Current Petrology / Tectonics Research Projects
My ongoing "hard rock" research focuses primarily on the petrology and geochronology of volcanic and plutonic rocks of the Cascade arc. This page contains brief summaries of these projects and links to some publications and posters.
How was the Modern Cascade Arc Established?
Igneous activity associated with the modern Cascade Range began ~46 million years ago, shortly after accretion of the Siletzia terrane. Following that collision the subduction zone was reestablished in a new location west of Siletzia, but the question is: how did that happen? It is commonly assumed that the subduction zone "jumped" westward, but this would require initiation of subduction in a place where the oceanic lithosphere was young, hot, and buoyant, which is problematic. Ken Clark and I have developed an alternative model in which subduction beneath Washington was re-established by northward movement of Farallon slab that was already subducting beneath California. We published this model in the following GEOLOGY paper:
Tepper & Clark (2024) Initiation of the Cascade Arc
Origin of the Burch Mountain Adakites
Burch Mountain, located just north of Wenatchee in central Washington, is the site of several small adakite intrusions of Miocene age. Although these rocks lie well east of the main Cascade arc, they are part of that magmatic province. However, they stand out as the most isotopically depleted rocks analyzed to date from anywhere in the Cascades (see plot at right) and in fact are among the most depleted rocks known from any arc on Earth.
The main questions we hope to answer in studying these adakites are how such depleted magmas were able to reach the surface and what their location toward the rear of the arc might tell us about flow patterns in the mantle wedge.
Here is a link to a 2024 GSA Abstract
Long-term Chemical Evolution of the Cascade Arc
The Cascades are an ideal laboratory in which to investigate how the chemical and isotopic compositions of subduction-related magmas change over space and time as an arc matures. This will be a long-term project that is only in its beginning stages. However we can already see intriguing patterns. For example, the plot to the right shows that Ba/Nb (a proxy for input of a slab-derived fluid) in Quaternary lavas increases from S to N along the arc in WA. The same pattern is seen in older plutonic rocks, indicating it is not controlled by properties of the oceanic plate.
Cryan & Tepper (2019) AGU Abstract - Early Cascades Lavas in Washington
Sullivan & Tepper (2021) GSA Abstract - Early Cascades Lavas in Central Oregon
Understanding the Cause(s) of the Challis Event
Between 52-45 million years ago the Pacific Northwest experienced widespread magmatism, faulting, and crustal extension that profoundly altered the regional geology. The cause(s) of this activity, referred to as the Challis Event, have been debated since the 1970's. More recently, based largely on new U-Pb dates from granites in eastern Washington, we have proposed a model that attributes the Challis Event to rollback and subsequent breakoff of the Farallon slab. This breakoff laid the groundwork for the subsequent establishment of the Cascade arc. Here are two papers that summarize this work:
Tepper et al (2023) Slab Rollback as the Cause of Challis Event
Kant et al (2018) Slab Breakoff Magmatism in the Washington Cascades