euhedral to partly embayed, unoxidized magnetite grains in groundmass. E. Felsic dike (Tfd8), euhedral, unoxidized magnetite grains in groundmass. F. Felsic dike (Tfd8), euhedral to partly embayed, unoxidized magnetite/ilmenite pair in groundmass. G. Proterozoic Estrella gneiss (p–Ce6), euhedral, unoxidized magnetite grains in plagioclase feldspar.

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significant. Therefore, we did not apply the outlier demagnetization; Fig. A3) closely correspond to than normal polarity specimens (Fig. A1, com- rejection method to specimens with PTRMs re- the laboratory unblocking temperature spectra of pare Tfd74 Bb and Ea). Conversely, in the dual lated to partial thermoremanent remagnetization, NRM. This demonstrates that for those sites with polarity mafic dike (Tmd50), normal polarity ma- isolated over a specific unblocking temperature well-defined, well-grouped magnetizations, the terial possesses a greater concentration of higher range and distance from an igneous contact. Also, entire population of ferromagnetic grains present coercivity grains than reverse polarity specimens contact test PTRM site means were calculated by carries equal fractions of the remanence and sup- (Fig. A1, compare Tmd50 Ba and Gb). There is giving unit weight to all individual specimens from ports the inference that the magnetizations are of no correspondence between remanence polarity a sample. thermoremanent origin. and dominance by either high or low coercivity Of the 115 sites demagnetized, we accepted 82 A more thorough understanding of the origin magnetite as the principal magnetic phase. Simi- for inclusion in group and grand means. Sites with of the ChRMs found in intrusive and Proterozoic larly, a modified Lowrie-Fuller test of normal and the following characteristics were rejected: (1) rocks was provided by optical inspection of mag- reverse polarity samples from the mafic dike in- sites with poorly grouped mean directions as in- netic oxides (Fig. A4, A–G). Examination of pol- dicates a slightly higher ratio of single domain to dicated by ␣95 of Ͼ20Њ (27 sites); (2) sites that ished thin sections with reflected light in air re- multidomain magnetite grains in the normal po- possessed anomalously high NRM intensities vealed the presence, in most rock types, of readily larity sample than in the reverse polarity sample (ϳ10–100 A/m) and random directions of mag- identifiable euhedral to subhedral, fresh to par- (Fig. A2). netizations, probably lightning induced (three tially oxidized magnetite grains. We interpret tex- Results from site Tfd51 offer a clue to the origin sites, located along the topographic crest of South tural associations between these magnetite grains of dual polarity magnetizations in dikes. The dike Mountains); and (3) sites that yielded well- and silicate phases, such as euhedral to subhedral sampled at this site is unique among dual polarity grouped mean directions considered outliers inclusions in feldspars and discrete grains in the dikes because two distinct rock types are present; when compared with other well-grouped site groundmass of porphyritic rocks, to indicate pri- a porphyritic phase (containing up to 50% feld- mean directions (three sites). We did not apply mary, magmatic origin of the magnetite. Except spar and biotite phenocrysts, Tfd51-1) and an the outlier rejection method to determination of for mafic dikes, magnetic oxide grains are rare in aphanitic phase (Tfd51-2). The porphyritic phase the group and grand means, where all accepted granitoids and felsic dikes (i.e., volume Ͻ0.05%). intrudes the aphanitic phase. The porphyritic site means were given unit weight. Of the 82 sites This is consistent with the relatively low magne- phase has a reverse polarity-affinity direction and accepted for analysis (possessing what we inter- tization intensities of these rocks (NRM intensi- the host aphanitic phase has a normal polarity- pret to be primary magnetizations), 24 are of re- ties of ϳ0.001 A/m). Hematite occurs in a range affinity remanence. Both Tfd51-1 and Tfd51-2, verse polarity and 58 are of normal polarity, with of associations, including alteration products of however, were rejected from inclusion in the 5 of these sites representing dual polarity dikes, 2 ferromagnesian silicate minerals and fracture fill- group mean because of high dispersion. We in- representing a dual polarity mylonite site, and 2 ings in nonferromagnesian silicate minerals terpret the data for site Tfd51 to indicate that representing a dual polarity granite site. (Fig. A4, B). magma intrusion occurred in multiple pulses We have addressed potential effects on the along this, and perhaps other, dikes. Multiple ROCK MAGNETISM remanence by penetrative extensional deforma- pulses of magma along the same dike, during op- tion of intrusions using anisotropy of magnetic posite polarities, may partially overprint previ- We conducted rock magnetic studies to more susceptibility data (AMS; R. F. Livaccari, unpub. ously acquired magnetizations of one polarity adequately assess the carriers of ChRMs found in data). Mylonite, granodiorite, and granite sites with the opposite polarity. This may explain why South Mountains footwall rocks and to demon- are characterized by well-developed magnetic in four of the six dual polarity dikes, only one of strate that these rocks are reliable recorders of fabrics with lineation anisotropy in agreement the two polarity magnetizations identified could the paleomagnetic field. Stepwise acquisition of a with orientations of finite extensional fabrics (to- be accepted for inclusion in the group mean, with direct current isothermal remanent magnetiza- tal anisotropy, based on Owens, 1974: Tsm ϭ the other polarity component characterized by tion (IRM) and back-field demagnetization of sat- 17.1%, n ϭ 123; Ttp ϭ 13.2%, n ϭ 80; Tm ϭ high dispersion. It is also possible that some in- uration IRM data was carried out on representa- 13.1%, n ϭ 23). The felsic and mafic dikes are trusions, with very short cooling times, recorded tive granodiorite, granite, felsic, and mafic dike characterized by weakly developed magnetic fab- transitional fields related to field reversals. We specimens (Fig. A1). All specimens, except Ttp1, rics consistent with a flow-related emplacement suggest that multiple intrusions along dilating acquired 90% of their saturation IRM in induced origin (total anisotropy, based on Owens, 1974: dikes, over dual field polarities, resulted in a com- fields between 0.2 and 0.3 T, indicating that prin- Tfd ϭ 4.4%, n ϭ 70; Tmd ϭ 3.9%, n ϭ 64). We plex thermal history and partial resetting of cipal magnetic phase present is magnetite. The interpret the well-developed magnetic fabric in thermoremanent magnetizations in dual polarity presence of subordinate amounts of hematite in the mylonites, granodiorite, and granite to have a dikes and adjacent host rocks. these specimens is indicated by the gradual ac- negligible effect on remanence properties. quisition of about 10% of the saturation IRM in The presence of dual polarity magnetizations of REFERENCES CITED direct fields up to 0.4–1.0 T (Fig. A1). 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Arizona: Arizona Bureau of Geology and Mineral Technol- REVISED MANUSCRIPT RECEIVED DECEMBER 13, 1994 Kirschvink, J. L., 1980, The least-squares line and plane analysis of ogy, Geological Survey Branch, Bulletin 195, 61 p. MANUSCRIPT ACCEPTED DECEMBER 20, 1994

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