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Current Research Projects
Most of my research continues to focus on either the petrology and geochronology of igneous rocks in the Pacific Northwest or on the geochemistry of water and sediment of lakes in the south Puget Sound area. This page contains brief summaries of these projects and links to some publications and posters.
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 and produced many of the features we see today, including the Cascades and the Coast Ranges. 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. 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
How was the Modern Cascade Arc Established? Igneous activity associated with the modern Cascade Range began ~45 million years ago, shortly after accretion of the Siletzia terrane. Following that collision the subduction zone shifted to a new location west of the Siletzia rocks 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.
Tepper & Clark (2024) Initiation of the Cascade Arc
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 time. This project is only in its beginning stages and is hampered somewhat by the lack of data from older Cascade rocks, but preliminary data suggest that over the past ~45 Ma the mantle wedge has become progressively more enriched in elements (e.g., LILE) that are transported in slab-derived fluids.
Cryan & Tepper (2019) AGU Abstract - Early Cascades Lavas in Washington
Sullivan & Tepper (2021) GSA Abstract - Early Cascades Lavas in Central Oregon
Behavior of Metals in Lake Sediment. For almost 100 years (1895 - 1984) Tacoma was the site of the ASARCO smelter, which for most of its history specialized in producing copper from arsenic-rich ores. Emissions from the smelter's 900-foot tall stack contaminated a >1000 km2 area with Cu, Pb, As, Zn, and other metals. Evidence of this air-borne metal pollution is clearly seen in sediment of local lakes but in many cases the highest metal concentrations are found in sediment that was deposited after the smelter had curtailed or even ended operations. I think this 'mismatch' is evidence that metals can be mobile within the sediment, migrating upwards toward the sediment-water interface as a result of changing redox conditions. Documenting that remobilization is responsible for the metal profiles we see and understanding why this process occurs in some lakes but not others is an ongoing project.
Nutrient Loading and Hazardous Algal Blooms in Local Lakes. Many lakes in the Tacoma area are experiencing more frequent and more severe blooms of toxic algae. The main cause of this problem is increased anthropogenic loading of nutrients - primarily phosphorus. However, the sources of nutrients and the response of the system varies greatly from one lake to another. My students and I have been working, mainly at Waughop Lake and Spanaway Lake, to understand where phosphorus comes from and also to assess methods that may be used to mitigate the problem. Currently I am doing experiments to measure the adsorption capacity of different materials that can be used to remove phosphorus from water.
Tepper et al (2013) WALPA Poster - Phosphorus Loading at Waughop Lake
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