A substantial portion of the Pacific basin is composed of seafloor formed during the Cretaceous Normal Superchron (CNS). Because this region lacks the magnetic lineations typically required to constrain tectonic reconstructions, we employ additional methods for interpreting CNS Pacific history, involving seafloor fabric, basement paleolatitudes, and age data. We utilize seafloor fabric, including fracture zones and the rift margins of large igneous provinces, to derive quantitative rotations. The timing of such rotations is constrained using rock ages, bounding magnetic isochrons, and estimates of interactions with surrounding terrains. The method relies on high-resolution shipboard bathymetry and rock ages, as much fine-scale seafloor fabric useful for reconstructions is not visible in satellite altimetry data. We show that the Ontong Java, Manihiki, and Hikurangi oceanic plateaus likely originated as one large superplateau, the Ontong Java Nui (OJN). Reconstructions of OJN at 123 Ma reveal large offsets between observed and predicted paleolatitudes. Observed paleolatitudes exhibit a systematic bias, which may be attributed to large-scale rotation of the entire plateau. Such a rotation would imply either that OJN was initially decoupled from the Pacific plate and able to rotate independently or that the orientation of the Pacific plate at 123 Ma differed from conventional model predictions. However, large uncertainties in absolute plate motion models prior to ca. 80 Ma preempt a conclusive interpretation for OJN formation. Given an ~10 km resolution limit for satellite altimetry, continued investments in seagoing research will be needed to investigate tectonic events in magnetic quiet zones.

The Manihiki Plateau in the western equatorial Pacific Ocean is a Cretaceous Large Igneous Province. Several studies have proposed that the Manihiki Plateau was formed by the same mantle plume that formed the Ontong Java and Hikurangi plateaus ca. 125 Ma. Recent multibeam bathymetric surveys of the Manihiki Plateau reveal the morphology of the Danger Islands Troughs (DIT), Suvarov Trough, which are systems of deep troughs within the plateau. The troughs divide the Manihiki Plateau into three distinct provinces, the North Plateau, the Western Plateaus, and the High Plateau. The DIT between the High Plateau and Western Plateaus comprises four en echelon troughs. With one exception, all segments of the DIT are bordered by steep escarpments, to 1500 m high. The basins of the DIT are smooth. Elongated northeast-southwest–striking scarps are common in the southernmost DIT and at the junction between the DIT and Suvarov Trough. The features revealed by the new bathymetric data indicate that a sinistral strike-slip tectonic environment formed the DIT during the break-up into the Manihiki and Hikurangi plateaus, whereas the Suvarov Trough developed after the formation of the DIT.

The few geological and geophysical studies of the Lyra Basin at the western margin of the Ontong Java Plateau (OJP; Pacific Ocean) revealed that it is underlain by thicker than normal oceanic crust. The unusually thick oceanic crust is attributed to the emplacement of massive lava flows from the OJP. Dredging was conducted to sample the inferred OJP crust on the Lyra Basin but instead recovered younger extrusives that may have covered the older plateau lavas in the area. The Lyra Basin extrusives are alkalic basalts with ( 87 Sr/ 86 Sr) t = 0.704513–0.705105, ( 143 Nd/ 144 Nd) t = 0.512709–0.512749, ε Nd (t) = +3.0 to +3.8, ( 206 Pb/ 204 Pb) t = 18.488–18.722, ( 207 Pb/ 204 Pb) t = 15.558–15.577, and ( 208 Pb/ 204 Pb) t = 38.467–38.680 that are distinct from those of the OJP tholeiites. They have age-corrected ( 187 Os/ 188 Os) t = 0.1263–0.1838 that overlap with the range of values determined for the Kroenke-type and Kwaimbaita-type OJP basalts, but their ( 176 Hf/ 177 Hf) t = 0.28295–0.28299 and ε Hf (t) = +7.9 to +9.3 values are lower. These isotopic compositions do not match those of any Polynesian ocean island volcanics. Instead, the Lyra Basin basalts have geochemical affinity and isotopic compositions that overlap with those of some alkalic suite and alnöites in the island of Malaita, Solomon Islands. Although not directly related to the main plateau volcanism at 120 Ma, the geochemical data and modeling suggest that the origin of the Lyra Basin alkalic rocks may be genetically linked to the mantle preserved in the OJP thick lithospheric root, with magmatic contribution from the Rarotongan hotspot.

The plate tectonic processes, or “plate,” model for the genesis of melting anomalies (“hotspots”) attributes them to shallow-sourced phenomena related to plate tectonics. It postulates that volcanism occurs where the lithosphere is in extension, and that the volume of melt produced is related primarily to the fertility of the source material tapped. This model is supported in general by the observation that most present-day “hotspots” erupt either on or near spreading ridges or in continental rift zones and intraplate regions observed or predicted to be extending. Ocean island basalt-like geochemistry is evidence for source fertility at productive melting anomalies. Plate tectonics involves a rich diversity of processes, and as a result, the plate model is in harmony with many characteristics of the global melting-anomaly constellation that have tended to be underemphasized. The melting anomalies that have been classified as “hotspots” and “hotspot tracks” exhibit extreme variability. This variability suggests that a “one size fits all” model to explain them, such as the classical plume model, is inappropriate, and that local context is important. Associated vertical motion may comprise pre-, peri-, or post-emplacement uplift or subsidence. The total volume erupted ranges from trivial in the case of minor seamount chains to ∼10 8 km 3 for the proposed composite Ontong Java–Manihiki–Hikurangi plateau. Time progressions along chains may be extremely regular or absent. Several avenues of testing of the hypothesis are being explored and are stimulating an unprecedented and healthy degree of critical debate regarding the results. Determining seismologically the physical conditions beneath melting anomalies is challenging because of problems of resolution and interpretation of velocity anomalies in terms of medium properties. Petrological approaches to determining source temperature and composition are controversial and still under development. Modeling the heat budget in large igneous provinces requires knowledge of the volume and time scale of emplacement, which is often poorly known. Although ocean island basalt–type geochemistry is generally agreed to be derived from recycled near-surface materials, the specifics are still disputed. Examples are discussed from the Atlantic and Pacific oceans, which show much commonality. Each ocean hosts a single, currently forming, major tholeiitic province (Iceland and Hawaii). Both of these comprise large igneous provinces that are forming late in the sequences of associated volcanism rather than at their beginnings. Each ocean contains several melting anomalies on or near spreading ridges, both time- and non-time-progressive linear volcanic chains of various lengths, and regions of scattered volcanism several hundred kilometers broad. Many continental large igneous provinces lie on the edges of continents and clearly formed in association with continental breakup. Other volcanism is associated with extension in rift valleys, back-arc regions, or above sites of slab tearing or break-off. Specific plate models have been developed for some melting anomalies, but others still await detailed application of the theory. The subject is currently developing rapidly and poses a rich array of crucial but challenging questions that need to be addressed.