82. Bird, P., and R. V. Ingersoll  Kinematics and paleogeology of the western United States and northern Mexico computed from geologic and paleomagnetic data: 0 to 48 Ma, Geosphere, XX(XX), doi: 10.1130/GES02474.1.
Abstract. Fault traces and offsets, cross-section length changes, paleomagnetic inclination and declination anomalies, and stress-direction indicators with ages back to 90 Ma are collected from the geologic literature on the western United States and northern Mexico. Finite-element program Restore simulates paleokinematics by weighted least squares and integrates displacements, strains, and rotations back in time, producing paleogeologic maps, as well as maps of velocity, heave rate, strain rate, and stress direction at 6 m.y. intervals. After calibrating three program parameters against neotectonic velocities from geodesy, all classes of data except inclination anomalies are fit reasonably well. The kink in the San Andreas fault near San Gorgonio Pass has been gradually restored by slip on adjacent faults and automated smoothing. Piercing-point pairs successfully restored along the San Andreas–Gulf of California plate boundary include the Pelona and Orocopia Schists at 6 Ma, the Pinnacles and Neenach Volcanics at 21 Ma, and the Jolla Vieja and Poway conglomerates adjacent to their Sonoran source at 48–42 Ma. During 18–6 Ma, rapid extension on the Oceanside detachment fault system was restored, placing present San Nicolas Island adjacent to present Rosarito, Baja California, at 18 Ma. Since ca. 18 Ma, the western Transverse Ranges have rotated 70° clockwise, restoration of which implies that sinistral faults in this province originated with NNE trends. The first contact between the Pacific and North America plates at ca. 28 Ma was not associated with any dramatic increase in dextral faulting on land; instead, the primary result was extension in the Plush Ranch–Vasquez-Diligencia basins and Colorado River corridor, probably driven by an unstable triple- junction and accelerated by heating and uplift of North America above enlarging slab windows.
Figure 1. Traces of all faults in our database, which includes most faults active (or potentially active) since 90 Ma. Color indicates dominant sense of offset in most recent phase of movement. Identifying serial numbers omitted for clarity. Dip directions of nonvertical faults omitted for clarity. Dashed faults were included in restoration but were assigned target offsets of zero. Two long sinistral faults in Mexico were included in restoration, although possibly not active after 90 Ma. Subduction megathrusts (westernmost purple traces) along the western edge of the continental shelf were neither included in our calculations nor restored. All traces shown are in Supplemental Material (see link above). Names of selected important faults in Figure 8. Polyconic projection about central meridian 110°W.
Figure 2. Distribution of 347 known throws (relative vertical offsets; N) on high-angle, essentially nonrotated normal faults active since 90 Ma in the western United States (WUS) and northern Mexico. (A) Cumulative distribution function (CDF) showing median throw of 2.51 km. (B) Probability density function (PDF) of the same distribution, compared to a Gaussian model with the same median that was chosen to represent this distribution in Table 1. The unfit peak in the PDF below 0.5 km of throw is due to inclusion of some large-scale field studies describing small faults in our database; however, the main peak is suggested to represent the throws of large normal faults such as those mapped by Stewart (1978) and Muehlberger (1992) using consistent criteria across the continent.
Figure 3. Target heaves (horizontal components of offsets) on modeled faults since 48 Ma, shown with ribbons whose widths are proportional to target heaves. Ribbon colors correspond to senses of offset. In western California and Baja California, center lines of overlapping ribbons are also shown, with 1-point black lines, to aid interpretation. It is important to note that this format does not show assigned uncertainties, and that offsets with large uncertainties that are not kinematically consistent with neighboring structures are not strongly enforced during restoration. The Pine Nut dextral fault (green ribbon) along the California-Nevada border is an example; its target dextral offset is 259 ± 115 km for 48–25 Ma, but offset of only 22 km was restored in reference model NI. All offsets shown are from Supplemental Material (see link above). Polyconic projection about central meridian 110°W.
Figure 4. Locations, present lengths, and former lengths of geologic sections in our database that were restored by previous authors using structural geology methods. Black brackets show present section locations. Colored ribbons show former lengths: blue where and/or when there has been net compression, and orange where and/or when there has been net extension. Tapered ends of ribbons show uncertainties in former lengths. Attached integer numbers give restoration times in Ma. Section identifier codes such as “C0017” tie to the data set presented in Supplemental Material (see link above). Polyconic projection about central meridian 110°W.
Figure 5. Paleomagnetic sampling localities yielding poles and/or virtual geomagnetic poles (VGPs) and their declination anomalies, which we interpret as net vertical-axis rotations since magnetization ages given by integers in Ma. Larger symbols are plotted where rotations are more significant (that is, greater than standard error, or greater than two standard errors). Some ages are not shown in areas of very dense sampling, but ages omitted are similar to those shown. Full details are contained in Supplemental Material (see link above). Polyconic projection about central meridian 110°W.
Figure 6. Paleomagnetic sampling localities yielding poles and/or virtual geomagnetic poles (VGPs) and their paleolatitude anomalies, which we interpret as net latitude changes since magnetization ages given by integers in Ma, in black. Colored bars connect each paleomagnetic locality (marked by circle) to nearest point on its nominal paleolatitude parallel; red bars indicate localities that appear to have moved north, and blue bars indicate localities that appear to have moved south. Colored bars with different magnetization ages are not parallel due to polar wander relative to stable North America. Thicker bars are plotted where anomalies are more significant (that is, greater than standard error, or greater than two standard errors). Some ages are not shown in areas of very dense sampling, but ages omitted are similar to those shown. Full details are contained in Supplemental Material (see link above). Polyconic projection about central meridian 110°W.
Figure 7. Overview of database of geologic paleostress-direction indicators, such as dated dikes, veins, fault sets, or stylolites. Each bar shows present (not restored) azimuth of most compressive horizontal principal axis of paleostress. Yellow butterfly symbols show 90% confidence ranges in azimuth. Red bars are relevant for 48–0 Ma; blue bars are relevant for 90–48 Ma; and white bars straddle 48 Ma boundary and are relevant for both periods. Full details are contained in Supplemental Material (link above). For a complete analysis of an earlier version of this data set, see Bird (2002). Polyconic projection about central meridian 110°W.
Figure 8. Location map for most faults, features, and locations in the greater California region that are mentioned in this paper. Named faults have traces colored according to dominant sense of offset, as in Figure 1. Other faults are represented by thin gray lines. Abbreviations: Mission R.-Ar.Parida-Sta.Ana = Mission Ridge–Arroyo Parida–Santa Ana; SA0n = San Andreas fault segment #0n; Santa Cruz Is.-Pt.Dume-S.M. = Santa Cruz Island–Point Dume–Santa Monica–Hollywood; Santa Rosa Is. = Santa Rosa Island; S.G.Pass-G.H. = San Gorgonio Pass–Garnet Hill. Eastern California shear zone outlined by heavy dashed green line. Colorado River extensional corridor outlined by heavy dashed red line. Polyconic projection about central meridian 120°W.
Figure 9. Paleogeologic map of greater southern California region at 6 Ma (Late Miocene) from reference model NI. Colored outcrops and black fault traces restored from the digital geologic map of Garrity and Soller (2009); colored fault traces restored from our database of Figure 1. See Paleogeologic Maps section of text for other conventions and known deficiencies. Two orange triangles adjacent to San Andreas fault in 118–119°W, 34–35°N represent Pinnacles and Neenach Volcanics, respectively, which had been moving apart since their co-development ca. 21 Ma. In area where San Gabriel Mountains block has been aligned with Orocopia and Chocolate Mountains (114–116°W, 32–33°N), five small tan outcrops of KTsv (“Cretaceous and Tertiary interlayered sedimentary and volcanic rocks” in the Explanation of Garrity and Soller, 2009) represent Pelona and Orocopia Schists exposed along an offset anticlinorium. The San Andreas fault trace between them would have been a new fault at this time. Dillon thrust fault discussed in text is blue trace at 116°W, 33.5°N. Polyconic projection about central meridian 116°W.
Figure 10. Model offsets on faults since 48 Ma in reference model NI (vertical axis) plotted relative to corresponding target offsets (horizontal axis). All offsets in kilometers. Uncertainty brackets on target offsets are one standard error. Black circles show “complete” restorations of fault offsets that began after 48 Ma. Gray circles show “incomplete” restorations of fault offsets that began before 48 Ma; in these cases, target value shown is prorated offset expected after 48 Ma. Points plotted along vertical axis represent cases where target offset was zero. Gray dashed line represents ideal of equal target and model offsets.
Figure 11. Misfits (errors) in model fault heaves, relative to geologic target heaves, and normalized by the standard errors of those targets. Results are shown for 2000 faults in model NI which had nonzero target heaves, and whose activity began at or after 48 Ma. There is a clear tendency to underfit heave targets. As discussed in text, this results from the conflicting influences of other constraints, especially the minimization of continuum strain. However, some part of the systematic misfit may be desirable because target rates are maximum fault offsets, but model predictions are mean offsets averaged along each fault trace.
Figure 12. Model extension (positive) or shortening (negative) along cross sections since 48 Ma in reference model NI (vertical axis); plotted relative to corresponding extension and/or shortening from published geologic models (horizontal axis). Black circles show “complete” length changes where sections had a simple initial geometry at or after 48 Ma. Gray circles show “incomplete” length changes where sections had a simple initial geometry before 48 Ma; in these cases, the “data” value shown is prorated length change expected after 48 Ma. Error bars on data values are ±σ, showing 67%-confidence range for each datum. Gray dashed line represents ideal of equal target and model length changes.
Figure 13. Fault heave rates and continuum strain rates at 3.1 Ma (late Pliocene) from reference model NI. Widths of colored ribbons are proportional to fault heave rates, and the average heave rate for most faults is also given by an adjacent number in units of mm/a. Discontinuities in heave rate along each trace are artifacts due to different heave-rate estimates in adjacent finite elements. Strain rates in non-faulting (continuum) finite elements are plotted as orientations of equivalent conjugate faults, with area (length squared) of each icon proportional to difference between most positive and most negative principal strain rates (E3 and E1). “X” symbol indicates conjugate strike-slip faults; rectangle symbol indicates a graben forming between conjugate normal faults, and barbell symbol indicates conjugate thrust faults. Sample symbols in legend are oriented as they would be with most compressive principal strain rate vertical (N-S) and most extensional horizontal principal strain rate horizontal (E-W). Most strain-rate tensors require two co-located symbols for full representation.
Figure 14. Natural strains accumulated since 6 Ma in reference model NI. Natural strain (also known as true strain) is the natural logarithm of stretch, which is the ratio of deformed dimension to original dimension. All rocks and coastlines are mapped in present positions. Our methods cannot integrate strain rate over geologic time in finite elements containing active faults, so strains within active fault corridors are not mapped here. Colors in triangles indicate natural log of the ratio of deformed area to original area (at 6 Ma). Superposed symbols show continuum strain tensors as sets of conjugate faults, with conventions similar to those in Figure 13.
Figure 15. Model vertical-axis rotations since 48 Ma in reference model NI (vertical axis) plotted relative to corresponding rotations indicated by paleomagnetic data (horizontal axis). Counterclockwise (CCW) rotations are plotted as positive; clockwise rotations are plotted as negative. Model rotations are accrued only back to age of magnetization for each pole and/or virtual geomagnetic pole (VGP), or to 48 Ma, whichever is younger. Black circles show “complete” rotations at localities where rocks were magnetized after 48 Ma. Gray circles show “incomplete” rotations at localities where rocks were magnetized before 48 Ma; in these cases, “data” value shown is prorated rotation expected after 48 Ma. Error bars on data values are ±σ, showing 67% confidence range for each datum. Gray dashed line represents ideal of equal target and model rotations.
Figure 16. Paleogeologic map of greater southern California region at 12 Ma (Middle Miocene) from reference model NI. See Paleogeologic Maps section of text for conventions and known deficiencies. This time is mid-way through extreme extension on the Oceanside detachment fault that exhumed crust of present Inner Borderland. Also at this time, a transition was under way from dextral Canton–San Gabriel fault (18–12 Ma) to dextral San Gabriel fault (12–6 Ma), both of which linked the San Andreas dextral fault of central California to the Cristianitos dextral fault and Oceanside dextral-transtensional detachment fault of southern California and Baja California. Two orange triangles adjacent to San Andreas fault in 117–118°W, 33–34°N represent Pinnacles and Neenach Volcanics, which had been moving apart since their co-development ca. 21 Ma. Polyconic projection about central meridian 116°W.
Figure 17. Left: figure 7A of Wilson et al. (2005) showing their reconstruction of the southern California region at 26.5 Ma (Late Oligocene). Right: our paleogeologic map of greater southern California region at 26.6 Ma from reference model NI. Heavy gray rectangle in right-hand map represents domain of left-hand map. These maps are very different because left map shows a reconstruction of present coastlines and tectonic block boundaries, whereas right map shows a reconstruction of present outcrops and fault traces. Still, enough common features can be found to contrast these reconstructions, as detailed in text. For reference, red outcrop of MZg Mesozoic granite at 114.2°W, 32.2°N in our reconstruction represents present eastern tip of Santa Monica Mountains. Yellow outcrop of Tv (Tertiary volcanics), next to teal outcrop of Jvi (Jurassic volcanics of intermediate composition) at 114.3°W, 31.1°N in our reconstruction represents present Santa Cruz Island. At 113.6°W, 30.2°N in our reconstruction, small tan outcrop of eT (Eocene sedimentary rocks) represents present San Nicolas Island.
Figure 18. Left: reconstruction at 36 Ma (late Eocene) from figure 9 of McQuarrie and Wernicke (2005). Right: our paleogeologic map of western U.S.A. and northern Mexico at 36 Ma from reference model NI. Heavy gray outline on right map shows position of left map in stable-North-America coordinates. These maps are very different because left map emphasizes reconstructed positions of Quaternary basins, whereas right map shows reconstruction of present outcrops and fault traces. Still, enough common features can be found to contrast these reconstructions, as detailed in text. Equidistant conic projections with central meridian 96°W and reference latitudes of 33°N and 45°N.
Figure 19. Paleogeologic map of greater southern California region at 21 Ma (Early Miocene) from reference model NI. See Paleogeologic Maps section in text for conventions and known deficiencies. At 117°W, 34°N, two adjacent orange triangles beside San Andreas fault represent Pinnacles Volcanics center (SW) and Neenach Volcanics center (NE), which have been interpreted (Matthews, 1976; Burnham, 2009) as having formed together at approximately this time. At 113.9°W, 30.25°N, small tan outcrop of eT (Eocene sedimentary rocks) represents present San Nicolas Island, restored adjacent to Peninsular Ranges block at a position corresponding to present Rosarito, B.C. Polyconic projection about central meridian 116°W.
Figure 20. Velocities at 21.1 Ma (Early Miocene) from reference model NI. This illustrates peak of medial phase of Tertiary extension, which followed early extension in northern Basin and Range (beginning 49–30 Ma) but preceded extension along coastal Oceanside detachment fault to form Inner Borderland (18–6 Ma). Fault traces shown in black; most active detachment faults were those in Colorado River extensional corridor (Fig. 8). These same fault traces are shown in corresponding colors in roughly coeval Figure 19. Polyconic projection about central meridian 116°W.
Figure 21. (A) Paleogeologic map of central southern California region at 48 Ma (Middle Eocene) from reference model NI. See Paleogeologic Maps section in test for conventions and known deficiencies. (B) Alternative reconstruction obtained by manually editing outcrop positions from map A, as described in text. Polyconic projections about central meridian 115°W.