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An Animated Tectonic Reconstruction of Southwestern North America Since 36 Ma

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An Animated Tectonic Reconstruction of Southwestern North America Since 36 Ma An animated tectonic reconstruction of southwestern North America since 36 Ma Nadine McQuarrie* Brian P. Wernicke Division of Geological and Planetary Sciences, MS 100-23, California Institute of Technology, Pasadena, California 91125, USA ABSTRACT tension east and west of the Sierra Nevada± ment vector ®elds of continental deformation Great Valley block and strain-compatible in order to test these models. The most dra- We present tectonic reconstructions and deformation between southern Arizona and matic recent improvement in obtaining such an accompanying animation of deformation the Mexican Basin and Range. Our results velocity ®elds has been the advent of space- across the North America±Paci®c plate show ;235 km of extension oriented based tectonic geodesy (especially using con- boundary since 36 Ma. Intraplate defor- ;N788W in both the northern (50% exten- tinuous global positioning systems [GPS]), mation of southwestern North America was sion) and central (200% extension) parts of which is yielding velocity ®elds that are un- obtained through synthesis of kinematic the Basin and Range. Following the initia- precedented in terms of both the scale of ob- data (amount, timing, and direction of dis- tion of east-west to southwest-northeast ex- servation and the accuracy of the velocities. placement) along three main transects tension at 15±25 Ma (depending on longi- These data have already been used as tests for through the northern (408N), central (368N± tude), a signi®cant portion of right-lateral physical models in southwestern North Amer- 378N), and southern (348N) portions of the shear associated with the growing Paci®c± ica (e.g., Bennett et al., 1999, 2003; Flesch et Basin and Range province. We combined North America transform jumped into the al., 2000) and elsewhere (e.g., Holt et al., these transects with ®rst-order plate bound- continent at 10±12 Ma, totaling ;100 km 2000). Substantial progress has also occurred ary constraints from the San Andreas fault oriented N258W, for an average of ;1 cm/ over the last decade in determining longer- and other areas west of the Basin and yr since that time. term velocity ®elds using the methods of plate Range. Extension and strike-slip deforma- tectonics and regional structural geology. tion in all areas were sequentially restored Keywords: Basin and Range, kinematic re- These longer-term displacement histories over 2 m.y. (0±18 Ma) to 6 m.y. (18±36 Ma) construction, extension, plate tectonics, ve- are essential for addressing the question of time intervals using a script written for the locity ®eld. how the lithosphere responds to major varia- ArcGIS program. Regions where the kine- tions in plate geometry and kinematics (e.g., matics are known constrain adjacent areas INTRODUCTION Houseman and England, 1986; England and where the kinematics are not well de®ned. Houseman, 1986; Bird, 1998) because such The process of sequential restoration high- The large-scale horizontal velocity ®eld at variations occur on the million-year time lighted misalignments, overlaps, or large Earth's surface is one of the main predictions scale. Plate tectonics is a precise method for gaps in each incremental step, particularly of physical models of lithospheric deforma- constraining the overall horizontal kinematics in the areas between data transects, which tion (e.g., England and McKenzie, 1982). of plate boundaries, using sea¯oor topograph- remain problematic. Hence, the value of the Two-dimensional, cross-sectional models of ic and magnetic data in concert with the geo- reconstructions lies primarily in highlight- ®nite deformation of mountain belts incorpo- magnetic time scale. For the diffuse defor- ing questions that might not otherwise be rating strong heterogeneity in rheologic pa- mation that characterizes the continental recognized, and thus they should be viewed rameters have been developed over the last de- lithosphere along plate boundaries, however, more as a tool for investigation than as a cade (e.g., Lavier and Buck, 2002; Braun and tectonic reconstruction at scales in the 100 km ®nal product. Pauselli, 2004). Owing to advances in com- to 1000 km range is not as straightforward. It The new sequential reconstructions show putation, fully three-dimensional models of is based primarily on structural geology and that compatible slip along the entire north- plate boundary deformation zones, incorpo- paleomagnetic studies and requires the iden- south extent of the inland right-lateral rating both horizontal and vertical variations ti®cation of large-scale strain markers and shear zone from the Gulf of California to in lithospheric rheology, will soon become consideration of plate tectonic constraints the northern Walker Lane is supported by common. Thus, a key observational frontier (e.g., Wernicke et al., 1988; Snow and Wer- available data and that the east limit of ac- will be the determination of precise displace- nicke, 2000; McQuarrie et al., 2003). Regional tive shear has migrated westward with re- strain markers within the continents may not spect to North America since ca. 10 Ma. *Current address: Department of Geosciences, exist in any given region, and even if they do, The reconstructions also highlight new Guyot Hall, Princeton University, Princeton, New they may not be amenable to accurate recon- problems regarding strain-compatible ex- Jersey 08544, USA. struction at large scales. Geosphere; December 2005; v. 1; no. 3; p. 147±172; doi: 10.1130/GES00016.1; 11 ®gures; 6 tables, 1 animation. For permission to copy, contact [email protected] q 2005 Geological Society of America 147 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/1/3/147/3332410/i1553-040X-1-3-147.pdf by guest on 02 October 2021 N. MCQUARRIE and B.P. WERNICKE In southwestern North America, a zone of of information on the kinematics of the dif- plate boundary deformation on the order of fusely deforming North American plate since 1000 km wide is developed along the plate 36 Ma, based on offsets of regional structural boundary. In mid-Tertiary time (36 Ma), this markers, and construct a strain-compatible ki- boundary was strongly convergent, with the nematic model of the horizontal motions at 2 Farallon plate subducting eastward beneath m.y. (0±18 Ma) and 6 m.y. (18±36 Ma) inter- the North American plate. Beginning at ca. 30 vals, presented as a continuous animation. The Ma, the Paci®c-Farallon ridge came in contact model is by no means a ®nal product, as new with the North American plate. Since then, the kinematic information and testing will require Paci®c±North America boundary has grown signi®cant modi®cations of the model. Rather, through the migration of triple junctions along the model is an attempt to be quantitatively the coast. Now, the entire margin from south- rigorous in a way that will be useful for com- ern Baja California to Cape Mendocino is a parison with large-scale, three-dimensional transform Paci®c±North America boundary, physical models and for the identi®cation of rather than a convergent Farallon±North issues regarding the structural kinematics that America boundary (Atwater, 1970). This might not otherwise be detected. Thus, in ad- change in the con®guration of the plate dition to the animation, we have constructed boundary is both relatively simple and pro- ``instantaneous'' velocity ®elds based on 2 found, making southwestern North America m.y. averages from 0 Ma to 18 Ma and 6 m.y. an ideal laboratory for investigating how con- averages from 18 Ma to 36 Ma. These results tinental lithosphere responds to changes in rel- are our best attempt at ``paleogeodesy,'' pre- ative plate motion. senting the geology-based kinematic model in Figure 1. Schematic diagram illustrating Re®ned plate tectonic reconstructions have a format similar to modern GPS velocity the method of using regional structural provided an improved kinematic model of the ®elds, which in turn may be quantitatively constraints to limit possible displacement change from convergent to transform motion compared to physically based model velocity paths in tectonic reconstructions. See text and have shown that there were signi®cant ®elds. for discussion and explanation of letters. variations in the obliquity of the transform af- ter it developed. In particular, during the in- METHODS terval ca. 16 to ca. 8 Ma, Paci®c±North Amer- path were in excess of AC (Fig. 1B), violating ica motion was highly oblique and included a By combining regional structural con- the condition of strain compatibility. There- margin-normal extensional component of as straints into a single model, the self-consistency fore, the displacement is constrained to be be- much as 2 cm/yr, coeval with a rapid pulse of of the model (i.e., its strain compatibility tween AB and AC, rather than some value Miocene extension that formed the Basin and through time) provides powerful additional greater than AB. Range province (Atwater and Stock, 1998; constraints on the kinematics in at least three A third and perhaps most useful additional Wernicke and Snow, 1998). At ca. 8 Ma, ways. The ®rst and most important is the fact constraint arises when local constraints con- Paci®c±North America motion changed to that high-quality local kinematic information tradict one another. For example, if recon- more purely coastwise motion, which appears imposes severe constraints on its surroundings struction along path q (Fig. 1A) required that to have changed the intraplate tectonic regime where information may not be available. As a point A restore to a position D, which is well from profound extension to a more complex hypothetical example, consider a large region within the other block, then the violation of mixture of extension, shortening, and trans- of oblique extension between two undeformed strain compatibility forces reevaluation of the form motion, responsible for the opening of blocks (Fig.
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