Evidence for Moho‐lower crustal transition depth diking and rifting of the Sierra Nevada microplate

Abstract

Lithospheric rifting most often initiates in continental extensional settings where “breaking of a plate” may or may not progress to sea floor spreading. Generally, the strength of the lithosphere is greater than the tectonic forces required for rupture (i.e., the “tectonic force paradox”), and it has been proposed that rifting requires basaltic magmatism (e.g., dike emplacement) to reduce the strength and cause failure, except for the case of a thin lithosphere (<30 km thick). Here we isolate two very similar and unprecedented observations of Moho‐lower crustal transition dike or fluid injection earthquake swarms under southern Sierra Valley (SV: 2011–2012) and North Lake Tahoe (LT: 2003–2004), California. These planar distributions of seismicity can be interpreted to define the end points, and cover ~25% of the length, of an implied ~56 km long structure, each striking N45°W and dipping ~50°NE. A single event at ~30 km depth that locates on the implied dipping feature between the two swarms is further evidence for a single Moho‐transition depth structure. We propose that basaltic or fluid emplacement at or near Moho depths weakens the upper mantle lid, facilitating lithospheric rupture of the Sierra Microplate. Similar to the LT sequence, the SV event is also associated with increased upper crustal seismicity. An 27 October 2011, Mw 4.7 earthquake occurred directly above the deep SV sequence at the base of the upper crustal seismogenic zone (~15 km depth).

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

“…There, the Walker Lane transtensional rift tip has propagated northward with time in the ancestral Cascades arc axis since ca. 12 Ma, in concert with northward migration of the Mendocino triple junction (Busby, 2013;Smith et al, 2016). The study presented herein, of two pull-apart basins in the ancestral Cascades arc, provides local-scale confirmation of this regional-scale interpretation.…”

“…Hildreth, 2007). The Lassen volcanic center is also a good modern analog, because it is a major arc magmatic focus at the rift tip (i.e., in the northernmost Walker Lane pull-apart basin; Busby, 2013;Smith et al, 2016).…”

GPS Imaging of vertical land motion in California and Nevada: Implications for Sierra Nevada uplift

“…Such a lithospheric strength contrast may focus tectonic uplift in the southern Sierra, irrespective of the source of forces that drive it, and possibly helps the SNCV respond vertically as a coherent block, i.e., providing a geographically coherent uplift from Central Valley to Sierra Crest rather than bending near a locked eastern edge. This strength contrast may persist northward along the length of the Sierra Nevada/Great Basin transition, where Moho depth seismicity swarms near Lake Tahoe and Sierraville (Figure ) reveal lower crustal diking that suggest active rifting along this boundary [ Smith et al , ]. This broken plate model could also help to explain the observations with only groundwater unloading, since disengaging the SNCV from areas further east could allow the rebound to be truncated at the Owens Valley.…”

Fluid Pressure‐Triggered Foreshock Sequence of the 2008 Mogul Earthquake Sequence: Insights From Stress Inversion and Numerical Modeling

“…Although the Nevada Seismological Laboratory dismissed volcanic activity as a cause for the Mogul sequence at the time, a magmatic intrusion deep beneath the Mogul area could provide extensive amounts of fluids. There is evidence for a deep magmatic intrusion to the west and northwest of the Mogul area (Smith et al, , ), but to date there is no supporting evidence (e.g., surface uplift) of such an intrusion beneath the Mogul area.…”