Roy A Johnson

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• Keynejad, S., Sbar, M. L., & Johnson, R. A. (2019). Assessment of machine-learning techniques in predicting lithofluid facies logs in hydrocarbon wells. Interpretation, 7(3), SF1-SF13. doi:doi:10.1190/INT-2018-0115.1
• Spencer, J. E., Richard, S. M., Lingrey, S. H., Bradford, J. J., Johnson, R. A., & Gehrels, G. E. (2019). Reconstruction of mid-Cenozoic extension in the Rincon Mountains area, southeastern Arizona, USA. Tectonics, 38(7), 1-20. doi:https://doi.org/10.1029/ 2019TC005565
• Spencer, J. E., Singleton, J. E., Strickland, J. S., Reynolds, S. J., Love, D., Foster, D. A., & Johnson, R. A. (2018). Geodynamics of Cenozoic extension along a transect across the Colorado River extensional corridor, southwestern USA. Lithosphere. doi:https://doi.org/10.1130/L1002
• Keynejad, S., Sbar, M. S., & Johnson, R. A. (2017). Comparison of model-based generalized regression neural network and prestack inversion in predicting Poisson's ratio in Heidrun Field, North Sea.. The Leading Edge (Society of Exploration Geophysicists), 36(11), 938-946. doi:https://doi.org/10.1190/tle36110938.1
• Olyphant, J. R., Johnson, R. A., & Hughes, A. N. (2017). Evolution of the Southern Guinea Plateau: Implications on Guinea-Demerara Plateau formation using insights from seismic, subsidence, and gravity data. TECTONOPHYSICS, 717, 358-371.
• Cohen, A. S., Campisano, C., Arrowsmith, R., Asrat, A., Behrensmeyer, A. K., Deino, A., Feibel, C. S., Hill, A., Johnson, R. A., Kingston, J., Lamb, H. F., Lowenstein, T., Noren, A., Olago, D. O., Owen, R. B., Potts, R., Reed, K., Renaut, R., Schabitz, F., , Tiercelin, J., et al. (2016). The Hominin Sites and Paleolakes Drilling Project: Inferring the Environmental Context of Human Evolution from Eastern African Rift Lake Deposits. Scientific Drilling, 21, 1-16. doi:10.5194/sd-21-1-2016
• Olyphant, J. R., Pelletier, J. D., & Johnson, R. A. (2016). Topographic controls on soil and regolith thickness from shallow-seismic refraction constraints across upland hillslopes in the Valles Caldera, New Mexico. Earth Surface Processes and Landforms, 41, 1684–1696. doi:10.1002/esp.3941
• DiMaggio, E. N., Arrowsmith, J. R., Campisano, C. J., Johnson, R. A., Deino, A. L., Warren, M., Fisseha, S., & Cohen, A. S. (2015). Tephrostratigraphy and depositional environment of the youngest (< 2.94 Ma) Hadar Formation (southern Afar, Ethiopia). Journal of African Earth Sciences. doi:http://dx.doi.org/10.1016/j.jafrearsci.2015.09.018
• Arca, M. S., Kapp, P., & Johnson, R. A. (2010). Cenozoic crustal extension in southeastern Arizona and implications for models of core-complex development. Tectonophysics, 488(1-4), 174-190.
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  Abstract: In conventional models of Cordilleran-style metamorphic core-complex development, initial extension occurs along a breakaway fault, which subsequently is deformed into a synform and abandoned in response to isostatic rebound and new faults breaking forward in the dominant transport direction. The Catalina core complex and associated geology in southeastern Arizona have been pointed to as a type example of this model. From southwest to northeast, the region is characterized by the NW-SE trending Tucson basin, the Catalina core complex, the San Pedro trough and the Galiuro Mountains. The Catalina core complex is bounded by the top-to-the-southwest Catalina detachment fault along its southwestern flank and the low-angle, northeast-dipping San Pedro fault along its northeastern flank. The Galiuro Mountains expose non-mylonitic rocks and are separated from the San Pedro trough to the southwest by a system of low- to moderate-angle southwest-dipping normal faults. This Galiuro fault system is widely interpreted to be the breakaway zone for the Catalina core complex. It is inferred to be folded into a synform beneath the San Pedro trough, to resurface to the southwest as the San Pedro fault, and to have been abandoned during slip along the younger Catalina detachment. This study aimed to test this model through analysis of field relations and geochronological age constraints, and reprocessing and interpretation of 2-D seismic reflection data from the Catalina core complex and San Pedro trough. In contrast to predictions of the conventional breakaway zone model, we raise the possibility of a moderate-angle, southwest-dipping detachment fault beneath the San Pedro trough that could extend to mid-crustal depths beneath the eastern flank of the Catalina Mountains. We present an alternative kinematic model in which extension was accommodated by a pair of top-to-the-southwest normal-fault systems (the Catalina and Galiuro detachment faults), with the only major difference between the two being the magnitude of displacement, which was greater for the Catalina detachment. © 2010 Elsevier B.V.
• H., F., & Johnson, R. A. (2010). Along-strike upper-plate deformation in response to metamorphic core complex emplacement, SE Arizona. Tectonophysics, 488(1-4), 162-173.
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  Abstract: Metamorphic core complexes represent concentrated zones of crustal extension; universally, the hanging walls of these systems are extensively fractured, attesting to the significant horizontal extension above the evolving detachment zone. Recent analysis of a large grid of 2-D seismic reflection lines within the Tucson Basin of southeastern Arizona has facilitated a nearly three-dimensional interpretation of subsurface features related to Cenozoic crustal extension. Within the northern Tucson Basin, the Catalina detachment fault dips 23-35° to the southwest from the western flank of the Catalina/Rincon Metamorphic Core Complex. In the southern portion of the basin, the NE-trending Santa Rita normal fault dips 15-20° to the northwest from the western flank of the Santa Rita Mountains and is cut by the Catalina detachment beneath the central Tucson Basin. The orientation of the Santa Rita fault is problematical in that its orientation is nearly perpendicular to extension directions in the region, while geologic and seismic reflection evidence indicates that the Catalina detachment and Santa Rita fault were active synchronously. One possible explanation is that the Santa Rita fault is accommodating along-strike upper-plate deformation in response to core complex emplacement. To test this, we employed a finite-element modeling approach. A two-dimensional model consisting of a homogenous elastic material undergoing uniform extension is used to study changes in stresses and displacements in proximity to a zone of weakness representing a detachment fault in the upper crust. Away from the detachment fault, primary principal stresses are oriented parallel to the regional extension direction as expected. Near the detachment, extension in the system produces a rotation of the primary principal stresses of nearly 60° with respect to the regional stress field around the end of the fault. Additionally, the model predicts mechanical failure of the upper-plate of the detachment system to the south of the Catalina core complex. This model supports our interpretation that the orientation and early displacement noted on the Santa Rita fault is the result of a perturbation in the regional stress field caused by the Catalina detachment and the associated brittle failure of the upper-plate from the extreme crustal extension associated with core complex emplacement. © 2009.
• Velasco, M. S., Bennett, R. A., Johnson, R. A., & Hreinsdóttir, S. (2010). Subsurface fault geometries and crustal extension in the eastern Basin and Range Province, western U.S.. Tectonophysics, 488(1-4), 131-142.
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  Abstract: We provide the first synthesis of seismic reflection data and active present-day crustal deformation for the greater Wasatch fault zone. We analyzed a number of previously unpublished seismic reflection lines, horizontal and vertical crustal velocities from continuous GPS, and surface geology to investigate the relationships between interseismic strain accumulation, subsurface fault geometry, and geologic slip rates on seismogenic faults across the eastern third of the northern Basin and Range Province. The seismic reflection data show recent activity along high-angle normal faults that become listric with depth and appear to sole into preexisting décollements, possibly reactivating them. We interpret these listric normal faults as reactivated Sevier-age structures that are connected at depth with a regionally extensive detachment horizon. These observations of subsurface structure are consistent with the mapped geology in areas that have experienced significant extension. We modeled the crustal deformation data using a buried dislocation source in a homogeneous elastic half space. The estimated model results include a low-angle dislocation (~. 8-20°) at a locking depth of ~. 7-10 km and slipping at 3.2 ± 0.2 mm/yr. Despite the model's relative simplicity, we find that the predicted location of the dislocation is consistent with the interpreted seismic reflection data, and suggests an active regionally extensive sub-horizontal surface in the eastern Basin and Range. This result may imply that this surface represents aseismic creep across a reactivated low-angle fault plane or the onset of ductile flow in the lower crust at or beneath the brittle-ductile transition zone under the present-day Basin and Range extensional regime. This result may also have implications for crustal rheology, and suggests that geodesy might, under some circumstances, serve as an appropriate tool for inferring deeper crustal structure. © 2009.
• Wagner, F. H., & Johnson, R. A. (2006). Coupled basin evolution and late-stage metamorphic core complex exhumation in the southern Basin and Range Province, southeastern Arizona. Tectonophysics, 420(1-2), 141-160.
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  Abstract: Records of lithospheric extension and mountain-range uplift are most continuously contained within syntectonic sedimentary rocks in basins adjacent to large structural culminations. In southeastern Arizona, metamorphic core complexes form mountain ranges with the highest elevations in the region, and supposedly much less extended terranes lie at lower elevations. Adjacent to the Santa Catalina-Rincon metamorphic core complex, within the Tucson Basin, stratigraphic-sequence geometries evident in a large suite of 2-D seismic reflection data suggest a two-phase basin-evolution model controlled by the emplacement and subsequent uplift of the core complex. In its earliest stage, Phase I of basin formation was characterized by extensive faults forming relatively small-scale proto-basins, which coalesced with the larger basin-bounding detachment fault system. Synextensional sedimentation within the enlarging basin is evidenced by sediment-growth packages, derived from adjacent footwall material, fanning into brittle hanging-wall faults. During this phase, volcanism was widespread, and growth packages contain interbedded sediments and volcanic products but, paradoxically, no mylonitic clasts from the adjacent metamorphic core complex. Phase II of basin evolution begins after a significant tectonic hiatus and consists of a symmetric deepening of the central basin with the introduction of mylonitic clasts in the basin fill. This is coupled with the activation of a series of high-angle normal faults ringing the core complex. These observations suggest a two-phase model for metamorphic core complex evolution, with an initial stage of isostatic core complex emplacement during detachment faulting that resulted in little topographic expression. This was followed, after a significant tectonic hiatus, by late-stage exhumation and flexural uplift of the Santa Catalina-Rincon metamorphic core complex through younger high-angle faulting. Moreover, the geometry of upper basin fill units suggests an extremely low effective elastic thickness in the region and that flexural uplift of the core complex induced asymmetric transfer of ductile mid-crustal rocks from beneath the subsiding Tucson Basin to the uplifting mountain range. © 2006 Elsevier B.V. All rights reserved.
• Marfurt, K. J., Johnson, R. A., & Pennington, W. D. (2003). An introduction - Solid-earth seismology: Initiatives from IRIS. Leading Edge (Tulsa, OK), 22(3), 218-219.
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  Abstract: Studies associated with the Incorporated Research Institutions for Seismology are discussed. IRIS is a university research consortium dedicated to exploring the earth's interior through the collection and distribution of seismographic data. Work done by IRIS groups in the field of passive seismic imaging, imaging of converted transmissions, and velocity analysis of long-offset diving waves are described.
• Morozova, E., Wan, X., Chamberlain, K. R., Smithson, S. B., Morozov, I. B., Boyd, N. K., Johnson, R. A., Karlstrom, K. E., Tyson, A. R., & Foster, C. T. (2002). Geometry of Proterozoic sutures in the central Rocky Mountains from seismic reflection data: Cheyenne belt and Farwell Mountain structures. Geophysical Research Letters, 29(13), 17-1 - 17-4.
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  Abstract: Seismic reflection data show ∼40° S-dipping reflectors that extend from the Cheyenne belt (CB-Archean-Proterozoic suture) to 20 km depths, and profound differences in crustal reflectivity across the suture zone, which is itself imaged as an interwedging boundary. Archean crust has a Moho reflection (at 40 km) and abundant upper crustal reflectivity; Proterozoic crust has no Moho reflection and is distinctly less reflective. Reflective zones south of the Cheyenne belt include 40° S- and N-dipping reflections that extend from the Farwell Mountain zone to 20 km depths; these are interpreted to be a complex cryptic suture zone between the 1.78-1.76 Ga Green Mountain block and 1.75-1.72 Ga Rawah block.
• Satarugsa, P., & Johnson, R. A. (2000). Contraints on crustal composition beneath a metamorphic core complex: Results from 3-component wide-angle seismic data along the eastern flank of the ruby mountains, Nevada. Tectonophysics, 329(1-4), 223-250.
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  Abstract: Metamorphic core complexes expose rocks deformed at deep upper to middle crustal levels during extreme crustal extension. However, mechanisms of crustal extension and exhumation of core complexes remain to be fully understood. Detailed study of crustal velocity structure and inferences about the composition of the crust beneath core complexes (and nearby) provide useful constraints on core-complex evolution. P- and S-wave velocity structures determined from seismic experiments along the eastern flank of the Ruby Mountains metamorphic core complex, Nevada, show that the crust can be divided into three main layers corresponding to the upper, middle and lower crust. We interpreted crustal composition by integrating results of P-wave velocities (Vp). S-wave velocities (Vs), Poisson's ratios (σ), seismic anisotropy, and reflection character with published geologic maps of the area. Near-surface estimates of Vp, Vs, σ, and anisotropy of 1.90-4.8 km/s, 1.01-2.75 km/s, 0.25-0.33, and 0.6-2.5%, respectively, are consistent with surface exposures of unconsolidated to consolidated sedimentary rocks, limestone, dolomite, siltstone, sandstone, porous sandstone, conglomerate, and weathered granite. Results from analysis of reflection responses, Vp, Vs, σ, and anisotropy also indicate that: (1) upper crustal rocks most likely consist of metaquartzite, schist, granite gneiss, and granite-granodiorite with Vp of 5.80-6.25 km/s, Vs of 3.20-3.72 km/s, σ of 0.22-0.25, and anisotropy of 0.6-2.5%: (2) possible middle crustal rocks are paragranulite, felsic granulite, felsic amphibolite gneiss, granite-granodiorite, and mica-quartz schist with Vp of 6.35-6.45 km/s, Vs of 3.70-3.75 km/s, and σ of 0.24; and (3) lower crustal rocks most likely consist of granulite-rather than amphibolite-facies rocks with Vp of 6.60-6.80 km/s, Vs of 3.85-3.92 km/s, σ of 0.24-0.25, and anisotropy of
• Mohapatra, G. K., & Johnson, R. A. (1998). Localization of listric faults at thrust fault ramps beneath the Great Salt Lake Basin, Utah: Evidence from seismic imaging and finite element modeling. Journal of Geophysical Research B: Solid Earth, 103(5), 10047-10063.
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  Abstract: Reflection seismic data from the Great Salt Lake Basin, Utah, show that the major basin-bounding normal faults decrease in dip from ∼60° at the surface to ∼10°-20° at depths as shallow as 4-6 km. This rapid decrease in fault dip at depths shallower than the brittle-ductile transition zone in the Basin and Range Province suggests an explanation other than a gradual change of rheology and stress orientations with depth. Using a dense grid of seismic data, gravity data, borehole data, and published geologic information from islands in the lake, we constrain the position of the Sevier age Willard thrust and a footwall imbricate and show their reactivation as normal faults during Tertiary extension. In the absence of surface geologic information, we use available subsurface information from the lake to draw an analogy with the Ogden duplex in the Wasatch Front, where Cenozoic normal faulting was superimposed on an earlier Sevier age thrust regime to give rise to listric normal faults. Our interpretations are consistent with finite element modeling results, which demonstrate that extensional slip on preexisting thrust ramps leads to the formation of energetically favored synthetic normal faults, some of which may merge with the thrust ramp and obtain listric geometries. Further slip on these listric faults gives rise to secondary synthetic and antithetic faults resulting in hanging wall grabens.
• Satarugsa, P., & Johnson, R. A. (1998). Crustal velocity structure beneath the eastern flank of the Ruby Mountains metamorphic core complex: Results from normal-incidence to wide-angle seismic data. Tectonophysics, 295(3-4), 369-395.
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  Abstract: P-wave velocity structure along the eastern side of the Ruby Mountains metamorphic core complex, determined using 2-D ray-inversion modeling, shows that: (1) the crust can be divided into three layers corresponding to the upper, middle, and lower crust; (2) from east of Ruby Dome to the northern end of the seismic profile, velocities of the upper crust range from 6.12 (±0.1) to 6.20 (±0.1) km/s and near-surface velocities range from 1.90 (±0.1) to 4.80 (±0.1) km/s. From this point to the southern end of the profile, upper crustal velocities range from 5.80 (±0.1) to 6.25 (±0.1) km/s and near-surface velocities range from 3.01 (±0.1) to 4.80 (±0.1) km/s; (3) middle crustal velocities range from 6.35 (±0.15) to 6.45 (±0.15) km/s; (4) lower crustal velocities range from 6.60 (±0.15) to 6.80 (±0.15) km/s; and (5) depths to the Moho vary irregularly between 30.5 and 33.5 km. From interpretation of these results, we conclude that: (1) the transition in metamorphic grade from deep upper crustal rocks south of Harrison Pass to middle crustal rocks north of Harrison Pass does not correlate with an increase in seismic velocity; near-surface basement seismic velocities increase in the central part of the range (near Ruby Dome) well north of the major metamorphic transition; (2) depths to the Moho do not reflect local surface relief; isostatic balance is achieved predominantly in the crust rather than by formation of a local crustal root; and (3) the crust has been modified during extension by intracrustal processes rather than by large-scale magmatic underplating.
• Johnson, R. (1996). SEG in support of science education: A recap of the 47th ISEF. Leading Edge (Tulsa, OK), 15(10), 1130-.
• Kruger, J. M., Johnson, R. A., & Houser, B. B. (1995). Miocene-Pliocene half-graben evolution, detachment faulting and late-stage core complex uplift from reflection seismic data in south-east Arizona. Basin Research, 7(2), 129-149.
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  Abstract: Reflection seismic data show that the late Cenozoic Safford Basin in the Basin and Range of south-eastern Arizona, is a 4.5-km-deep, NW-trending, SW-dipping half graben composed of middle Miocene to upper Pliocene sediments, separated by a late Miocene sequence boundary into lower and upper basin-fill sequences. Extension during lower basin-fill deposition was accommodated along an E-dipping range-bounding fault comprising a secondary breakaway zone along the north-east flank of the Pinaleño Mountains core complex. This fault was a listric detachment fault, active throughout the mid-Tertiary and late Cenozoic, or a younger fault splay that cut or merged with the detachment fault. Most extension in the basin was accommodated by slip on the range-bounding fault, although episodic movement along antithetic faults temporarily created a symmetric graben. Upper-plate movement over bends in the range-bounding fault created rollover structures in the basin fill and affected deposition within the half graben. Rapid periods of subsidence relative to sedimentation during lower basin-fill deposition created thick, laterally extensive lacustrine or alluvial plain deposits, and restricted proximal alluvian-fan deposits to the basin margins. A period of rapid extension and subsidence relative to sediment influx, or steepening of the upper segment of the range-bounding fault at the start of upper basin-fill deposition resulted in a large downwarp over a major fault bend. Sedimentation was restricted to this downwarp until filled. Episodic subsidence during upper basin-fill deposition caused widespread interbedding of lacustrine and fluvial deposits. Northeastward tilting along the south-western flank of the basin and north-eastward migration of the depocentre during later periods of upper basin-fill deposition suggest decreased extension rates relative to late-stage core complex uplift. © 1995 Blackwell Science Ltd.
• Kruger, J. M., & Johnson, R. A. (1994). Raft model of crustal extension: evidence from seismic reflection data in southeast Arizona. Geology, 22(4), 351-354.
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  Abstract: An archlike zone of seismic reflectivity, interpreted as an uplifted zone of ductilely deformed middle and lower crust, is imaged below the Pinaleno Mountains core complex in southeast Arizona. The top of the reflective zone coincides with the base of an inferred mid-Tertiary detachment fault beneath the Safford basin but diverges from the detachment fault as an apparent mylonite front to form a culmination at ~1.9 s (~4 km) beneath the Pinaleno Mountains. From this culmination, the zone of reflectivity dips to the southwest below the Eagle Pass detachment fault and flattens at ~4.8 s (~13.5 km) beneath the relatively unextended upper crust of the Galiuro Mountains. The data suggest that mylonite zones form not only as the continuation of detachment faults into the brittle-ductile transition, but also along a regional zone of decouping between the middle and upper crust. -from Author
• Yarnold, J. C., Johnson, R. A., & Sorenson, L. S. (1993). Identification of multiple generations of crosscutting "domino- style' faults: insights from seismic modeling. Tectonics, 12(1), 159-168.
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  Abstract: Seismic modeling and geometric analysis provide clues for identification of multiple generations of cross-cutting planar rotational faults in seismic reflection profiles. Many elements characteristic of this structural geometry are apparent, although contorted, at shallow levels in unmigrated and migrated synthetic seismograms. Structural features potentially signaling an earlier generation of rotational faulting within a tilt-block terrain include first-generation fault segments, intrablock terminations of horizons, steep dips of prefault strata, anomalously high fault-strata intersection angles, "composite' basin geometry, truncations of early synextension sedimentary wedges, and anomalous crustal thinning. Evaluation of published profiles from the Bay of Biscay and Galicia Bank indicates that the crosscutting "domino' fault model probably is not applicable in these areas as previously suggested. While the model may be appropriate for the basement structure of Spring Valley, Nevada, seismic evidence is equivocal. -Authors
• Johnson, R. A., & Loy, K. L. (1992). Seismic reflection evidence for seismogenic low-angle faulting in southeastern Arizona. Geology, 20(7), 597-600.
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  Abstract: Focal mechanisms for large (M>6) earthquakes in extensional terranes suggest that seismogenic normal faults have dips that range from ~30° to ~70°. Geologic relations suggest that low-angle faults have accommodated large-scale upper-crustal extension. Seismic reflection data from the Tucson basin in southeast Arizona image a low-angle normal fault (the Santa Rita fault) that crops out along the trend of late Quaternary fault scarps caused by large-magnitude (M ~6.7-7.6) earthquakes. Velocity-independent dip analysis from shot records of the Santa Rita fault indicates that it has a true dip of ~20° to a depth of at least 6 km. This observation suggests that low-angle extensional faults may be seismogenic and that actual mechanisms for accommodation of upper-crustal extension depend on local conditions of stress and preexisting geologic structure. -from Authors
• Greene, L. C., Richards, D. R., & Johnson, R. A. (1991). Crustal structure and tectonic evolution of the anza rift, northern Kenya. Tectonophysics, 197(2-4), 203-211.
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  Abstract: The Anza trough is a Mesozoic rift located in northern Kenya that appears to be the failed third arm of a paleo-triple junction which allowed the separation of Madagascar from Africa during the Jurassic. The rift is oriented NW-SE and its tectonic evolution is related to that of the Mesozoic southern Sudan rift system. We analyzed seismic and gravity data from the southwestern side of the Anza rift including the Chalbi Desert to gain a better understanding of rift structure. Gravity data delineate the main rift basins as well as a small sub-basin on the southwest side of the main rift. Normal faulting evident on the NW end of a 42-km-long, NW-SE oriented Vibroseis® profile, marks the western boundary of the sub-basin. This sub-basin is offset from the trend of the main Anza trough; the western boundary may be a complex fault zone accommodating a change in direction of the main rift trend. Gravity values increase to the NW in the faulted area, suggesting shallowing of basement. A strong NW-dipping reflection from 0.5 s to almost 3 s is interpreted as a pre- to mid-Cretaceous unconformity. The configuration of the unconformity and the normal faulting strongly resembles the half-graben geometry imaged in the East African Rift. Numerous discontinuous reflections can be seen deeper in the section between 6 and 9 s, but a distinct reflection Moho cannot be interpreted with certainty. In addition to seismic and gravity data, regional geologic and well data lead us to conclude that there are probably Jurassic marine sediments in the bottom of the Anza rift. © 1991.
• Wilson, J. M., McCarthy, J., Johnson, R. A., & Howard, K. A. (1991). An axial view of a metamorphic core complex: crustal structure of the Whipple and Chemehuevi Mountains, southeastern California. Journal of Geophysical Research, 96(B7), 12,293-12,311.
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  Abstract: A 135-km-long, NW-SE trending, seismic refraction/wide-angle reflection profile provides a unique along-strike view of the crustal structure of a belt of metamorphic core complexes in southeastern California: the Whipple, Chemehuevi, and Sacramento mountains metamorphic core complexes. Observations support greater uplift and a slightly deeper midcrustal origin for the rocks now exposed in the core of the Whipple Mountains compared to rocks in the Chemehuevi and Sacramento mountains. Despite the enhanced uplift and extension in the Whipple Mountains, the crust is thicker here (30 km) than anywhere else along the Colorado River extensional corridor. Inflation of the crust during Tertiary extension is suggested as the dominant mechanism. Both mantle derived magmatism and lateral ductile inflow in the crust are proposed. -from Authors
• Smithson, S. B., & Johnson, R. A. (1989). Chapter 26: Crustal structure of the western US. based on reflection seismology. Memoir of the Geological Society of America, 172(1), 577-612.
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  Abstract: Interpretation of crustal reflection profiles shows contrasting crustal styles and Moho from the craton to the Cordilleran belt. Crustal deformation, however, may be determined from highly reflective ductile fault (mylonite) zones that occur in many geologic settings, and fractures and chemical alteration may cause shallow detachments to reflect, e.g., the Sevier Desert and the Picacho Mountains detachments. The oldest Archean crust in Minnesota consists of a stack of nappes 30 km thick, and the underlying Moho, which is nonreflective, may be gradational. Younger Archean is sutured to this along a complex, moderately dipping zone marked by a mylonite. Proterozoic crust in Kansas and the Colorado Plateau is characterized by arcuate crossing reflections that can be caused by a combination of folding and intrusion. The Mojo is generally nonreflective except in the extended terranes of the Basin and Range, Rio Grande rift, northwest Cordillera, and Mojave-Sonoran Desert. The best crustal reflections are found in the extended terrane of the Basin and Range, where the subhorizontal reflection geometry is probably caused by ductile flow (metamorphism) under simple shear, resulting in a strong compositional layering and lensing of the deep crust and Moho. This young Moho contrasts sharply with the nonreflective Moho under the craton that may represent a gabbro-eclogite phase change. Underplating by gabbroic magma may have taken place in such diverse tectonic settings as the Minnesota Archean, the Oklahoma Proterozoic, and the Basin and Range Cenozoic crust. No evidence for Moho offset has been found in any of the areas studied. A complex crustal reflection pattern and distinct Moho reflection in the northwest Cordillera may be related to moderate extension conditioned by less (than Basin and Range) thermal input to the crust. Compression caused folding and thrust faulting in the Wyoming foreland. Basaltic magma chambers may have been detected beneath the Rio Grande rift and Death Valley; apparently, basaltic magma is more likely to generate reflections than granitic magma. Questions exist about how much of the exposed Precambrian crust in southern California, where a midcrustal detachment has been proposed, is allochthonous. Reflection profiling reveals a crust that is both vertically and horizontally heterogeneous.
• Smithson, S. B., Johnson, R. A., Hurich, C. A., Valasek, P. A., & Branch, C. (1987). Deep crustal structure and genesis from contrasting reflection patterns: an integrated approach.. Geophysical Journal - Royal Astronomical Society, 89(1), 67-72.
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  Abstract: An integrated approach including technique development into data acquisition, processing and modeling is used to aid the final interpretation of crustal reflection data by minimizing the many unknowns. Thus, a project may start out with a series of experiments involving data acquisition designed to attack a particular problem and conclude with modeling as a test to the interpretation of a problem. -Authors
• Johnson, R. A., & Smithson, S. B. (1985). Thrust faulting in the Laramie Mountains, Wyoming, from reanalysis of COCORP data. Geology, 13(8), 534-537.
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  Abstract: Detailed analysis of COCORP seismic reflection data from the Laramie Mountains, Wyoming, shows that processing artifacts produce confusing patterns in a profile that crosses the mountain front. Elimination of artifacts in reprocessing allows for interpretations of residual reflections that are consistent with mapped geology. Reprocessing and model analysis image a near-surface fault-zone reflection and constrain its apparent west dip to be 30°-35°W. Bands of east- and west-dipping seismic events from basement rocks do not appear to offset the basement surface and therefore probably are not from faults directly related to uplift of the range. Multiple Laramide structures east of the exposed fault contact between Precambrian and Paleozoic rocks contribute to uplift of the range, and an exposed basement arch, which extends in the subsurface below Line 3, provides evidence for macroscopic folding of crystalline basement rocks. Lack of continuous fault-zone reflections extendable to depth on the seismic data suggests the absence of major mylonite zones analogous to those associated with the Wind River thrust. Such mylonites are probably better reflectors than brittle deformation zones associated with faults beneath the Laramie Mountains. Although fault-zone reflectivity depends on the physical properties of individual fault zones, the Wind River thrust and Laramie Mountains frontal fault zone may approach end members of fault-zone reflectivity. © 1985 Geological Society of America.
• Smithson, S. B., Pierson, W. R., Wilson, S. L., & Johnson, R. A. (1985). Seismic reflection results from Precambrian crust.. The Deep Proterozoic Crust in the North Atlantic Provinces, 21-37.
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  Abstract: Results of crustal reflection profiling in the Precambrian can provide decisive information on such features as faulting, extension vs compression and crustal scale deformation in general, vertical vs horizontal movements, intrusions, mafic differentiates, underplating and plate tectonics, even though targets in the Precambrian are complex. Mylonite zones are probably the best crustal reflectors so that a moderately-dipping reflector separating the ancient Archaean gneiss terrain from younger Archaean greenstone belts in Minnesota is probably a thrust fault marking the suture zone between the two terrains. This structure implies the operation of plate tectonics by at least the late Archaean. The 3600 m.y. Minnesota gneiss terrain is underlain by a thick (10 km) sequence of layered deformed rocks, whose presence restricts early Archaean events. A Proterozoic suture in SE Wyoming is marked by a thick mylonite zone and a change in crustal structure that has persisted through time. The Adirondack anorthosite and granulite terrain is underlain by a thick layered sequence in the lower crust. Formation of a late Proterozoic basin filled with 10-12 km of interlayered sedimentary rocks in Texas and Oklahoma may have been accompanied by crustal underplating. (Authors' abstract)-J.M.H.
• Smithson, S. B., Pierson, W. R., Wilson, S. L., & Johnson, R. A. (1985). Seismic reflection results from Precambrian crust.. The deep Proterozoic crust in the North Atlantic Provinces. Proc., Moi, 1984, 21-37.
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  Abstract: Results of crustal reflection profiling in the Precambrian can provide decisive information on such features as faulting, extension vs. compression and crustal scale deformation in general, vertical vs. horizontal movements, intrusions, mafic differentiates, underplating, and plate tectonics even though targets in the Precambrian are complex. Mylonite zones are probably the best crustal reflectors so that a moderately dipping reflector separating the ancient Archean gneiss terrain from younger Archean greenstone belts in Minnesota is probably a thrust fault marking the suture zone between the 2 terrains. This structure implies the operation of plate tectonics at least by the late Archean. The 3.6b.y. Minnesota gneiss terrain is underlain by a thick (10km) sequence of layered deformed rocks, whose presence restricts early Archean events. A Proterozoic suture in SE Wyoming is marked by a thick mylonite zone and a change in crustal structures that has persisted through time. The Adirondack anorthosite and granulite terrain is underlain by a thick layered sequence in the lower crust. Formation of a late Proterozoic basin filled with 10-12km of interlayered sedimentary rocks in Texas and Oklahoma may have been accompanied by crustal underplating.-Authors
• Johnson, R. A., Karlstrom, K. E., Smithson, S. B., & Houston, R. S. (1984). Gravity profiles across the Cheyenne Belt, a precambrian crustal suture in southeastern Wyoming. Journal of Geodynamics, 1(3-5), 445-472.
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  Abstract: Geologic discontinuities across the Cheyenne Belt of southeastern Wyoming have led to interpretations that this boundary is a major crustal suture separating the Archaean Wyoming Province to the north from accreted Proterozoic island arc terrains to the south. Gravity profiles across the Cheyenne Belt in three Precambrian-cored Laramide uplifts show a north to south decrease in gravity values of 50-100 mgal. These data indicate that the Proterozoic crust is more felsic (less dense) and/or thicker than Archaean crust. Seismic refraction data show thicker crust (48-54 km) in Colorado than in Wyoming (37-41 km). We model the gravity profiles in two ways: 1) thicker crust to the south and a south-dipping ramp in the Moho beneath and just south of the Cheyenne Belt; 2) thicker crust to the south combined with a mid-crustal density decrease of about 0.05 g/cm3. Differences in crustal thickness may have originated 1700 Ma ago because: 1) the gravity gradient is spatially related to the Cheyenne Belt which has been immobile since about 1650 Ma ago; 2) the N-S gradient is perpendicular to the trend of gravity gradients associated with local Laramide uplifs and sub-perpendicular to regional long-wavelength Laramide gradients and is therefore probably not a Laramide feature. Thus, gravity data support the interpretation that the Cheyenne Belt is a Proterozoic suture zone separating terrains of different crustal structure. The gravity "signature" of the Cheyenne Belt is different from "S"-shaped gravity anomalies associated with Proterozoic sutures of the Canadian Shield which suggests fundamental differences between continent-continent and island arc-continent collisional processes. © 1984.
• Kubichek, R. F., Humphreys, M. C., Johnson, R. A., & Smithson, S. B. (1984). Long-range recording of VIBROSEIS data: simulation and experiment.. Geophysical Research Letters, 11(9), 809-812.
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  Abstract: Long-range recording of COCORP vibrators was undertaken in the Adirondacks using a radio repeater to retransmit the vibrator start-up code at ranges up to 100 km. A number of events were recorded at a range of 47 km; these events correspond to reflections from depths of about 9, 13, 16, 20, and 24 km and a possible Moho reflection from 40 km might be present. Simulation of VIBROSEIS recording demonstrates that low-level signals with a S/N as low as 0.05 may be extracted from noise even though reflections have amplitudes less than the least significant bit. Such wide-angle VIBROSEIS recording is practical and necessary in crustal reflection studies for defining the Moho and other deep reflectors.-Authors
• Smithson, S. B., Johnson, R. A., & Wong, Y. K. (1981). Mean crustal velocity: a critical parameter for interpreting crustal structure and crustal growth. Earth and Planetary Science Letters, 53(3), 323-332.
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  Abstract: Mean crustal velocity is a critical parameter for genesis of continental crystalline crust because it is a function of mean crustal composition and therefore may be used to resolve continental crustal growth in space and time. Although the best values of mean crustal velocity are determined from wide-angle reflection measurements, most studied here necessarily come from vertical averages in crustal refraction determinations. The mode of 158 values of mean crustal velocity is 6.3 km/s, a velocity which corresponds to a mean crustal composition of granodiorite to felsic quartz diorite; Archean crust may be slightly more mafic. Mean crustal velocities range from 5.8 to 7.0 km/s. The lowest values invariably are found in thermally disturbed rift zones and the highest values correspond to velocities in gabbro. Velocities in island arcs may be as low as 6.0 km/s but are typically 6.5-6.9 km/s which corresponds to andesitic composition; estimates of island arc composition are andesitic. If values of mean crustal velocity are not biased, this observation suggests that continental crust did not grow simply by addition of island arc material. Possibilities are that crust formed from fusion of island arcs and was later changed to more felsic composition by addition of material from the mantle or that the late Archean episode of major crustal growth did not involve processes similar to younger island arcs. Some crustal blocks might be changed in composition and thickness by such processes as underplating, interthrusting, necking and sub-crustal erosion. Specially designed experiments are suggested to determine this parameter so critical for understanding genesis of continental crust. © 1981.

Others

• Johnson, R. A., Keynejad, S., & Kardell, D. A. (2017, July). Appendix D: Vintage Seismic Analysis Report. in: Gootee, B.F., Cook J., Young, J.J., and Pearthree, P.A., 2017, Subsurface Hydrogeologic Investigation of the Superstition Vistas Planning Area, Maricopa and Pinal Counties, Arizona. Arizona Geological Survey, University of Arizona..
• Arca, M. S., & Johnson, R. A. (2010, May). Compilation Geologic Map from the Baboquivari Mountains to the Transition Zone of the Colorado Plateau. Arizona Geological Survey, Contributed Map Series, CM-10-A: 1-CD Rom with browse graphics of 2 map sheets at 1:250,000 scale. CM-10-A,S, Two color map sheets, map scale 1:250,000..