Graduate Thesis Or Dissertation
 

Resurgent Uplift of Samosir Island in Toba Caldera, Indonesia : Insights from Lake Sediment Paleomagnetism and C-14 Age Determination

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/dv140031s

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  • Around 74 ka, a supervolcano, Toba Caldera in Sumatra, Indonesia erupted, producing the Youngest Toba Tuff and its associated caldera. After this catastrophic eruption, a lake filled the caldera, sedimentation within the lake occurred, and the process known as resurgence began. Today, the resurgent dome, Samosir Island, is uplifted 700 m above the lake with the upper 100 m composed of post-eruption lake sediments. These sediments and their ages offer insight to the resurgent uplift history. To constrain sediment chronology, uplift style and rates, we collected 173 discrete paleomagnetic 8 cm3 cubes and 45 radiocarbon samples from 11 sites around the island. Bulk organic 14C ages provide an initial chronostratigraphic framework, which is improved by correlating paleomagnetic signals between site sections. 24 radiocarbon samples have been dated, with the oldest dating at ~46 ka. Natural and laboratory magnetizations on discrete samples were studied using alternating field (AF) demagnetization at the Oregon State University P-Mag Lab. Although there is variability in magnetic susceptibility between study sites and natural remanant magnetization intensities are often relatively low (~10-4 (A/m)), AF demagnetization behavior suggests a primary magnetization is recorded. Characteristic remanent magnetizations are reasonably well-defined using a principal component analysis with maximum angular deviation values < 10°, though stronger samples typically have better resolved magnetizations. An age model is created from 6 sites across the island with 14C ages ranging from ~12 ka to 46 ka. Low inclination values, averaging around -7.16°, are observed, shallower than the geocentric axial dipole prediction for the site location of approximately 4°. However, this is consistent with the negative inclination anomaly associated with this region. The age model and 14C ages suggest uplift rates are faster to the east and slower to the west indicating that Samosir was uplifted as a trapdoor that was locally segmented into fault blocks that moved differentially to each other.
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