Structural Geology Of The Santa Rita Mountains, Southeast .
Structural Geology of theSanta Rita Mountains,Southeast of Tucson,ArizonaGEOLOGICALSURVEYPROFESSIONALPAPER 748
Structural Geology of theSanta Rita Mountains,Southeast of Tucson,ArizonaBy HARALD DREWESGEOLOGICALSURVEYPROFESSIONALPAPER 748A description of the abundant and locallycomplex systems of faults, folds, andstructurally controlled intrusives andan analysis of their developmentUNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972
UNITED STATES DEPARTMENT OF THE INTERIORROGERS C. B. MORTON, SecretaryGEOLOGICAL SURVEYV. E. McKelvey, DirectorLibrary of Congress catalog card No. 72 600231For sale by the Superintendent of Documents, U.S. Government Printing Oflfice, Washington, D.C. 20402Stock No. 2401-00239
CONTENTSPagePageAbstract.- ---- - - .Introduction. - - ---.Location and objectives.Acknowledgments.Structural features.Southwestern structural unitSan Cayetano block.Grosvenor Hills block.Montosa Canyon block.Salero block.Santa Rita fault scar.Central structural unitMount Wrightson blockAdobe Canyon block. .Sawmill Canyon fault zone.Northeastern structural unit.Greaterville block1223366791113141415161717Structural features ContinuedNortheastern structural unit ContinuedRosemont block.Helvetia block. . - - .Tear faults.Thrust faultsFolds-DikesRange-front faultsStructural development-Pre-Laramide deformation. -- --.Laramide Orogeny,Piman phase. - -- ---- .Mid-Laramide quiescent timeHelvetian phase. - - -- -Post-Laramide deformation.References Plates are in pocket]PLATE 1.2.3.4.Tectonic map of the Santa Rita Mountains.Geologic map of the Montosa Canyon-Glove mine area, west flank of the Santa Rita Mountains.Geologic map of the Adobe Canyon-Sawmill Canyon fault zone area, east flank of the Santa Rita Mountains.Geologic map of the Rosemont-Helvetia area, north end of the Santa Rita Mountains.Page228FIGURE 1. Index map of part of southeastern Arizona. ' .2. Diagram summarizing the tectonic record of the Santa Rita Mountains,TABLEPage4TABLE 1. Rocks of the Santa Rita MountainsIII
STRUCTURAL GEOLOGY OF THE SANTA RITA MOUNTAINS,SOUTHEAST OF TUCSON, ARIZONABy HARALD DREWESABSTRACTThe Santa Rita Mountains and the hills to the southwest areunderlain by an extensive sequence of sedimentary, volcanic, andmetamorphic rocks of Precainbrian to Holocene age. This sequenceis more complete than any similar sequence in other mountainsof southeastern Arizona. The structural record of the area is alsoextensive. Faults are the most abundant structural features;intrusive bodies are numerous, and many of them are structurallycontrolled; and folds are common in some parts of the mountains.The structural features, however, have not obliterated the stratigraphic record, because areas of complexly deformed rocks areseparated by areas of only slightly deformed rocks. The successionof structural events is thus decipherable, and many of the eventsare geologically closely dated by radioinetric ages of the igneousrocks and by a few faunal ages of the sedimentary rocks. TheSanta Rita Mountains are, therefore, a key area for understandingthe regional tectonic development.The major structural features of the Santa Rita Mountains arethe northwest-trending high-angle fault zones the Santa Ritafault zone to the southwest and the Sawmill Canyon fault zone tothe northeast. These fault zones are of Triassic to about Paleoceneage. They separate three major structural units whose generalattitudes and internal minor structural features differ considerably from each other. The Santa Rita fault zone contains a recordof great vertical movement and of possible lateral movement;however, successive plutonic intrusions have obliterated all butsmall remnants of this zone. The Sawmill Canyon fault zonecontains a record of repeated vertical movement, some thrustfaulting, and possibly some left-lateral movement.The southwestern major structural unit, lying southwest of theSanta Rita fault zone, contains two areas in which faulting andmagmatic activity are closely related. The first of the two areas,the San Cayetano Mountains-Grosvenor Hills area, was both intruded and covered by a granodiorite-rhyodacite complex of late(?)Oligocene age. After the time of magmatic activity, deep-levelintrusives of the San Cayetano Mountains were raised along alarge normal fault to the level of the shallow intrusives of theGrosvenor Hills area and the Grosvenor Hills Volcanics. Abundantsmall normal faults border some of the shallow intrusives, chieflylaccoliths, that were emplaced in the volcanic pile. Several grabenblocks foundered into parts of the volcanic pile. In the second ofthe areas, the Montosa Canyon area, remnants of a thrust plateand tear fault show evidence of both northeastward and southwestward transport during Late Cretaceous to early Tertiary time.The younger southwestward movement was accompanied by theemplacement of an extensive igneous sheet containing abundantand large exotic blocks, believed to be largely foundered remnantsof the roof of a shallow sill that was emplaced beneath the thrustplate about the time that the thrust plate moved southwestward.Large stocks intruded the Montosa Canyon area volcanic complexonly a few million years after the last major volcanic eruption.The central major structural unit, between the Santa Rita andSawmill Canyon fault zones, is for the most part a simple eastwarddipping homoclinal block. In the northeast corner of the unit,Upper Cretaceous rocks are tightly folded along northwestwardtrending axes. Several swarms of dikes and one of quartz veins, allof early Tertiary age, trend east to northeast across the unit.The northeastern major structural unit, northeast of the Sawmill Canyon fault zone, is cut by many northwest-trending tearand normal faults, as well as by thrust faults of two ages. Therocks in thrust plates of late Late Cretaceous age are disharmonically folded, chiefly along northwest-trending axes and southwest inclined axial planes, showing that the plates were transported northeastward. Small stocks cut some of these thrustfaults. Other thrust faults and northwest-trending left-lateral tearfaults of late Paleocene age show evidence of a northwest transportdirection. Some of these faults cut the stocks. Plugs and dikesassociated with ore deposits intrude many of the younger faultsand the axial planes of some folds. Structural features are especiallycomplex where younger faults that are associated with northwesttransport are superposed on segments of older faults or foldsassociated with northeast transport. Inconspicuous or short rangefront faults lie along both flanks of the northern part of the mountains.The tectonic development of the area, indicated by an analysisof the structural record presented here and the sedimentary recordpresented in supplementary reports, is similar to that of the surrounding region, and only the local record of the developmentduring the Mesozoic is more complete than the regional record.The most ancient rocks, the Pinal Schist and Continental Granodiorite, show effects of the Mazatzal Revolution, which are typicalof central Arizona. Alternating upward and downward epeirogenicmovements throughout the Paleozoic Era are recorded by amarine sequence whose continuity is interrupted by several disconformities. Strong vertical movements, largely on faults, occurred at intervals from the Triassic to the Early Cretaceous; twostocks were injected into the rocks of the area, at about the end ofthe Triassic and during Middle Jurassic time.During the Laramide Orogeny, which lasted from about 90to 53 ni.y. (million years) ago, the area was severely deformed.The orogeny apparently took place in two phases, an early (Piman)phase and a late (Helvetian) phase, which were separated by aperiod of tectonic quiescence 10-20 ni.y. in duration. The Pimanphase began with folding in the early Late Cretaceous and culminated with northeast-directed thrust faulting of what was probably a single relatively thin plate mainly composed of beddedrocks. Abundant subsidiary tear faults and folds deformed theplate during the Piman phase, and some favorably oriented segments of older faults were reactivated. The Piman phase ended
SANTA RITA MOUNTAINS, SOUTHEAST OF TUCSON, ARIZONAwith the emplacement of several large stocks in the late LateCretaceous. The Helvetian phase of late Paleocene age comprisednorthwest-directed thrust and tear faulting and emplacement ofsmall stocks. At the close of the Helvetian phase, plugs and dikesof quartz latite porphyry intruded the faults, and mineralizingfluids associated with these intrusive rocks spread along the faults.In post-Laramide time the area was deformed largely by normalfaults. Many of these faults were associated with late(?) Oligocenevolcanism; others were related to intrusion of several dike andvein swarms into east- to northeast-trending tension fractures.During the late Tertiary the area was tilted gently southeast on arange-front fault, along which the youngest movement was latePleistocene.INTRODUCTIONLOCATION AND OBJECTIVESThe geology of the Santa Rita Mountains was intensively investigated during the years 1962-69. Geologic mapping of the Mount Wrightson and Sahuaritaquadrangles, lasting a total of about 350 days during1962-68, showed the mountains to be underlain by astratigraphic sequence that is unusually complete forsoutheastern Arizona. Consequently, the structuraldevelopment of the area may be interpreted in greaterdetail than is possible elsewhere in this part of theState. An understanding of the succession of structuralevents in the Santa Rita area can assist similar studiesthroughout the region and may be especially useful innearby areas where the geologic record is less completeand the interest in reconstructing that record is greaterbecause of the mining activity.The Santa Rita Mountains are the first range southeast of Tucson. They extend more than 25 miles (40 km)southward, from Pantano Wash, along which the mainhighway and railroad east of Tucson lie, to SonoitaCreek, about 12 miles (20 km) from the Mexicanborder (fig. 1). The mountains commonly reach heightsof 6,000-7,000 feet (2,000 m), but Mount Wrightsonreaches an elevation of 9,453 feet (2,881 m). Thebroad valley to the east of the mountains lies at about110-00'110 30'Y LITTLE'[J'RA'GOONMOUNTAINS32 00'31 30'UNITIMEXICO10I1020 MILES20 KILOMETERSFIGURE 1. Location of the Santa Rita Mountains in southeastern Arizona. Shaded areas show location of Sahuarita and MountWrightson quadrangles.
STRUCTURAL FEATURES4,500 feet (1,400 m), whereas that to the west is only3,000 feet (900 m) high. The mountains thus aresufficiently high to receive enough orographic rainfallto support extensive, largely scrubby forests, whichcontrast markedly with the grasslands to the east andthe Sonoran Desert vegetation to the west.The objectives of this report are to describe andinterpret the structural features of the Santa RitaMountains. More specifically, the area studied is theMount Wrightson quadrangle (Drewes, 1971b) andthe Sahuarita quadrangle (Drewes, 1971a); thus thearea includes the smaller San Cayetano Mountainsand Grosvenor Hills, which lie to the southwest of theSanta Rita Mountains, but it does not include thenortheast tip of the Santa Rita Mountains. Detailedsystematic descriptions of rocks are avoided here byreferring to the topical reports, chiefly by Drewes(1971c, 1972a) and to unpublished data by Drewes;other reports are cited in the text. The local stratigraphic section is summarized in table 1. Some pertinentdetails, however, are repeated or expanded in this textbecause of the frequent close connection between thesedimentary, plutonic, and volcanic records and thestructural record. The interpretation of the regionaltectonics will be presented in a separate report.The geologic investigation of the Santa Rita Mountains is part of a larger program of the U.S. GeologicalSurvey to map and interpret the geologic history of allranges roughly between Tucson and Bisbee. Manygeologists are involved in this program, and most ofthe field studies are completed. J. R. Cooper has mappedthe Sierrita Mountains, T. L. Finnell the EmpireMountains, Creasey (1967) the Whetstone Mountains,R. B. Raup the Canelo Hills, F. S. Simons the PatagoniaMountains, Hayes and Raup (1968) the Huachuca andMustang Mountains, respectively, and Hayes andLandis (1961, 1964) the Mule Mountains.ACKNOWLEDGMENTSThe geologic investigation of the Santa Rita Mountains was facilitated by many people. Foremost amongthese were my colleagues, J. R. Cooper, T. L. Finnell,P. T. Hayes, R. B. Raup, and F. &. Simons, whosework in the adjacent areas progressed concurrentlywith the Santa Rita study. The development of a localstratigraphic sequence, so important to a structuralstudy, benefited directly from some data obtained fromadjacent areas. Without any doubt, some structuralinterpretations also benefited from frequent discussionswith my colleagues; the interpretations presented here,however, are my own responsibility. Additional discussions in the field with J. H. Courtright, S. C. Creasey,M. D. Crittenden, Jr., P. H. Pickard, and R. E.Wallace helped to clarify some structural problemsand to raise others, for which I am particularly grateful.Fieldwork was facilitated by many other people.Invaluable assistance in mapping and sampling wasprovided by G. C. Cone, Bruce Hansen, C. W. Norton,F. W. Plut, J. R. Riele, R. G. Rohrbacher, ArthurSutheimer, and W. M. Swartz during the years 1962through 1968. The courtesies of George Bradt, RoyGreen, Dewey Keith, and George Yakobian of thecommunities around the Santa Ritas are recalled withpleasure; and those of Professors John Anthony, D. L.Bryant, and Evans Mayo of the University of Arizonaare likewise acknowledged.Further invaluable support was obtained from manylaboratories. Some radiometric dates were obtainedfrom Prof. P. E. Damon before their publication, andmany other dates were provided by R. F. Marvin,T. W. Stern, S. C. Creasey, and their colleagues. Theefforts of many other analysts, whose work is acknowledged more specifically in the collateral reports, areindirectly reflected in this study. G. C. Cone also ablyassisted in many phases of the preparatory work.The results of a few earlier geologic studies provideduseful background information. Schrader (1915) madea pioneer reconnaissance of the area. Creasey andQuick (1955) mapped some of the mines and structuralfeatures of the Helvetia district. Several theses describeparts of the Santa Rita Mountains, of which those byAnthony (1951), Heatwole (1966), Heyman (1958),Lutton (1958), and Michel (1959) contribute considerably to the understanding of certain local structuralunits.STRUCTURAL FEATURESThe rocks of the Santa Rita Mountains are abundantly faulted and less commonly folded. Most faultsare high-angle structures that comprise many normalfaults and some reverse and tear faults. Other faultsare low-angle structures, which are mainly thrust faultsbut which include some faults whose genetic environment is unknown. A few thrust faults are steeply inclined owing to tilting or to local irregularities alongthe fault plane. Most folds are small open flexures ordrag folds, but a few are nearly isoclinal and of largeamplitude.The Santa Rita Mountains consist of three majorstructural units, whose internal characters vary fromunit to unit and whose boundaries are marked by majorfault zones. The units are referred to here as the southwestern, central, and northeastern units (pi. 1). Therocks in the southwestern unit are typically gentlyinclined to the south and are cut by many normalfaults. The rocks in the central unit are inclined moderately to the east and are cut by only a few iaults,
SANTA RITA MOUNTAINS, SOUTHEAST OF TUCSON, ARIZONATABLE 1. Rocks of the Santa Rita Mountains, Arizona[Broken lines indicate that the adjacent formations are contemporaneous or that theur relations are uncertain]Groups, formations, and membersDescriptionEstimated thickness(ft)Youngest gravel and low-level terrace depositsGravel and intercalated sand; infantile graysoil on low-level terrace gravel.0-300 LatePleistoceneIntermediate-level pediment and terrace depositGravel and sand; capped by weakly developedred soil.0-400 MiddlePleistoceneHigh-level pediment depositsGravel and sand; capped by well-developedred soil.0-50 PleistoceneandPlioceneBasin-fill gravelGravel, sand, and silt, commonly pinkish-grayand slightly indurated; locally includes tuffaceous beds.0-2, 000 PlioceneandMiocene(?)Gravel of NogalesGravel, sand, and silt; rich in volcanic clasts;commonly pale red, poorly sorted, and slightlyindurated.0-1,000 Rhyolite intrusives of the northern Santa Rita MountainsRhyolite porphyry of a plug and of the GardnerCanyon and Box Canyon dike scomplexsouthernSanta RitaMountainsTertiaryDikes and stock of the San CayetanoMountainsRhyodacite porphyry dike swarm and mediumcoarse-grained light-gray granodiorite.Dikes and laccoliths in the Grosvenor HillsareaRhyodacite vitrophyre, medium-gray to eneRhyodacite memberRhyolite memberRhyolite tuff; a little welded tuff and lava.500 Gravel and silt; a little limestone and shale.0-200Mineralized quartz veins of the southern SantaRita Mountains.Rhyolitic volcanicsRhyolitic tuff and lava of Wasp Canyon.Olivine andesite plugsVesicular and amygdaloidal plugs at DeeringSpring.Andesite dikes and sillsSmall andesite intrusives; include some daciteand diorite intrusives.Greaterville intrusivesQuartz latite porphyry dikes and plugs, lightgray to grayish-orange-pink; contains stubbybipyramidal quartz phenocrysts; associatedwith mineralization.Helvetia stocksGranodiorite to quartz monzonite stocks and aquartz diorite stock, medium coarse grained.Cottonwood Canyon dike swarmQuartz latite porphyry, finely porphyritic tocoarsely porphyritic.Gringo Gulch plugsHornblende dacite porphyry; partly verycoarsely porphyritic; microgranodiorite in coreof one plug.Volcanics of Red MountainRhyolitic and andesitic pyroclastic rocks, intensely altered.Rhyolitic to dacitic tuff, sandstone, and a capping andesite lava.900 Lower memberRhyolitic and dacitic pyroclastic rocks and someflows; contains intercalated epiclastic rocks.700 Elephant Head Quartz MonzoniteStocks of coarse-grained quartz monzonite andan aplitic phase.Madera Canyon GranodioriteStock of coarse-grained hornblende granodiorite,and porphyritic and leucocratic phases.Josephine Canyon DioriteLarge stock of fine-grained diorite and quartzdiorite and a late quartz monzonite phase.LateCretaceousUpper memberSedimentary and volcanic rocks.Arkose memberArkose and conglomerate; largely facies ofwelded tuff member.3,500 500 Welded tuff memberRhyodacite welded tuff.1,200 Exotic block memberDacitic volcanics containing large exotic blocks.1,000 Lower memberFort CrittendenFormation0-100Upper memberGringo Gulch VolcanicsSaleroFormation150-2,200Gravel and silt memberQuartz vein swarmPaleocene(?)CretaceousRhyodacite lava flows, agglomerate, tuff, andwelded tuff.Dacitic volcanics.400 Upper red conglomerate memberVolcanic conglomerate; some sandstone andsiltstone.1,400 Rhyolitic tuff memberTuff, in part intercalated in upper red conglomerate member.0-650Brown conglomerate memberArkosic conglomerate; some sandstone andsiltstone.2,000 Lower red conglomerate memberVolcanic conglomerate; some sandstone andsiltstone.800-1,200Shale memberFossiliferous gray shale; some sandstone andconglomerate.4,550
STRUCTURAL FEATURESTABLE 1. Rocks of the Santa Rita Mountains, Arizona Continued[Broken lines indicate that the adjacent formations are contemporaneous or that their relations are uncertain]AgeGroups, formations, and membersTurney Ranch FormationEstimated thickness(ft)Description1,500 Sands
blocks foundered into parts of the volcanic pile. In the second of the areas, the Montosa Canyon area, remnants of a thrust plate and tear fault show evidence of both northeastward and southwest- . Mountains, Hayes and Raup (1968) the Huachuca and . SANTA RITA MOUNTAINS, SOUTHEAST OF TUCSON, ARIZONA Mountains Rita Mountains.
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