Cratonic, circumcratonic and oceanic mantle domains beneath the western United States

Abstract

Seismic tomography, heat flow data, and xenolith thermobarometry allow one to broadly define the architecture of the lithosphere beneath the western United States. More precise information about domain geometry is forthcoming from an integration of the isotopic data from basalt‐borne and kimberlite‐borne xenoliths and volcanic rocks. Basalt‐borne and kimberlite‐borne xenoliths provide fundamentally important information about the timing of chemical “processes” responsible for the formation of domains and, as such, constrain their temporal evolution. On the basis of isotopes the following compositional analogs can be identified in the mantle beneath the western United States: (1) two enriched mantle domains (EM1 and EM2), (2) depleted mid‐ocean ridge basalt (MORB) mantle (DMM), and (3) a domain similar to the source of ocean island basalts (OIB). In the western United States the EMl domain appears to be restricted to sub‐Archean terrains (<2 HFU) (HFU = μcal cm−2 s−1) and may record processes unique to that time. DMM does not apparently exist under Archean terrains and may be restricted to Proterozoic (and Phanerozoic) subcrustal locations. The EM2 domain is more widespread beneath Proterozoic terrains and appears to be related to suprasubduction or recycling processes, a feature that is consistent with the replacement of the DMM domain by the EM2 domain as observed in basalt‐borne xenoliths. Upwelling of the OIB domain in the southern Basin and Range has led to partial replacement of both the DMM and EM2 domains. Volcanic rocks provide an additional regional monitor of domain continuity and distribution. In areas of Archean (>2.7 b.y.) crust (heat flow <2 HFU and >40 km thick), volcanic rocks tap an Archean subcratonic domain (EM1), whereas in areas of Proterozoic crust (heat flow >2 HFU and <40 km thick), volcanic rocks tap circumcratonic (EM2) and oceanic (OIB) domains. No evidence exists for the presence of domain EMl in Proterozoic regions, perhaps indicating that it is unique to the Archean cratons. Interplay of domains is apparent throughout the region except in the southern Basin and Range, where OIB domains predominate. This is consistent with the existence of (1) upwelled asthenosphere in the south, (2) thicker lithosphere (EM2) elsewhere beneath the Proterozoic crust, and (3) an Archean keel (EMl) beneath Wyoming and Montana. The depleted MORB mantle (i.e., DMM) is believed to represent the sub‐Proterozoic lithospheric wedge that existed prior to its modification by subduction processes during production of one of the enriched mantle domains (i.e., EM2). This is consistent with the close spatial association of the DMM and EM2 domains in the xenolith record. Certain aspects of tectonic history are believed to have been responsible for the lateral continuity of mantle domains. Stabilization of crustal regions in the Archean may have helped mature EMl domains in the underlying lithosphere. Under Proterozoic terrains, subduction may have been responsible for the production of EM2 domains at the expense of preexistent DMM domains. Similarly, asthenospheric upwelling (OIB) in the Phanerozoic appears to have locally transformed the sub‐crustal lithosphere with the partial eradication of the DMM and EM2 domains.