Mesozoic tectonics and metamorphism in the Pequop Mountains and Wood Hills region, northeast : Implications for the architecture and evolution of the Sevier orogen: Discussion and reply

Discussion

Jim Wise Independence Mining Company, Inc., HC 31 Box 78, Elko, Nevada 89801

Camilleri and Chamberlain (1997) do an foliation by the thrusts? In Figure 10 of Camilleri technique valid? Cite a reference for this technique excellent job showing increasing pressure and and Chamberlain (1997), the great-circle girdles or mention that this is a new method. What is the temperature toward the Ruby Mountains–East defined by bedding and metamorphic foliation in- source of uranium and lead incorporated into the Humboldt metamorphic core complex in northeast dicate shallow-plunging fold axes to the northeast metamorphic sphene? Why were unmetamor- Nevada. However, the presented structural inter- believed by the authors to represent southeast phosed equivalents of the metacarbonate rock not pretation of hinterland Sevier deformation, in the transport direction for the thrusts. Do lineation analyzed to address this problem? I wonder if ra- Wood Hills and Pequop Mountains, is not sub- data from the thrusts exist to verify the southeast diometric values from metamorphosed rocks are stantiated by the data. Both crosscutting relation- transport direction? The fold orientations reported given greater significance than warranted. These ships and radiometric dates described by Camilleri are part of a larger regional pattern that includes interpretations may limit a metamorphic event, but and Chamberlain (1997) lacked evidence to pre- the Adobe Range, Snake Mountains, and the I am not convinced that a one-to-one correlation clude earlier deformations. The calculated 69 km southern end of the Pequop Mountains (Ketner, between thrusting and metamorphism exists. of shortening due to their inferred Sevier thrusting 1984; Coats, 1987, Plate 1). Were the thrusts Camilleri and Chamberlain (1997) reported events was not supported by the field data and in- formed prior to metamorphism and the intrusion 69 km of shortening across an inferred and un- terpreted cross sections. Herein, I detail the con- of the dike and then reactivated or folded after the exposed fault they named the Windermere cerns I have about crosscutting relationships, ra- metamorphic event? The youngest rock unit cut by thrust. The authors suggested that the thrust jux- diometric dates for age constraints, and the the Independence thrust is a Permian carbonate taposed different facies of the Roberts Moun- introduction of large structures that lack exposure. unit (Coats, 1987, Plate 1). Crosscutting relations tains Formation composed of dolomitic siltstone Camilleri and Chamberlain (1997) described do not clearly preclude Jurassic or early Mesozoic (inner shelf) and platy limestone (outer shelf). three main thrust faults, the mapped Independence orogenic events. These two compositions typically vary in many thrust, an unnamed thrust with mappable expo- Radiometric age constraints presented by vertical sections in the Great Basin, and the fa- sures, and an interpreted and unexposed fault they Camilleri and Chamberlain (1997) may be correct. cies are often laterally discontinuous (Nichols named the Windermere thrust. The crosscutting re- However, they have taken the treatment of this and Silberling, 1977). Dolomite may be pro- lationships for thrust timing were based on the in- type of data to a degree that may not be justified. duced by a variety of processes that do not nec- ference of the thrusts cutting a metamorphic fab- The two samples reported had dates of 154 ± 5 Ma essarily reflect the original depositional environ- ric, and speculation that the thrusts cut a dike with (U-Pb of zircon from a metamorphosed dike, sam- ment (Dunham and Olson, 1978). A facies map a U-Pb age date interpreted by the authors to be ple 151P), and 84.1 ± 0.2 Ma (U-Pb of metamor- was not presented to support this inferred thrust 154 Ma. The authors did not present orientation phic sphene from metacarbonate, sample 127AP). juxtaposition of facies. A thrust is not required data for the dike and the fabrics within. Map rela- Five zircon grains were analyzed to date the dike, for the rock-type change because the region is at tions do not show the thrusts cutting the dike. The resulting in dates ranging from 156 Ma to 1753 a previously interpreted facies boundary (Matti dike was reported to have a single metamorphic Ma. Does the degree of grain abrasion influence and McKee, 1977). The regional reconstructions foliation, and the authors believe that the Indepen- the final result? What date would an unabraded presented by the authors are not physical pierc- dence thrust truncates a similar fabric. Fault rocks grain yield? How can ±5 m.y. uncertainty be as- ing points. The inferred Windermere thrust was were not documented in the paper, nor were strike signed to samples that have undergone selective interpreted by the authors from metamorphic and dip measurements for the thrusts shown on treatment for abrasion, and then two of the five an- isograds, for which the authors invoked a south- their maps (their Figures 3, 4, and 5), suggesting alyzed grains be chosen to represent the crystal- east-tapering thrust wedge as an explanation. that the fault either was not examined or was not lization age variance? Could the dike be coeval However, the folds the authors believe to be re- exposed. Perhaps the fault rocks contain the meta- with the thrust? For the metacarbonate sphene lated to the thrust include a much larger area morphic fabric? Without this information what can date, what other ways can the slope of the sphene- than the model of southeast tapering thrust be utilized to show truncation of the metamorphic whole rock mixing line be interpreted and is this wedge can account for. The metamorphic iso-

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Matti, J. C., and McKee, E. H., 1977, Silurian and Lower De- grads parallel the modern-day topographic axis points or offset markers to support their estimate vonian paleogeography of the outer continental shelf of the of the Ruby Mountains– East Humboldt meta- of crustal shortening. Cordilleran miogeocline, central Nevada, in Stewart, J. H., morphic core complex. The metamorphic pat- Stevens, C. H., and Fritsche, A. E., eds., Paleozoic paleo- geography of the western : Pacific coast pale- terns could be from an unidentified process at REFERENCES CITED ogeography symposium—1: Society of Economic Paleon- midcrustal depths preceding exhumation of tologists and Mineralogists, Pacific Section, v. 7, these rocks, a process completely unrelated to Camilleri, P. A., and Chamberlain, K. R., 1997, Mesozoic tec- p. 181–215. tonics and metamorphism in the Pequop Mountains and Nichols, K. M., and Silberling, N. J., 1977, Depositional and the Sevier orogeny. Wood Hills region, northeast Nevada: Implications for the tectonic significance of Silurian and lower Devonian Constraining the age limits of a fault by inter- architecture and evolution of the Sevier orogen: Geologi- dolomites, Roberts Mountains and vicinity, east-central Nevada, in Stewart, J. H., Stevens, C. H., and Fritsche, preting a correlation between a metamorphic cal Society of America Bulletin, v. 109, p. 74–94. Coats, R. R., 1987, Geology of Elko County, Nevada: Nevada A. E., eds., Paleozoic paleogeography of the western event of questionable time and a thrust of uncer- Bureau of Mines and Geology Bulletin 101, 111 p. United States: Pacific coast paleogeography sympo- tain orientation is unsupported by the data pre- Dunham, J. B., and Olson, E. R., 1978, Diagenetic dolomite sium—1: Society of Economic Paleontologists and Min- eralogists, Pacific Section, v. 7, p. 217–240. sented. I request that the authors elaborate on the formation related to Paleozoic paleogeography of the Cordilleran miogeocline in Nevada: Geology, v. 6, accuracy of their dating methods, present more p. 556–559. substantial crosscutting relations, consider that Ketner, K. B., 1984, Recent studies indicate that major struc- tures in northeastern Nevada and the Golconda thrust in the thrusts may have a protracted history predat- north-central Nevada are of Jurassic or Cretaceous age: MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 10, 1997 ing the dike intrusion, and document piercing Geology, v. 12, p. 483–486. MANUSCRIPT ACCEPTED JULY 11, 1997

Reply

Phyllis A. Camilleri* Department of Geology and Geography, Austin Peay State University, Clarksville, Tennessee 37044-4418 Kevin R. Chamberlain Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071-3006

In Camilleri and Chamberlain (1997), we conclusions regarding the structural history and cordia at a high angle (Camilleri and Chamber- presented a tectonic reconstruction of the reconstruction of the middle to upper Mesozoic lain, 1997, Fig. 12) and is interpreted to reflect Mesozoic middle to upper crust in the Sevier crust in this region. Below we discuss five main mixtures of magmatic zircon (ca. 154 Ma) and hinterland in Pequop Mountains and Wood issues: thermochronology, crosscutting rela- varying amounts of inherited zircon. Data from a Hills region in northeast Nevada. This region is tionships and the Independence thrust, the Win- select fraction of grains that contained visible in- structurally complex because it underwent dermere thrust, the origin of metamorphism, clusions (#4) support this interpretation, because Mesozoic metamorphism and polyphase thrust and the age of the Windermere thrust. they plot far to the right of the other data, and faulting and was then overprinted by late Meso- yielded the oldest Pb/Pb date of 1753 Ma. The zoic to Tertiary polyphase normal faulting. In U-Pb THERMOCHRONOLOGY data from all five fractions are not statistically lin- our reconstruction we integrated new and pub- ear, however, and scatter of the data is interpreted lished data, including geologic and meta- U-Pb data from three samples were reported in to be due to recent Pb loss. Support for this sec- morphic mapping, thermobarometry, ther- Camilleri and Chamberlain (1997); these data ond interpretation comes from the observation mochronology, crosscutting relationships, and help refine the metamorphic and structural his- that U-Pb data from fraction #1, a selection of distribution of sedimentary facies. Wise re- tory in the Wood Hills and Pequop Mountains re- whole, euhedral, unabraded grains, are more dis- quests more detailed information regarding gion. The data come from zircon analyses from a placed from possible mixing lines than the data Mesozoic contractional phases of deformation granitic dike (151P) that contains the metamor- from fractions #2, #3, and #5, which were air and metamorphism. Specifically, he questions a phic fabric, and analyses of metamorphic sphene abraded to remove the outer parts of the grains mapped thrust fault named the Independence from metacarbonate rocks collected in the Wood prior to dissolution. Recent Pb loss may still be thrust and the existence of an inferred thrust Hills (97C) and Pequop Mountains (127AP). present in the analyses because none of the points fault named the Windermere thrust and voices (Note that the tables published with the original are concordant, so estimating the magmatic age concern with our interpretation of U-Pb ther- article were misprinted; corrected tables have and uncertainty requires assessing the possible mochronologic data bearing on ages of meta- been reprinted as an errata in Geological Society range of this effect. The lower intercept of the morphism and thrust faulting. We welcome this of America Bulletin, v. 109, no. 4, p. 504.) Wise mixing line shown in Figure 12 of Camilleri and opportunity to clarify and elaborate on relations asks whether the data from two of the samples Chamberlain (1997) may be a minimum age for that we believe substantiate and support our (151P and 127AP) have been over-interpreted. dike crystallization; the true mixing line could U-Pb data from zircon from the granitic dike plot above this line and be subparallel. On the ba- *E-mail: [email protected] plot in a broadly linear array that intersects con- sis of the near linearity of the data from the

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abraded grains and our experience with discor- lated from both the 238U-206Pb and 235U-207Pb wonders if the Independence thrust could have dance of young zircons, we estimated that the in- data is interpreted to indicate that these mixing formed prior to metamorphism and the intrusion tercept of 154 Ma is no more than 5 m.y. younger lines are isochrons. The higher uncertainty in the of the 154 Ma dike and also if the fabric could be than the true magmatic age. Conversely, the in- 235U-207Pb dates is due to the lower abundance of superimposed on the thrust. The thrust surface is tercept could also be too old if the slope of the 235U relative to 238U, and less spread in the not exposed. However, other evidence can be true mixing line is steeper than shown. The steep- 235U/204Pb vs. 207Pb/204Pb data. Consequently, the used to ascertain that the Independence thrust est possible chord would go through the point 238U-206Pb sphene plus whole-rock date of 84.1 ± postdates the metamorphic fabric. The dike pre- with the least inheritance (#5) and the Pb-Pb date 0.23 Ma is interpreted as the age of metamorphic dates metamorphism, as discussed above. Meta- of the point with the most inheritance (1753 Ma). sphene growth. Coupled with experimentally de- morphism in both the hanging wall and footwall The lower intercept of this chord is 151.8 ± 0.9 termined diffusion parameters for Pb in sphene, of the Independence thrust dies out stratigraphi- Ma, or no more than 3 m.y. younger than our and metamorphic limits on peak temperatures, we cally upward; Lower Cambrian strata are lower stated mean. It must be emphasized that our esti- interpret this age of sphene growth as the timing amphibolite facies, whereas middle Mississip- mates of the possible range of magmatic age for of peak metamorphism in the Pequop Mountains pian strata are unmetamorphosed (Camilleri and this sample are conservative. It is impossible for rather than simply a cooling age during unroofing. Chamberlain, 1997, Figs. 4 and 6). Foliation in the dike to be younger than 151 Ma, and in our Our interpretation of correlations between the both the hanging wall and footwall of the Inde- opinion, it is highly unlikely that the dike is older timing of metamorphism and timing of thrust- pendence thrust is consistently at a very low an- than 159 Ma. ing are based on structural and metamorphic ar- gle to bedding or parallel to bedding, but the atti- Metamorphic sphene from the Pequop Moun- guments and are summarized in the sections tude of foliation varies above and below the tains (127AP) has relatively low proportions of ra- that follow. thrust. Bedding and foliation in the footwall dip diogenic Pb to initial Pb; consequently, calculated uniformly to the southeast. In marked contrast, concordia coordinates (radiogenic Pb/U) are 154 Ma DIKE, THE METAMORPHIC bedding and foliation in the hanging wall of strongly influenced by the choice of initial Pb iso- FABRIC, AND THE INDEPENDENCE the Independence thrust are deformed by back topic compositions. In contrast, U-Pb isochron di- THRUST thrusts and folds, and hence have many different agrams (238U/204Pb vs. 206Pb/204Pb and 235U/204Pb attitudes (cf. Camilleri and Chamberlain, 1997, vs. 207Pb/204Pb) avoid the need to make a correc- In the second paragraph of his discussion, Figs. 5 and 10). Our foremost piece of evidence tion for initial Pb isotopic compositions. On these Wise questions whether the thrust faults could that the Independence thrust postdates the meta- diagrams, linearity of data implies that the U-Pb predate metamorphism and emplacement of the morphic fabric is that it displaces the metamor- data represent simple mixtures of two compo- dike dated by U-Pb methods as 154 Ma. He also phic gradient; it emplaces higher grade rocks nents, ideally a nonradiogenic, initial Pb isotopic wonders if we have lineation data to substantiate over lower grade rocks and low-grade rocks reservoir (U/204Pb = 0) and a radiogenic Pb iso- our inference of a top-to-the-southeast sense of over unmetamorphosed rocks (Camilleri and topic reservoir (U/204Pb approaches infinity). A slip for the Independence thrust. We address Chamberlain, 1997, Fig. 4). For example, the test of whether the linear mixtures are isochrons these issues here, but first must elaborate on structurally highest level of the exposed part of comes from comparing the dates calculated from Wise’s statement that “The crosscutting relation- the Independence thrust is present on the eastern the two independent U to Pb decay chains ships for thrust timing were based on the infer- flank of the Pequop Mountains. In this area, the (238U-206Pb and 235U-207Pb). As with concordant ence of the thrusts cutting a metamorphic fabric, thrust juxtaposes low-grade, foliated Ordovician data on a concordia plot, it is nearly impossible to and speculation that the thrusts cut a dike [in the Pogonip Group strata over unmetamorphosed, naturally form U-Pb reservoirs that yield overlap- Pequop Mountains] with a U-Pb age date inter- unfoliated Mississippian Chainman Shale and ping 238U-206Pb and 235U-207Pb dates unless the preted by the authors to be 154 Ma.” This state- Diamond Peak Formation rocks (cf. Camilleri reservoirs are chronologically related. ment is not entirely true and warrants clarifica- and Chamberlain, 1997, Figs. 4 and 5). The In the case of the sphene from the Pequop tion. The Independence thrust is the only thrust implication is that, although the thrust is not Mountains (127AP), 238U/204Pb vs. 206Pb/204Pb that has an age constrained by crosscutting rela- exposed, the foliation in the hanging wall is for only the 5 sphene analyses yields a good linear tionships with the metamorphic fabric, and we truncated by the thrust fault. Furthermore, the fit with a mean square of weighted deviates of 1.2, inferred that the Windermere thrust predates discordance in the orientation of foliation across and a calculated 238U-206Pb date of 84.8 ± 0.9 Ma. metamorphism. The 154 Ma dike is exposed in the thrust in many places implies that the folia- Data from the whole rock lie on this line, suggest- the hanging wall of the Independence thrust tion predates the thrust. For visualization pur- ing that the sphene and whole rock were in Pb iso- (Camilleri and Chamberlain, 1997, p. 86) and, poses, we have modified our cross section C–C′ topic equilibrium at the time of sphene formation. along with the enveloping Cambrian strata, is de- (Camilleri and Chamberlain, 1997, p. 81) of the Adding the whole-rock data to the sphene data re- formed into a prograde metamorphic S-tectonite. Independence thrust by removing unit patterns sults in a linear regression with a mean square of The foliation in the dike and surrounding rocks and superimposing the trace of foliation (Fig. 1). weighted deviates of 1.39 and a date of 84.1 ± strikes north-northwest and dips 35° to 40° to the In summary, on the basis of our observations 0.23 Ma. Both the sphene-only and sphene-plus- northeast. Because the metamorphic fabric is su- that (1) the Independence thrust cuts isograds and whole rock dates overlap within error; including perimposed on the dike, we concluded that meta- the metamorphic gradient, and (2) folds in the the whole rock data simply lowers the uncertainty. morphism must postdate intrusion of the dike. hanging wall of the Independence thrust fold the Similar results come from the 235U/204Pb foliation, we conclude that the thrust postdates vs. 207Pb/204Pb data, with linear regression of only Crosscutting Relationships Between the metamorphic fabric. Combining this conclu- the sphene data yielding a mean square of the Independence Thrust and the sion with the 84 Ma age of the metamorphic fab- weighted deviates of 0.40, date of 93 ± 12 Ma, Metamorphic Fabric ric in the Pequop Mountains leads us to further and sphene plus whole rock yielding a mean conclude that the Independence thrust postdates square of weighted deviates of 1.44 and a date of We have interpreted the Independence thrust 84 Ma, and hence is not Jurassic or Early Creta- 80 ± 3.6 Ma. Agreement of dates that are calcu- as postdating the metamorphic fabric, but Wise ceous in age.

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Figure 1. Simplified northwest-southeast cross section through the Pequop Mountains depicting metamorphic grade and orientation of folia- tion. This cross section is the same as cross section C–C′ in Camilleri and Chamberlain (1997, p. 81). Zm—Proterozoic McCoy Creek Group; ZC– u—Proterozoic and Cambrian strata, undifferentiated; –Cnp—Cambrian Notch Peak Formation; Ope—Ordovician Pogonip Group and Eu- reka Quartzite, undifferentiated; OSDu—Ordovician, Silurian, and Devonian dolomite, undifferentiated; Dg—Devonian Guilmette Formation; Mu—Mississippian Joana Limestone and Chainman and Diamond Peak formations, undifferentiated; PPu—Pennsylvanian and Permian strata, undifferentiated; TQu—Tertiary and Quaternary surficial deposits.

Independence Thrust and Sense of Slip ble that all of these folds are related. The precise tinuous but regionally metamorphosed Paleozoic ages of the folds in the areas referred to by Wise section of the exhumed footwall of the Winder- Wise wonders if we have lineation data to sub- are unknown, but they must be younger than the mere thrust. Two specific examples of low-angle stantiate our inference of a top-to-the-southeast youngest strata that are folded (upper Paleozoic normal faults that duplicate sections are (1) the sense of slip for the Independence thrust. Wise strata to strata of Triassic age). Consequently, Mary’s River fault in the northwestern part of the notes that “Fault rocks [related to the Indepen- these folds could be the same age as the folds in Wood Hills and southernmost Windermere Hills dence thrust] were not documented in the paper, the hanging wall of the Independence thrust, that contains an unmetamorphosed Devonian to nor were strike and dip measurements for the which we infer to have formed between 84 and Permian section in its hanging wall, which lies di- thrusts shown on their maps, . . . suggesting that 75 Ma (Camilleri and Chamberlain, 1997). rectly above exposed regionally metamorphosed the fault either was not examined or was not ex- Cambrian to Permian strata in its footwall; and posed.” Orientation data for the fault and discus- EVIDENCE BEARING ON THE (2) the Pequop fault in the Pequop Mountains sion of fault rocks were not presented in our pa- REQUIREMENT FOR THE (Camilleri and Chamberlain, 1997, Figs. 3, 4, and per because no exposures of the fault surface WINDERMERE THRUST 5). The presence of duplicated section across the were found, hence no lineation data exist. How- normal faults implies that the Paleozoic section ever, in most places the fault can be located Wise notes that we “suggested that the [Win- was duplicated by thrust faulting prior to normal within 7 m slope distance, and in one place, dermere] thrust juxtaposed different facies of the faulting. Furthermore, it is barometric data, not where the thrust crosses the topographic divide in Roberts Mountains Formation” and further sug- isograds, that require a southeast-tapering wedge the range, the thrust can be located within about gests that “A thrust [Windermere] is not required of thrusted material above the footwall of the 1 m. There, the thrust cuts low-grade Ordovician for the rock-type change because the region is at a Windermere thrust (see Camilleri and Chamber- carbonate rocks in its hanging wall and unmeta- previously interpreted facies boundary . . .” Fur- lain, 1997, p. 89). Regarding the juxtaposition of morphosed Mississippian clastic rocks in its foot- thermore, he states that “The inferred Winder- different facies of the Silurian Roberts Mountains wall. Rocks adjacent to the contact are fractured. mere thrust was interpreted by the authors from Formation across the Windermere thrust, first, we In the absence of fault-rock fabric data due to metamorphic isograds, for which the authors in- emphasize that the different Silurian rocks men- lack of exposure, we used the regional northeast voked a southeast-tapering thrust wedge as an ex- tioned by Wise are currently juxtaposed by the trends of folds and thrusts in the hanging wall of planation.” Parts of these comments need clarifi- low-angle normal faults. A western platy lime- the Independence thrust to infer that thrust trans- cation. The existence of Windermere thrust is not stone facies of the formation lies in the unmeta- port was to the southeast. based on metamorphic isograds sensu stricto or morphosed hanging walls of the normal faults and Wise further states in his second paragraph the juxtaposition of different facies. The existence a more eastern dolomitic facies lies in the struc- that “The fold orientations reported [in the hang- of this thrust is based on (1) barometric data from turally underlying exhumed footwall of the Win- ing wall of the Independence thrust] are part of a metasedimentary rocks that require burial depths dermere thrust. Wise is correct in stating that east- larger regional pattern [of folds of similar orien- that greatly exceed stratigraphic depths, and northeast–trending facies boundaries lie along the tation that crop out outboard of the area studied (2) the presence of duplicated Paleozoic sections region of our area of study (e.g., Miller et al., by Camilleri and Chamberlain] that includes the across low-angle normal faults. These normal 1991); however, it is not possible to map a deposi- Adobe Range, Snake Mountains, and the south- faults separate unmetamorphosed Paleozoic sec- tional boundary between the facies in this area be- ern end of the Pequop Mountains . . .” It is possi- tions in their hanging walls from the nearly con- cause they are structurally juxtaposed one on top

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of the other. As an example, in Figure 2 we show an east-west cross section through the hanging wall and footwall of the Pequop low-angle normal fault in the Pequop Mountains. Unmetamor- phosed Ordovician to Permian strata, including the platy limestone facies, compose the hanging wall of the Pequop fault. Metamorphosed Cam- brian and Ordovician strata and texturally un- metamorphosed Silurian and Devonian strata, in- cluding the dolomitic facies of the Roberts Mountains Formation, are in the footwall. In ad- dition to the contrast in Silurian rocks across the Pequop fault, there is a marked contrast in the na- ture of Ordovician strata: the Ordovician section (unit Ope) in the footwall is regionally metamor- phosed, whereas this same unit in the hanging wall of Pequop fault is not. The limestone facies in the hanging wall of the Pequop fault currently lies structurally to the east or southeast of its orig- inal site of deposition, but probably the present stratigraphic offset of the two facies is not as great as it was prior to normal faulting. This logic fol- lows from the geometric argument that, in order to get the present structural juxtaposition of the two facies across the Pequop normal fault, the more western limestone facies of the formation must have been thrust to the east-to-southeast along the inferred Windermere thrust, but was subsequently extended back to the west-to-northwest along the Pequop fault. We specifically used the juxtaposi- tion of the different Silurian facies as one piece of evidence to infer the sense of slip on the Winder- mere thrust, not for justifying the former existence of the Windermere thrust. One other statement in Wise’s fourth para- graph needs clarification. Wise states that “the folds the authors believe to be related to the [Windermere] thrust include a much larger area than the model of a southeast-tapering thrust wedge can account for.” In the context of Wise’s Figure 2. (A) Simplified map of the northern Pequop Mountains depicting the locations of the fourth paragraph, the meaning of the foregoing Independence and unnamed thrusts, the Pequop fault, and distribution of metamorphosed strata statement is unclear. We believe that Wise has and different facies of the Silurian Roberts Mountains Formation. (B) West-east cross section confused structures formed in two different A–A′ through the Pequop fault, which is cut by a series of high-angle normal faults. Location of events because we did not state that the folds in section is shown in A. Cross section is vertically exaggerated to highlight units in the hanging wall our area of study were related to the Windermere of the Pequop fault. With the exception of the Roberts Mountains Formation, units in the hang- thrust. Rather, we stated that we interpret them to ing wall of the Pequop fault are collectively shown as gray. Units composing the footwall of the Pe- be related to the younger Independence thrust quop fault are unpatterned where unfoliated, whereas the trace of foliation is illustrated in cleaved (Camilleri and Chamberlain, 1997, p. 84). The rocks. All Paleozoic strata in the hanging wall of the Pequop fault are unmetamorphosed, whereas folds Wise is referring to fold the prograde meta- in the footwall Ordovician and older strata are regionally metamorphosed and Silurian and morphic fabric that records an increase in pres- younger strata are texturally unmetamorphosed. A 1:24 000 geologic map of the northern Pequop sure. We attributed the increase in pressure Mountains and a field guide that discusses and compares the metamorphosed units and a partial recorded by the metamorphic rocks to tectonic (faulted) section of the Roberts Mountains Formation in the footwall of the Pequop fault with the burial by the Windermere thrust; hence we reason same units in the hanging wall of the Pequop fault is available in Camilleri (1994). –C u—Upper that the folds that fold the metamorphic fabric Cambrian strata, undifferentiated; –C np—Cambrian Notch Peak Formation; Ope—Ordovician postdate the tectonic burial event. Pogonip Group and Eureka Quartzite, undifferentiated; OSd—Ordovician and Silurian Ely Springs and Laketown dolomite, undifferentiated; Srm—Silurian Roberts Mountains Forma- Piercing Points and Reconstruction tion; SDd—Silurian and Devonian Lone Mountain and Simonson dolomite, undifferentiated; Dg—Devonian Guilmette Formation; Mu—Mississippian Tripon Pass Limestone and Chainman Wise notes that our reconstruction of the in- and Diamond Peak formations, undifferentiated; Pu—Permian strata, undifferentiated. ferred Windermere thrust did not involve utiliza-

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tion of piercing points and requested us to sup- directly underlying the region. We prefer the first rane, and indicates that at least modest tectonic port our estimate of shortening by documenting option because there is evidence for structural bur- loading began at or by 153 Ma. Hence we used piercing points or offset markers (cut-offs). Our ial, the distribution of isograds appears to be depth this age as a probable older age bracket for the minimum estimate of shortening accommodated dependent, and apparent thermal gradients during Windermere thrust. by the Windermere thrust utilized structural, metamorphism do not appear to be excessive (see In conclusion, regionally metamorphosed Pale- barometric, and metamorphic data and involved discussion in Camilleri and Chamberlain, 1997, ozoic strata in the Sevier hinterland in the Wood some assumptions, a list of which was given p. 89). The second possibility would invoke a Hills and Pequop Mountains region record at least (Camilleri and Chamberlain, 1997, p. 90–91). large batholith below exposures of the metamor- two main phases of Mesozoic thrust faulting. The This is the best estimate derived from the avail- phosed Paleozoic section and at present there is earliest phase was accommodated by the inferred able data. No piercing points were found in the no evidence for this. However, we noted that Windermere thrust, the former existence of which field area and therefore they could not be used in there are granitic bodies documented to be ap- is required by barometric data that imply burial in reconstruction. However, we did utilize a cut-off proximately coeval with peak metamorphism in excess of stratigraphic depths and by the presence in our reconstruction. the . Some of these granitic of duplicated Paleozoic sections. The character of rocks are derived from in situ partial melting and regional metamorphism in the footwall of the ORIGIN OF METAMORPHISM others compose intrusive bodies; rocks of this age Windermere thrust is consistent with metamor- are geochemically regarded as partial melting of phism due to tectonic burial. A U-Pb age of 84 Ma We interpreted the Barrovian regional meta- the crust and are inferred to be a result of crustal on sphene yields an age of peak metamorphism morphism in the Wood Hills and Pequop Moun- thickening (e.g., Wright and Wooden, 1991; and a younger age limit on the Windermere thrust. tains region as a byproduct of tectonic burial. McGrew, 1992). We concluded that metamor- The second phase of thrust faulting involved slip Wise stated that “The [Barrovian] metamorphic phism is largely a byproduct of tectonic burial along the Independence thrust and small-scale patterns could be from an unidentified process at and that burial probably induced partial melting thrust faulting and folding in its hanging wall. The midcrustal depths preceding exhumation of these in the deep crust, resulting in rising melts that Independence thrust and structures in its hanging rocks, a process completely unrelated to the Se- transferred and contributed some heat necessary wall deform or cut the 84 Ma fabric and thus are vier orogeny.” On the basis of our U-Pb data and for midcrustal metamorphism. At this time this younger than this age. work of McGrew (1992), we have documented interpretation best fits all of the available data. that peak metamorphism is regionally Late Cre- REFERENCES CITED taceous in age (see discussion in Camilleri and AGE OF THE WINDERMERE THRUST Chamberlain, 1997), which is in part coeval with Camilleri, P. A., 1994, Mesozoic and Cenozoic tectonic and metamorphic evolution of the Wood Hills and Pequop Early Cretaceous to Tertiary contractional defor- We bracketed the age of the Windermere thrust Mountains, Elko County, Nevada [Ph.D. dissert.]: mation in the Sevier orogen. The Barrovian meta- and tectonic loading as ca. 153 Ma to 84 Ma. Our Laramie, Wyoming, University of Wyoming, 196 p. Camilleri, P. A., and Chamberlain, K.. R., 1997, Mesozoic tec- morphism records an increase in both pressure younger bracket is a tight constraint but the older tonics and metamorphism in the Pequop Mountains and and temperature. The only evidence that can ex- age bracket, which Wise is concerned about, is not Wood Hills region, northeast Nevada: Implications for the plain the increase in pressure is the presence of precise. The younger limit reflects the age of peak architecture and evolution of the Sevier orogen: Geologi- cal Society of America Bulletin, v. 109, p. 74–94. duplicated Paleozoic sections across the low-an- metamorphism and is based on the inference that Hudec, M. R., 1992, Mesozoic structural and metamorphic his- gle normal faults, which implies that the Paleo- the tectonic load must have been emplaced prior tory of the central Ruby Mountains metamorphic core zoic section was duplicated by thrust faulting to the peak of metamorphism that records the high complex, Nevada: Geological Society of America Bul- letin, v. 104, p. 1086–1100. prior to normal faulting. The Paleozoic sections pressures. Our placement of the older age limit is McGrew, A. J., 1992, Tectonic evolution of the northern East above the low-angle normal faults are unmeta- less precise and warrants elaboration. The Win- Humboldt Range, Elko County, Nevada [Ph.D. dissert.]: Laramie, University of Wyoming, 191 p. morphosed, whereas the Paleozoic section be- dermere thrust must predate the inception of bur- Miller, D. M., Repetski, J. E., and Harris, A. G., 1991, East- neath underwent metamorphism; this suggests ial metamorphism, and the only age constraint we trending Paleozoic continental margin near Wendover, that only the structurally buried Paleozoic section have on the inception of metamorphism is that it , in Cooper, J. D., and Stevens, C. H., eds., Paleozoic paleogeography of the western United States—II: Society underwent Barrovian regional metamorphism. began some time after 154 Ma (the age of the dike of Economic Paleontologists and Mineralogists, Pacific We offer two reasonable end-member possibili- that is deformed by the metamorphic fabric). We Section, v. 67, p. 439–461. ties for the origin of heat necessary for metamor- used data from the southern Ruby Mountains, Wright, J. E., and Wooden, J. L., 1991, New Sr, Nd, and Pb iso- topic data from plutons in the northern Great Basin: Im- phism. One possibility is that heating is solely a which regionally is within the Late Cretaceous plications for crustal structure and granite petrogenesis in natural consequence of burial; rocks heat up as Barrovian metamorphic terrane, to infer an older the hinterland of the Sevier thrust belt: Geology, v. 19, p. the geotherm reequilibrates following burial (i.e., age limit. Hudec (1992) documented a modest 457–460. thermal relaxation). The other possibility is that amount of tectonic burial at 153 Ma (Late Juras-

heat was entirely derived from deep-middle to sic) in the southern Ruby Mountains, which is the MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 5, 1997 lower crustal intrusions of batholithic proportions earliest record of tectonic burial within this ter- MANUSCRIPT ACCEPTED AUGUST 12, 1997

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