It has been suggested that the Shatsky Rise oceanic plateau formation began simultaneously with a reorganization of spreading at a triple junction bordering the northern Pacific plate, and this coincidence has led to speculation about the connections between the two events. We present new marine geophysical data that constrain the seafloor spreading history of the Pacific-Izanagi-Farallon triple junction just before the birth of the Shatsky Rise. Bathymetric data reveal en echelon, abandoned spreading centers trending northwest-southeast located adjacent to the southwest flank of the Shatsky Rise. Magnetic anomalies and bathymetry are interpreted to indicate that segments of the Pacific-Farallon Ridge near the triple junction propagated northwest from chron M23 (153 Ma) to chron M22 (151 Ma) during a spreading ridge reorganization at the edge of a likely microplate. Our detailed examination of bathymetric and magnetic anomaly lineations also shows that the strike of the Pacific-Izanagi Ridge changed gradually on the west side of the triple junction around chron M22. Our observations indicate that the plate boundary reorganization began several million years before the formation of the Shatsky Rise, implying that the eruption of the plateau did not cause the reorganization.

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 Lyra Basin is believed to be a contiguous part of the Ontong Java Plateau (OJP), based on geophysical studies. Volcaniclastic rocks dredged at two sites in the Lyra Basin document another post-plateau episode of magmatism on the OJP; they are olivine-titanaugite-phyric alkali basalts with as much as ~30% modal phenocrysts. Lyra Basin basalts have compositions that vary from picritic (MgO ~22 wt%) to more evolved (MgO ~5 wt%) and have low SiO 2 (41–46 wt%), high TiO 2 (2–4 wt%), and high Na 2 O + K 2 O (1–5 wt%) contents that are distinctly different from tholeiites that compose the main OJP. The 40 Ar- 39 Ar weighted mean age of Lyra Basin basalts is 65.3 ± 1.1 Ma, determined using a single-grain laser fusion method of the ground-mass from the least altered alkali basalt and of biotite separates from differentiated samples. This age is interesting because it is much younger than the main stage of OJP formation (122 Ma) and no ca. 65 Ma alkaline basalts have been found previously near or on the OJP. Incompatible trace element modeling suggests that the volcanic rocks of the Lyra Basin may have been formed by a low degree of partial melting (~3%), predominantly at the garnet-lherzolite stability field from the same OJP mantle source preserved in its thick lithospheric root. However, major and trace elements and isotopic compositions can be better explained by magma mixing of Rarotongan alkali magma and magma derived from OJP-source mantle melting (12% partial melting at garnet stability field) in the ratio of 1:2. Although the trace element compositions of Lyra basalts can be reproduced by OJP-source mantle melting with or without contribution from the Rarotongan hotspot, the lower potassium content of the calculated Rarotongan hotspot-influenced melt is more compatible with that of an average composition of Lyra basalt. These results are consistent with previous reconstruction of the OJP path from 120 Ma to its present position, indicating that it may have passed over the Rarotongan hotspot at 65 Ma. In either case, the petrogenesis of Lyra Basin basalts highlights the role of the thick lithospheric root of the OJP in the late-stage development of the plateau. Additional evidence for episodic late-stage magmatic activity on the OJP helps to elucidate the magmatic evolution of the plateau and may provide insights into the origins of other large igneous provinces.

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.

Abstract Comparison of oceanic anomaly block models in the M0-M29 interval from the Japanese, Phoenix, Hawaiian and Keathley lineations indicates that the Hawaiian block model represents the closest approximation to a constant spreading rate record. The new Hawaiian block model differs slightly from that of Larson and Hilde (1975). Currently popular numerical age estimates for polarity chrons, base CM0 (121 Ma), CM16-CM15 (137 Ma) and top CM25 (154 Ma), are consistent with constant spreading rate in the new Hawaiian block model but inconsistent with constant spreading in the Larson and Hilde (1975) block model. A new time scale (CENT94) is based on the above ages and constant spreading in the new Hawaiian block model. Land section magnetostratigraphy, mainly from Italy and Spain, has provided direct correlations of polarity chrons to stage boundaries through ammonite biozones, and indirect correlation through nannofossil and calpionellid biozonations: Barremian-Aptian (base of CM0), Hauterivian-Barremian (upper part of CM4), Valanginian-Hauterivian (base of CM1 In), Berriasian-Valanginian (CM15n), Tithonian-Berriasian (base of CM18), Kimmeridgian-Tithonian (CM22A) and Oxfordian-Kimmeridgian (top CM25). These correlations yield the following stage boundary ages using CENT94: Barremian-Aptian (121 Ma), Hauterivian-Barremian (126 Ma), Valanginian-Hauterivian (131.5 Ma), Berriasian-Valanginian (135.8 Ma), Tithonian-Berriasian (141.6 Ma), Kimmeridgian-Tithonian (150 Ma), and Oxfordian-Kimmeridgian (154 Ma).