Basalt of Summit Creek: Eocene Magmatism Associated with Farallon Slab Break Off

Abstract:

In the Pacific Northwest the Early-Middle Eocene was a time of widespread magmatism and tectonic reorganization that included accretion of the Siletzia terrane, Challis volcanism, and establishment of the modern Cascade arc. Although individual events are well documented our knowledge of the underlying tectonic framework is incomplete. To better understand the tectonic changes that occurred during this interval we studied the ~48 Ma Basalt of Summit Creek (BSC), a 1500m section of lavas located south of Mt. Rainier that erupted during the critical time period between the docking of Siletzia and the initiation of the modern Cascade arc.

The BSC consists mainly of tholeiitic basalts (wt. % SiO₂ = 45.54-63.45, Mg# = 0.68-0.30) with EMORB traits (La/Yb_N = 1.2-5.9; ²⁰⁶Pb/²⁰⁴Pb = 19.005-19.102; ²⁰⁷Pb/²⁰⁴Pb = 15.538-15.593; ²⁰⁸Pb/²⁰⁴Pb = 38.560-38.714). These lavas lack arc signatures (e.g., HFSE depletions) but overlap in elemental and isotopic composition with oceanic basalts of the Crescent Formation (part of Siletzia) located ~100 km to the west.

We suggest that emplacement of lavas that lack arc traits in what was the forearc was a response to break off of the Farallon slab, which occurred as a result of the accretion of Siletzia at ~49 Ma (Wells et al., 2014). Break off opened a gap in the subducted slab, allowing upwelling and subsequent decompression melting. BSC lavas are consistent in age, location and composition with this model.

After break off subduction resumed outboard of Siletzia, initiating the Cascade arc. Thus, BSC provides evidence of Farallon slab break off and furthers our understanding of the tectonic transition from widespread magmatism of the Early-Middle Eocene to the Cascade arc.