AAPG Bulletin, V. 82 (1998), No. 12 (December 1998), P. 2272-2276.

Late Paleozoic Deformation of Interior North America: The Greater Ancestral Rocky Mountains: Discussion¹

Charles F. Kluth²

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received May 8, 1997; revised manuscript received September 2, 1997; final acceptance May 27, 1998.
²Chevron, P.O. Box 5046, San Ramon, California 94583.

INTRODUCTION

I commend Ye et al. (1996) for their thought-provoking interpretation of the late Paleozoic deformation of western and southern North America. I agree with them on several points of their interpretation, such as the inclusion of features from Oklahoma and Nebraska to northern Mexico as part of a "unified system of deformation" (Ye et al., 1996, p. 1418) of the "Greater Ancestral Rocky Mountains." I comment here on some other aspects of their interpretation.

STRATIGRAPHIC DETAIL AND TIMING

I agree with Ye et al. (1996) that interpreting the exact age of movement on faults is often difficult because of lack of age control on key stratigraphic horizons and geologic relationships. They emphasized that the first motion on several large-scale structures within the Ancestral Rocky Mountains occurred at approximately the same time (Late Mississippian or Early Pennsylvanian), and motion ended regionally at approximately the same time (Late Pennsylvanian or Early Permian); however, it is possible to interpret more detail of the timing and relative rates of structural differentiation of individual uplifts and basins from what is known of the stratigraphic record. There are uncertainties in the relationship between maximum rate of structural differentiation and maximum sediment thickness in the adjoining basin; however, rapid increases in sedimentation rates and orders-of-magnitude changes in thickness of synorogenic deposits for succeeding, relatively brief time periods (McKee and Crosby, 1975; Kluth, 1986) can be interpreted to reflect changes in the rate of structural differentiation and development of accommodation space.

The Ancestral Rocky Mountain basins were not starved for sediment, as implied by Ye et al. (1996, p. 1419). The Ancestral Rocky Mountain basins were filled as they subsided by synorogenic sediments eroded from nearby uplifts. The synorogenics are interbedded with shallow-marine limestones, which include subaerial exposure surfaces, even in the centers of basins. This relationship indicates that basin filling kept pace with subsidence, so that the basin surface was near sea level throughout the history of rapid development (Baars et al., 1988; T. Lawton, 1997, personal communication). The exception to this general relationship may be the well-known starved basins in the southern part of the system (see Ye et al., 1996, p. 1400-1403). An example of the greater detail of the stratigraphic record than that cited by Ye et al. (1996) is provided by the Paradox basin/Uncompahgre uplift in southwestern Colorado. The Early Pennsylvanian rocks include shallow-marine carbonates and are approximately 100 m in the center of the Paradox basin. The Desmoinesian rocks in the Paradox basin are over 3000 m thick (Frahme and Vaughn, 1983) and include evaporites and shales that grade into arkosic sandstones along the mountain front. In the later Pennsylvanian, the arkoses extended farther out into the basin, indicating that the rate of subsidence was decreasing relative to erosion and deposition from the uplift. (De Voto, 1980). The data, taken altogether, indicate that the rate of structural differentiation, although not precisely constrained in absolute

End page 2272----------------

terms, can be interpreted to have a maximum in the Desmoinesian and gradually decrease in the Late Pennsylvanian.

This type of analysis is possible in other basins cited by Ye et al. (1996). In the Anadarko basin, the most intense thrusting is interpreted to have been in the Atokan (Brewer et al., 1983). The Red River uplift and Matador arch were emergent in the Morrowan and submerged in the Missourian (McKee and Crosby, 1975; Kluth, 1986; Ye et al., 1996). The Wolfcampian section in the Delaware basin is over 4 km thick and proximal synorogenic facies first appear along the flanks of the Central Basin uplift in the lower part of the Wolfcampian section (McKee and Crosby, 1975; Kluth, 1986; Ye et al., 1996, p. 1410). Rapid tectonism in the Pedregosa basin began approximately during the Desmoinesian and continued into the late Wolfcampian (Ye et al., 1996). Uplift of the Sierra del Nido uplift in northern Mexico is late Wolfcampian-Leonardian, and subsidence in the Las Delicias basin [Chihuahua trough of Ye et al. (1996)] does not end until the Guadalupian or Ochoan (Ye et al., 1996). Similar interpretations have been made (Kluth, 1986; Baars et al., 1988) for basins not cited by Ye et al. (1996).

The pattern of maximum tectonic activity for late Paleozoic basins in the western United States, as seen in the stratigraphic record, is spatially diachronous and correlates more closely with the development of features within the Ouachita-Marathon orogene to the southeast (Graham et. al., 1975; McKee and Crosby, 1975; Kluth, 1986) than was inferred by Ye et al. (1996). Their narrative included statements that support this interpretation.

FAULT GEOMETRY

Ye et al. (1996) emphasized the thrust-faulted mountain fronts on uplifts that have been recognized for some time within the Greater Ancestral Rocky Mountains (Frahme and Vaughn, 1983, White and Jacobson, 1983; McConnell, 1989); however, the references that Ye et al. cited (Baars and Stevenson, 1984; Baars et al., 1988) suggest that the bounding fault zone between the Uncompahgre uplift and Paradox basin, for instance, is more complicated in terms of the geometry and types of faults than they portray. The geometry of the fault zones that separates the Uncompahgre uplift from both the Central Colorado trough to the north and the Paradox basin to the south are thrust faults in some areas, but are a complicated array of high-angle faults in other areas (Baars et al., 1988).

Other examples include the Gore fault (not cited by Ye et al., but a major bounding fault for the eastern side of the Central Colorado trough) on the west-central side of the Frontrange uplift. This fault dips westward at a high angle (>75°) (Tweto and Lovering, 1977) and has approximately 3000 m of late Paleozoic structural relief. This fault was a high-angle fault during the Ancestral Rocky Mountains deformation and is oriented subparallel to the bounding thrust on the Uncompahgre uplift to the southwest. West of the Gore fault, in the Central Colorado trough, seismic and other data (De Voto et al., 1986; Wachter and Johnson, 1986) indicate that the deformation of that region and the northwestern side of the Uncompahgre uplift was by high-angle faulting. De Voto et al. (1971) documented late Paleozoic normal faults bounding the Apishapa uplift.

The northern part of the eastern Frontrange was an easterly dip slope into the late Paleozoic Denver basin (Tweto, 1980; Kluth, 1997). The data indicate that this region did not have thrust geometry as asserted by Ye et al. (1996, figure 1).

Little data are published for the fault patterns active within the cores of most of the uplifts during the late Paleozoic. The data presented by Ye et al. (1996, p. 1410) do not support the statement that the Central Basin platform is "strongly folded and faulted." A detailed analysis by Yang and Dorobek (1995a) suggested the presence of normal and reverse faults within the core of the Central Basin platform, and along its margins. The style of faulting observed in that region appears to relate to the area examined. The subsidence profile of the Delaware and Midland basins of Yang and Dorobek (1995b) are similar to those shown by Ye et al. (1996). Yang and Dorobek (1995b) have interpreted the Delaware and Midland basins have a different structural model for their generation. Thus, the subsidence models must be considered nonunique support for their thrust loading hypothesis for basin development.

In summary, existing data from throughout the region indicate that fault geometries are more complex than that depicted by Ye et al. (1996) and that they change geometry along strike. Ye et al. (1996) have understated the diverse geometry of the faults and mountain fronts for which there are published data; therefore, because there is "little or no direct evidence of basement-involved overthrusting in other basin systems" (Ye et al., 1996, p. 1419), their statement that it "seems highly likely that all of these basins have a common origin related to thrusting of basement over the margins of the basins" (Ye et al., 1996, p. 1419) is unjustified.

SUBSIDENCE

The argument of Ye et al. (1996, p. 1426) about the general subsidence of the Greater Ancestral

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Rocky Mountains region is overstated. The southern uplifts, near the Ouachita/Marathon orogene (the Central basin and Diablo platforms in west Texas, Wichita uplift), were covered by sedimentary rocks in the Permian. Northward in the Ancestral Rocky Mountain system and farther from the Ouachita/Marathon region, however, the basement in the cores of the uplifts remained exposed for approximately 100 m.y. after deformation ended and they (Frontrange, Wet Mountains, Uncompahgre, San Luis uplifts) were not covered until the Late Jurassic (De Voto et al., 1971; Tweto, 1979).

SUBDUCTION ZONE

I agree with Ye et al. (1996, p. 1420) that tectonic events along the western margin of the United States "offer little possibility of a cause" for the development of the Greater Ancestral Rocky Mountains (Kluth and Coney, 1981; Kluth, 1986). I also agree with Ye et al. (1996, p. 1425) that the plate boundary activity in northeastern Mexico is not known from the geologic record. Ye et al. gave no evidence for truncation or "other events" of late Paleozoic age (1996, p. 1417), nor did they elaborate on what the "other events" might be. Ye et al.’s (1996, p. 1399) Figure 1A suggests that restoration of the Mojave-Sonora megashear (Anderson and Schmidt, 1983) would place the western plate margin of northern Mexico in the late Paleozoic even farther away from the site of the Greater Ancestral Rocky Mountains; therefore, the depiction of the areal distribution of the late Paleozoic features in Ye et al.’s figure 10 is difficult to evaluate.

The Late Paleozoic arc-related rocks in northeastern Mexico do not "indicate" (Ye et al., 1996, p. 1425) the presence of an east-dipping subduction zone along the southwestern margin of North America. Handschy et al. (1987) interpreted the arc-related volcanic rocks as a suspect terrane, accreted to North America from the east. The data are presently insufficient to choose from an array of possible alternatives.

If a shallowly dipping subduction zone had existed in the late Paleozoic beneath western North America, at least some evidence of it would be expected to remain within the Ancestral Rocky Mountain basins. For example, Late Cretaceous and early Tertiary igneous activity is well known in the Rocky Mountain region and the pattern of igneous ages and chemistry is important evidence for a shallowly dipping subduction zone to have existed below the Laramide Rocky Mountains (Coney and Reynolds, 1977; Dickinson and Snyder, 1978).

In contrast, there is no evidence in the Ancestral Rocky Mountain late Paleozoic geologic record for volcanism or intrusive activity (see Curtis, 1958, De Voto, 1980). That is, there is no evidence of Pennsylvanian igneous rocks within the synorogenic rocks of the Ancestral Rocky Mountains that would suggest that a low-dipping subduction zone was active somewhere on the western margin of Mexico; furthermore, it is unclear how an easterly dipping subduction zone could be located in "northeastern Mexico" (Ye et al., 1996, p. 1424, see their figure 1B), or that the subduction zone included a low east-dipping subducted slab beneath western North America during the late Paleozoic.

In summary, there is no evidence in the synorogenic rocks deposited in the Ancestral Rocky Mountain basins that a low-dipping subduction zone existed off of the western margin of Mexico in the late Paleozoic. One of the main features of the hypothesis of Ye et al. (1996) thus must be regarded as completely unsupported by the rock record.

HYDROCARBON EXPLORATION

Ye et al. (1996) suggested that the recognition of the thrusts that occur along the margins of some of the Ancestral Rocky Mountain uplifts offer a new opportunity for the exploration for hydrocarbons. Exploration wells, some cited by Ye et al. (1996), have drilled through the basement overhangs along the late Paleozoic mountain fronts. Drilling through overhangs at the margins of basement-cored uplifts is expensive and includes high risk; therefore, a venture such as this requires at least some data and a well thought-out interpretation. One may then assume that the overhangs were recognized some time before the wells were drilled and that the geometry of the mountain fronts has been recognized within the hydrocarbon industry for some time. My experience has been that the risk of exploring for hydrocarbons in the late Paleozoic mountain fronts has not been in the recognition that some of them had thrust geometry. The biggest structural challenge in exploring beneath basement overhangs of any age is that the geometry cannot usually be imaged well on seismic data; therefore, trap risk cannot be accurately assessed.

CITATIONS

I am unfamiliar with anyone else with the last name of "Kluth" being involved in interpreting the Ancestral Rocky Mountains. I, therefore, suspect that the references by Ye et al. (1996, p. 1419) to personal communications by "P. Kluth" were, in

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fact, probably to me. Suggestions by my colleagues that I am P. Kluth’s evil twin, as far as I know, are unfounded.

CONCLUSIONS

In summary, the lack of any data in the extensive rock record for even the presence of a subduction zone, much less of its dip, makes it impossible to evaluate the main hypothesis of Ye et al. (1996) that a low-dipping subduction zone in southwestern Mexico was related in some way to the Greater Ancestral Rocky Mountains. The evidence for the close timing of events in the Greater Ancestral Rocky Mountains and the Ouachita-Marathon orogeny (previously cited) (see Kluth, 1986) suggests a close link between these two events. The high angle between the trend of most of the Greater Ancestral Rocky Mountains is problematic. Kluth (1986) interpreted the uplifts and basins as representing a zone of distributed crustal shear, with a northward component of movement of the southern blocks during the collision between North America and South America. This working hypothesis must be preferred to one in which there is evidence that local regions, some of which were cited by Ye et al. (1996), are more complex than they have been interpreted to be and in which there is no evidence in the rock record for several of the key features. This lack of evidence makes it impossible to assess the value of the comparisons of Ye et al. (1996, p. 1423) between the late Paleozoic deformation of southern and western North America and other orogenes. 

REFERENCES CITED

Anderson, T. H., and V. A. Schmidt, 1983, The evolution of middle America and the Gulf of Mexico-Caribbean Sea region during Mesozoic: Geological Society of America Bulletin, v. 94, p. 941-966.

Baars, D. L., and G. M. Stevenson, 1984, The San Luis uplift, an enigma of the Ancestral Rockies: The Mountain Geologist, v. 21, p. 57-67.

Baars, D. L., et al., 1988, basins of the Rocky Mountain region, in L. L. Sloss, ed. Sedimentary cover—North American craton: The geology of North America: Geological Society of America Decade of North America, v. D-2, p. 109-220.

Brewer, J. A., R. Good, J. E. Oliver, L. D. Brown, and S. Kaufman, 1983, COCORP profiling across the southern Oklahoma aulacogen: overthrusting of the Wichita Mountains and compression within the Anadarko basin: Geology, v. 11, p. 109-114.

Coney, P. J., and S. J. Reynolds, 1977, Cordilleran Benioff zones: Nature, v. 270, p. 403-406.

Curtis, B. F., 1958, Pennsylvanian paleotectonics of Colorado and adjacent areas, in B. F. Curtis, ed., Symposium of Pennsylvanian rocks of Colorado and adjacent areas: Denver, Rocky Mountain Association of Geologists, p. 9-12.

De Voto, R. H., 1980, Pennsylvanian stratigraphy and history of Colorado, in H. C. Kent and K. W. Porter, eds., Colorado geology: Rocky Mountain Association of Geologists Symposium Volume, p. 71-101.

De Voto, R. H., F. H. Peel, and W. H. Pierce, 1971, Pennsylvanian and Permian stratigraphy, tectonism, and history, northern Sangre de Cristo Range, Colorado, in H. L. James, ed., Guidebook of the San Luis basin, Colorado: New Mexico Geological Society 22nd Field Conference Guidebook p. 141-163.

De Voto, R. H., B. L. Bartleson, C. J. Schenk, and N. B. Waechter, 1986, Late Paleozoic stratigraphy and syndepositional tectonism, northwestern Colorado, in D. S. Stone, ed., New interpretations of northwest Colorado geology: Rocky Mountain Association of Geologists Symposium Volume, p. 37-49.

Dickinson, W. R., and W. S. Snyder, 1978, Plate tectonics of the Laramide orogeny: Geological Society of America Memoir 151, p. 355-366.

Ellis, C. H., 1966, Paleontologic age of the Fountain Formation south of Denver, Colorado: The Mountain Geologist, v. 3, p. 155-160.

Frahme, C. W., and E. B. Vaughn, 1983, Paleozoic geology and seismic stratigraphy of the northern Uncompahgre front, Grande County, Utah, in J. D. Lowell, ed., Rocky Mountain foreland basins and uplifts: Rocky Mountain Association of Geologists Symposium Volume, p. 201-211.

Graham, S. A., W. R. Dickinson, and R. V. Ingersoll, 1975, Himalayan-Bengal model for flysch dispersal in the Appalachian-Ouachita system: Geological Society of America Bulletin, v. 86, p. 273-286.

Handschy, J. W., G. R. Keller, and K. J. Smith, 1987, Ouachita system in northern Mexico: Tectonics, v. 6, p. 323-330.

Jordan T. E., and R. C. Douglass, 1980, Paleogeography and structural development of the Late Pennsylvanian to Early Permian Oquirrh basin, northwestern Utah, in T. D. Fouch and E. R. Megathan, eds., Paleozoic paleogeography of west-central United States: Rocky Mountain Section SEPM Symposium 1, p. 217-238.

Kluth, C. F., 1986, Plate tectonics of the Ancestral Rocky Mountains, in J. A. Peterson, ed., Paleotectonics and sedimentation in the Rocky Mountain region, United States: AAPG Memoir 41, p. 353-369.

Kluth, C. F., 1997, Comparison of the location and structure of the late Paleozoic and Late Cretaceous-early Tertiary Front Range uplift, in D. W. Bolyard and S. A. Sonnenberg, eds., Geologic history of the Colorado Front Range: Rocky Mountain Association of Geologists Guidebook 7, p. 31-42.

Kluth, C. F., and P. J. Coney, 1981, Plate tectonics of the Ancestral Rocky Mountains: Geology, v. 9, p. 10-15.

Mack, G. H., L. J. Suttner, and J. R. Jennings, 1979, Permo-Pennsylvanian climatic trends in the Ancestral Rocky Mountains, in D. L. Baars, ed., Permianland: Four Corners Geological Society Guidebook, p. 7-12.

McConnell, C. A., 1989, Determination of offset across the northern margin of the Wichita uplift, southwest Oklahoma: Geological Society of America Bulletin, v. 101, p. 1317-1332.

McKee, E. D., and E. J. Crosby, coordinators, 1975, Paleotectonic investigations of the Pennsylvania System in the United States: U.S. Geologic Survey Professional Paper 853, pt. 1, 349 p.

Tweto, O., 1979, Geologic map of Colorado: U.S. Geological Survey, scale 1:500,000.

Tweto, O., 1980, Tectonic history of Colorado, in H. C. Kent and K. W. Porter, eds., Colorado geology: Rocky Mountain Association of Geologists, Symposium Volume, p. 5-9.

Tweto, O., and T. S. Lovering, 1977, Geology of the Minturn 15-minute quadrangle, Eagle and Summit counties, Colorado: U.S. Geological Survey Professional Paper 956, 96 p.

Wachter, N. B., and W. E. Johnson, 1986, Pennsylvanian-Permian paleostructure and stratigraphy as interpreted from seismic data in the Piceance basin, northwest Colorado, in D. S. Stone, ed., New interpretations of northwest Colorado geology: Rocky Mountain Association of Geologists Symposium Volume, p. 51-64.

Weimer, R. J., 1996, Guide to the petroleum geology and Laramide

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orogeny, Denver basin and Front Range, Colorado: Colorado Geological Survey Bulletin 51, p. 97-102.

White, M. A., and M. I. Jacobson, 1983, Structures associated with the southwest margin of the ancestral Uncompahgre uplift, in W. R. Avert, ed., Northern Paradox basin-Uncompahgre uplift: Grand Junction Geological Society Guidebook, p. 33-39.

Yang, K. M., and S. L. Dorobek, 1995a, The Permian basin of west Texas and New Mexico: tectonic history of a "composite" foreland basin and its effects of stratigraphic development, in S. L. Dorobek and G. M. Ross, eds., Stratigraphic evolution of foreland basins, SEPM Special Publication no. 52, p. 149-174.

Yang, K. M., and S. L. Dorobek, 1995b, The Permian basin of west Texas and New Mexico: flexural modeling and evidence for lithospheric heterogeneity across the Marathon foreland, in S. L. Dorobek and G. M. Ross, eds., Stratigraphic evolution of foreland basins, SEPM Special Publication no. 52, p. 38-50.

Ye, H., L. Royden, C. Burchfiel, and M. Schuepbach, 1996, Late Paleozoic deformation of interior North America: The Greater Ancestral Rocky Mountains: AAPG Bulletin, v. 80, no. 9, p. 1397-1432.

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AAPG Bulletin, V. 82 (1998), No. 12 (December 1998), P. 2277-2279.

Late Paleozoic Deformation of Interior North America: The Greater Ancestral Rocky Mountains: Reply¹

Hongzhuan Ye,² Leigh Royden,³ Clark Burchfiel,⁴ and Martin Schuepbach⁴

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received March 19, 1998; final acceptance May 27, 1998.
²University of Texas at Dallas, P.O. Box 688, Richardson, Texas 75083-0688.
³Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.
⁴Danube International Petroleum Company, 2651 North Harwood Street, Dallas, Texas 75201.

We are largely in agreement with Charles Kluth on the observational data that constrain the late Paleozoic deformation of western and southern North America. Perhaps the major differences in our interpretation reflect different approaches to analysis of regional tectonic systems. Coming from a background that includes extensive analysis of a wide variety of tectonic systems (including many that are active or recently active), we have abstracted a few basic principles that seem to be generally useful in analyzing regional tectonic systems and that helped to guide our interpretation of the late Paleozoic deformation of North America (Ye et al., 1996). (1) Events must be evaluated over a geographic region that is sufficiently large so that one is sure one is looking at the entire system of deformation, not at an isolated piece. (2) In so far as possible, it is important to identify the detailed timing of events across the entire tectonic system and to compare only events that are contemporaneous. (3) In many instances, the regional pattern of events is most diagnostic of the larger scale kinematics and dynamics of tectonic systems rather than local complexities along individual fault zones or the details of localized stratigraphic features in poorly exposed regions; tectonic systems always contain a lot of "noise." (4) Most tectonic systems have obvious tectonic analogs within the geologic record; comparison with similar looking systems, particularly young ones, can reveal much insight into the evolution of older systems. From this perspective, we address the specific points contained in the discussion by Kluth (1998). (With a few exceptions we have omitted references from this reply because appropriate references are already given in the relevant sections of our original paper.)

STRATIGRAPHIC DETAIL AND TIMING

To our knowledge, our paper (Ye et al., 1996) was the first published work to attempt correlation of stratigraphic and structural features across the entire system of late Paleozoic deformation of southern and western North America. Although we agree with Kluth that the timing of peak tectonic activity within the Ancestral Rocky Mountains was spatially variable, we felt it beyond the resolution of the available data to emphasize shifts in the locus of maximum deformation through time. Dating of peak tectonic activity is considerably more difficult than dating onset and cessation of deformation, and data because peak tectonic activity in some parts of the system are uncertain or simply not available. To the extent that it was possible for us to discern periods of peak tectonism throughout the Greater Ancestral Rocky Mountains, there appears to have been a modest reorganization of deformation during the Desmoinesian, with some regions undergoing peak activity in the Morrowan-Desmoinesian (e.g., the Anadarko region), some areas undergoing peak activity in the Desmoinesian-Wolfcampian (e.g., the Pedregosa basin), and some regions undergoing large magnitude deformation throughout the Pennsylvanian (e.g. the Delaware basin).

We agree with Kluth (1998) that most of the sediment-starved basin regions occur in the southern part of the Greater Ancestral Rocky Mountain system and did not mean to imply otherwise (see Ye et al., 1996, figure 2, for example); however, the importance of noting that some of the basins in this system are starved is highlighted by Kluth’s comment about the Wolfcampian section in the Delaware basin. Because the deeper portions of the Delaware basin were starved until the Wolfcampian, we believe that one can place only broad limits on the period of peak tectonism in this region. For example, the 4-km-thick Wolfcampian section in the Delaware basin is mainly posttectonic and probably represents the passive filling of an approximately 1-km-deep sea created by earlier tectonism. In addition, the proximal sedimentary sequences that would be most likely to contain synorogenic facies

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along the eastern part of the basin are missing (perhaps overthrust?), again making it difficult to constrain the precise timing of deformation. Because there is strong evidence for Desmoinesian deformation and denudation within the Central Basin platform, considerable deformation likely also occurred prior to the Wolfcampian along the eastern margin of the Delaware basin. In any case, the point is that the detailed timing of peak deformation in the Delaware basin is highly uncertain, thereby illustrating some of the difficulties in correlating peak tectonic events across a large region.

In an effort to keep this reply brief, we will simply say that we believe it unlikely that uplift of the Sierra del Nido region did not occur until the Wolfcampian because subsidence in the adjacent Pedregosa basin occurred throughout most of the Pennsylvanian; however, the necessity of inferring the timing of uplift from subsidence of the adjacent basin again illustrates the difficulties and large uncertainties in trying to bracket too tightly the ages of peak tectonism.

FAULT GEOMETRY

The structural expression of convergence within continental crust is generally complex; many orogenes that accommodate dominantly orogene-normal convergence contain high-angle reverse faults, normal faults, and strike-slip faults, in addition to thrust faults. In our opinion, the presence of high-angle and normal faults within the Ancestral Rocky Mountains is compatible with the largest scale structures (basins and adjacent uplifts) being the result of crustal scale shortening and overthrusting along basement involved faults. What is most significant is that in several geographically distant places within the system the presence of large-scale overthrusting is clear. Even a cursory look at the Laramide Rocky Mountains, which we argued (Ye et al., 1996) are tectonically analogous to the Ancestral Rocky Mountains, shows that steep, normal, and strike-slip faults are present along with the basement-involved overthrusts that define the basins and adjacent uplifts (see, for example, Oldow et al., 1989, and references contained therein). Note that until the acquisition of good subsurface data to constrain the geometry of Laramide structures at depth, arguments about the deformation of the Laramide Rocky Mountains were similar to the current arguments of Kluth (1998) (and many others) against the widespread occurrence of basement-involved overthrusting in the Ancestral Rocky Mountains. Identification of Paleozoic structures also is more difficult than identification of Laramide structures because of extensive reworking of Paleozoic features by Cretaceous and Tertiary deformation and because of the subsequent burial of Paleozoic features in many parts of the system.

Truncation of our remarks as quoted by Kluth (1998) lends a somewhat different flavor than we had intended when we wrote (Ye et al., 1996, p. 1419.) "As yet, we have little or no direct evidence for basement-involved overthrusting in the other basin systems, but the orientation, geometry, timing, and observable style of deformation are so similar throughout the entire region . . . that it seems highly likely that all of these basins have a common origin related to thrusting of the basement over the margins of the basin." Perhaps the fact that Kluth believes this statement to be unjustified is related to our different approaches to analyzing regional systems in which we emphasize the importance of regional patterns of deformation and comparison to tectonically analogous systems over the local complexities of individual fault systems in areas of poor exposure.

SUBSIDENCE

First, we agree completely that subsidence data are commonly, if not usually, nonunique with respect to subsidence mechanism. The main point of our flexural analysis based on subsidence of the Midland and Fort Worth basins was to show that the two-dimensional geometry of this complexly subsided region is compatible with subsidence related to flexural loading from two sides. Second, Kluth (1998) is correct in stating that only the southern one-half of the Greater Ancestral Rocky Mountains subsided significantly immediately after the cessation of tectonism. We emphasized this regional subsidence only because we were attempting to outline some of the ways in which the Paleozoic deformation appears to have been different from the Laramide deformation of the Rocky Mountains; we should have been explicit as to which part of the system we were referring.

SUBDUCTION ZONE

Abundant evidence is in the literature for the truncation of the Paleozoic continental margin of the southwestern United States in Permian-Early Triassic (e.g., Burchfiel and Davis, 1981; Walker, 1988). The lack of citations to this effect was an oversight on our part. We are puzzled by Kluth’s (1998) comment that truncation would place the western plate margin of northern Mexico even farther away from the site of the Greater Ancestral Rocky Mountains because all of the data used in our paper come from sites inboard of the truncation

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zone. Because margin truncation is likely to have involved large left-lateral strike-slip displacements along a zone trending northwest-southeast, the truncated portions of the arc may be present today in Central America, although we believed that this was too speculative to include in our paper (Ye et al., 1996).

The Paleozoic rock record from north-central Mexico is poorly known, but clearly volcanic rocks and volcanoclastic sediments are present over much of this region, volcanism was active in the Desmoinesian (or possibly pre-Desmoinesian), and in the Permian and Desmoinesian volcanic rocks were present immediately west of and were shed into the Chihuahua basin. The composition of these volcanic rocks and inclusions within volcanoclastic debris flows indicate that they were erupted as arc volcanic rocks developed on continental crust or on an older highly evolved arc. We believe that our statements (Ye et al., 1996) accurately reflected the uncertainty in interpretation of these rocks, as in, for example (Ye et al., 1996, p. 1424),"this . . . volcanic arc testifies to the presence of a nearby subduction boundary of late Paleozoic age. In our interpretation, this arc probably is an Andean-type volcanic arc erupted through the continental crust and implies northeast-dipping subduction of oceanic lithosphere along a subduction boundary southwest of . . . the Chihuahua basin" and "because the subduction boundary itself is not known from the geologic record, [its tectonic style] is essentially unknown and must be inferred from its volcanic arc and from coeval deformation in its hinterland" (Ye et al., 1996, p. 1425). True, we cannot prove, and do not claim to prove, that there was late Paleozoic subduction beneath northeastern and north-central Mexico; nevertheless, we propose that this is a reasonable interpretation of the data and that it is especially intriguing in light of the coeval deformation occurring in interior North America. In this regard, our emphasis on the importance of regional-scale patterns of coeval deformation and on comparison to young tectonic analogs leads us to assign a greater regional importance to these arc rocks than does Kluth (1998).

Kluth (1998) greatly overstates the case that we make for flat-slab subduction beneath western North America in the late Paleozoic. Nowhere in the abstract do we refer to flat-slab subduction, and in the body of the text (Ye et al., 1996, p. 1426) the discussion of flat-slab subduction is mainly confined to one paragraph, prefaced by the remark that "because late Paleozoic events along the southwestern margin of North America are so poorly known, detailed speculations about the exact relationship between events along this plate boundary and events within the Greater Ancestral Rocky Mountains are probably not warranted" and then go on to note that the two best known examples of intraplate shortening in front of a subduction zone involve regions of flat-slab subduction. There may be problems with applying a flat-slab model to Paleozoic subduction beneath southwestern North America, but the absence of volcanism within interior North America is not necessarily one of them (for example, arc volcanism is absent in the Sierra Pampeanas in Argentina).

CONCLUSIONS

In summary, we have not attempted to prove that the late Paleozoic deformation of interior North America is caused by Andean-style subduction in northeastern Mexico. Rather, we attempted to marshal the available structural and stratigraphic data for most of late Paleozoic southern and western North America, to examine the spatial relationship of coeval events, and to propose what we believe is a sensible, novel, and (we hope) thought-provoking interpretation of these data. In any event, we expect that our basic hypothesis will be examined and tested as more constraints on the nature and timing of late Paleozoic events become available. 

REFERENCES CITED

Burchfiel, B. C., and G. A. Davis, 1981, Triassic and Jurassic evolution of the Klamath Mountains-Sierra Nevada geologic terrane, in W. G. Ernst, ed., The geotectonic development of California, Rubey v. 1: Englewood Cliffs, New Jersey, Prentice-Hall, p. 50-70.

Kluth, C. F., 1998, Late Paleozoic deformation of interior North America: the Greater Ancestral Rocky Mountains: discussion: AAPG Bulletin, v. 82, p. 2278-2282.

Oldow, J. S., A. W. Bally, H. G. Avé Lallemant, and W. P. Leeman, 1989, Phanerozoic evolution of the North American cordillera: United States and Canada, in A. W. Bally and A. R. Palmer, eds., The geology of North America—an overview: Geological Society of America, v. A, p.139-232.

Walker, J. D., 1988, Permian and Triassic rocks of the Mojave desert and their implication for timing and mechanisms of continental truncation: Tectonics, v. 7, p. 685-709.

Ye, H., L. Royden, C. Burchfiel, and M. Schuepbach, 1996, Late Paleozoic deformation of interior North America: the Greater Ancestral Rocky Mountains: AAPG Bulletin, v. 80, no. 9, p. 1397-1432.

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Late Paleozoic Deformation of Interior North America: The Greater Ancestral Rocky Mountains: Discussion & Reply