JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. B1, PAGES 511-522, JANUARY 10, 1996 Thermal and unroofing history of a thick, tilted Basin-and-Range crustal section in the Tortilia Mountains, Arizona Keith A. Howard u.s. GeologicalSurvey,Menlo Park, California David A. Foster Victorian Institute of Earth and Planetary Sciences, Schoolof Earth Sciences, La Trobe University Bundoora, Victoria, Australia Abstract.We estimate here a geothermal gradient of only 17 __+ 5øC km-• for thetilted Grayback fault block in southeastern Arizona when extension began -25 Ma. This gradient is lower than preextension gradients estimated elsewhere in the Basinand Range, is only about 50% of typicalgradients in the Basin and Range today, and needs to be accounted for in modelsof continental extension. The Grayback block exposes a 12-km- thick crustalsection of Proterozoic and Cretaceous granitoids, which was tilted 90 ø during extension between25 and 15 Ma. Zircon fission-track agesdecrease structurally downward (westward) across the blockand were all within a zone of partial track annealing prior to tilting and quenching. The zircon age gradientsuggests that the 220ø-240øC isotherm migrateddownward 5-6 km duringPaleogene erosion and regionalcooling. Apatite fission-track ages decrease westward from -83 Ma in the structurally highest crystalline rocks to -24 Ma at -6-km paleodepth and then to -15 Ma another 6 km farther west. Track-length analysis confirms that apatites abovethe break in slopein age at -5.7-km paleodepth residedin a zone of partial annealing prior to tilting, and deeper apatites record rapid coolingupon tilting and unroofing beginning-25 Ma. At that time the 110 _+10øC isotherm determined by the depth at which tracksin apatitewere fully erased was at a basement paleodepth of-5.7 km, and the 220 __+ 30øCisothermas estimated from zircon data residedat a pretilting basement depth of-12.15 km. From consistent values of paleogeothermal gradient for two depth intervals we estimate the pretilt gradient was 17 +_ 5øC km -•.From 25 to 15 Ma the rotating Grayback block cooled rapid•ly as higher,westward movingblocks unroofed it tectonically at a rate of-1 km m.y. . Introduction Tilted crustal blocks found in somehighlyextended parts of the Basinand Range province expose unbrokendepth profiles up to 15 km thick and expose cross sections of basement-rock sections. These blocksform setsof tilted dominolike panels that rotated progressively along with their bounding listric and (or) planar normal faults(Figure1). Where structurally intact, originally thick tilted blocks provide exceptional cross- sectional viewsof crustalstructure [Howardet al. 1982;Fryxell et al., 1992; Holm and Wernicke,1990]. They also provide opportunities to study thermal history of a crustal section [Fitzgerald et al., 1991; Gans et al., 1991; Fosteret al., 1990, 1994]. This paper summarizes evidencethat the Grayback fault block in the Tortilla Mountains, southern Basin and Range province,exposes an essentially intact 12-km-thicksectionof granitoidrocksand dikes,tilted 90 ø. Fission-track thermochro- nology of apatite and zirconfrom a paleodepth traverse across the Graybackblock allowsus to assess an Oligoceneto Mio- cene timing of extensional tilting and unroofingand to study Copyright1996 by the American Geophysical Union. Paper number 95JB02909. 0148-0227/96/95JB-02909505.00 thermal structure before and during extension. We estimate here a geothermal gradient of 17 _+ 5øC km-• when extension began at -25 Ma. This gradient is lowerthan estimated for the onsetof extension elsewhere in the Basin and Range and only about 50% of typicalgradients in the Basinand Range today. Geologic Setting The Tortilla Mountains lie in the San Pedro trough, an extensional belt that separates the exhumed Catalina core complex of Dickinson [1991] from a breakaway zoneto the east in the Galiuro and Dripping Springs Mountains (Figure 2). West dipping extensional faultsin the troughboundsediment- filled basins and east tilted fault blocks composed of pre- Tertiary rocks. The fault blocks in the Tortilla Mountains consist largelyof the Middle ProterozoicRuin Granite, youngerProterozoicto early Tertiary intrusions,Middle Proterozoic to Mesozoic strata that overlie the Ruin Granite, and synextensional Ter- tiary clastic strata (San Manuel and Cloudburst Formations) (Figures 3 and 4). Stratal dips and stratigraphic relations among synextensional deposits show that the fault blocks were tilted eastward to their present steepdipsduringlatestOligo- ceneand early Mioceneextension [Dickinson, 1991]. To the northeast of the highly extended rocks in the Tortilla 511