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Segmentation of the Laramide Slab—Evidence from the Southern Segmentation of the Laramide SlabÐevidence from the southern Sierra Nevada region Jason Saleeby² Division of Geological and Planetary Sciences, California Institute of Technology, M.S. 100-23, Pasadena, California 91125, USA ABSTRACT ®ned by plate edge relations and the cor- 1992). A commonly cited plate tectonic mech- responding trajectory of Farallon±North anism for the orogeny is intensi®ed traction During the latest Cretaceous-early Paleo- American relative plate motions when and tectonic erosion of the subcontinental gene Laramide orogeny, the lithosphere be- viewed on a pre-Neogene palinspastic base. mantle lithosphere due to ¯attening of the sub- neath the southernmost Sierra Nevada The plate interior is suggested to have been ducted slab (Coney and Reynolds, 1977; batholith and the adjacent Mojave Desert deformed ®rst by end loading as the shal- Dickinson and Snyder, 1978; Bird, 1988). The region batholith was sheared off and dis- low slab segment initially descended be- response of the craton was deformation and placed deeper into the mantle. The litho- neath the plate edge, and then by greater uplift along a north-northeast±trending corri- sphere beneath the greater Sierra Nevada basal traction components as the shallow dor extending from southwest Arizona batholith to the north was left intact until segment progressed beneath the cratonic through Wyoming (Fig. 1). This intracratonal mid-Miocene time, when fragments of it region. The subcontinental mantle litho- deformation zone is for the most part inboard were entrained as volcanic xenoliths. The sphere beneath the cratonic deformation of the Cordilleran (Sevier) foreland fold-thrust Laramide slab was evidently segmented zone remained intact through Laramide belt, thereby calling for special circumstances into a shallow ¯at segment to the south and time. relative to much of Cordilleran tectonic his- a deeper segment to the north. Shearing off Similar segmented slab topologies have tory. Slab ¯attening at Laramide time seem- of the upper mantle to the south was fol- been resolved beneath the modern Andean ingly offers such a special circumstance, al- lowed by the tectonic underplating of orogen where the shallow segments are cor- though the dynamics responsible for slab schists derived from Franciscan subduction related with the subduction of aseismic ¯attening are not well understood, nor is the complex and possibly forearc basin proto- ridges. A number of researchers have sug- reason for the geographic restriction of Lar- liths. The overlying batholithic crust was gested that the Laramide orogeny arose amide deformation relative to the much more deformed, deeply denuded, and tectonically from the subduction of a counterpart of the regionally extensive Cordilleran active mar- breached westward across its forearc re- Hess-Shatsky large igneous province of the gin. Such a geographic focusing of Laramide gion while the schists were underplated. northwest Paci®c basin. Fragments of rock deformation has been suggested to have re- Westward breachment resulted from a assemblages that are correlative to the sulted from the subduction of a counterpart of combination of west-directed thrusting and Hess-Shatsky province that were accreted the Hess-Shatsky large igneous province of extensional collapse. The westernmost to the Franciscan complex in Laramide the northwest Paci®c basin, which was em- breached rocks were tectonically removed time support this view. Recognition of the bedded in the Farallon plate (Livaccari et al., by a combination of trench linked trans- resulting shallow slab segment and its tra- 1981; Henderson et al., 1984; Barth and form and subduction erosion processes. jectory beneath North America explains the Schneiderman, 1996). The analysis offered Subsequent tectonic erosion by the Neogene geographic focusing of Laramide deforma- here ®nds much merit in this view, although San Andreas transform has left the Salinia tion, relative to the rest of the Cordilleran the main proposition presented here is not and the San Gabriel terranes as dispersed orogen, and the relationships between plate contingent on this view. This proposition as- residual fragments of the westward edge and plate interior deformational serts that the Laramide slab possessed a shal- breached arc segment. regimes. low slab segment and that the subduction of The shallow slab segment appears to this shallow segment can be correlated with have been ;500 km in width, measured Keywords: tectonics, subduction, delami- both plate edge and interior Laramide along the plate edge. To the south, the slab nation, Cordillera, Laramide. deformation. descent appears to have remained deep be- Geologic events along the southwest Cor- neath the Peninsular Ranges batholith. INTRODUCTION dilleran plate edge that have been attributed to Classic Laramide structures of the craton Laramide tectonics include: 1) the cessation of are concentrated in a corridor that corre- The Laramide orogeny is recognized as a arc magmatism in the Sierra Nevada batholith sponds to the shallow slab segment as de- regional compressional event that deformed and associated modi®cation of the subbatho- the southwest North American craton in latest lith geotherm by conductive cooling from be- ²E-mail: [email protected]. Cretaceous-early Paleogene time (Miller et al., low (Dumitru et al., 1991); 2) the low-angle GSA Bulletin; June 2003; v. 115; no. 6; p. 655±668; 4 ®gures. For permission to copy, contact [email protected] q 2003 Geological Society of America 655 J. SALEEBY 656 Geological Society of America Bulletin, June 2003 SEGMENTATION OF THE LARAMIDE SLAB thrust emplacement of Franciscan-af®nity greywacke-basalt assemblages and their meta- morphism in high-pressure greenschist± amphibolite facies (Rand-Pelona-Orocopia schists); and 3) the spatially related deforma- tion, denudation, and westward breachment of an ;500-km-long segment of the Cordilleran batholith belt above the underplated schists (Silver, 1983; May, 1989; Jacobson et al., 1996; Malin et al., 1995; Barth and Schnei- derman, 1996). The disrupted segment of the batholith belt corresponds to much of the Mo- jave Desert, Salinia and the San Gabriel ter- rane as restored to their pre-Neogene positions along the San Andreas fault, and the adjacent southern Colorado River Desert region. The northern end of the disrupted segment of the batholith belt coincides with a zone of deep denudation of the southernmost Sierra Nevada batholith that has rendered a structurally con- tinuous oblique crustal section (Saleeby, 1990; Pickett and Saleeby, 1993). The southern end of the disrupted segment of the batholith belt corresponds to the northern Peninsular Ranges batholith. For discussions below, it is useful to symbolize these three segments of the bath- olith belt as: SNB 5 Sierra Nevada batholith; MSB 5 Mojave Desert±Salinia batholith, in- cluding the San Gabriel terrane and adjacent southern Colorado River Desert basement complexes; and PRB 5 Peninsular Ranges batholith. Insights regarding the deep structure of the SNB in Laramide time are augmented by pet- rologic and geochemical data on lower-crust± upper-mantle xenoliths that were entrained in mid-Miocene volcanic rocks from the San Joaquin volcanic ®eld, the location of which is shown in Figure 2 (Dodge et al., 1986, Figure 2. Map of California region showing in generalized form a number of key tectonic 1988; Ducea and Saleeby, 1996, 1998; Ducea, and geographic features that are referred to in text. The San Andreas transform system, 2001; Lee et al., 2000; Saleeby et al., 2002). Transverse Ranges transrotational deformation, Neogene extension, and the eastern Cal- Constraints on the lower-crust±upper-mantle ifornia shear zone constitute main superposed deformations that were restored in the structure posed by the oblique crustal section, California region for the construction of the Figure 1 palinspastic base. along with seismic and the xenolith data, to- gether carry important implications regarding the topology and some of the effects of the Much of the discussion will focus on Figure geological setting in California, as well as the Laramide slab. These implications and their 1, which is a reconstruction of the principal major Neogene tectonic systems that are re- relations to a spectrum of geologic features tectonic elements of the southwest U.S. region stored on Figure 1. The key restorations are: attributed to the Laramide orogeny are the at late Laramide time. Figure 2 shows some 1) Salinia and the San Gabriel terrane along subject of this paper. of these elements as they occur in their current the San Andreas transform system (Ross, Figure 1. Map showing selected tectonic and basement features of the southwest Cordillera at the close of the Laramide orogeny on a pre-Neogene palinspastic base. This map shows how the breached segment of the southwest Cordilleran batholithic belt, expressed primarily by Salinia and the San Gabriel terrane, lies in a north-northeast±trending deformation corridor that includes classic cratonal structures of the Laramide orogeny, and that this corridor is generally parallel to the Farallon±North American relative plate motions for Laramide time. The restored trace of the Garlock fault is used as a surface tracer for the position of the in¯ection in the slab. This tracer and its north-northwest projection bound the northwest margin of the deformation corridor that is consistent with the slab segmentation model for Laramide deformation. References given in text. N Geological
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