Tectonic evolution of the transform margin of western North America has attracted great interest. This boundary formed as the fault-fault-trench type Mendocino triple junction has moved north. The tectonic transition is expected to have a significant thermal signature because the subduction zones are quite distinct from any other plate boundary zones with their depressed heat flow in the outer arc regions.

There is a significant amount of heat flow data in most of California covering the Great Valley and Sierra Nevada regions where were part of the outer arc of the Farallon subduction was located. The heat flow in a very large area covering all over the Great Valley and western Sierra Nevada shows the consistent pattern of a recent subduction. The unique pattern of the heat flow distribution indicates the tectonic origin of the data which has a potential to reveal the nature of the tectonic transition after the cessation of subduction 30 Ma. In this collection, the first two papers deal with the interpretation of heat flow data using forward thermal models of the lithosphere.

The two proposed tectonic scenarios, namely the slab window and the stalled slab models, were compared in terms of the thermal consequences. The low heat flow in the Great Valley and the narrow transition zone toward the Coast Ranges rules out opening of a slab window in a conventional sense and is therefore indicative of a stalled slab type transition. Therefore, the high heat flow in the Coast Ranges must be explained by a mechanism other than opening of a slab window beneath the Coast Ranges.

The low (reduced) heat flow in the Western Sierra Nevada which extends to the south end of the region where the subduction ceased more than 15 My ago is important in understating the thermal history of the lithosphere after the cessation of subduction. The thermal data are also in close agreement with the seismic cut-out depth where sufficient seismic activity is present. The numerical thermal model of the region shows that there is a significant lateral component of the heat flow in to the Sierra Nevada due to Basin and Range province and due to basal heating. The model further suggests that the lateral heating results in considerable thermal uplift at the eastern edge of the Sierra Nevada region. The model is also applicable in the Northern Baja California since this region was part of the same tectonic setting as the Sierra Nevada arc before the inland jump of the San Andreas Fault.

The Coast Ranges are interesting with having spots of magmatic and volcanic arc activity likely associated with the cessation of subduction. The third paper is a study related to one of these spots 90 km north of San Francisco, characterized by elevated heat flow in an area of 2500 km² (The Geysers anomaly). The geothermal system is associated with a very young (∼2 My) bimodal volcanism and magma intrusion at crustal levels. Taking advantage of a sealed, vapor dominated geothermal system due to rocks of very low permeability, forward and inverse models of the deeper magmatic source were constructed. We used extensive heat flow data that were collected over more than 20 years time period. The models revealed that the magmatic source in the Geysers must be as shallow as 7-8 km in order to satisfy the thermal data. Furthermore, the magma system must cover most of the thermally anomalous region.

Another type of geothermal system is characterized by rising of hot waters by buoyancy forces without the necessity of a magmatic source at depth. We studied one of these systems in interior Alaska, called Chena Hot Springs, in the fourth paper. The explored system is 1 km long and temperatures are only 74°C. Although the system is moderate in temperature, the low-temperature surface conditions enable the system to be exploited for production of electricity. The geochemical analyses show that the source temperatures are around 121°C. We analyzed the temperature data from 17 exploration wells in order to understand the deep geothermal system better and the location of the target temperatures of 121°C. This system is interesting being a small compared to most Basin-and-Range type of geothermal systems but showing the similar complex flow characteristics.