|Title||Evidence for Moho-lower crustal transition depth diking and rifting of the Sierra Nevada microplate|
|Publication Type||Journal Article|
|Year of Publication||2016|
|Authors||Smith K.D, Kent G.M, von Seggern D.P, Driscoll NW, Eisses A.|
|Journal||Geophysical Research Letters|
|Type of Article||Article|
|Keywords||basin; Basin and Range; beneath; california; deformation; delamination; diking; fault; mendocino triple junction; Moho-transition depth seismicity; northern walker lane; plate boundary; rifting; Sierra microplate; western united-states|
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 (<30km 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 similar to 25% of the length, of an implied similar to 56km long structure, each striking N45 degrees W and dipping similar to 50 degrees NE. A single event at similar to 30km 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, M-w 4.7 earthquake occurred directly above the deep SV sequence at the base of the upper crustal seismogenic zone (similar to 15km depth).