Reanalysis of the COCORP Utah Line 1 Deep Seismic Reflection Profile

Reanalysis of the COCORP Utah Line 1 deep seismic reﬂ ection proﬁ le: Toward an improved understanding of the Sevier Desert detachment question

John H. McBride1,*, William J. Stephenson2, and Eleanor I. Prussen McBride1 1Department of Geological Sciences, Brigham Young University, P.O. Box 24606, Provo, Utah 84602, USA 2U. S. Geological Survey, Box 25046, MS 966, Denver, Colorado 80225, USA

ABSTRACT INTRODUCTION 1983; Von Tish et al., 1985). For example, recent papers by Christie-Blick et al. (2007a), Hintze The Sevier Desert detachment (or “refl ec- Understanding the mechanism of low-angle- and Davis (2003), and Niemi et al. (2004) repro- tion,” SDR), which underlies the Sevier Des- normal-fault (LANF) formation remains an duce the travel-time section or a line drawing of ert basin along the eastern margin of the USA enduring puzzle in earth science. One of the it. Yet, the expression of the SDR on the origi- Basin and Range, is commonly cited as a “type most oft-cited examples of a continental LANF nally processed common-depth-point (CDP) example” of a low-angle normal fault (LANF) is the Sevier Desert refl ection (SDR) or “detach- seismic profi le is far from simple. Key geo- in continental crust. We present the results of ment,” along which crustal extension has pur- physical factors that might be used to challenge reanalyzing the SDR on the COCORP (Con- portedly been accommodated in the eastern the detachment origin include the poor continu- sortium for Continental Refl ection Profi ling) Basin and Range in west-central Utah, USA ity of the SDR, apparent changes in refl ection deep seismic profi le crossing the Sevier Des- (Allmendinger et al., 1983; Smith and Bruhn, character, variations in structural slope, and ert basin (Utah Line 1). We employ a strategy 1984; Von Tish et al., 1985; Planke and Smith, apparent offsets or major structural irregulari- of showing how shallow crustal velocity mod- 1991; Coogan and DeCelles, 1996; Carney and ties of the refl ection. Previous workers have not els, derived from fi rst-break analysis of the Janecke, 2005; DeCelles and Coogan, 2006) necessarily misinterpreted such features, but the shot records, may be used to reduce the effect (Figs. 1 and 2). The SDR has achieved a sort unavailability of a depth section for the actual of lateral velocity variations on imaging the of iconic status as one of the type examples of a seismic data may present a hindrance to pursu- SDR. Our results imply that the irregulari- LANF that developed in continental crust (e.g., ing a more detailed interpretation of the SDR. ties and discontinuities along the SDR are Wernicke, 1981; Coleman and Walker, 1994). The purpose of this paper is to, fi rst, present likely caused by overlying lateral velocity However, the validity of the detachment inter- the results of a reprocessing effort, highlight- variations. The reprocessed versions of the pretation for the SDR has been discussed at ing the sources of the distortion of the seismic section reveal a smoother, simpler, and more length in the literature (Mitchell and McDonald, image. From a purely technical point of view, continuous SDR, lacking most of the appar- 1986; Allmendinger and Royse, 1995; Anders we wished to test the hypothesis that applying ent large offsets and structural variations on et al., 1995, 1998; Wills et al., 2005; Christie- a static correction based on a two-layer velocity the currently available version of the profi le. Blick and Anders, 2007; Coogan and DeCelles, model derived from fi rst-break analysis would Seismic attribute and structural analyses of 2007; Christie-Blick et al., 2007a). The leading smooth and simplify the expression of the SDR. the refl ection indicate signifi cant variations alternative interpretation is that the SDR rep- Second, we demonstrate the use of seismic attri- along the profi le, which are likely related in resents a shallow crustal unconformity aligned butes in order to examine continuity and consis- part to the fi eld acquisition, but may also sug- with a deeper Mesozoic thrust fault (Anders tency of the SDR. Last, our study does not seek gest lateral variations in the physical origin and Christie-Blick, 1994). A key data set for to settle the debate on the SDR, but to suggest of the SDR. Such lateral variations may be interpreting the SDR is the COCORP (Consor- how the new results refocus some issues for the consistent with previous studies that chal- tium for Continental Refl ection Profi ling) Utah controversy of the detachment interpretation lenge the LANF interpretation of the SDR Line 1 deep seismic refl ection profi le (Fig. 2), beneath the eastern Basin and Range. As part on the basis of attributing different origins acquired in 1982. Despite the intense focus of our presentation, we assess the limitations of to different parts of the refl ector; however, a on the SDR, relatively little interest has been the methodologies, which are important due to smoother and more continuous SDR points shown in the actual seismic refl ection data set varying subsurface and attenuation conditions, to a tectonically uniform origin and is thus itself, with most studies (but see also Smithson to the limited frequency bandwidth of the seis- interpreted to be more consistent with a and Johnson [1989]) citing the original early mic data, and to the lack of good seismic veloc- LANF explanation. 1980s version of the data (Allmendinger et al., ity control along the profi le.

*Corresponding author e-mail: [email protected].

Geosphere; December 2010; v. 6; no. 6; p. 840–854; doi: 10.1130/GES00546.1; 8 ﬁ gures.

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/6/840/3339836/840.pdf by guest on 27 September 2021 Sevier Desert detachment — Cambrian; — Cretaceous; J Cretaceous; — Ordovician; C Tertiary; K Tertiary; — — Silurian; O — re labeled with unit symbols. Selected volcanic re as black solid lines (including faults from the as black solid lines (including faults from Quaternary; T — 2). Devonian; S — Mississippian; D — Pennsylvanian; M — le is shown by CDP numbers (green dots) and the vibrator point locations by squares. A more detailed more A point locations by squares. dots) and the vibrator numbers (green le is shown by CDP Pennsylvanian–Permian; P — Permian; PP — Triassic; P1 and P2 Triassic; — Precambrian. The reader is directed to Hintze et al. (2000) for further explanation of the symbols. Selected parts map a further to Hintze et al. (2000) for is directed The reader Precambrian. — PC proﬁ COCORP The location of the reprocessed noted. are outcrops rock by Hintze and Davis (200 of faults, is provided shown in the map, including interpretation version of a portion the area Quaternary fault and fold database of the U.S. Geological Survey [U.S. Geological Survey, 2006]). Geologic units are coded: Q 2006]). Geologic units are Quaternary fault and fold database of the U.S. Geological Survey [U.S. Survey, Figure 1. Geologic map (prepared from digital data in Hintze et al. [2000]) for the Sevier Desert basin area. Faults are shown Faults are Desert basin area. the Sevier digital data in Hintze et al. [2000]) for from 1. Geologic map (prepared Figure Jurassic; Tr

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GEOLOGICAL AND GEOPHYSICAL Mesozoic–Eocene structure of the Sevier fold- that thicken to the west (Planke and Smith, BACKGROUND and-thrust belt (Planke and Smith, 1991; Hintze 1991) (Fig. 1). Post-Sevier Orogeny erosion and Davis, 2003). The basin is bordered on the is expressed locally by early Cenozoic conti- The Sevier Desert basin in west-central east and west by the Pavant and Canyon ranges nental sediments that overlap Cretaceous and Utah, USA, resulted from Cenozoic crustal and the Cricket Mountains, respectively, and is early Tertiary synorogenic strata (Hintze and extension superimposed over the older, late ﬁ lled with 4.0 ± 0.6 km of sedimentary strata Davis, 2003). Widespread volcanic deposits of

Figure 2. (A) the unmigrated stacked section for the eastern portion of the COCORP Utah Line 1 profi le displayed with the original data processing (the digital data are available publicly from Cornell University as a SEG-Y fi le). Only the fi rst 6 s of traveltime are displayed. The CDP numbers are from the original processing performed at Cornell University. The exact locations of the CDPs are not available, and do not match those from the present reprocessing as shown in Figure 1. Vibrator point locations (VP) are also shown (see Fig. 1) as taken directly from the printed section (Nelson, 1988). For this and all seismic sections, 0 traveltime represents the processing datum of 1900 m above sea level, except as noted otherwise. (B) migrated section from Von Tish et al. (1985). Yellow dashed line traces the interpreted SDR. AAPG©1985, reprinted by permission of the AAPG whose permission is required for further use. (C) simplifi ed redrawing of preliminary depth section of Allmendinger et al. (1983) showing only their interpretation of the SDD and associated refl ectors. On this cross section we plot the approximate correspondence of the CDPs from the reprocessed section (Fig. 4) and the VPs (projected perpendicular to the CDP profi le) so that the previous sections can be compared with the reprocessed versions.

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variable thickness began to accumulate in the 2003]). The break-away zone for the detach- subsurface boundary. This is similar to the basin beginning in the middle Eocene and are ment has been suggested to be along the west- classic refraction statics approach; except that likely related to the episode of crustal extension ern side of the Canyon Range (e.g., Otton, 1995) deeper and longer-wavelength lateral veloc- (Hintze and Davis, 2003). Recent tectonism is (Fig. 1), although this has been disputed (Wills ity variations are targeted rather than the usual evidenced by Holocene faulting (Fig. 1) that is and Anders, 1999) (see also review in Mor- shallow “weathered zone” velocity variations. mapped in the vicinity of the COCORP profi le; ris and Hebertson, 1996). Although numerous In this way, long-wavelength velocity distor- some of the more prominent faults (e.g., the petroleum industry seismic profi les have been tions originating in the crust above the SDR are Holocene Clear Lake fault [Hintze and Davis, used to describe the detachment, most of these suppressed when the seismic data are stacked 2003]) may be projected into the profi le. The only show the detachment as a subhorizon- in the CDP domain. Because the source-to- area along the seismic profi le where the SDR tal boundary in the shallow sedimentary crust fi rst receiver offset is ~400 m, it is probably not appears (approximately the central part of the (Planke and Smith, 1991; Coogan and DeCelles, possible to resolve very shallow lateral veloc- eastern half) traverses only Quaternary surfi - 1996; Wills et al., 2005). The COCORP Utah ity variations. But long-wavelength effects can cial Lake Bonneville deposits (Fig. 1) and thus Line 1 (Fig. 2) deep seismic data set provides be potentially modeled and removed. Possible outcrop constraints for interpreting subsurface the only profi le with a broad, full-crustal-scale problems with this approach include interpret- refl ections are lacking. perspective (Smith and Bruhn [1984] show an ing fi rst arrivals accurately on the shot records The Sevier fold-and-thrust belt is one of the ~5 s seismic industry profi le that approximately and distortion of refl ection hyperbolae due to most studied fold-and-thrust belts in the world follows the COCORP profi le). Thus the debate large static shifts, which limits the effectiveness and constitutes a transitional zone located just on the meaning of the refl ector tends to pivot on of CDP stacking. The resulting stacked section west of the Basin and Range-Colorado Plateau the single COCORP profi le. also has the disadvantage of introducing distor- boundary (e.g., Burchfi el and Davis, 1975; tions into the travel-time section above the SDR, Planke and Smith, 1991; Hodges and Walker, METHODOLOGY: REPROCESSING OF i.e., within the interval in which the distortions 1992; Wernicke, 1995; DeCelles and Coogan, COCORP UTAH LINE 1 originate, and possibly smoothing over some 2006). However, understanding the preexten- shallow offsets. sional history of this region requires that one Discussion of the data acquisition and The static correction procedure consisted fi rst understand the amount of extension that original processing for COCORP Utah Line of automatic fi rst-break picking of direct- and may have displaced Sevier-aged and older 1 (Fig. 2) can be found in Allmendinger et al. head-wave arrivals, which were then corrected structures (DeCelles and Coogan, 2006). In (1983). For the present study, just those aspects manually for every record. In many instances, particular, DeCelles and Coogan (2006) have of the data processing relevant to the SDR are a direct arrival cannot be observed, in which argued that “large-magnitude extensional res- mentioned. Only the eastern half (approxi- case a minimum value of 1500 m/s was used. A toration” is required along Sevier thrust faults mately) of the profi le was analyzed intensively velocity model was derived in which the upper- in order to maintain a critical taper commonly since this is the controversial region for the layer velocity varied laterally, the depth to the observed in fold-and-thrust belts throughout the Sevier Desert detachment (SDD) interpreta- fi rst refractor varied, and the velocity of the sec- world. Interpretations of industry and academic tion and the area for which a signifi cant change ond, head-wave-producing layer (infi nite half- seismic refl ection profi les from the Sevier Des- in the stacked section resulted from the repro- space) also varied laterally (Fig. 3). Varying the ert basin have suggested a regional detachment cessing. The reprocessing began with the velocity of the half-space (from the head wave) (“Sevier Desert detachment”) that extends for vibroseis-correlated shot records, after which was a critical step in order to account for the over 70 km beneath the basin and to the west three-dimensional (3D) geometry was assigned signifi cant lateral variation in the rigid bedrock to the Utah-Nevada border (Allmendinger et to the traces. Frequency bandpass fi lters were geology (Fig. 3). A surface-consistent residual al., 1983; Smith and Bruhn, 1984; Von Tish tested, but deemed destructive due to the limited static correction was also applied to account for et al., 1985; Planke and Smith, 1991) (Fig. 2). bandwidth of the vibroseis source (8–31.25 Hz, very short-wavelength velocity variations, but Planke and Smith (1991) give estimates of at with a fi eld anti-alias fi lter). Starting at about which did not impose signifi cant changes on least 80–130 km for the north-south width of CDP 2950 (Fig. 1), the profi le begins to bend the fi nal image. Upper-layer velocities for the the interpreted detachment. Palinspastic res- to the southeast, which causes the SDR to be portion of the COCORP profi le shown in Fig- torations have been used to suggest that the depicted partly as a strike section. ure 4 averaged ~2400 m/s, whereas velocities total horizontal displacement on the interpreted Lateral velocity variations are expected from beneath the refractor averaged ~5200 m/s detachment lies between 28 and 38 km (Sharp, within the central Sevier Desert basin due to (Fig. 3). Planke and Smith (1991) reported aver- 1984; Von Tish et al., 1985) or more (Coogan the thickening of Tertiary sedimentary strata age velocities for Cenozoic formations from and DeCelles, 1996). Further review of the directly above the SDR and to the presence of well logs in the Sevier Desert basin ranging stratigraphic, structural, and tectonic back- Pliocene basalts as shown, for example, by Von from 2 km/s near the surface to ~4.5 km/s at ground of the Sevier Desert basin can be found Tish et al. (1985) from drill-hole data (volcanic ~2 km depth below ground level. Paleozoic and in Hintze and Davis (2003), Wills et al. (2005), rocks crop out as close as less than a kilometer Precambrian rocks were reported with average and DeCelles and Coogan (2006). from the CDP profi le [Fig. 1]). The combination velocities ranging from 5 km/s near the surface of relatively low-velocity Tertiary sedimentary to ~5.5 km/s at 2 km depth and increasing to THE SEVIER DESERT REFLECTION strata and high-velocity volcanic rocks makes over 6 km/s deeper. The velocities derived from INTERPRETED AS A DETACHMENT for a complicated velocity structure above the fi rst-break analysis are more or less consistent SDR that could cause interruptions of the refl ec- with these well log observations. One conse- The Sevier Desert detachment is not exposed tion (velocity push-down or pull-up). Thus the quence of applying a refraction static solution in in outcrop and is inferred to exist almost entirely most important processing step was to derive this manner is to effectively “replace” the upper on the basis of seismic refl ection profi les source- and receiver-domain static corrections part of the earth in the stacked section (Fig. 4) (mostly 1970s vintage data [Hintze and Davis, based on the depth to a long-wavelength rigid with material having a velocity equal to that of

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the replacement velocity (Fig. 3). This has the the COCORP profi le. We attempted to derive a of traces stretched more than 50% were zeroed). effect of shifting upward (in traveltime) shal- velocity model based on tomographic ray trac- NMO velocity analysis provided a stacking low refl ections as if they were refl ecting within ing (see McBride et al. [2010] for an example velocity function; however, due to the scarcity a medium with the replacement velocity func- of applying this technique) in order to correct of coherent refl ected arrivals and the general tion. These effects are more pronounced where for the effect of the Sevier Desert basin on the complexity of the crust, a reliable interval veloc- the difference between the replacement velocity SDR image. The resulting stacked section was ity function is diffi cult to obtain. We applied an and the upper layer velocity is relatively high not appreciably different from the original ver- apparent velocity fi lter (“tau-p” [vertical travel- (e.g., CDPs 2400–2800, between traveltimes sion. We believe this is because steep velocity time-slowness] fi lter) in the shot domain and 200 and 700 ms [Fig. 4A]). The refracting gradients in the model restricted ray penetration a 5-trace mix in the stack domain (Fig. 4A) interface used in the static correction (Fig. 3) is depth for the source-receiver offsets involved, in order to reduce noise. These processes are shown plotted on the travel-time CDP-stacked thereby limiting the ability of the tomographic intended to reduce the effect of noise caused by section (Fig. 4C), which shows a good correla- technique to resolve the critical long-wavelength scattering, but may smooth over small structural tion between the onset of refl ectivity (Fig. 4C) statics problem in these data. details. The reader is directed to McBride et al. and the head-wave-generating surface for most The velocity model (Fig. 3) was used to com- (2005) for a discussion of this fi lter applied to of the section. A previous study applied an pute source and receiver domain statics prior to another COCORP data set. Amplitude balanc- early form of model-based static corrections, normal move-out (NMO) correction and CDP ing with a 1 s window automatic gain control analogous to our approach (Branch, 1985; stacking. Muting fi rst breaks was performed was applied before and after CDP stacking. In Smithson and Johnson, 1989) for a portion of using an automatic CDP stretch mute (portions order to reduce noise further, the stacked section is also shown with a post-stack coherency fi l- ter (Fig. 4B) as developed by LITHOPROBE at the University of Calgary (e.g., van der Velden and Cook, 2005). This version of the stack is post- stack processed to strongly emphasize apparent breaks in continuity of the SDR. The most notable breaks appear beneath CDPs 2400–2500 and beneath CDPs 1990–2000. Migration trials on the stacked data did little to alter the main geometrical relationships, due to the low dip of the refl ector (≤11°) (see also Fig. 2). In order to attempt a view of the SDR in a depth section, we devised a simple, vertically and laterally varying average velocity function using the variable replacement velocity applied at 0 traveltime and increasing downward to 6 km/s at ~10 s, where sporadic refl ections from the Moho discontinuity arrive (not shown herein; see Allmend- inger et al. [1983]). As an independent check on this result, we then computed a depth conversion based on a smoothed version of the rms (root mean square) velocity function used to stack the data. The results of the depth conversions are shown in Figure 4B, which indicate a moderate degree of consistency between the two methods. Although such a simple approach is limited by a lack of detail, it is consistent with available regional seismic refraction data. Chulick and Mooney (2002) show an average crustal velocity for the study area of between 5.8 and 6.0 km/s, consistent with our simplifi ed function. As a simple fi rst-order check on the applicability of our velocity conversion, the average velocity function results in the Moho refl ection arrival times on Utah Line 1 being converted to ~30 km depth, which agrees well with compilations based on modeling seismic refraction profi les (Braile et al., 1989; Chulick Figure 3. Results of the refraction statics modeling for the eastern portion of and Mooney, 2002) (this exercise only indicates the COCORP Utah Line 1 profi le as reprocessed for this study. Top is the depth consistency of our depth conversion with previous (below the ground surface) to a head-wave–generating surface. Bottom is the results, not a proof). Results from an expanding seismic velocity (P-wave) of the earth just below the head-wave–generating sur- spread experiment along COCORP Utah Line 5 face (commonly referred to as the “replacement velocity”). (Liu et al., 1986), which intersects Utah Line 1

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/6/840/3339836/840.pdf by guest on 27 September 2021 Sevier Desert detachment B A ection. Note that none of the le. Note that all seismic sections in this depth to the SDR as predicted by Liu et al. depth to the SDR as predicted Sevier Desert refl Sevier e elevation static correction. For the reprocessed the reprocessed For e elevation static correction. Fig. 6). (B) As in Figure 4A, but with a post-stack As in Figure Fig. 6). (B) — proximation. lter applied. This section has been converted from time to depth as described in the text. The red dashes show the results of dashes show the results The red time to depth as described in the text. This section has been converted from applied. lter ). (A) As in Figure 2, but reprocessed as described in the text. See Figure 1 for location of the profi 1 for as described in the text. See Figure 2, but reprocessed As in Figure ). (A) lter, as described in the text, and displayed as a variable-area no wiggle section with a negative trace bias. SDR as described in the text, and displayed a variable-area lter, continued on following page other reprocessed sections has had a coherency fi sections has had a coherency reprocessed other marks the cross The green independently converting the SDR to depth using a smoothed version of stacking velocity function. that this is a model depth only and thus is, at best, an ap is reminded The reader explanation). more (1986) (see text for paper have an individual trace scaling applied. The original processing (Fig. 2) used a replacement velocity of 4500 m/s for th velocity of 4500 m/s for (Fig. 2) used a replacement The original processing have an individual trace scaling applied. paper has been smoothed (as in static correction 2209, the source of CDP West velocity was varied (Fig. 3). versions the replacement fi coherency Figure 4 ( Figure

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near CDP 2425 (Fig. 1), provided a generalized depth model in which the SDR was reported to be 4.8 km deep below their processing datum of 1432 m above sea level. For our processing of datum of 1900 m, this translates to a subdatum depth of 5.3 km, which matches well our model- C

ectors are likely too ectors are based estimate in Figure 4B. We stress that our depth conversion should not be considered robust because good velocity constraints are lacking. In particular, the depths of some shallow refl ectors in the center of the profi le are likely underestimated. Allmendinger et al.’s (1983) preliminary depth conversion is similar in some respects to our solution, but also shows important differences (Fig. 2). ned by seismic and structural attributes tion (Fig. 3). Note that problems with the tion (Fig. 3). Note that problems defi r, the shallow bright refl r,

n the static correction (Fig. 3), recomputed as (Fig. 3), recomputed n the static correction RESULTS OF REPROCESSING COCORP UTAH LINE 1 ed as variable area without the “wiggle trace” and ed as variable area

Original Processing

lowest-reflection strength–low heterogeneity strength–low lowest-reflection In order to compare the reprocessing results with the version of COCORP Utah Line 1 that has been the basis of most crustal-scale studies of the SDR (the version usually cited in the literature), we have accessed the digital SEG-Y coherency-fi ltered stacked section available from Cornell University (Nelson, 1988). This section is displayed in Figure 2 with no further processing. The originally processed version shows the SDR as an irregular and discontinuous surface. Anders and Christie-Blick (1994) and Christie-Blick et al. (2007a) refer to offsets and misalignments of the SDR as detracting from a detachment interpretation. Signifi cant apparent high-reflection strength–low heterogeneity high-reflection strength–low structural complexity (Fig. 2) can be seen, for example, along the shallow portion of the SDR between CDP 3000 and the eastern end of the profi le; beneath the northward projection of the with a 1 7, but with amplitude balancing (automatic gain control to the section in Figure similar ection strength, Clear Lake fault (CDP 2510); and expressed as apparent structural highs and lows (e.g., CDPs 2320, 1990, and 1800). Beneath CDP 3010 and again beneath CDP 3240, the seismic image of

low-reflection strength–low heterogeneity strength–low low-reflection the SDR is interrupted by sharp breaks across which the apparent dip of the reﬂ ector abruptly changes. These features are likely related at least in part to the sharp bends in the line of sources and receivers. In any case, the main interpretive focus of the paper lies mainly to the west.

Reprocessing

A principal goal of the reprocessing was

low-reflection strength–high heterogeneity low-reflection to determine if a smoother and more continu- ). (C) Stacked CDP section processed as reﬂ section processed ). (C) Stacked CDP ous travel-time image of the SDR could be obtained using conventional processing. The reprocessing strategy and parameters were thus

continued focused on improving upper-to-middle crustal reﬂ ection coherency and not on the shallow basin reﬂ ectivity, which does not accordingly Figure 4 ( Figure This version of the section, display noise and amplitude loss effects. to reduce and post-stack in order s window) applied pre- segments of the SDR, as The four arrivals. with a negative trace bias, emphasizes only the highest amplitude and most coherent (Fig. 8), are displayed across the top of the section. The green dashed line represents the head-wave–generating surface used i dashed line represents The green the top of section. displayed across (Fig. 8), are velocity func time to depth using the replacement datum (1900 m above sea level) and converted from traveltime below processing the shallow section above SDR. In particula the range of CDPs 2400–2600 for solution seem to persist over static correction velocity in the static solution (Fig. 5). layer the upper shallow in depth, as affected by a low value for show improvement. In fact, distortion has been

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introduced into parts of the shallow section due time section (Fig. 4C), whereas the shallower shifts and the velocity model. Smoothing may to an imperfect shallow crustal velocity model. portion (east of CDP 2600) dips less steeply. be warranted due to poor fi rst-break picking The shallow velocity structure above the SDR The degree of contrast between the “basinal” or to unaccounted-for complexity; however, (including the Sevier Desert basin) is likely to be upper-layer velocity and the lower half-space too much smoothing may allow static shifts to vertically and laterally complex, especially due replacement velocity signifi cantly impacts the remain in the stack that otherwise should have to volcanic units interlayered with lower-velocity amount of time shift (and smoothing) of the been removed. Figure 6 shows an example of sedimentary strata, which could be expected to SDR as well as of the shallow bright refl ection the effects of smoothing versus no smoothing produce sharp velocity gradients. Thus the irreg- above it, as seen across the middle length of the where a small apparent irregularity in the SDR ular appearance of the stack above the SDR may profi le. In order to understand this effect better, appears with smoothing. be affected by a trade-off between layer thick- we produced a spectrum of results using dif- Altogether, the reprocessed CDP section indi- ness and velocity uncertainties in the model. The ferent upper layer velocities (1000–4450 m/s) cates a smoother and more continuous SDR reprocessed version is shown in Figure 4. The while maintaining a constant replacement between ~0.4 and 4.5 s, along a map distance of corresponding velocity-depth model (Fig. 3) velocity (4500 m/s). The spectrum (Fig. 5) over 50 km. For example, the image of the SDR indicates that most signifi cant static effects will demonstrates how the shape and traveltime beneath the area of the Clear Lake fault projec- be applied between about CDPs 2000 and 3000. of the refl ections depend on this velocity con- tion is simpler on the reprocessed section (dis- Thus the apparent structural complexity of the trast. As the upper layer velocity approaches cussed below). A nearly fl at, ~4-km-long seg- SDR (in the area of interest, which more or less the replacement velocity, the SDR becomes ment of the SDR on the original section (centered corresponds to this CDP range) likely arises from “pushed down” and generally uneven compared beneath CDP 2400, Fig. 2) now appears as part of the laterally varying velocity structure above the to the effect of using a lower upper layer veloc- a smoothly dipping refl ection (centered beneath refl ector. As can be seen from comparing the ity (e.g., 2000 m/s), which produces a smoother CDP 2350, Fig. 4A). A broad antiformal shape, original and reprocessed stacks (Figs. 2 and 4A, result while decreasing refl ection arrival times. almost 20 km long, on the SDR (between CDPs respectively), an improvement in the continuity The shallow bright refl ection above the SDR 2050 and 2450, Fig. 2) is transformed to almost and linearity of the SDR has been achieved by is shifted “upward” in time as the upper layer a straight refl ection on the new section (between the new processing. Applying a migration to the velocity decreases. It is likely that the fi nal CDPs 2000 and 2400). Areas of poor continuity uncorrected time section (Von Tish et al., 1985) result (Fig. 4) has shifted the shallow refl ector remain on the reprocessed version especially cen- tends to accentuate the apparent undulations on too far upward, compared to what is interpreted tered on CDP 2460, which appears to correspond the time section (Fig. 2). The deeper portion of from well data in the area (Planke and Smith, with the northward projection of the Clear Lake the SDR (west of CDP 2600) now follows a 1991). We also experimented with the applica- fault (Figs. 1, 4A, and 4B). Small offsets (~100 ms) mostly straight west-dipping line on the travel- tion of different levels of smoothing the static of the SDR persist on the reprocessed stack (e.g., below CDPs 2330, 2580, and 2630; Fig. 4A). The complexity in refl ector geometry is probably not fully accounted for by the static correction, which was aimed at longer-wavelength lateral velocity effects. As pointed out by Von Tish et al. (1985), such offsets may represent unaccounted-for velocity effects originating in the shallow crust or could also represent minor faulting. Another signifi cant difference observed within the shallow crust on the reprocessed section is the position of an apparent “hinge” marking the change in dip located at about CDP 2600, across which the SDR increases in slope to the west and plunges deeper into the middle crust (Fig. 4A). This hinge corresponds to a gentle bend in the line of CDPs (Fig. 1), and thus seems likely to be at least partly an effect of the acquisition geometry. Furthermore, the position of such a “hinge” depends strongly on the contrast in the upper- and lower-layer velocities (Fig. 5). A prominent hinge was also observed on the original section further to the Figure 5. Results of refraction statics solutions in a portion of the CDP stack using east (Fig. 2), which corresponds to a major bend varying upper-layer velocities (1000–4450 m/s) while keeping the replacement (lower- in the survey. Between this “hinge” and the layer half space) velocity constant (4500 m/s). The datum (1900 m) is identical to the eastern end of the reprocessed profi le, the maxi- other stacks. The stacking used a simple vertical velocity gradient for NMO correction. mum relief on the SDR, where it is most likely Using the same NMO velocity function will obviously produce inconsistent results in to correspond to an unconformity, is ~750 ms the stacking coherency, but not signifi cantly affect general arrival times. Colored lines (or 2160 m, using the very simple time-to-depth show the arrival times for the SDR and the shallow bright refl ection above it between conversion [Fig. 4C]). Other than the one major CDPs 2200 and 2900 (see section in Figure 4A for context). Vertical arrow indicates slope change at the “hinge,” the SDR lacks projection of Clear Lake fault (Fig. 1). most of the apparent structural variations on the

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Figure 6. Example of stacking with a smoothed vs. an unsmoothed source statics solution. This section is a portion of that shown in Fig- ure 4A. Note the somewhat smoother appearance of the SDR between CDPs 2400 and 2500 on the unsmoothed version (top) as well as the slight shingling of the SDR beneath about CDP 2580 on the smoothed version (bottom). Arrows indicate areas where small offsets of the SDR appear differently on the two versions of the stack.

original version of the CDP stack. For exam- SDR is not so clearly offset and may in fact be zone (Oviatt, 1989), which intersects the profi le ple, beneath CDP 3000 on the original section unaffected by faulting. A close-up of the pos- with two fault strands (Fig. 2). This part of the (Fig. 2), the SDR shows a reversal in apparent sible offset of the SDR can be observed on both profi le previously has been interpreted as small dip, accompanied by a complex pattern of over- the smoothed and unsmoothed versions of the half-graben with a west-dipping fault (Allmend- lapping refl ections. The reprocessed section stacked section (Fig. 6), although the offset is inger et al., 1983), which only can be interpreted (Fig. 4A) shows a smoother and simpler image expressed somewhat differently depending on from the original profi le (Fig. 2). The west- with a shallower apparent dip, but also with a the static solution. Moving east along the pro- dipping fault has been interpreted to sole into loss of amplitude below this point due possibly fi le, a notable offset appears on the shallowest another detachment, but not to interact with the to stronger muting of refracted arrivals. refl ection (likely from volcanic strata) where SDR (Allmendinger et al., 1983). In summary, Although neither the reprocessing param- the Clear Lake fault trend intersects the profi le the reprocessed section lessens the evidence eters nor the original acquisition parameters (CDP 2460), as remarked above. Although this for offset of the SDR by high-angle normal were optimized for resolving shallow structure, fault is not mapped even close to the profi le, faults; however, the poor resolving power of we note some features of possible neotectonic its length and relatively straight strike make it the COCORP data in the shallow section points interest. The best possible vertical seismic reso- possibly the most interesting neotectonic fea- to the need for high-resolution seismic surveys lution (using the Rayleigh criterion), assuming ture that could be expressed on the profi le. On (e.g., with 3 m as opposed to 100 m station spac- 1500 m/s and 32 Hz would be ~12 m; however, the reprocessed profi le (Fig. 4A), the vertical ing) in order to study geologically recent fault- a more realistic estimate, based on 4500 m/s and downward projection of this offset corresponds ing in the Sevier Desert. a peak frequency of 14 Hz, gives ~80 m. Smith- to a zone of poor continuity along the SDR, but son and Johnson (1989) interpret a west-dipping without a clear offset of the SDR. This is in con- Reprocessing: Seismic Attributes (in the plane of the section) fault cutting a Plio- trast to the image on the original section (Fig. 2), cene basalt refl ection (their fi g. 17) beneath which shows a strong apparent disruption and In order to quantify the structural and seismic CDP 2580 (Fig. 4A) that appears to also cut, to narrow synformal feature beneath the fault pro- property variability of the SDR on the COCORP a lesser degree, the SDR. Such a fault cutting jection. The only mapped (Fig. 1) Quaternary profi le, several seismic attributes (e.g., refl ection shallow refl ectors may also be inferred from faults that actually are known to cross the seis- strength) were tested (Figs. 7 and 8). Employing the reprocessed section (Fig. 4A), although the mic profi le are part of the Drum Mountains fault a suite of attributes will more likely produce a

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reliable characterization (e.g., see review in Chopra and Marfurt, 2006). This is especially important for the COCORP profi le, due to the limited bandwidth and also due to the likelihood of varying noise and attenuation along the profi le (e.g., Fig. 7). Such variation could arise from a number of factors such as near-surface geology variation and localized areas of low CDP fold. The changing orientation of the CDP profi le (top, Fig. 8) and the nonuniqueness in stacking events also introduce uncertainty in the use of attributes. In an effort to reduce some of these effects, the attributes were computed from the CDP stack after correcting for spherical diver-

n these zones may frustrate the interpreta- gence and anelastic attenuation (as a function of Figure 8B. The color scale is arbitrary with The color 8B. Figure

es in noise character as expressed by mottled as expressed es in noise character traveltime) without the usual short-window gain control. Further, the results were smoothed so as to reduce the effect of possible meaningless variation (Fig. 8). As with any use of seismic attributes, the nonuniform effects of attenuation

lowest-reflection strength–low heterogeneity strength–low lowest-reflection and distortion from the overlying media cannot be fully accounted for, reducing the accuracy of

spheri- seismic attribute without gain balancing or ection strength the attribute. The most useful attributes were average reﬂ ection strength (Sheriff and Geldart, 1995), slope of instantaneous frequency (Sheriff and Geld- art, 1995), and reﬂ ection heterogeneity (e.g., lter and a post-stack trace mix, as discussed in the text. In order to represent to represent and a post-stack trace mix, as discussed in the text. In order lter Levander et al., 1994; Bean et al., 1999). The attributes were computed for a 150 ms window

ection. Green dashed line is as in Figure 4C. Individual trace scaling has been applied dashed line is as in Figure ection. Green centered over the SDR interpreted directly from the reprocessed stacked section. Average refl ection strength provides a measure of the variation in absolute amplitude of a refl ection, and thus furnishes a qualitative indication of the seismic impedance (i.e., rock properties) contrast relative Sevier Desert refl Sevier high-reflection strength–low heterogeneity high-reflection strength–low to the surrounding media. The slope of instanta- — neous frequency is a less common seismic attribute and may be considered to provide a qualitative indication of heterogeneity. The refl ection heterogeneity attribute is based on the square lters except for a pre-stack “tau-p” fi a pre-stack lters except for root of 1 plus the vertical derivative of the amplitude squared computed in a sliding window. The as the refl le (same portion as shown in Fig. 4) processed refl ection strength attribute was also computed for the entire stacked section without amplitude

low-reflection strength–low heterogeneity strength–low low-reflection balancing (Fig. 7). Lastly, a function showing the “instantaneous slope” of the SDR was extracted in order to investigate structural variations that may be too subtle to detect by visual inspection. The “instantaneous slope” was measured on a tracing of the reﬂ ection, after time-to-depth conversion, following the peak (positive) amplitude directly from the stacked data. This function does not show apparent geologic dip in the usual sense, but rather relates to the change in the depth of the SDR between each CDP sample

low-reflection strength–high heterogeneity low-reflection (a nominal distance of 50.3 m) as a function of distance along the proﬁ le. The reﬂ ection strength seismic attribute can

ection strength more robustly, the attribute was computed pre-stack. Note that this attribute is not the same as shown in the attribute was computed pre-stack. robustly, more ection strength be used to examine the distinctiveness of the to the display (each trace has had amplitude divided by mean absolute value of trace). Note lateral chang noise i The increased black above and below the SDR (e.g., CDPs 1900–2060 2910–3200, respectively). zones replacing colored tion of seismic attribute patterns (Fig. 8). hot colors representing high values and cold representing low. SDR low. high values and cold representing hot colors representing Figure 7. Portion of the COCORP Utah Line 1 profi 7. Portion of the COCORP Figure and with no fi attenuation corrections cal divergence or refl SDR relative to the general refl ectivity of the

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section. Figure 4C shows this attribute, but with the SDR over much of its length as well as its (Fig. 8B). Four regions of refl ection character amplitude balancing applied so as to indicate the highly discrete expression (relative to an other- may now be recognized (Fig. 8). To begin with, envelope of high strength relative to other arriv- wise diffuse crustal refl ectivity) between about from the eastern end of the profi le to about CDP als in a 1000 ms automatic gain control win- CDPs 2600 and 1990. 2600, the “instantaneous slope” changes from dow. Further, the computation of the refl ection In order to integrate the results of the attribute high positive values (apparent east-dipping) strength attribute in the prestack domain with- analysis and the reprocessed seismic data, the of degrees (~9° from the smoothed curve) out amplitude balancing or frequency fi ltering CDP stacked section is redisplayed with the SDR to negative values (apparent west- dipping) (Fig. 7) demonstrates the striking continuity of fl attened and registered with the attribute suite (Fig. 8A). At about CDP 2600, a second area

A degrees

Figure 8 (continued on following page). Several attributes derived from the eastern portion (matching the portion shown in Fig. 4) of the reprocessed COCORP Utah Line 1 (arrows on the CDP scale show bends in the profi le). All attributes are derived from a surface interpreted through the center of the SDR on the reprocessed stacked section. This surface was fi rst picked automatically following the peak (positive) amplitude (i.e., the picked surface was “snapped” to the peak), after which the surface was occasionally manually shifted to correct for cycle skipping away from the interpreted SDR. Numbers 1–4 refer to interpreted segmentation in the SDR, based on these attributes. The top of the diagram shows the north-south excursion of the UTM (NAD 1927) station coordinates in order to track bends in the profi le that might impart an effect on the attributes. The CDP fold and elevation variations are also shown. (A) Computation of “instantaneous slope” (i.e., the change in depth to the SDR between CDP traces, divided by the average CDP interval, converted to an angle using the average velocity function applied in the time-to-depth conversion in Fig. 4B). Note that this represents an unmigrated, apparent dip. Positive values indicate a dip to the east; negative values indicate a dip to the west. (B) Portion of the stacked section (matching the length of profi le in Fig. 4) with the SDR shown fl attened to an arbitrary traveltime in order to depict lateral changes in refl ectivity and to indicate the quality of picking the surface.

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can be defi ned where the slope settles to a mini- two peaks (Fig. 8A). This region (“3”) corre- not always marked by sharp boundaries. Per- mum value of −25° to −20° and then maintains sponds to declining average refl ection strength haps not surprisingly, the boundaries between an average value of roughly −10°, but with a values (Fig. 8C). On the other hand, the slope of the regions sometimes correspond to bends in high variability until about CDP 2210. The instantaneous frequency and refl ection hetero- the CDP profi le, although the correspondence is expression of these easternmost two regions geneity begin to show some distinct variability not one-to-one (i.e., eight bends are recognized, (“1” and “2”) approximately matches a pat- (Figs. 8D and 8E, respectively). The western- Fig. 8). The bends likely affect the slope compu- tern on the average refl ection strength attribute most region (“4”) displays a distinct refl ection tation in places, but do not quite so easily explain (Fig. 8C). Average refl ection strength abruptly character descending to smaller absolute values the attribute variation. A vexing problem is the increases from the east beginning around CDP of dip, fi nally leveling off to near zero (Fig. 8A), area of low CDP fold just over a critical area of 2650 and then generally describes a region (“2”) while average refl ection strength changes to interest at the “1-2” boundary (top, Fig. 8). of unevenly high strength until about CDP 2210. a zone of intermediate, but variable, values. The SDR for regions “1” and “2” has relatively Both the slope of instantaneous frequency and DISCUSSION little expression on the slope of instantaneous refl ection heterogeneity attributes (Figs. 8D and frequency or refl ection heterogeneity (Figs. 8D 8E, respectively) for region “4” now indicate Statement on the Controversy and 8E, respectively), although a subtle increase the maximum variability relative to the other in values can be observed at the 1–2 bound- regions. The fl attened SDR section (Fig. 8B) A statement of the controversy, in its most ary, especially for the latter attribute. A third also suggests higher heterogeneity for the west- elemental form, is between the refl ection being region of refl ection character (“3”) can next be ern part of region “4.” In summary, the three a LANF or being a chance apparent alignment defi ned between CDPs 2210 and 1990 where seismic attributes and the “instantaneous slope” of the basinal unconformity and an unrelated the “instantaneous slope” averages roughly can be interpreted to suggest four regions of thrust fault. As pointed out by Christie-Blick et −15°, is less variable, and is situated between refl ection character, albeit in different ways and al. (2007a), the reevaluation of the Sevier Des-

Figure 8 (continued). (C) Average refl ection strength attribute computation for the SDR computed with only a post-stack 12 s (data length) automatic gain control in order to balance all amplitudes for each trace. (D) Slope of instantaneous frequency attribute for the SDR. (E) Refl ection heterogeneity attribute for the SDR. The seismic attributes were all computed with no gain D balancing (except in C) but with a trace amplitude recovery function applied in order to adjust the amplitudes for the effects of spherical divergence and inelas- tic attenuation. The attribute computations were applied with a three-point median despiker (to remove noise bursts) followed by averaging over seven CDP traces. The results were normalized to fi t between 0 and 1000 (thus the arbitrary scale for C–E). A 150 ms window was chosen centered over E the interpreted SDR. Different window lengths were tested (50 and 200 ms), which gave qualitatively similar results. Note that the apparently near-zero regions for the last two attributes show signifi cant variability, but are scaled down due to the high values in region “4.”

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ert detachment (SDD) interpretation has now Christie-Blick et al. (2007a) cite drill-hole data In general, the smoother and more continuous appeared to have run its course in the literature; and geological arguments against the need for structure of the SDR could be taken to support however, most of the reevaluations have been large extensional strains in the basin in order to an interpretation involving a single geological based in part on a data processing result now over advocate that the SDR is really two spatially and feature, such as a LANF. A simpler structure a quarter-century old. Nevertheless, the signifi - genetically distinct segments: to the east, merely might be especially expected for a geologically cance of the SDD interpretation is hard to over- an unconformity and, to the west, a fortuitously young (Von Tish et al., 1985) feature that lacks state. Christie-Blick et al. (2007a) appropriately aligned deep refl ection that plunges into the a complex history as a Mesozoic thrust fault. cite Axen (2004): “Even one compelling exam- middle crust and can be interpreted as a thrust Our reanalysis also supports the conclusion of ple of a primary LANF (low-angle normal fault) fault related to the Sevier Orogeny. Anders et Von Tish et al. (1985) that the SDR is not sub- or of LANF slip is suffi cient to prove that they al. (2001) have stressed the fact that the SDR, stantially offset by high-angle normal faults as may form and slip at low dip, respectively.” And where observed from drill-hole observations, shown by some authors and thus permits large among data sets that are invoked to confi rm the does not show deformation fabrics that might be displacements along the SDD. existence of LANFs, at least in continental crust, expected for a major fault, as seen in other well- The application of structural and seismic attri- the COCORP Utah Line 1 deep seismic profi le is documented detachments in the area (e.g., Cave butes should be a critical part of any analysis of foremost. A reprocessing of the profi le is timely Canyon detachment). Although the validity and refl ection character. This is especially crucial for due to renewed interest in the SDR as a potential relevance of the drill-hole correlations have been the thrust-and-unconformity hypothesis since it deep drilling target (Christie-Blick et al., 2007b). criticized (e.g., Allmendinger and Royse, 1995; requires separate histories for the chance align- The initial observation of the westward- Wernicke, 1995; Coogan and DeCelles, 2007), ment of two separate refl ectors; however, we cau- dipping Sevier Desert refl ection (SDR) was we are unaware of any defi nitive evidence of a tion that the use of attributes may be fraught with made by McDonald (1976), who interpreted deformation zone associated with the SDR. ambiguity associated with the acquisition itself, the refl ection as a Mesozoic thrust fault that especially the changing CDP profi le orientation had been reactivated as a low-angle normal-slip Relevance of the Reanalysis (Fig. 8), as well as other factors such as varying detachment (see also Mitchell and McDonald, attenuation. Four distinct refl ector segments can 1987), based on several industry seismic pro- From a seismic exploration perspective, the be classifi ed based on combining the results of fi les. Allmendinger et al. (1983) likewise inter- detachment hypothesis seems more likely to the attributes (Fig. 8), with two segment bound- preted the same refl ection on the COCORP Utah predict a consistent expression of the SDR, as aries being the most striking (between “1” and Line 1 deep seismic profi le as a detachment that well as predict a relatively smooth structure for “2” and between “3” and “4”) (Fig. 4C). The for- may have originated as a Mesozoic thrust fault. the refl ector itself (detachments shown as “car- mer boundary is manifested both as a change in Many authors have suggested that the detach- toons” tend to depict a smooth feature, although, “instantaneous slope” (steadily changing east of ment is probably a splay of the Sevier-age (Cre- in reality, faults need not be so simple). Such an CDP 2600–2650 and relatively uniform on aver- taceous) Pavant thrust fault (McDonald, 1976; expectation of a smooth structure would be bol- age to the west) and as a prominent increase in Allmendinger and Royse, 1995; Christie-Blick stered by the conclusion of some workers that average refl ection strength to the west where the et al., 2007a). Von Tish et al. (1985) further inter- the SDD is actually a relatively recent (Ceno- refl ector shows less slope variation (Fig. 8). The preted the COCORP and other seismic profi les zoic) feature of the crust. On the other hand, slope stabilization and the increase in refl ection to conclude that the SDR is just as likely a new the thrust-and-unconformity hypothesis more strength to the west of CDP 2600–2650 are sug- Cenozoic low-angle normal fault (i.e., not a reac- probably predicts heterogeneity, especially with gestive of an important down-dip transformation tivated thrust) over much of its extent. Anders et respect to the idea that the SDR not only has in the physical properties of the surface defi ning al. (2001) point out that the detachment is usu- a polyphase history, but is really two (at least) the SDR, although the bend and low CDP fold in ally considered to have developed with a dip distinct geological features. Seen this way, the the profi le here call for caution. The hypothesis not much different from its current value of two hypotheses may be testable by relating the that the SDR does not represent a detachment 11°. Planke and Smith (1991) and Coogan and results of our reanalysis to the geological pre- requires that, at some particular point moving DeCelles (1996) used the COCORP profi le and dictions made by each. from east to west across the Sevier Desert basin, other industry seismic profi les to add support to The originally processed COCORP Utah Line the refl ector ceases to be an unconformity and the detachment interpretation by noting trunca- 1 (the version most cited in the literature) shows becomes a Mesozoic thrust fault. If one accepts tions of dipping strata by the SDR above the the SDR as an irregular and somewhat discon- this hypothesis, then the area around CDP 2600 refl ector. Proponents of the detachment hypoth- tinuous surface, even though interpretive line could function as that point. On the other hand, esis point to the presence of high-angle normal drawings tend to depict a smooth feature. The proponents of the detachment hypothesis may faults on seismic profi les that sole into the SDR reprocessing (Figs. 4A and 4B) demonstrates interpret this same area in a structural sense, (e.g., McDonald, 1976; Allmendinger et al., that such irregular features could be artifacts of as a zone where steep faults displacing Tertiary 1983; Coogan and DeCelles, 1996), while oppo- a laterally varying velocity structure above the sediments dip down and sole into the detachment nents have suggested that the listric faulting is SDR. The smoother structure is also apparent in (e.g., analogous to that shown by Allmendinger related to salt withdrawal features. the gentle rolling over and fl attening out of the et al. [1983] and Von Tish et al. [1985] [Fig. 2]). Alternative interpretations that seek to negate SDR as it rises in the crust (to a traveltime less On their detailed geological cross section located the detachment explanation point out that the than 500 ms) and attains an apparent dip to the north of the COCORP profi le, DeCelles and only direct, geological observations of the SDR east where the eastern part of the seismic profi le Coogan (2006) show a bend in the SDD that (from drill holes) indicate “an unconformity bends around to the south (Fig. 1). This contrasts projects approximately into that observed at our between loosely consolidated Tertiary valley with the markedly rougher apparent structure CDP 2600. fi ll and the underlying dense Paleozoic bed- expressed on the original seismic section (Fig. 2) From a purely seismic attribute point of view rock” (Anders et al., 2001; Hintze and Davis, that showed abrupt changes in the apparent dip (using the heterogeneity measures of slope of 2003). Anders and Christie-Blick (1994) and of the SDR (e.g., beneath CDPs 3010 and 3240). instantaneous frequency and refl ection hetero-

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geneity), the “3-4” boundary is the most promi- can be observed along the refl ector. However, that a large amount of crustal extension in the nent along the SDR (Figs. 8 and 4C). The abrupt each of these conclusions is predicated on critical Sevier Desert basin is not in evidence and, on increase in refl ection heterogeneity west of this assumptions as described above. a global scale, weakens the case for LANFs boundary is consistent with the refl ection under- The resulting seismic cross section shows the in continental crust. An important question for going a distinct change to the west, including SDR to be a nearly continuous geological fea- the unconformity-thrust interpretation is where a break in coherency of the SDR (Fig. 4B). An ture over much of its length. The model depth exactly would the strong refl ection from the inspection of the fl attened section at this bound- section (Fig. 4B) recreates some of the structure unconformity “merge” with or be replaced ary (Fig. 8B) suggests an increase in the com- of the SDR in the hand-drawn depth section pre- by a Mesozoic thrust fault in the image? The plexity of the SDR, changing from a relatively sented by Allmendinger et al. (1983); however, increased smoothness and continuity of the SDR simple and coherent refl ection to one less coher- as can be seen by comparing Figures 2 and 4, on the new image and lack of an obvious lateral ent and including more cycles. On the repro- this older section shows two prominent bends in discontinuity separating an unconformity and a cessed section (Fig. 4C), this boundary appears the SDR at VP 1200 and VP 1490, whereas the thrust may be problematic for such an interpre- to separate two structurally distinct parts of the new section shows a smoother structure for the tation. We thus conclude that the continuity of SDR, suggesting either a chance co-alignment of SDR in these areas. The SDR lacks large struc- the SDR, including the lack of large offsets, is unrelated dipping features or a marked change in tural offsets apparent on the originally processed more consistent with the detachment hypothesis the underlying process creating the refl ector (or section where crossed by Quaternary fault trends than with the alternative view. Nevertheless, simple lateral variations in acquisition param- (some small offsets are visible). The smoother geologists may need to explain the signifi cant eters, surface geology, processing, or greater and more continuous expression of the SDR, variations in the SDR in order to make a detach- complexity of travel paths with increasing after reprocessing, may point to a uniform origin ment hypothesis more credible. Undoubtedly depth). On the refl ection strength stacked section for the refl ector, as concluded by Von Tish et al. the signifi cance of the SDR will likely continue (Fig. 7), the SDR almost disappears as a discrete (1985), and thus supports a LANF interpretation. to be debated, but the results of the reprocess- and well-defi ned refl ection west of CDP 1990 We acknowledge that two domains of structural ing will need to be incorporated into subsequent (but note the increased noise in this region). The dip may be consistent with previous studies that discussions of the geophysical evidence for a “3-4” boundary corresponds to where the profi le challenge the LANF interpretation of the SDR, LANF beneath the eastern Basin and Range. bends southwest, becoming more of a strike pro- in which the shallow portion of the refl ector is fi le, which may account for some of the decrease deemed to be an unconformity, while the steeper ACKNOWLEDGMENTS in slope, but less so the change in heterogeneity. and deeper portion is thought to be an unrelated The authors gratefully acknowledge software In fact, the “3-4” boundary may mark a merg- Mesozoic thrust fault. But these two domains grants to Brigham Young University from the Land- ing of a more discrete refl ecting surface higher are not inconsistent with a LANL interpretation, mark (Halliburton) University Grant Program (Pro- in the crust with a more diffuse refl ection pattern particularly noting the complexity of the geology MAX2D™) and from Seismic Micro-Technology deeper. Similar patterns have been described on and acquisition parameters. The defi nition of (The Kingdom Suite™). J.H. McBride fi rst presented many examples of deep seismic profi les, espe- multiple domains along the SDR is supported by preliminary results of this study on July 17, 2008, at the workshop “Testing the Extensional Detachment cially from Paleozoic compressional orogens seismic attribute computations, which suggest Paradigm: Scientifi c Drilling in the Sevier Desert (Cook and Varsek, 1994; McBride and Knapp, distinct segments based on physical property Basin,” which was sponsored by the National Science 2002; Krawczyk et al., 2008) where they have variations. Such variations in refl ection character Foundation. He expresses appreciation to Nick Chris- been interpreted as representing a mechanical would not be unexpected for a detachment that tie-Blick and the other organizers of the workshop for the invitation to participate and the insight gained into change from more brittle conditions higher in has experienced a complex history, including a the controversy surrounding the interpretation of the the crust to more ductile at greater depth. On steeper segment that is a reactivated Mesozoic Sevier Desert detachment. Funding from the National the originally processed section (Fig. 2) the area thrust as put forth by Allmendinger et al. (1983). Earthquake Hazards Reduction Program contributed immediately around and west of this boundary is DeCelles and Coogan (2006), in their geologi- to this project. Reviews of a preliminary version of the distorted by overlying lateral velocity variations cal cross section, show several very different paper by M.H. Anders and L.F. Hintze are much appreciated. Geosphere reviews by R.L. Bruhn, C.H. Jones, and thus could not be easily observed by early kinds of geological contacts across the SDR. We and R.A. Johnson are greatly appreciated, pointed out interpreters of the data. also demonstrate that the SDR image is highly signifi cant problems, and improved the fi nal version dependent on the details of the velocity model, of the paper. Internal reviews by the U.S. Geologi- CONCLUSIONS including the degree of smoothing; more accu- cal Survey (J.J. Miller and R.A. Williams) were also benefi cial. Much appreciation is expressed to Kris rate knowledge of upper crustal velocity struc- Vasudevan (University of Calgary), who applied the A complete reprocessing of the COCORP ture would undoubtedly improve the accuracy of coherency fi ltering as shown in Figure 4B. Apprecia- Utah Line 1 deep seismic refl ection profi le pro- the SDR image as well as of the shallow section, tion is expressed to E.H. Christiansen for sharing his vides an alternative image for interpreting the which has been distorted due to poor velocity insights on the geology of the Sevier Desert basin. SDR and the tectonics of the Sevier Desert basin. constraints. Use of trade names is for descriptive purposes only and does not represent a product endorsement of the We have applied state-of-the-art data processing More expansively, our results have implica- U.S. Geological Survey. techniques, including refraction static corrections tions for the Cenozoic history of crustal exten- in the receiver and shot record domains, noise sion along the eastern Basin and Range and REFERENCES CITED reduction, coherency fi ltering, and seismic attri- Colorado Plateau transition. Arguing for the bute analysis. 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American Association of Petroleum Geologists Bul- Coleman, D.S., and Walker, J.D., 1994, Modes of tilting dur- letin, v. 70, p. 1015–1021. MANUSCRIPT RECEIVED 16 SEPTEMBER 2009 ing extensional core complex development: Science, Mitchell, G.C., and McDonald, R.E., 1987, Subsurface Ter- REVISED MANUSCRIPT RECEIVED 31 MARCH 2010 v. 263, p. 215–218, doi: 10.1126/science.263.5144.215. tiary strata, origin, depositional model, and hydrocar- MANUSCRIPT ACCEPTED 3 APRIL 2010

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