Evidence for Moho‐lower crustal transition depth diking and rifting of the Sierra Nevada microplate. Issue 20 (30th October 2016)

• Record Type: Journal Article
• Authors: Smith, Kenneth D.; Kent, Graham M.; von Seggern, David P.; Driscoll, Neal W.; Eisses, Amy
• Is Part Of: Geophysical research letters. Volume 43:Issue 20(2016)
• Page Start: 10, 738
• Page End: 10, 744
• Publication Date: 2016-10-30
• Subjects: Moho‐transition depth seismicity -- Sierra microplate -- rifting -- diking -- Basin and Range; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1002/2016GL070283
• Languages: English
• ISSNs: 0094-8276

Abstract

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). Key Points: Two 7 × 7 km planar volcanic like swarms align along and suggest a N45W striking 50 degree NE dipping 56 km long Moho depth structure The interpreted structure supports a mechanism for rifting of the mircroplate and footwall unloading and flexural uplift