Fossil slabs attached to unsubducted fragments of the Farallon plate

Abstract

As the Pacific–Farallon spreading center approached North America, the Farallon plate fragmented into a number of small plates. Some of the microplate fragments ceased subducting before the spreading center reached the trench. Most tectonic models have assumed that the subducting oceanic slab detached from these microplates close to the trench, but recent seismic tomography studies have revealed a high-velocity anomaly beneath Baja California that appears to be a fossil slab still attached to the Guadalupe and Magdalena microplates. Here, using surface wave tomography, we establish the lateral extent of this fossil slab and show that it is correlated with the distribution of high-Mg andesites thought to derive from partial melting of the subducted oceanic crust. We also reinterpret the high seismic velocity anomaly beneath the southern central valley of California as another fossil slab extending to a depth of 200 km or more that is attached to the former Monterey microplate. The existence of these fossil slabs may force a reexamination of models of the tectonic evolution of western North America over the last 30 My.

Complexity of the deep San Andreas Fault zone defined by cascading tremor

“…The geometry is consistent with the hypothesis that the San Andreas Fault in central California soles into the top of the stalled Monterey Microplate, a remnant of the subducted Farallon Plate that was captured by the Pacific Plate [35][36][37] . If so, the offset LFE families may be generated by strike slip on the shallowly dipping interface between the stalled slab and the North American Plate.…”

“…In particular, LFE sources south of Parkfield that are offset up to 10 km northeast from the main trend perhaps hint at the presence of a fossil Farallon slab remnant at the base of the crust. If so, then the mantle San Andreas Fault zone may be deflected eastward along the slab surface, as some have proposed [35][36][37] . Future work should endeavour to constrain the sense of slip of these offset sources, which would further inform the configuration of the San Andreas beneath the crust.…”

Seismic structure beneath the Gulf of California: a contribution from group velocity measurements

“…Previous studies (Zhang et al 2009;Wang et al 2013) interpret the high-velocity anomaly beneath west-central Baja as a fossil slab and this is also justified by the distribution of the post-subduction volcanism in the area. In our study, this feature is not as laterally continuous, nor does it cover as large an area as that observed by Wang et al (2013), although we note that their map extends further westwards than ours. We follow their interpretation of this feature as a stalled remnant of the Guadalupe and/or Magdalena microplates since this feature persists at depths of ∼60 to ∼110 km (Figs 9 and 12).…”

“…A relatively highvelocity anomaly was also found by Zhang et al (2009) at depths between 120 and 160 km. Considering the distribution of volcanic rocks in Baja, Wang et al (2013) interpret this fast velocity anomaly as a fossil slab attached to the unsubducted Magdalena microplate. The fast velocity zone is also evident in the east-west profiles in Fig.…”

Review of Late Jurassic-early Miocene sedimentation and plate-tectonic evolution of northern California: illuminating example of an accretionary margin

“…It appears to have fragmented into several smaller east-dipping plate segments (Wang et al 2013 (Ingersoll 1978(Ingersoll , 1979(Ingersoll , 1983Linn et al 1992 A few pre-Mesozoic, multicycle grains were reported from basal GVG sandstones by DeGraaff- Surpless et al (2002) and Wright and Wyld (2007). These zircon ages provide Precambrian peaks at 1,000-1,200, 1,400, 1,800-2,000, and 2,600 Ma, suggesting that minor ultimate sources including the Grenville, Mazatzal-Yavapai, and Wyoming or Superior provinces.…”