Basin and Range volcanism as a passive response to extensional tectonics

Abstract

A long-standing issue in Cordilleran geology involves the nature of Basin and Range (western USA) volcanism, and whether such magmatism provides a trigger for extensional deformation, or if volcanic activity is a passive response to extension. We use space-timecomposition patterns across the central and southern Basin and Range, and where appropriate, reconstructed latitudes and longitudes of volcanic rocks, to show that volcanism is fundamentally a passive process. Our analysis suggests that Basin and Range volcanism is initiated by the transition from a subduction to a transform boundary (now manifest as the San Andreas fault), which causes a slab window to open, as the subducting Farallon plate falls away. Accordingly, volcanic activity follows the northward-migrating Mendocino Triple Junction (MTJ). In the wake of the MTJ, continental mantle lithosphere is heated over a time scale of 10-12 Ma; it then rapidly degrades (or is removed) 17-20 Ma after MTJ arrival at any given latitude, and is replaced by asthenosphere. In the central Basin and Range, MTJ migration triggers the well-documented structural migration of the Sierra Nevada away from the Colorado Plateau. Not only is volcanism triggered by the tectonic transition, but in the central Basin and Range volcanism also migrates west, following the initiation of upper crustal extensional faulting; the lag between the onset of extension and initiation of volcanism is a remarkably consistent 2 Ma. Within this framework, the initial stage of eastward-concentrated volcanism is dominated by felsic magmatism, which then gradually degrades to a more mafi c composition. This pattern, and temporal relations indicate that once lithospheric extension begins, between 2 and 5 Ma are needed to develop conduits through which nonviscous magmas can transit the crust, and that especially high amounts of extensional strain favor the eruption of K 2 Orich, low-degree partial melts. Our observations indicate that it is unlikely that mantle plume processes initiated Basin and Range volcanism within the study area, and that decreases in SiO 2 and increases in incompatible element abundances to the east within the Cordillera are best explained by continental mantle lithosphere that thickens to the east, which reduces average melt fractions.

Birth of a plate boundary at ca. 12 Ma in the Ancestral Cascades arc, Walker Lane belt of California and Nevada

“…It may instead represent slab window magmatism related to the passage of the Mendocino triple junction (MTJ; Fig. 3), as suggested by Putirka and Platt (2012). The SWNVF lies between 36.5°N and 37.5°N, and the MTJ passed those latitudes at 17.6 and 13.0 Ma (Keith Putirka, 2013, personal commun.).…”

“…17-20 Ma post-MTJ Putirka and Platte, 2012). These authors inferred that the MTJ infl uenced magmatism and tectonics at 27 Ma in southeast California (33°N) and at 17-18 Ma near Lake Mead (36°N), southeast of the region shown on Figure 1, and at 15-16 Ma at Death Valley (36.5°N), at the south end of Figure 1 (Putirka and Platt, 2012). However, it is important to emphasize that the enriched arc signal (e.g., high La, Ba) can last for as long as 17 m.y.…”

“…Extensive Cenozoic volcanic and sedimentary rocks, excellent exposure, and abundant previous geological mapping in the Walker Lane make it an excellent region in which to study rifting processes (e.g., Fig. 2; Henry and Perkins, 2001;Trexler et al, 2012;Putirka and Busby, 2007;Busby et al, 2008aBusby et al, , 2008bFaulds and Henry, 2008;Busby and Putirka, 2009;Cashman et al, 2009Cashman et al, , 2012Jayko, 2009aJayko, , 2009bNorton, 2011;Cousens et al, 2012;Jayko and Bursik, 2012;Putirka and Platt, 2012;Riley et al, 2012;Trexler et al, 2012;Wakabayshi, 2013).…”

Pleistocene volcanism and shifting shorelines at Lake Tahoe, California

“…These compositions suggest that the magmas were produced by subduction-related melting that was later affected by conductive heating (Farmer et al, 2013). Putirka and Platt (2012) suggest that this type of volcanism is a passive response to extension and that degradation of the submantle root may continue up to 10 Ma after the cessation of subduction.…”

A tale of two Walker Lane pull-apart basins in the ancestral Cascades arc, central Sierra Nevada, California

“…We discovered a basalt feeder dike for this (sample Qdike 8-19-08; Table 1). These mafic rocks form part of the regional suite of postsubduction magmas (Farmer et al, 2013), which have been interpreted as continental rift magmas (Lange and Carmichael, 1996;Cousens et al, 2011Cousens et al, , 2012John et al, 2015b;Putirka and Platt, 2012).…”