Gravity-Induced Folding Off a Gneiss Dome Complex, Rincon Mountains, Arizona

Gravity-Induced Folding off a Gneiss Dome Complex, Rincon Mountains, Arizona

GEORGE H. DAVIS Department of Geosciences, University of Arizona, Tucson, Arizona 85721

ABSTRACT rocks crop out in isolated, shallow-dipping Most of them suggested a northerly sheets (1 to 25 km2 in extent) around the directed thrusting during early to middle Detached isoclinal folds, overturned base of the Rincon Mountains (Fig. 2). The Tertiary time. Drewes (1971, 1973) pro- asymmetric folds, and unbroken cascades sedimentary rocks (and their metasedimen- posed that the Paleozoic and Mesozoic of recumbent folds pervade sheets of tary equivalents) rest subconcordantly rocks were transported at least 16 to 32 km sedimentary and metasedimentary rocks of upon the gently to moderately steeply dip- northeastward by regional thrusting during Paleozoic and Mesozoic age that occur ping surface of the granitic gneiss (Fig. 3). the Laramide orogeny. McColly (1961) and around the base of the Rincon Mountains This surface, which conforms in attitude to Arnold (1971) suggested that the folds and near Tucson, Arizona. The sheets of folded the foliation in the gneiss, locally is marked faults that they mapped in sedimentary rocks rest subconcordantly on the gently by grooves and slickensides and was rocks on the west and south flanks of the dipping surface of the granitic gneiss that mapped by Pashley (1966) as the Catalina Rincon Mountains formed during gravity composes much of the range. This surface, fault. gliding of the rocks off the granitic gneiss known as the Catalina fault, parallels the Low-angle tectonic movement has oc- during Miocene time. attitude of the foliation in the gneiss and is curred within the sheets of Paleozoic and The focus of this study is a determination folded about two macroscopic upright an- Mesozoic sedimentary rocks. The dynamics of the tectonic transport direction(s) of the tiforms and an intervening synform. of the low-angle displacement have been in- sheets of sedimentary rocks based on slip- The low-angle tectonic displacement terpreted both by regional compressional line orientations calculated for folds in reflected in the folds was brought about by overthrusting and local gravitational tec- those sheets. The results are compared to local gravitational tectonics. The slip-line tonics. Darton (1925), Moore and others existing transport models to determine directions inferred from the geometry of the (1941), Brennan (1957), Layton (1957), whether the fold-forming event(s) resulted fold arrays define a radial pattern centered Acker (1958), Kerns (1958), Weidner from regional overthrusting, from gravity on the Rincon Mountains. The forms of the (1958), and Pashley (1966) attributed the tectonics related to emplacement of the folds are consistent with the characteristics folds and faults in the foothills of the Rin- granitic gneiss, or from a combination of of gravity-induced folds. con Mountains to regional overthrusting. the two processes. Most of the gravity-induced folding is interpreted to have accompanied the 28- to 24-m.y. uplift that ended the Tertiary metamorphism of gneiss in the Rincon Mountain complex. The Catalina fault is interpreted to be a décollement, above which the sedimentary and metasedimentary rocks folded independently of their substratum. Key words: structural geology, gravity tectonics, folds, gneiss dome, structural analysis.

INTRODUCTION

The Rincon Mountains near Tucson, Arizona (Fig. 1), which herein include the Tanque Verde Mountains, are composed predominantly of gneissic granitic rocks that have been folded into two large, open antiforms and an intervening synform. The limbs of these N. 60° E.—trending upright symmetrical folds dip on the average 15° to 20° (Fig. 2; Pashley, 1966). A north-striking high-angle normal fault of Miocene(?) to Pleistocene age (Drewes, 1971) truncates the folds to the east and defines the pre- cipitous eastern boundary of the Rincon Mountains (Fig. 2). Paleozoic and Mesozoic sedimentary Figure 1. Location map.

Geological Society of America Bulletin, v. 86, p. 979-990, 11 figs., July 1975, Doc. no. 50715.

979

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MAJOR ROCK UNITS Rincon Valley Granite ronment in which the folds formed and thus be useful in distinguishing among the fold- Catalina Gneiss The Rincon Valley Granite was described forming tectonic processes mentioned by Moore and others (1941) as a medium- above. The term "Catalina Gneiss" was intro- to coarse-grained, greenish granitic rock The properties of folds in sedimentary duced by DuBois (1959) to refer to the composed of sericitized potassium feldspar, and metasedimentary rocks of Paleozoic gneissic granitic rocks that make up the oligoclase, and interstitial quartz with and Mesozoic age at five locations around core of the Rincon Mountains and the chloritized biotite. It is distinctive in the the Rincon Mountains were examined in Santa Catalina Mountains (Fig. 1). DuBois field because of its light greenish-brown/ this investigation. The areas range in areal (1959) divided the Catalina Gneiss into gray color on weathered surfaces and its extent from 1 to 25 km2 and include banded augen gneiss, augen gneiss, and highly shattered condition. The rock has Saguaro National Monument (Paleozoic granitic gneiss/gneissic granite. Mayo the composition of granodiorite and locally rocks), Loma Alta (Mesozoic rocks), Col- (1964) studied the cataclastic foliation that grades into quartz monzonite (Acker, ossal Cave (Paleozoic rocks), Martinez characterizes the banded gneiss in the 1958; Arnold, 1971). K-Ar age deter- Ranch (Paleozoic rocks), and Bear Creek forerange of the Catalina Mountains; he minations of 1,540 ± 60 m.y. obtained (Paleozoic and Mesozoic rocks) (Fig. 2). described it as consisting of aligned mica by Marvin and others (1973) indicate that Each represents a separate structural do- books and quartz and feldspar augen, ac- the Rincon Valley Granite is Precambrian. main characterized by essentially uniform centuated by layers and lenses of pegmatite, Drewes has mapped the "Rincon Valley bedding strike. Specific methods employed quartz lenses and sheets, dark coarse- Granodiorite" and considers it to represent in this study are described in Appendix 1. grained mica schist, and bands of porphy- an allochthonous plate, the middle of three riticorporphyroblastic rock. Drewes (1973, regional thrust plates (Drewes, 1971). Geometric Analysis personal commun.) has disclosed that the core rocks in the Rincon Mountains are Phanerozoic Sedimentary Rocks Saguaro National Monument. Sedi- petrologically more complex than previ- mentary rocks on the west side of the ously recognized. Among the mappable The Paleozoic sedimentary rocks in the Rincon Mountains within Saguaro Na- units that Drewes (1972) has distinguished foothills of the Rincon Mountains consist tional Monument (Fig. 2) consist of lime- are gneissic biotite-muscovite quartz mon- chiefly of limestone interbedded with lesser stone, dolomite, shale, and limestone con- zonite, porphyritic biotite-rich meta- amounts of siltstone and shale. An account glomerate of Permian(?) age (McColly, granodiorite, schist, and metadiorite or of the Paleozoic formations cited in this 1961), as well as remnants of Pennsylva- metadiabase. "Catalina Gneiss" is used paper is provided by Bryant (1968). nian, Cretaceous, and possibly Mississip- herein merely as a general term for the crys- Mesozoic rocks are sparse in the Rincon pian formations (Drewes, 1974, personal talline, granitic-gneissic rocks in the Rincon Mountain area (Fig. 2). They belong to the commun.). The rocks strike northeast, dip and Santa Catalina Mountains. Bisbee Group of Early Cretaceous age and northwest, and are essentially parallel to The age(s) of the rocks from which the have been mapped by Arnold (1971) in the upper surface of the underlying Cata- Catalina Gneiss was derived, the age(s) of Rincon Valley and by Drewes (1972) in lina Gneiss, which lies in the northwest metamorphism, and the age of macroscopic Happy Valley and in Saguaro National limb of the Tanque Verde antiform. Lime- folding in the Catalina Gneiss are uncer- Monument (Drewes, 1974, personal com- stone units are locally metamorphosed tain. Damon and others (1963) have con- mun.). along the sedimentary rock-gneiss contact. cluded, on the basis of (1) the spatial rela- Sedimentary rocks of the Tertiary Pan- Much of the field work centered on tion of the Catalina Gneiss to its mantle of tano Formation are exposed along much of analyzing folds in limestone and shale ex- younger Precambrian Apache Group rocks the margin of the Rincon Mountains, where posed on a steep south-facing hillside in the in the Catalina Mountains (Waag, 1968) they are characteristically moderately central part of the domain. McColly (1961) and (2) the 1,660-m.y. age derived from the steeply dipping and broken by abundant interpreted the structure there as a single lead isotopic content of zircon in the gneiss, normal faults (Brennan, 1957; Arnold, large recumbent fold. However, the rocks that most of the Catalina Gneiss was de- 1971). Dating demonstrates that the Pan- are replete with unbroken cascades of re- rived from older Precambrian rocks. The tano Formation includes rocks that are cumbent and overturned folds (Fig. 4). In- characteristic K-Ar ages of approximately pre—lower Oligocene and post—lower terbedded shale units are pervaded by a 27 m.y., obtained from mica in the gneiss, Miocene in age (Metz, 1963; Damon and bedding-plane cleavage and contain root- further suggested to Damon and others Bikerman, 1964; Damon and others, 1965, less isoclinal folds and S-shaped minor folds (1963) that the core rocks of the Santa 1966; Finnell, 1970). that are asymmetric basinward. Catalina—Rincon complex had been sub- Visual harmonic analysis of 23 individual jected to a Cretaceous-Tertiary metamor- FOLDS surfaces of folds revealed the relatively phic event that ended by late Oligocene broad shape-to-amplitude distribution to early Miocene time. Mayo attributed the The basis for the fold analysis used in this shown in Figure 5 (see Appendix 1). The information of the banded variety of Catalina investigation is Sander's principle of sym- terlimb angles for these folds range on the Gneiss to metasomatism of the Pinal Schist metry (Sander, 1948, 1950). Specifically, I average from 15° to 80°. Fold hinge zones brought about by ingress of heat along assumed that (1) the (statistical) symmetry are typically subangular to subround, with reactivated east-northeast—trending Pre- of the fold elements faithfully reflects the P i values from approximately 2.0 to 7.0. cambrian zones of weakness. He suggested kinematics of low-angle movement of the Analysis of the shapes of fold layers indi- that the upright macroscopic folds de- sheets within which the folds are found; (2) cates that folds in Saguaro National Mon- veloped through vertical uplift, citing as the absolute orientations of the folds, com- ument generally belong to Class 1C of evidence flowage in the gneiss away from bined with knowledge of the asymmetry of Ramsay (1967) and thus are intermediate axes of uplift (Mayo, 1964; Peterson, the folds, can be used to define the actual between ideal parallel and ideal similar 1968). Implicit in Mayo's model is that the movement direction(s) for the sheets; and folds (Fig. 5). 28- to 24-m.y. K-Ar ages record the termi- (3) careful study of the fold profiles might The folds in limestone and shale in nation of metasomatism and vertical uplift. contribute information regarding the envi- Saguaro National Monument are typically

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/7/979/3433802/i0016-7606-86-7-979.pdf by guest on 30 September 2021 FOLDING OFF A GNEISS DOME COMPLEX, RINCON MOUNTAINS, ARIZONA 981 inant rock type and typically displays a SAGUARO well-developed bedding-plane cleavage. NATIONAL MONUMENT Along the northern perimeter of the sheet, the sedimentary rocks rest in fault contact on the Catalina Gneiss (Fig. 2). The gneiss and sedimentary rocks have approximately the same attitude, striking west- northwest and dipping approximately 20° S. Limestone is metamorphosed within a stratigraphic interval of approximately 15 m. Along the southern perimeter of the sheet, the sedimentary rocks are in low- angle fault contact with Rincon Valley Granite. The sedimentary rocks at Loma Alta contain abundant mesoscopic folds (Fig. 7). No systematic variations as a function of rock type could be detected through visual harmonic analysis (Fig. 5). The interlimb angles for most of the 43 surfaces examined range from 10° to 45° and herein are considered "tight" to "close." Pt for the most part varies from 0.6 to 7.0, indicating that the hinge zones are subround to subangular. The shapes of 15 fold surfaces (Fig. 5) conform to Class 1C, but they approach the form of ideal similar folds (Class 2). Stereographic projection of elements of 136 folds in the Loma Alta domain indicate that the folds in general are recumbent to overturned, characterized by subhorizontal to gently plunging axes and gently inclined r~~1 QUATERNARY ALLUVIUM WM MESOZOIC SEDIMENTARY ROCKS axial surfaces (Fig. 6). The average axial- I 1 CATALINA GNEISS •• PALEOZOIC SEDIMENTARY ROCKS surface attitude is approximately N. 70° 1 1 -AGE UNCERTAIN- W., 16° NE. Orientations of axes vary im PRECAM8RIAN RINCON VALLEY F!wl TERTIARY GRAVEL GRANITE widely but reveal maxima at approximately 10° N. 20° W. to N. 20° E. 15° S. 60° E. to 5 KM east, and 10° S. 15° E. The axes of 30 une- CONTOUR INTERVAL EQUALS 300 m quivocal asymmetric folds were plotted Figure 2. Generalized geologic map of the Rincon Mountain area. Adapted from Pashley (1966), stereographically (Fig. 6) and were sys- Arnold (1971), and Drewes (1972). tematically distributed. The S-shaped folds range in trend from north-northeast to overturned, with moderately inclined axial Loma Alta. Cretaceous shale and lime- northwest, whereas the Z-shaped folds surfaces and gently to moderately plunging stone with intercalated siltstone and lime- range in trend from northeast to south. axes. Lower hemisphere equal-area net pro- stone conglomerate are exposed at the base Colossal Cave. Sedimentary rocks jections based on 120 folds (Fig. 6) reflect of the southeast flank of Tanque Verde an- studied within the Colossal Cave domain an average axial-surface orientation of N. tiform in the vicinity of Loma Alta (Fig. 2). are Paleozoic in age and form a sheet ap- 20° W., 40° NE. The attitude of fold axes The distribution of Cretaceus sedimentary proximately 150 m thick that rests on the averages 28° N. 20° E., with the plunge rocks there is restricted to two subcircular southeast limb of the Rincon Peak antiform ranging from 0° to 85°. Folds that were un- areas, 3 and 2 km in diameter. The sedi- (Fig. 2). The rocks consist of limestone with equivocally asymmetric have S-shaped mentary rocks within these areas form a interbedded shale and include formations profiles. sheet less than 90 m thick. Shale is the dom- of Cambrian through Permian age. The shale characteristically has a well-devel- ssw NNE oped bedding-plane cleavage. The distribution of the Paleozoic formations is shown on geologic maps compiled by Acker (1958), Layton (1957), Arnold (1971), and Drewes (in prep.). In general, .the strata strike east and dip 20° N. However, Arnold (1971) noted that the strike of the rocks in the northern part of the Colossal Cave domain defines a convex-southwestward arc. Along the northern edge of the Colossal Figure 3. Geologic cross section showing sheets of folded sedimentary rocks and a sliver of granite Cave domain, the foliation in the Catalina separated by the Catalina fault from the underlying granite gneiss, Colossal Cave domain. Gneiss strikes west-northwest and dips 30°

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Figure 4. Fold structures in the Saguaro National Monument area, looking northeast. A, recumbent fold in thin-bedded limestone and intercalated shale; B, recumbent fold in limestone; C, slightly overturned synform in limestone; D, recumbent fold in thinly laminated limestone.

SW. Its upper surface is subparallel to the Profile analysis of the folds in the Co- sults shown in Figure 5. The folds in lime- overlying sedimentary rocks. At one local- lossal Cave domain was difficult because of stone have layer boundaries that conform ity a 20-m-thick tabular slice of Precam- the large size of the structures. Few Vi to the geometry of Class 1C folds, but they brian Rincon Valley Granite occurs along wavelengths of the folds could be photo- tend to approach Class IB (ideal parallel the contact between the gneiss and overly- graphed in normal section, and thus the folds). In contrast, the shale layers are ing limestone (Fig. 3; Drewes, 1971; Davis methods used for profile studies in the other marked by thickened hinge zones of the and others, 1974a). The granite-limestone domains could be used only sparingly. Con- folds, and they approach the form of ideal contact is a low-angle fault marked by sequently, the discussion of fold profiles similar folds. southwest-plunging grooves and slicken- that follows must be considered tentative Orientation relations of folds in the Co- sides. The Rincon Valley Granite is also ex- and, at best, approximate. lossal Cave domain indicate that the folds posed in windows within the central part of The profiles of eighteen folds were are overturned to recumbent with gently the Colossal Cave domain and is in low- analyzed using Hudleston's visual method, plunging axes and gently inclined axial sur- angle fault contact with the overlying and they have the distribution of properties faces. The axial surfaces (Fig. 6) strike an sedimentary rocks. shown in Figure 5. Fourteen of the folds average of N. 15° W. and dip 20° NE. The Spectacular macroscopic recumbent and have interlimb angles of less than 30° primary mode for axial orientations is 20° overturned folds pervade the sedimentary (tight), and four have interlimb angles be- E., but a secondary mode reflects an at- rocks in the Colossal Cave domain. The tween 30° and 60° (close). The range in Pi titude of 15° N. 20° W. The secondary folds are best exposed in Posta Quemada values for the eighteen folds is from 0.5 to north-northwest mode is based on fold at- Canyon in the northern part of the area 00, and thus the hinge zones vary in form titudes measured exclusively in the extreme (Fig. 8). Like most folds examined in this from subround through subangular to northwestern part of the domain. The ex- project, the folds in the Colossal Cave do- chevron, with most being subangular or posed portions of the limbs of the asymmet- main have unbroken hinge zones. The chevron. Ramsay's methods for determin- ric macroscopic folds are parts of Z-shaped topographic relief of 150 to 300 m affords ing changes in orthogonal thickness were asymmetric folds (viewed from west to east; an excellent vantage for viewing and map- applied to only two normal profiles of folds Davis and others, 1974a). ping the unbroken cascades of folds. in the Colossal Cave domain with the re- Martinez Ranch. The Martinez Ranch

domain is at the base of the exposed part of rocks, predominantly limestone, represent east-northeast, dip from 20° to 40° S., and the southern limb of Rincon Mountain an- parts of the Horquilla Limestone, Escab- are concordant to the underlying surface tiform and consists of a narrow sheet con- rosa Limestone(P), Abrigo Formation, and atop the Catalina Gneiss. Limestone in the taining approximately 75 m of gently Bolsa Quartzite (Drewes, 1972; Liming, lower 50 m of the exposed section is mar- dipping Paleozoic rocks (Fig. 2). These 1974). The sedimentary rocks strike east to bleized. The rocks are truncated to the east by a north to north-northwest-striking normal fault (Fig. 2). A B C D E F The folds in the Martinez Ranch domain 1.0 3_CLASS are chiefly mesoscopic and are best exposed along the steep northern edge of the Mar- t' tinez Ranch sheet. There the folds are over- SAGUARO ì, CLASS turned and knee-shaped with fold size vary- NATIONAL 3 0.5-1 IC ing (in part) as a function of layer thickness MONUMENT CLASS 2- (Fig. 9). Locally, metamorphosed limestone CLASS 3 units crop out in which intraformational 0 0- folds are defined by siliceous interbeds. I oc 90° Visual harmonic analysis of 35 folds in A B C D E F limestone reveals the distribution shown in 1.0- -CLASS IB-i Figure 5. Most range in interlimb angle " CLASJC S t' from 20° to 80° and thus may be considered close to open folds. The hinge zones are for LOMA the most part subangular to chevron, as in- 0.5- CLASS 2 - ALTA dicated by the dominant range in P, values CLASS 3 from 3.0 to The plot of variations in orthogonal thickness of 13 layers for folds 0.0 in limestone reveals that the folds in the 0° OC 90° Martinez Ranch area are transitional be- 1.0 CLASS IB—i tween Class 1C and Class 2 (Fig. 5). CLASS Stereographic projections of elements of t' IC folds in the Martinez Ranch domain indicate that the folds are overturned and COLOSSAL 0.5- characterized by gently plunging axes and CAVE CLASS 2' gently inclined axial surfaces. Specifically,

CLASS 3 axial surface orientations for the folds average N. 50° W., 25° NE., and axes plunge on the average 20° S. 75° E. (Fig. 6). The A B C D E F plot of axes, according to asymmetry (Fig. 6), is systematic and indicates that the east-plunging folds are Z-shaped, whereas the west-plunging folds are S-shaped. MARTINEZ Bear Creek. The Paleozoic and RANCH Mesozoic sedimentary rocks herein assigned to the Bear Creek domain are exposed in foothills along the eastern face of the Rincon Mountains (Fig. 2). The rocks generally strike northeast, dip gently to moderately southeast, and rest on the Catalina Gneiss whose upper surface and foliation bear the same approximate attitude. Folds are exceptionally well exposed BEAR in one subarea within the domain. CREEK Specifically, a steep north-facing slope along a 1-km segment of Bear Creek contains hundreds of isoclinal and tight folds in marble and calc-silicate rocks. The metasedimentary rocks in which the folds Figure 5. Fold profile analysis for the domains under consideration. Diagrams on left are based on are contained belong to the Horquilla For- Hudleston's visual profile analysis of individual surfaces defined between inflexion and hinge points. mation (Drewes, 1972). The discussion that Columns A through F represent increasingly chevron shapes; rows 1 through 5 correspond to follows is based almost exclusively on folds folds of decreasing V« wavelength amplitude. Profiles on margins are intended to give morphologic exposed at this locality. meaning to the respective boxes and are not meant to imply average forms. Solid squares = limestone; The steep nature of the north-facing open squares = calc-silicate rocks and marble; solid circles = shale; open circles = siliceous interbeds; solid triangles = siltstone. Diagrams on right reveal forms of individual layers and are based on Ram- slope affords excellent exposure of the folds say's orthogonal thickness parameters, t' = proportional change in orthogonal thickness throughout (Fig. 10). As in the other domains, the folds the fold; a = measure of limb inclination. Class IB = ideal parallel layer; Class 2 = ideal similar are, for the most part, contained between layer; Class 1C = intermediate between ideal parallel and ideal similar. For the Bear Creek domain, homoclinally dipping layers and may be re- the solid lines represent marble layers; dotted lines represent calc-silicate layers. garded as intraformational. The folds are

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characterized by unbroken hinge zones and ideal parallel and ideal similar folds and Movement within individual flexural-slip by cascades of overturned and recumbent thus are characterized by some hinge-zone and flexural-flow folds generally takes place forms. Tabular bodies of aplite and pegma- thickening. The scarcity of axial plane along planes subparallel to bedding and tite locally have been intruded along the cleavage, the abundance of bedding-plane along linear paths approximately normal to axial surfaces of some of the folds. Most of cleavage, and the obvious influence of layer- the hinge lines. Because most of the folds the folds recognized in this part of Bear ing on the morphology of the folds suggest examined in this study are tight or nearly Creek domain are mesoscopic. that hinge-zone thickening was accom- isoclinal, it is possible to approximate the The profiles of 61 individual surfaces of plished by flow within the layers. Accord- movement plan during the final stage of de- folds in the Bear Creek domain are ex- ingly, the folds may be classed kinemati- velopment of individual folds by specifying amined (Fig. 5). The interlimb angle for cally as flexural-slip and flexural-flow folds that (1) the movement occurred in planes most of the folds is less than 30°; in fact, that evolved through the combination of subparallel to the axial surface for a given many of the folds display interlimb angles slippage between layers and flow within fold, and (2) the line of relative movement of less than 10°. Accordingly, the folds may layers (Donath and Parker, 1964). was contained in the axial surface and be labeled "tight" to "near isoclinal." The SAGUARO NAT'L MON hinge zones are generally subround, as indi- BEAR CREEK cated by the dominance of i\ values of less than 5.0. It is evident that marble layers in folds in the Bear Creek domain display ap- preciably greater proportional changes in orthogonal thickness than the calc-silicate layers. This difference is graphically shown in the plot of changes in orthogonal thickness (Fig. 5). The folds in marble approximate the geometry of ideal similar folds (Class 2), whereas the folded calc-silicate layers are Class 1C folds. The folds examined in the vicinity of the Bear Creek domain are overturned and characterized by gently plunging axes and moderately inclined axial surfaces (Fig. 6). The axial surface orientations average N. 20° E., 35° SE., and the axes plunge an average of 20° S. 25° E. The fold axes, when stereographically plotted according to asymmetry (Fig. 6), reveal a nonsystematic distribution; that is, the southeast-plunging folds are both Z- and S-shaped. LOMA ALTA Summary COLOSSAL CAVE MARTINEZ RANCH

Table 1 is a summary of the rocks and structures in the five domains. Each is characterized by thin sheets of sedimentary (and metasedimentary) rocks that rest subconcordantly on the upper surface of the Catalina Gneiss. With the exception of the Bear Creek domain, the gneiss in each domain composes part of a limb of one of the major northeast-trending upright folds in the Rincon Mountains. The folds within the sedimentary sheets are typically overturned to recumbent, asymmetric down the flanks of the plunging basement antiforms, and characterized by gently plunging axes and gently to moderately dipping axial surfaces. IN0E TERMINATE ISEE TEXTI Kinematic Analysis

In attempting to define the direction(s) of low-angle tectonic transport for the sheets of folded rocks, it is necessary to consider the kinematics involved in the development Figure 6. Lower-hemisphere equal-area net projections showing orientation attributes of folds in of the individual folds in these sheets. It has the domains under consideration. For locations of the domains, see Figure 2 and inset relief map. A, been demonstrated that most of the folds poles to axial surfaces; B, fold axes; C, axes of unequivocal asymmetric folds. Open circles — axes of that were analyzed are transitional between S-shaped folds. Solid circles = axes of Z-shaped folds.

Figure 7. Examples of fold structures in the Loma Alta area. A, detached isoclinal recumbent fold in siltstone, looking east; B, overturned antiform in thin-bedded limestone and shale sequence, looking east; C, recumbent fold in dolomite, looking north; D, recumbent fold in limestone, looking east.

TABLE I. SUMMARY OF FOLD ATTRIBUTES oriented perpendicular to the axis. It is assumed in the analysis that follows that this Saguaro Lorna Alta Colossal Martinez Bear Creek Cave Ranch "approximate movement plan," when defined statistically for the array of folds in Dom1nant Limestone Shale and Limestone Limestone Marble and rock (shale) siltstone (marble) calc-silicate each domain, directly reflects and (or) is (dolomite) rocks symmetrically related to the direction of Sheet 60 m 90 m 150 m 75 m 90 m(?) thickness low-angle displacement for each sheet of Structural Northwest South limb of South limb of South limb of East of folded rocks. position Umb of Tanque Verde Rineon Peak Rincon Peak boundary fault Tanque Verde antiform antiform antiform The trends of slip-line directions for the antiform folds in the Saguaro National Monument, Surface Tight, close, Tight to close, Tight with sub- Close to opening Tight to nearly Colossal Cave, Martinez Ranch, and Bear profile and open, with with subround angular to chevron with subangular isoclinal with subangular to to subangular hinge zones to chevron hinge subround hinge Creek domains are shown in Figure 11. Be- subround hinge hinge zones zones zones zones cause fold-axis orientations for each of Layer form Class 1C Class 1C, Class 1C Transitional Marble, Class 2; these domains tend to cluster about a point approaching between Class 1C Calc-s1l1cate maximum ("b"), it was assumed that the Class 2 and Class 2 rocks, Class 1C mean slip-line direction ("a") for each do- Attitude of N. 40° E., N. 70° W., N. 70° W., N. 80° W., N. 20° E., underlying 25° NW. 20° SW. 30° SW. 30° SW. 30° SE. main is oriented approximately 90° from gneiss "b" and contained in the modal axial sur- Orientation Overturned. Recumbent- Recumbent to Overturned. Overturned. of folds Axes: 28° N., overturned. overturned. Axes: 20° S., Axes: 20° S., face ("ab"). The actual sense of movement 20° E. Axes: subhori- Axes: 15° E. 75° E. 25° E. Axial surfaces: zontal to gently Axial surfaces: Axial surfaces: Axial surfaces: could be determined for the rocks in the N. 20° W., 40° NE. plunging. N. 15° W., 20° NE. N. 50° W., 25° NE. N. 20° E., 35° SE. Saguaro National Monument, Colossal Axial surfaces: N. 70° W., 16° NE. Cave, and Martinez Ranch domains be- Asymmetry Overturned Overturned Overturned Overturned Nonsystematic cause of the systematic asymmetry of the basinward basinward basinward basinward folds in those domains. Specifically, the

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Figure 8. Examples of fold structures in the Colossal Cave area. A, macroscopic cascade of recumbent folds in limestone and shale, looking northeast; B, macroscopic isoclinal recumbent fold defined by massive limestone units, looking east; C and D, hinge zones of recumbent folds in limestone, looking east. folds in sedimentary rocks in Saguaro Na- tionship indicates that the slip-line direction longed fold-forming event (Davis and tional Monument formed as a response to for Loma Alta may be determined by treat- others, 1974b). A correspondence exists in northwest-directed movement, whereas the ing the folds geometrically as slip folds surface profile, layer form, and inclination folds in the Colossal Cave and Martinez (Hansen, 1971, p. 51). The stereographic magnitude of the fold elements from do- Ranch domains evolved as a response to projection of axis orientations plotted ac- main to domain (Figs. 5, 6; Table 1). This south-directed movement. cording to asymmetry yields a separation suggests that in spite of differences in fold The orientations of fold axes in the Loma angle of approximately 30° (Fig. 6). Using morphology attributable to such factors as Alta domain vary widely in trend (Fig. 6B). the methods outlined by Hansen (1971), layer thickness and degree of metamorph- According to Hansen (1971), such varia- the slip-line direction is interpreted here as ism, the environment and dynamics of fold- tion in axial trend is not unexpected in the bisector of this separation angle, namely ing in each of the domains were essentially sheets that have undergone low-angle tec- N. 18° E., S. 18° W. Asymmetry of folds in equivalent. tonic transport. The variation at Loma Alta the sedimentary sheet at Loma Alta indi- The orientations of the slip-line direc- is interpreted to reflect the rotation of fold cates that the specific sense of low-angle tion!; (Fig. 11) are interpreted to reflect the hinges following their formation but during tectonic transport was to the south- approximate transport direction of the continued low-angle movement. An alter- southwest and hence basinward. sheets under consideration. Taken as a native is that the variation reflects complex whole, these transport directions are in- movement. That the fold axes show such ORIGIN OF FOLDS compatible with the transport directions great spread in orientation in the Loma Alta explicit in the regional overthrust models domain alone may be due to the lack of The folds in the five domains are inter- proposed to date. In particular, the south- supportive strength of the shale that con- preted to have formed during a common directed transport proposed for the Loma tains most of the folds. The orientations of episode of low-angle displacement. No evi- Alta, Colossal Cave, and Martinez Ranch the axes, although varied in trend, tend to dence for superimposed folding was ob- domains is opposed to the north and define a great circle whose orientation is served except for several refolded folds northwest-directed transport specified in subparallel to the northwest-striking, within the Saguaro National Monument overthrust models by Brennan (1957), northeast-dipping modal axial surface for domain, and these are the expression of in- Layton (1957), Kerns (1958), Weidner the domain (Fig. 11). This orientation rela- cremental movements during a single, pro- (1958), and Pashley (1966). The inferred

Figure 9. Examples of folds in the Martinez Ranch area. A, overturned antiform in limestone, looking east; B, Z-shaped fold defined by siliceous bed in marble, looking east; C, Z-shaped overturned folds in thinly laminated limestone, looking east; D, hinge zones of overturned antiforms in limestone, looking east.

northeast transport direction in the Bear The forms of the folds are in accord with sheets and (2) asymmetry in the direction of Creek domain is parallel to that obtained the characteristics of gravity folds outlined gliding. Assuming that the Catalina Gneiss by Drewes for Laramide thrust plates by de Sitter (1954). Geologic cross sections in the Rincon Mountains represents a struc- (Drewes, 1973), but the south-directed by Schneegans (1938), Heim (1921), Men- tural high from which the sedimentary transport within the Loma Alta, Colossal gaud (1939), and de Sitter (1949) are pre- sheets descended, both of these conditions Cave, and Martinez Ranch domains is sented by de Sitter (1954) to illustrate the are satisfied by most of the folds in the strongly discordant to regional northeast- common attributes of gravity folds. The study area. ward transport. The northwest transport folds presented in these cross sections are Several factors suggest that gravity- direction for rocks in the Saguaro National identical in morphology to many of the induced folding in the Rincon Mountain Monument domain is inconsistent with the folds in the Rincon Mountain area. area was a response to aspects of the northeastward-directed thrusting proposed Specifically, the gravity structures illus- dynamic history of the Catalina Gneiss. by Moore and others (1941). trated by de Sitter (1954) are characterized These factors include the spatial association The slip-line and tectonic transport direc- by unbroken cascades of overturned and of the recumbent and overturned folds in tions (Fig. 11) are consistent with low-angle recumbent folds ("cascade piles") and the sheets with the underlying Catalina displacement accompanying local gravita- "complete reversed flanks" of folds. Cas- Gneiss, the presence of anomalously highly tional tectonics. The slip-line directions de- cade piles containing complete reversed folded and metamorphosed sedimentary rived stereographically define a radial pat- flanks are abundant in the sedimentary rocks along the Catalina Gneiss contact tern centered on the Rincon Mountains. In rocks in the Rincon Mountain area and zone, and the geometric correspondence of four of the five domains, the asymmetry of probably evolved through gravity-induced the inferred slip-line directions to the dip the folds was unequivocally basinward. folding. Furthermore, the geologic cross direction of limbs of macroscopic folds in Such asymmetry can be explained by sections in de Sitter (1954) connote that the gneiss. The slip-line directions corre- gravity-glide folding away from the struc- gravity-induced fold structures are, for the spond strikingly to the attitude of the folia- tural high now represented by the Rincon most part, characterized by (1) axial planes tion in the Catalina Gneiss. The slip line for Mountains. that dip toward the source area for the glide folds in Saguaro National Monument is

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Figure 10. Examples of folds in the Bear Creek area, looking south-southeast. A, tight recumbent folds in calc-silicate layers (white) and marble (dark gray); B, large isoclinal recumbent fold defined by siliceous interbeds in marble; C, isoclinal overturned fold in calc-silicate rock; D, recumbent folds outlined by calc-silicate layers (white) in marble.

approximately normal to the axial trace of (1963, p. 118), uplift of the Catalina Gneiss and Range province (Damon and Mauger, the Tanque Verde antiform and subparallel in the Rincon Mountain area took place be- 1966). Major Basin and Range faulting in to the local dip direction of the Catalina tween 28 and 24 m.y. ago. Specifically, they southern Arizona was not initiated until Gneiss (Fig. 2). The inferred south- interpreted the 28- to 24-m.y. K-Ar dates post-early Miocene time (Damon, 1968b) southwest sense of slip for folds at Loma for mica from the Catalina Gneiss as an and therefore probably was not responsible Alta suggests that the source area of the indication that "the uplift of the gneiss had for the uplift recorded in the 28 to 24 m.y. sedimentary sheet was approximately coin- progressed to the point that [the event. cident with the culmination of the Tanque Cretaceous-Tertiary] metamorphism had It is suggested that most of the gravity- Verde antiform. The direction of slip is terminated by Upper Oligocene-Lower induced folding accompanied the 28 to 24 subparallel to the dip direction of the upper Miocene time." Damon (1974, personal m.y. uplift that ended the Tertiary meta- surface of the gneiss. The south-directed commun.) believes that a steep temperature morphism in the Santa Catalina—Rincon slip determined for folds in the Colossal gradient existed prior to 28 m.y. ago with complex. This interpretation is consistent Cave and Martinez Ranch areas is sub- the gneiss at temperatures between 400° with stratigraphic constraints that can be parallel to the dip direction of the south and 500°C (Hedge, 1960). Between 28 and placed on the timing of folding. Paleozoic limb of Rincon Peak antiform. The calcu- 23 m.y. ago, the presently exposed gneiss and Cretaceous rocks are deformed by lated slip-line direction for the Bear Creek cooled from 400° to 500°C to about 100°C gravity-induced folds. In addition, the domain is discordant by approximately 60° (Damon, 1968a, 1968b). Damon suggested early Tertiary Pantano Formation locally to the dip direction of the gneiss to the that cooling of the gneiss between 28 and has been deformed by folding and low- north of the domain (Fig. 2), but it is sub- 24 m.y. ago was brought about by domal angle faulting (Brennan, 1957; Layton, parallel to the dip direction of the gneiss to uplift, presumably involving the diapiric 1957; Pashley, 1966; Drewes, 1974, per- the west-southwest. ascent of gneissic domes and arches sonal commun.; Pierce, 1974, personal The specific timing of gravity-induced (Mayo, 1964). The timing of this period of commun.). It is probable that some low- folding is not known for certain because the inferred doming coincides with that of an angle displacement and gravity-induced history of formation, emplacement, and up- intense period of magmatism not only in folding accompanied emplacement and (or) lift of the Catalina Gneiss is not fully under- the area near Tucson (Damon and Biker- incipient uplift of the gneiss during the stood. According to Damon and others man, 1964), but also throughout the Basin "Cretaceous-Tertiary metamorphism" it-

self, prior to the 28 to 24 m.y. uplift re- stratum. The dip angles of the curviplanar viewed the manuscript and provided help- corded in the K-Ar clock. For example, a décollement range up to 30°, providing ful suggestions for its improvement. Cretaceous gravity-induced folding event from a dynamic standpoint a sufficient The Department of Geosciences, Univer- affecting Pinal Schist and Apache Group slope whereby gravity gliding could take sity of Arizona, defrayed field, computer, rocks has been documented by Davis and place. The elevated temperatures postulated and drafting expenses. others (1975) in the Tortolita Mountains, a by Damon (1968a, 1968b) for the gneiss APPEND DC 1. METHODS OF STUDY northwest continuation of the Catalina- during the inferred time of uplift influenced Rincon gneiss complex. The folding ac- the dynamics of gravity gliding. The specific The orientations of approximately 540 folds companied intrusion of a gneissic effects included a decreased viscosity and were determined. Axis and axial-surface orienta- granodiorite body that, in the late stage of increased ductility of the sedimentary and tions usually were measured directly, but in cases emplacement, was profoundly cataclastic- metasedimentary rocks adjacent to the where the folds were extremely large and (or) ally deformed. The Class 2 folds in marble décollement, which rendered creep an poorly exposed, axis and axial surface were de- and calc-silicate rocks in the Bear Creek efficient mechanism for tectonic transport rived stereographically through construction of tt domain might be another example of early of the overlying units (Kehle, 1970). Kehle's and /3 diagrams. Fold orientation data for each synmetamorphic folding. multiple décollement model of gravity glid- domain were converted by computer into ing by distributed simple shear readily ex- lower-hemisphere equal-area net projections The layer forms of the folds in the do- using programs devised by Davis (1972). Slip- mains studied are dominantly Ramsay plains the configuration of gravity-induced line directions based on the statistically preferred Class 1C and 2, indicating that the rocks folds in Saguaro National Monument modes were then defined using methods outlined were moderately ductile at the time of fold- (Davis and others, 1974b), Loma Alta, in Hansen (1971). In each of four of the do- ing. Hinge-zone thickening and the de- Martinez Ranch, arid Bear Creek. mains, the fold axes define a unimodal pole dis- velopment of close to tight to isoclinal folds tribution ("b"), and the slip-line direction ("a") would indicate that the folding took place was interpreted to be 90° from "b" and con- ACKNOWLEDGMENTS under substantial cover, possibly 3 to 4 km. tained within the slip plane ("ab," parallel to the The maximum inclined distance of gravity modal axial surface) (Hansen, 1971, p. 172). In the Loma Alta domain, the fold axis orientations gliding for Paleozoic and Mesozoic rocks Evans B. Mayo introduced me to the define a plane whose orientation is approxi- now exposed in the Rincón Mountain area geology in the Tucson area and kindled my mately parallel to "ab" and the modal axial sur- is approximately 9 km. interest in the problems outlined in this face for the domain. As indicated by Hansen It is suggested that during uplift and grav- paper. Assistance and constructive input (1971, p. 51), folds with such orientation rela- ity gliding, the Catalina fault served as a was provided by students at the University tions may be treated geometrically as slip folds. décollement separating the underlying of Arizona, particularly Eric Frost, Susan Accordingly, for the array of folds in the Loma Catalina Gneiss from the overlying Hunt, Brett Liming, and John Schloderer. Alta domain, the slip-line orientation was deter- sedimentary rocks that — through gliding mined by bisecting the separation angle (Hansen, Harald Drewes, Paul Damon, Evans Mayo, 1971, p. 33-38), that is, "the planar angle that — folded independently of their sub- Robert Scholten, and Barry Voight re- separates fold axes by orientation into groups of SAGUARO NAT'L. MONUMENT BEAR CREEK opposite asymmetry." Normal profiles of many of the folds were photographed so that meausrements of fold morphology could be made readily in the laboratory. Shapes of individual fold surfaces and layers were selected as fundamental criteria for defining fold morphology. The "visual" Fourier profile analysis described by Hudleston (1973) was chosen to define the shapes of indi- 10-15% vidual fold surfaces. Accordingly, individual surfaces of natural folds between inflexion and hinge points were compared to the 30 idealized fold forms constructed by Hudleston and corres- ponding to specific amplitudes (1 to 5) and shapes (A to F) (Fig. 5). On the basis of this comparison, each natural fold was assigned a LOMA ALTA MARTINEZ RANCH shape to amplitude alphanumeric index. Average ,Q o "tightness" of folds within each domain was determined by measuring the interlimb angle of the modal individual surface profile(s) and then emp- loying the nomenclature suggested by Fleuty (1964), that is, 0° interlimb angle = isoclinal; 0° COLOSSAL CAVE to 30° interlimb angle = tight; 30° to 60° inter- a. limb angle = close. As Ramsay (1967, p. 349) pointed out, apparent tightness is in part a func- 3-6-9% 5-10-15% tion of the extent of the hinge zone. Thus, in this study, Ramsay's measure of the hinge zone breadth (P,) was calculated from modal surface profiles so that the significance of the interlimb angle as a measure of tightness could be better evaluated.1 The forms of individual folded layers 10-15-20% 1 P, = length of projection of limbs on the join of the inflexion points /length of projection of hinge zone of the Figure 11. Interpreted slip-line directions (a-a) for domains under consideration. Inset relief map join of the inflexion points. For the purposes of this shows locations of domains. Each lower-hemisphere projection contains (1) solid-line contours of fold study, hinge zones were classified on the following basis: axes, (2) dashed-line contours of poles to axial surfaces, and (3) great circle representing modal P, = chevron; <=° > P, > 5, subangular; S > P, > 0, axial-surface orientation. subround.

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were analyzed using Ramsay's method (1967, p. and chronology of ore deposits and vol- and orogenic translation: Geol. Soc. 359-362) based on orthogonal thickness canic rocks: Ann. Rept. to U.S. Atomic America Bull., v. 81, p. 1641-1664. parameters. The proportional change in or- Energy Comm., C00-689-50, Table 26, p. Kerns, J. R., 1958, Geology of the Agua Verde thogonal thickness (i') in selected folds was de- 50. Hills, Pima County, Arizona [M.S. thesis]: termined as a function of limb dip (a) relative to 1966, Correlation and chronology of ore Tucson, Univ. Arizona, 69 p. the axial surface normal (Fig. 5). Each natural deposits and volcanic rocks: Ann. Rept. to Layton, D. W., 1957, Stratigraphy and structure fold was assigned to a specific "Ramsay" class U.S. Atomic Energy Comm., C00-689-60, of the southwestern foothills of the Rincon 2 and (or) subclass (Ramsay, 1967, p. 365-372; Table 14, p. 26. Mountains, Pima County, Arizona [M.S. Fig. 5). Darton, N. H., 1925, A resume of Arizona geol- thesis]: Tucson, Univ. Arizona, 87 p. To clarify the configuration of the folds and ogy: Arizona Bur. Mines Bull. 119, 298 p. Liming, R. B., 1974, Geology and kinematic fold patterns, details of the structural geology Davis, G. H., 1972, Deformational history of the analysis of deformation in the Martinez were mapped in parts of the Saguaro National Caribou strata-bound sulfide deposit, Ranch area, Pima County, Arizona [M.S. Monument and Colossal Cave domains. In par- Bathurst, New Brunswick, Canada: Econ. thesis]: Tucson, Univ. Arizona, 86 p. ticular, a 20,000 m2 area containing complexly Geology, v. 67, p. 634-655. Marvin, R. F., Stern, T. W., Creasey, S. C., and folded limestone and shale in Saguaro National Davis, G. H., Eliopulos, G. J., Frost, E. G., Mehnert, H. H., 1973, Radiometric ages of Monument was mapped at a scale of 1:120 Goodmundson, R. C., Knapp, R. B., Lim- igneous rocks from Pima, Santa Cruz, and (Davis and others, 1974b; Davis and Frost, in ing, R. B., Swan, M. M., and Wynn, J. C., Cochise Counties, southeastern Arizona: prep.). Also, cascade and recumbent folds within 1974a, Recumbent folds — Focus of an in- U.S. Geol. Survey Bull. 1379, 27 p. part of the Colossal Cave domain were mapped vestigative workshop in tectonics: Jour. Mayo, E. B., 1964, Folds in gneiss beyond North at a scale of 1:1,200 (Davis and others, 1974a). Geol. Education, v. 22, p. 204-208. Campbell Avenue, Tucson, Arizona: Davis, G. H., Frost, E. G., and Schloderer, J. P., Arizona Geol. Soc. Digest, v. 7, p. 1974b, Scrutiny of folded gravity-glide 123-145. sheets in Saguaro National Monument, McColly, R. A., 1961, Geology of the Saguaro Arizona: Geol. Soc. America Abs. with National Monument, Pima County, 2 Subclass IB, parallel folds characterized by constant Programs, v. 6, p. 439. Arizona [M.S. thesis]: Tucson, Univ. orthogonal thickness; subclass 1C, orthogonal thickness Davis, G. H., Anderson, Phillip, Budden, R. T., Arizona, 80 p. on the flank of the fold is less than at fold hinge, but t' Keith, S. B., and Kiven, C. W., 1975, Origin Mengaud, L., 1939, Etudes géologiques dans la always exceeds cos a\ Class 2, ideal similar fold; Class of lineation in the Cataiina-Rincon- région de Gavarnie et du Mont Perdu: 3, layer thickness on flank of fold measured parallel to axial trace is less than at hinge. Tortolita gneiss complex, Arizona: Geol. France Service Carte Géol. Bull. 199, p. Soc. America Abs. with Programs, v. 7 (in 197-223. REFERENCES CITED press). Metz, Robert, 1963, The petrography of the Pan- de Sitter, L. U., 1949, Le style Nord-Pyrénéen tano beds in the Cienega Gap area, Pima Acker, C. J., 1958, Geologic interpretation of a dans les Alpes Bergamasques: Soc. Géol. 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Engineers Trans., v. 235, p. thesis]: Tucson, Univ. Arizona, 29 p. 99-112. Heim, A., 1921, Geologie der Schweiz, Band II: Damon, P. E., Erickson, R. C., and Livingston, Leipzig, Tauchnitz, p. 367-369. MANUSCRIPT RECEIVED BY THE SOCIETY D. E., 1963, K-Ar dating of Basin and Hudleston, P. J., 1973, Fold morphology and DECEMBER 17, 1973 Range uplift, Catalina Mountains, Arizona: some geometrical implications of theories REVISED MANUSCRIPT RECEIVED NOVEMBER 18, Nuclear Geophysics-Nuclear Sci. Ser., no. of fold development: Tectonophysics, v. 16, 1974 38, p. 113-121. p. 1-46. Damon, P. E., and Associates, 1965, Correlation Kehle, R. O., 1970, Analysis of gravity sliding Printed in U.S.A.

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