Description: Continental breakup represents the successful process of rifting and thinning of the continental lithosphere, leading to plate rupture and initiation of oceanic crust formation. Magmatism during breakup seems to follow a path of either excessive, transient magmatism (magma-rich margins) or of igneous starvation (magma-poor margins). The latter type is characterized by extreme continental lithospheric extension and mantle exhumation prior to igneous oceanic crust formation. Discovery of magma-poor margins has raised fundamental questions about the onset of ocean-floor type magmatism, and has guided interpretation of seismic data across many rifted margins, including the highly extended northern South China Sea margin. Here we report International Ocean Discovery Program drilling data from the northern South China Sea margin, testing the magma-poor margin model outside the North Atlantic. Contrary to expectations, results show initiation of Mid-Ocean Ridge basalt type magmatism during breakup, with a narrow and rapid transition into igneous oceanic crust. Coring and seismic data suggest that fast lithospheric extension without mantle exhumation generated a margin structure between the two endmembers. Asthenospheric upwelling yielding Mid-Ocean Ridge basalt-type magmatism from normal-temperature mantle during final breakup is interpreted to reflect rapid rifting within thin pre-rift lithosphere.

Description: Drill cores recovered during several ODP and IODP Expeditions offshore Central America contain an extensive Early Cenozoic ash layer record. These ash layers have been deposited by plinian eruptions that originated either at the Central American Volcanic Arc (CAVA) or at the Galápagos Hot Spot. While plinian eruptions are well known from the CAVA, volcanism from the Galápagos region is dominantly recorded in effusive and strombolian deposits from subaerial and submarine eruptions although rare large explosive eruptions of evolved trachytic or dacitic compositions did occur in the Pleistocene (e.g., Geist et al., 1994).We have established a tephrostratigraphy from recent through Miocene times from the unique archive of ODP/IODP sites offhore Central America in which we identify tephra source regions by geochemical compositions of the glass shards. Thus we found numerous CAVA-derived tephra layers characterized by typical arc signatures (e.g., Nb-Ta troughs, LILE enrichments), but more surprisingly also an extensive record of tephra layers mostly of Miocene age featuring ocean island geochemical compositions (e.g., low La/Nb and Ba/La ratios, high Nb/Rb ratios). At this geographical setting the only plausible source for these layers is the Galápagos archipelago. Such Miocene ash layers occur in the cores of ODP Sites 1039, 1241, and 1242. At IODP Site U1381, on the Cocos Ridge offshore Costa Rica, 67 primary Miocene (~8 Ma to ~16.5 Ma) fallout ash layers have been recovered. Inferred transport distances of at least 50to 450 km from their vents imply Plinian eruptions, although two-thirds of the ash beds formed from basaltic magmas and only one-third from rhyolitic magmas that are typically associated with plinian eruptions. Our age model for Site U1381 based on sediment accumulation rates, 40Ar/39Ar dating and biostratigraphic ages, reveals a distinct increase in eruption frequency at around 14 Ma. We interpret this as an increase in magma production rates due to changes in interactions between Galápagos plume and spreading ridge.

Description: Highlights • Dating 400 ka paleoclimate record of Neotropics. • Revision and new eruptive volumes for large Central American eruptions. • Age models for Petén Itzá sediments. • Linking lacustrine ash inventory to eruptions from Central America and Mexico. Abstract Lake Petén Itzá, northern Guatemala, lies within a hydrologically closed basin in the south-central area of the Yucatán Peninsula, and was drilled under the auspices of the International Continental Scientific Drilling Program (ICDP) in 2006. At 16°55′N latitude, the lake is ideally located for study of past climate and environmental conditions in the Neotropical lowlands. Because of its great depth (〉160 m), Lake Petén Itzá has a record of continuous sediment accumulation that extends well into the late Pleistocene. A key obstacle to obtaining long climate records from the region is the difficulty of establishing a robust chronology beyond ∼40 ka, the limit of 14C dating. Tephra layers within the Lake Petén Itzá sediments, however, enable development of age/depth relations beyond 40 ka. Ash beds from large-magnitude, Pleistocene-to-Holocene silicic eruptions of caldera volcanoes along the Central American Volcanic Arc (CAVA) were found throughout drill cores collected from Lake Petén Itzá. These ash beds were used to establish a robust chronology extending back 400 ka. We used major- and trace-element glass composition to establish 12 well-constrained correlations between the lacustrine tephra layers in Lake Petén Itzá sediments and dated deposits at the CAVA source volcanoes, and with their marine equivalents in eastern Pacific Ocean sediments. The data also enabled revision of eight previous determinations of erupted volumes and masses, and initial estimates for another four eruptions, as well as the designation of source areas for 14 previously unknown eruptions. The new and revised sedimentation rates for the older sediment successions identify the interglacial of MIS5a between 84 and 72 ka, followed by a stadial between 72 and 59 ka that corresponds to MIS4. We modified the age models for the Lake Petén Itzá sediment sequences, extended the paleoclimate and paleoecological record for this Neotropical region to ∼400 ka, and determined the magnitude and timing of CAVA eruptions.

Description: International Ocean Discovery Program (IODP) Hole U1436A (proposed Site IBM-4GT) lies in the western part of the Izu fore-arc basin, ~60 km east of the arc-front volcano Aogashima, ~170 km west of the axis of the Izu-Bonin Trench, 1.5 km west of Ocean Drilling Program (ODP) Site 792, and at 1776 meters below sea level (mbsl). It was drilled as a 150 m deep geotechnical test hole for potential future deep drilling (5500 meters below seafloor [mbsf]) at proposed Site IBM-4 using the D/V Chikyu. Core from Site U1436 yielded a rich record of Late Pleistocene explosive volcanism, including distinctive black glassy mafic ash layers that may record large-volume eruptions on the Izu arc front. Because of the importance of this discovery, Site U1436 was drilled in three additional holes (U1436B, U1436C, and U1436D), as part of a contingency operation, in an attempt to get better recovery on the black glassy mafic ash layers and enclosing sediments and to better constrain the thickness of the mafic ash layers. IODP Site U1437 is located in the Izu rear arc, ~330 km west of the axis of the Izu-Bonin Trench and ~90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll, at 2117 mbsl. The primary scientific objective for Site U1437 was to characterize “the missing half of the subduction factory”; this was because numerous ODP/Integrated Ocean Drilling Program sites had been drilled in the arc to fore-arc region (i.e., ODP Site 782A Leg 126), but this was the first site to be drilled in the rear part of the Izu arc. A complete view of the arc system is needed to understand the formation of oceanic arc crust and its evolution into continental crust. Site U1437 on the rear arc had excellent core recovery in Holes U1437B and U1437D, and we succeeded in hanging the longest casing ever in the history of R/V JOIDES Resolution scientific drilling (1085.6 m) in Hole U1437E and cored to 1806.5 mbsf. The stratigraphy at Site U1437 was divided into seven lithostratigraphic units (I–VII) that were distinguished from each other based on the proportions and characteristics of tuffaceous mud/mudstone and interbedded tuff, lapilli tuff, and tuff breccia. The section is much more mud rich than expected, with ~60% tuffaceous mud for the section as a whole (89% in the uppermost 433 m) and high sedimentation rates of 100–260 m/My for the upper 1320 m (Units I–V). The proportion (40%) and grain size of tephra are much smaller than expected for an intra-arc basin, composed half of ash/tuff and half of lapilli tuff of fine grain size (clasts 〈 3 cm). These were deposited by suspension settling through water and from density currents, in relatively distal settings. Volcanic blocks are only sparsely scattered through the lowermost 25% of the section (Units VI and VII, 1320–1806.5 mbsf), which includes hyaloclastite, in situ quench-fragmented blocks, and a rhyolite peperite intrusion (i.e., proximal deposits). The transition from unconsolidated to lithified rocks occurred progressively; however, sediments were considered lithified from 427 mbsf (top of Hole U1437D) downward. Alteration resulted in destruction of fresh glass from ~750 mbsf downward, but minerals are less altered. Because of the alteration, the deepest biostratigraphic datum was at ~850 mbsf and the deepest paleomagnetic datum was at ~1300 mbsf. Additional age control deeper than this depth is provided by an age range of 10.97–11.85 Ma inferred from a nannofossil assemblage at ~1403 mbsf and a preliminary U-Pb zircon concordia intercept age of 13.6 +1.6/–1.7 Ma, measured postcruise on a rhyolite peperite in Unit VI at ~1390 mbsf. Based on the seismic profiles, the Miocene–Oligocene hiatus (~17–23 Ma) was predicted to lie at ~1250 mbsf, but strata at that depth (Unit V, 1120–1312 mbsf) are much younger (~9 Ma), indicating that we recovered a thicker Neogene section of volcaniclastics and associated igneous rocks than anticipated. Our preliminary interpretation of shipboard geochemistry is that arc-front versus rear-arc sources can be distinguished in the upper, relatively distal 1320 m of section (Units I–V), whereas the lower, proximal 25% of the section (Units VI–VII) may be geochemically heterogeneous, suggesting that the rear-arc magmas only fully compositionally diverged after ~13 Ma.

Description: This thesis is a cumulative work based on four independent manuscripts. All studies use tephrostratigraphy and geochemistry as tools for identifying the emplacement processes and/or provenance of marine tephra layers. The study areas are located at the Pacific Ring of Fire and used drill core samples from ODP and IODP Legs. Papers 1 and 2 (Schindlbeck et al., 2013 and Kutterolf et al., 2014) deal with tuffaceous sandstones, which were recovered during IODP Expedition 322 in the Nankai Trough. These packages are part of the Late Miocene middle Shikoku Basin Facies. Geochemical homogeneity and density-graded structures (pumice enrichment at the top, lithic enrichment at the bottom) support the emplacement by large volume, high-energy turbidity currents, which are each the product of a major volcanic eruption. Major and trace element, and isotope compositions of glass shards indicate a Japanese mainland source. Therefore we propose the collision zone of Paleo-Honshu and the Izu-Bonin-arc as possible source region, which implies ~350 km transport distance from the source to Site 322. Manuscripts 3 and 4 (Schindlbeck et al., submitted and Schindlbeck et al., in preparation) study ODP and IODP cores offshore the southern part of the Central American Volcanic Arc (CAVA). Six sites offshore Costa Rica recovered a well-preserved tephrostratigraphy from the Miocene to the Pleistocene. The subduction-related volcanism at the CAVA is known for highly explosive Plinian eruptions. These eruptions generate eruption columns, which rise up to 40 km into the stratosphere. When reaching their level of neutral buoyancy, they spread laterally with the prevailing wind and generate widely dispersed ash deposits covering areas up to 〉106 km2. If an ash cloud travels over oceans, the ash is deposited as marine ash layers, which are best preserved, since the marine environment is relatively non-erosive. The marine tephra inventory is therefore a very good archive for highly explosive Plinian eruptions. ODP and IODP cores recovered a well-preserved stratigraphy of Plinian eruptions derived from the CAVA from Miocene to Pleistocene, but also an extensive record of mainly Miocene tephra layers originated at the Galápagos hotspot. These layers from Plinian eruptions from the Galápagos are particularly interesting, since such abundant Plinian eruptions from this oceanisland setting were not described before, and the onland tephra record on the Galápagos islands is limited to Quaternary deposits. To conclude, the manuscripts presented in this study emphasize the advantage of marine tephrostratigraphy as a useful tool in volcanology. The well-preserved marine tephra layers (1) complement onland volcanic records, (2) expose previously undiscovered volcanic activity (e.g., Galápagos), (3) provide insights and comprehension of geological processes and (4) help to understand the reasons of variability of volcanic activity in time.