ABSTRACT The mid-Cenozoic succession in the northeast limb of the Mount Diablo anticline records the evolution of plate interactions at the leading edge of the North America plate. Subduction of the Kula plate and later Farallon plate beneath the North America plate created a marine forearc basin that existed from late Mesozoic to mid-Cenozoic times. In the early Cenozoic, extension on north-south faults formed a graben depocenter on the west side of the basin. Deposition of the Markley Formation of middle to late? Eocene age took place in the late stages of the marine forearc basin. In the Oligocene, the marine forearc basin changed to a primarily nonmarine basin, and the depocenter of the basin shifted eastward of the Midland fault to a south-central location for the remainder of the Cenozoic. The causes of these changes may have included slowing in the rate of subduction, resulting in slowing subsidence, and they might also have been related to the initiation of transform motion far to the south. Two unconformities in the mid-Cenozoic succession record the changing events on the plate boundary. The first hiatus is between the Markley Formation and the overlying Kirker Formation of Oligocene age. The succession above the unconformity records the widespread appearance of nonmarine rocks and the first abundant appearance of silicic volcanic detritus due to slab rollback, which reversed the northeastward migration of the volcanic arc to a more proximal location. A second regional unconformity separates the Kirker/Valley Springs formations from the overlying Cierbo/Mehrten formations of late Miocene age. This late Miocene unconformity may reflect readjustment of stresses in the North America plate that occurred when subduction was replaced by transform motion at the plate boundary. The Cierbo and Neroly formations above the unconformity contain abundant andesitic detritus due to proto-Cascade volcanism. In the late Cenozoic, the northward-migrating triple junction produced volcanic eruptive centers in the Coast Ranges. Tephra from these local sources produced time markers in the late Cenozoic succession.

Abstract The Mesozoic of Nuevo Leon is composed of more than 10,000 m of sedimentary rocks displaying abrupt physical changes and containing abundant planktonic foraminifera, allowing precise chronostrati-graphic determinations. This succession has been deposited in the western extension of the ancestral Gulf of Mexico (Mexican Sea). Its paleogeographic setting corresponds to the oblique subduction of the Kula-Far-allon plate under the North American plate. Active oblique subduction of the western margin of Mexico has resulted in the structural features of the Mesozoic sedimentary cover, forming a folded belt made of an intricate array of mountain ranges corresponding to the Nuevo Leones Cordillera characterized by: (1) kilomet-ric-scale anticlinal ridges and narrow synclinal valleys of the Jurassic-Cretaceous sedimentary cover commonly displaying box (fan-shaped) folds; (2) a well-developed pattern of en echelon anticlinal folds; (3) juxtaposition of tectonostratigraphic domains; (4) asymmetrical, overturned, doubly plunging anticlines evolving into faulted anticlines; (5) disrupted, long and sinuous fold trends; (6) lack of large horizontal displacement due to overthrusting; (7) folding being predominant over faulting; (8) local thrusting with opposite vergence; and (9) lack of volcanism and regional metamorphism. Those features are the result of transpressional tectonics since the mid-Jurassic including a greater basement involvement in the tectonic deformation. Analysis of the megastructures exposed in the region using SIR-A and LANDSAT imagery of northeast Mexico revealed that the megastructures of the Mesozoic Cordillera between parallels 22°00' and 26°00' and meridians 99° 00’ and 101°00’ can be referred to deformation of the thin sedimentary cover above transcurrent faults following the morphological pattern of the lab experiment by Odonne and Vialon. The folding styles of the Nuevo Leones Cordillera were analyzed in light of tectonic transpression and wrench-fault tectonics driven by transcurrent faults in the basement as established by Harland, Lowell, and Beck, resulting in the recognition of subsurface faults in the basement. We associate the geometric arrangement of the megastructural trends to sense of displacement of strike-slip (transcurrent) faults and fold patterns as demonstrated by Odonne and Vialon (1983) and the well-known strike-slip tectonic setting of New Zealand as described by Bishop (1967).

Current interpretations of Cretaceous tectonic evolution of the northwest Pacific trace interactions between the Pacific plate and three other plates, the Farallon, Izanagi, and Kula plates. The Farallon plate moved generally eastward relative to the Pacific plate. The Izanagi and Kula plates moved generally northward relative to the Pacific plate, with Izanagi the name given to the northward-moving plate prior to the Cretaceous normal polarity superchron and the name Kula applied to the postsuperchron plate. In this article I suggest that these names apply to the same plate and that there was only one plate moving northward throughout the Cretaceous. I suggest that the tectonic reorganization that has previously been interpreted as formation of a new plate, the Kula plate, at the end of the superchron was actually a plate boundary reorganization that involved a 2000 km jump of the Pacific–Farallon–Kula/Izanagi triple junction. Because this jump occurred during a time of no magnetic reversals, it is not possible to map or date it precisely, but evidence suggests mid-Cretaceous timing. The Emperor Trough formed as a transform fault linking the locations of the triple junction before and after the jump. The triple junction jump can be compared with an earlier jump of the triple junction of 800 km that has been accurately mapped because it occurred during the Late Jurassic formation of the Mesozoic-sequence magnetic lineations. The northwest Pacific also contains several volcanic features, such as Hawaii, that display every characteristic of a hotspot, although whether deep mantle plumes are a necessary component of hotspot volcanism is debatable. Hawaiian volcanism today is apparently independent of plate tectonics, i.e., Hawaii is a center of anomalous volcanism not tied to any plate boundary processes. The oldest seamounts preserved in the Hawaii-Emperor chain are located on Obruchev Rise at the north end of the Emperor chain, close to the junction of the Aleutian and Kamchatka trenches. These seamounts formed in the mid-Cretaceous close to the spreading ridge abandoned by the 2000 km triple junction jump. Assuming that Obruchev Rise is the oldest volcanic edifice of the Hawaiian hotspot and thus the site of its initiation, the spatial and temporal coincidence between these events suggests that the Hawaii hotspot initiated at the spreading ridge that was abandoned by the 2000 km jump of the triple junction. This implies a tectonic origin for the hotspot. Other volcanic features in the northwest Pacific also appear to have tectonic origins. Shatsky Rise is known to have formed on the migrating Pacific-Farallon-Izanagi triple junction during the Late Jurassic–Early Cretaceous, not necessarily involving a plume-derived hotspot. Models for the formation of Hess Rise have included hotspot track and anomalous spreading ridge volcanism. The latter model is favored in this article, with Hess Rise forming on a ridge axis possibly abandoned as a result of a ridge jump during the superchron. Thus, although a hotspot like Hawaii could be associated with a deep mantle plume today, it would appear that it and other northwest Pacific volcanic features originally formed as consequences of shallow plate tectonic processes.

Reconstruction of the tectonic evolution of the Pacific basin indicates a direct relationship between intraplate volcanism and plate reorganizations, which suggests that volcanism was controlled by fracturing and extension of the lithosphere. Middle Jurassic to Early Cretaceous intraplate volcanism included oceanic plateau formation at triple junctions (Shatsky Rise, the western Mid-Pacific Mountains) and a diffuse pattern of ocean island volcanism (Marcus Wake, Magellan seamounts) reflecting an absence of any well-defined stress field within the Pacific plate. The stress field changed in the Early Cretaceous when accretion of the Insular terrane to the North American Cordillera and the Median Tectonic arc to New Zealand stalled migration of the Pacific-Farallon and Pacific-Phoenix ocean ridges, leading to the generation of the Ontong Java, Manahiki, Hikurangi, and Hess Rise oceanic plateaus. Plate reorganizations in the Late Cretaceous resulted from the breakup of the Phoenix and Izanagi plates through collision of the Pacific-Phoenix ocean ridge with the southwest margin of the basin and development of island arc–marginal basin systems in the northwestern part of the basin. The Pacific plate nonetheless remained largely bounded by spreading centers, and intraplate volcanism followed preexisting lines of weakness in the plate fabric (Line Islands) or resulted from fractures generated by ocean ridge subduction beneath island arc systems (Emperor chain). The Pacific plate began to subduct under Asia in the Early Eocene as inferred from the record of accreted material along the Japanese margin. Further changes to the stress field at this time resulted from abandonment of the Kula-Pacific and the North New Guinea (Phoenix)–Pacific ridges and from development of the Kamchatkan and Izu-Bonin-Mariana arcs, leading to the generation of the Hawaiian chain as a propagating fracture. The final major change in the stress field occurred in the Late Oligocene as a result of breakup of the Farallon into the Cocos and Nazca plates, which caused a hiatus in Hawaiian volcanism; initiated the Sala y Gomez, Foundation, and Samoan chains; and terminated the Louisville chain. The correlations with tectonic events are compatible with shallow-source models for the origin of intraplate volcanism and suggest that the three principal categories of volcanism, intraplate, arc, and ocean ridge, all arise from plate tectonic processes, unlike in plume models, where intraplate volcanism is superimposed on plate tectonics.

Cretaceous through Paleogene dominantly calc-alkaline magmatism in the Lake Clark region, southwestern Alaska, can be divided into three main periods of activity based on previous work and new 40 Ar/ 39 Ar and U-Pb geochronology. These episodes follow a protracted history of arc magmatism related to the Talkeetna arc. The first episode is Albian based on new 40 Ar/ 39 Ar dates from hornblende in dikes cutting the Tlikakila complex of 101–97 Ma. These rocks have relatively flat REE patterns and Th/Nb values lower than typical arc rocks but higher than average mid-ocean ridge basalt (MORB). These data are consistent with rift-related magmatism in a region that had previously experienced arc magmatism. The second episode is represented by a granodiorite with a SHRIMP U-Pb zircon age of 64.5 ± 0.9 Ma and is similar to widespread 70–60 Ma volcanic rocks to the west of the study area. These calc-alkaline rocks are enriched in the light REE and have high Th/Nb values. They were generated during northward subduction of the Kula plate beneath Alaska. The Eocene episode is also calc-alkaline with enriched light rare earth element (REE) and high Th/Nb values. 40 Ar/ 39 Ar equilibria in 100 Ma dikes were reset at 44–42 Ma, and biotite from a rhyolite indicates an eruptive age of 43.0 ± 0.2 Ma. These 43 Ma rocks formed during initiation of Aleutian magmatism on the continent in response to north or northwest subduction of the Pacific plate and may be related to a change in the direction or velocity of subduction at this time.