PUTIRKA, KEITH Dhttp://hdl.handle.net/10211.3/2001122024-03-29T07:43:11Z2024-03-29T07:43:11ZIntroduction: Origin and Evolution of the Sierra Nevada and Walker LanePutirka, Keith D.Busby, Cathy J.http://hdl.handle.net/10211.3/2014902020-04-20T21:54:07Z0012-01-01T00:00:00ZIntroduction: Origin and Evolution of the Sierra Nevada and Walker Lane
Putirka, Keith D.; Busby, Cathy J.
From Geosphere (2011) 7 (6): 1269-1272.
0012-01-01T00:00:00ZCenozoic volcanism in the Sierra Nevada and Walker Lane, California, and a new model for lithosphere degradationPutirka, Keith D.Jean, MarlonCousens, BrianSharma, RohitTorrez, GerardoCarlson, Chadhttp://hdl.handle.net/10211.3/2014892020-04-20T21:54:07Z0004-01-01T00:00:00ZCenozoic volcanism in the Sierra Nevada and Walker Lane, California, and a new model for lithosphere degradation
Putirka, Keith D.; Jean, Marlon; Cousens, Brian; Sharma, Rohit; Torrez, Gerardo; Carlson, Chad
Volcanic rock and mantle xenolith compositions in the Sierra Nevada (western United States) contradict a commonly held view that continental crust directly overlies asthenosphere beneath the Sierran range front, and that ancient continental mantle lithosphere (CML) was entirely removed in the Pliocene. Instead, space-time trends show that the Walker Lane is the principle region of mantle upwelling and lithosphere removal in eastern California, that lithosphere loss follows the migration of the Mendocino Triple Junction (MTJ), and that the processes of lithosphere removal are not yet complete beneath the Sierra Nevada and its range front. Key evidence is provided by volcanic rock compositions. The 87Sr/86Sr ratios for mafic volcanics of the Sierra (MgO > 8%) are mostly > 0.705, and 143Nd/144Nd < 0.5127, much unlike eastern sub-Pacific asthenosphere (where 87Sr/86Sr < 0.7027 and 143Nd/144Nd > 0.5129), but very much like CML. Similarly, Sierran volcanics carry CML-like trace element ratios, with La/Nb > 3 and Th/Nb > 1, values that are significantly higher than asthenosphere-derived melts (La/Nb < 1.5, Th/Nb < 0.08). Spinel-bearing mantle xenoliths contained in Pleistocene–Holocene volcanics from the Sierran range front also have a CML composition, with 87Sr/86Sr and ɛNd ratios that range to 0.7065 and –3.6, respectively. New estimates of melt extraction depths using Si activity and mineralogy-sensitive trace element ratios (Sm/Yb, Lu/Hf) show that CML extends from the base of the crust (40 km) to depths of 75 km beneath the range front, and to 110 km for Pliocene volcanics of the southern Sierra. This means that garnet-bearing lithologies could not have been dislodged from beneath the southern Sierra until after the Pliocene. Only in the Walker Lane do young (0.18 Ma) volcanic rocks, from the Coso volcanic field, approach asthenosphere-like compositions, which occurs only 20 Ma after MTJ arrival. Temporal trends show that MTJ arrival at any given latitude south of 37°N signals lithosphere heating, probably due to asthenosphere that upwells to replace the sinking Farallon plate. Partial melts of the asthenosphere, and perhaps the asthenosphere itself, intrude into and cause heating and partial melting of overlying CML; this culminates after 10 Ma. After 20 Ma, CML becomes highly degraded and asthenosphere-derived melts are dominant. North of 37°N, volcanic rocks approach asthenosphere-derived compositions to the west, not the east, and 87Sr/86Sr ratios increase from 18 to 0 Ma, indicating that this region has entered a phase of lithosphere heating, but not yet a phase of lithosphere removal. We propose a new model of lithosphere degradation, where asthenosphere or its partial melts pervasively invade CML beneath the Walker Lane. This process is now nearly complete beneath Coso, and is migrating west, so that it is only partly complete at the southern Sierra range front, or within the Sierra Nevada, at any latitude. This model of intermixed asthenosphere and lithosphere better explains the compositions of volcanic rocks and their included xenoliths, and the remarkably consistent S-wave receiver function data, which show a 70-km-thick lithosphere beneath the Sierra Nevada. If the upper mantle is warm CML, permeated by partial melts, this model may also explain low P- and S-wave velocities.
From Geosphere (2012) 8 (2): 265-291.
0004-01-01T00:00:00ZBasin and Range volcanism as a passive response to extensional tectonicsPutirka, Keith D.Platt, Bryanthttp://hdl.handle.net/10211.3/2014882020-04-20T21:54:07Z0010-01-01T00:00:00ZBasin and Range volcanism as a passive response to extensional tectonics
Putirka, Keith D.; Platt, Bryant
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-time-composition 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 mafic 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 K2O-rich, 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 SiO2 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.
From Geosphere (2012) 8 (6): 1274-1285.
0010-01-01T00:00:00ZFormation of the Sierra Nevada batholith: magmatic and tectonic processes and their temposPaterson, Scott R.Lackey, Jade StarMemeti, ValiMiller, Robert B.Miller, Jonathan S.Mundil, RolandPutirka, Keith D.http://hdl.handle.net/10211.3/2014872020-04-20T21:54:07Z2012-09-01T00:00:00ZFormation of the Sierra Nevada batholith: magmatic and tectonic processes and their tempos
Paterson, Scott R.; Lackey, Jade Star; Memeti, Vali; Miller, Robert B.; Miller, Jonathan S.; Mundil, Roland; Putirka, Keith D.
The article focuses on the Geological Society of America (GSA) Penrose Field Forum held in Sierra Nevada in California and Nevada from September 1-8, 2012. The forum aimed at understanding the magmatic and tectonic processes and their tempos in arcs. During the forum, the geologists recognized the variations in degree of magmatic interaction at the emplacement level in different intrusive suites. It also informs about activities and awards distributed during the forum.
From GSA Today, 2013, Vol.23(2), pp.15-17.
2012-09-01T00:00:00Z