ABSTRACT The North American Cordillera experienced significant and varied tectonism during the Triassic to Paleogene time interval. Herein, we highlight selected questions and controversies that remain at this time. First, we describe two tectonic processes that have hindered interpretations of the evolution of the orogen: (1) strike-slip systems with poorly resolved displacement; and (2) the closing of ocean basins of uncertain size, origin, and mechanism of closure. Next, we divide the orogen into southern, central, and northern segments to discuss selected controversies relevant to each area. Controversies/questions from the southern segment include: What is the origin of cryptic transform faults (Mojave-Sonora megashear vs. California Coahuila transform fault)? Is the Nazas an arc or a continental rift province? What is the Arperos basin (Guerrero terrane), and did its closure produce the Mexican fold-and-thrust belt? How may inherited basement control patterns of deformation during subduction? Controversies/questions from the central segment include: Can steeply dipping mantle anomalies be reconciled with geology? What caused high-flux events in the Sierra Nevada batholith? What is the origin of the North American Cordilleran anatectic belt? How does the Idaho segment of the orogen connect to the north and south? Controversies/questions from the northern segment include: How do we solve the Baja–British Columbia problem? How big and what kind of basin was the Early Cretaceous lost ocean basin? What connections can be found between Arctic geology and Cordilleran geology in Alaska? How do the Cretaceous tectonic events in the Arctic and northern Alaska connect with the Cordilleran Cretaceous events? What caused the Eocene tectonic transitions seen throughout the northern Cordillera? By addressing these questions along the length of the Cordillera, we hope to highlight common problems and facilitate productive discussion on the development of these features.

ABSTRACT This field trip highlights evidence of late Pliocene–early Pleistocene submarine and subaerial volcanism coeval with marine and marginal-marine sedimentation in the Santa Rosalía Basin in Baja California. The best exposures of these rocks occur at the El Álamo Canyon, which exhibits outcrops of the Tirabuzón and Infierno formations interbedded with submarine and subaerial volcanic and volcaniclastic deposits that are covered by the subaerial ignimbrites of Reforma and Aguajito calderas. Extensive field mapping and stratigraphy carried out in this canyon, aided with 40 Ar/ 39 Ar, and U-Pb geochronology, allowed us to divide the stratigraphy into three main sequences that, from base to top, are: (1) Santa Rosalía succession, (2) the Reforma caldera complex, and (3) El Aguajito caldera. This refined stratigraphy indicates that eight episodes of volcanism occurred between 2.5 and 1.36 Ma, during marine sedimentation in an internal continental shelf and in estuaries, coastal lagoons, or protected bays as supported by the fossil record. This sedimentary and volcanic interbedding suggests transgressions and regressions of the sea level, as well as tectonic uplifting. After the inception of volcanism in the Reforma caldera complex (1.29 Ma), the area emerged from these shallow seas followed by the formation of the Aguajito caldera (1.1 Ma), and then later on by the Tres Vírgenes volcanic complex (0.3 Ma). The last complex erupted a lava flow ca. 22 ka that so far stands as the youngest activity in the region. The magmatic evolution in the region is characterized by post-subduction calc-alkaline magmatism. Such magmatism is expressed as pure calc-alkaline rocks from El Aguajito–Reforma calderas, and as hybrid transitional magmas formed by adakitic rocks from the Tres Vírgenes volcanic complex.

The crustal structure of the Peninsular Ranges batholith can be divided geophysically into two parts: (1) a western mafic part that is dense, magnetic, and characterized by relatively high seismic velocities (>6.25 km/s), low heat flow (<60 mW/m 2), and relatively sparse seismicity, and (2) an eastern, more felsic part that is less dense, weakly magnetic, and characterized by lower seismic velocities (<6.25 km/s), high heat flow (>60 mW/m 2), and abundant microseismicity. Potential-field modeling indicates that the dense, mafic part of the batholith extends to depths of at least 20 km and likely to the Moho. The magnetic anomalies of the western part of the batholith extend south beyond the spatially extensive exposures of the batholith to the tip of the Baja California peninsula, which suggests that the mafic part of the batholith projects beneath Cenozoic volcanic cover another 400 km. The linearity and undisrupted nature of the magnetic belt of anomalies suggest that the western part of the batholith has behaved as a rigid block since emplacement of the batholith. The batholith may have influenced not only the development of the Gulf of California oblique rift, but also strike-slip faulting along its northern margin, and transtensional faulting along its western margin, likely because it is thermally and mechanically more resistant to deformation than the surrounding crust.

At 12.5 Ma, after subduction below the North American plate stops, right-lateral transform motion occurs along the margin between the Pacific and North American plates. The Tosco-Abreojos fault zone, located along the western margin of southern Baja California, has been interpreted as the main transform boundary between both plates until early Pliocene, when the plate boundary was transferred to the Gulf of California, leading to the capture of Baja California Peninsula by the Pacific plate. However, the morphology and the seismic activity of the Tosco-Abreojos fault zone suggest this right-lateral strike-slip motion is still active. The Tosco-Abreojos fault zone is characterized by bathymetric scarps and asymmetric basins filled by recent sediments which are deformed. These observations are compatible with the hypothesis that the motion of the Pacific plate with respect to the North American plate is partitioned, as indicated by kinematic data (GPS versus global models) between the still active Tosco-Abreojos fault zone and the Gulf of California where most of the motion is accommodated. The Baja California Peninsula can thus be considered as an independent block limited to the west by the Tosco-Abreojos and San Benito fault zones and to the east by the Gulf of California transform boundary.

Recent studies on shallow submarine hydrothermal vents (at water depths <200 m below sea level [mbsl]) suggest that their activity could have been responsible for the formation of oxide, sulfide, and precious metal–bearing ores. The boundary between shallow and deep hydrothermal vents has been established at a depth of 200 mbsl, which represents an abrupt change in the environmental parameters and in the structure of the biotic communities. Shallow submarine vents support complex biotic communities, characterized by the coexistence and competition of chemosynthetic and photosynthetic organisms. Some biogeochemical and biomineralizing processes related to chemosynthesis are similar to those described in deep hydrothermal vents and in cold seeps. Frequently, hydrothermal shallow vent water has lower salinity than seawater. This fact, together with the isotopic compositions, is evidence of a meteoric component in vent water. Venting of exsolved gas, evidenced by continuous bubbling, is a striking feature of shallow submarine hydrothermal systems. In most cases, vent gas is rich in CO 2 , but occasionally it can be rich in N 2 , CH 4 , and H 2 S. In México, shallow submarine hydrothermal venting has been studied in Punta Banda and Bahía Concepción, Baja California Peninsula, and in Punta Mita, Nayarit. The tectonic setting of those hydrothermal systems corresponds to continental margins affected by extension, with high geothermal gradients. These vents do not show obvious links with volcanic activity. Their study has contributed to the understanding of mineralogical and geochemical processes in shallow submarine hydrothermal vents. Those systems could be a potential source of geothermal energy.