Abstract The objective of this review is to introduce some organic analytical methods that may be used to study organic matter in ore deposits. It is not a comprehensive guide to all techniques. The underlying theme is to guide the interested reader toward an analytical strategy that is appropriate for the information sought. Some of the background issues that an analyst has to consider are initially introduced, with a brief summary of the thermal maturation of organic matter. Following an introduction to sample preparation, the remainder of the chapter summarizes general and spectroscopic methods that provide a general indication of organic composition, followed by physical techniques that ultimately yield information at a molecular scale. Each analytical technique will be introduced, followed by its applications. Where applicable, a case study will be described. The hydrothermal alteration of organic matter from immature precursors to mature products is used as an analog of a process responsible for the maturation of organic matter found in ore deposits. The intention is to indicate how information obtained from these analytical methods can be used to solve problems stemming from the occurrence of organic matter in ore deposits. In order to understand the applicability and limitations of organic analytical techniques, it is first necessary to understand the forms in which organic matter can be present in and around ore deposits (see Giże, 2000; and Leventhal and Giordano, 2000). The simple reason is that all organic analytical methods are constrained by the organic matter form, such as phase (solid, liquid, or gas) and, often, by the molecular structure and molecular weight. To date, a single analytical method that provides a complete analysis of organic matter exists only in science fiction films. Consequently, part of the analyst' s consideration is in rendering the organic material into a form that can be analyzed by the techniques and instruments available without losing the information sought.

Abstract The first draft of this chapter was written in 1977. I had intended it to be an introduction to my doctoral dissertation on the tectonic evolution of the southern Gulf of California, but I had the good sense not to include it there because my academic minor was in the history of science and I didn’t want to defend the historical rigor of what follows here. When the opportunity arose to include it in this memoir I thought, why not? This will probably be as good a place as any to hide it from the eyes of serious historians. So I rewrote it and added to it, but it is still more sea story than history. I will briefly sketch about 600 years of history, including about 200 years of the history of Spanish exploration off northwest Maxico and there are entire libraries that specialize in that subject. I visited one such library at Berkeley and found it to be frequented by scholars. They seemed to be quiet, rather serious people carrying around huge boxes full of 3x5 reference cards. But, my interest in Spanish exploration only began in 1975, just 2 years before I wrote the first draft of this chapter, and I don’t feel competent to analyze or criticize conflicting historical details found in the various sources. So, since this story isn’t likely to be of much use to historians, the obvious question becomes: What do 16th and 17th century Spaniards have to do with plate tectonics or

Abstract A new bathymetric map of the Gulf of California and surrounding region has been produced. The map incorporates previously published data along with a large volume of new data collected on cruises conducted as part of a cooperative effort between Oregon State University and the General Directorate of Oceanography of the Mexican Navy. It provides some new detail on the west side of the peninsula and offshore of southern Mexico. The map consists of two sheets, which can be spliced together. The tracklines for the various cruises are printed on the backs of the maps and can be viewed by using a light table. The map encompasses the entire province at one scale, on one kind of projection, and at one contour interval.

Abstract We have prepared gravity anomaly maps of the onshore-offshore region of northwest Mexico and southern Alta California. Plates 3 and 4 of this volume show free-air gravity anomalies at sea and simple Bouguer anomalies on land, contoured at a 10 mGal interval. We present eight modeled geophysical sections which cross the peninsula of Baja California, the Gulf of California, the Salton Trough, the Laguna Salada, and mainland Mexico. Three additional geophysical sections cross the Rivera Fracture Zone. The northern sections show a crust about 12 km thick beneath the outer Continental Borderland. The crustal thickness increases to about 17 km beneath the inner Borderland and reaches about 28 km maximum thickness beneath the northern Peninsular Ranges. The maximum crustal thickness beneath the central and southern Peninsular Ranges is about 22 km. The crust beneath the Salton Trough and the Laguna Salada thins to about 21 km minimum. Beneath the northern Gulf of California the crust thins to 13 km minimum and under the central Gulf it is about 10 km thick. In both the northern and central Gulf the thinnest crust is found near the peninsular coast. Beneath the southern Gulf, the crust is about 8 km thick and is no longer asymmetrical. A low density upper mantle (3.10 to 3.15 gm/cm 3 ) is found beneath the entire Gulf. Its volume increases to the south. Based on onshore geology, section densities, and the mapped anomalies, we trace Franciscan-like rocks continuously beneath the western margin of the peninsula and borderland. Similarly, Mesozoic metamorphic and batholithic rocks that make up the northern Peninsular Ranges continue under the Tertiary volcanics of the central and southern Peninsular Ranges. High density units (2.82 to 2.90 gm/cm 3 ) are emplaced in the upper crust beneath parts of the borderland and Bahia Sabastiân Vizcaino. Mapped anomalies show continuations of transform faults beneath the sediment cover on the flanks of the Gulf, and reveal linear basement highs and lows under the west side of the peninsula. Mapped anomalies also emphasize the continuity of basement highs and lows in the borderland. The ridges and troughs of the Rivera Fracture Zone are reflected in the crustal structure at depth. The crust thins underneath the ridges and thickens beneath the troughs, probably caused by alteration of the crust and upper mantle by faulting and hydrothermal activity.

Abstract Using Oregon State University and Mexican Navy cruise data, along with maps previously published by other workers, we have compiled a set of regional maps of the marine magnetic anomalies and oceanic crustal isochrons off northwest Mexico and southern Alta California. We then examined magnetic anomaly profiles obtained in and near the mouth of the Gulf of California to determine spreading rates between the North American, Pacific, Rivera, and Cocos plates. The post-4 Ma Pacific-North American spreading rate in the mouth of the Gulf of California, measured using magnetic anomaly profiles, is either 66 or 49 km/m.y., depending on which of two sets of subjective anomaly correlations is preferred. The post-4 Ma Pacific-Rivera spreading rate over the Rivera Rise increases along the ridge from 51 km/m.y. near the Tamayo Fracture Zone to 56 km/m.y. near the Rivera Fracture Zone. From this linear rate-of-change of spreading we determine that the Pacific-Rivera instantaneous pole of relative motion is about 21 arc-degrees northeast of the middle of the Rivera Rise. Early Gulf spreading on the northern Rivera plate at the Maria Magdalena Rise may have been diffuse, characterized by multiple axes of extension. The post-1.9 Ma Pacific-Cocos spreading rate south of the Rivera Fracture Zone is about 88 km/m.y. After a ridge-jump at about 5 Ma and prior to 1.9 Ma, the spreading rate here was only about 69 km/m.y. Prior to about 5 Ma, the spreading rate was about 76 km/m.y. Thus, although there may have been a change in the overall spreading rate coincident with the ridge-jump, it appears that about 3 m.y were still required to completely transfer Pacific-Cocos spreading from the Mathematicians Rise to the East Pacific Rise.

Abstract Lineaments in marine gravity anomalies, bathymetry, and magnetic anomalies, along with previously mapped onshore faults, have been used to construct a Seismo-Tectonic Map of the region extending from Lázaro Cardenas in Michoacan, Maxico, to north of Point Conception in Alta California. Earthquake epicenters taken from the NOAA/NGDC Hypocenter Data File for all events with M b >4.2 occurring between 1963 and 1985 are also shown on the map. We have tried to identify the principal faults and lineaments of the province. The structural pattern that emerges for the Gulf of California, the California Borderland, and the Pacific margin of Baja California is similar to that seen onshore in southern Alta California and northern Baja California. Present-day shear between the Pacific and North American plates is distributed across a fairly wide zone of subparallel crustal slices. In Mexico, where this zone includes most of the Gulf crust, more than one extensional axis commonly occurs at the same plate rotational latitude. Earlier faulting along the Pacific side of the peninsula and in the outer Borderland appears to have occurred in a similar style. In the southernmost Gulf the shear is presently less widely distributed; in this area several horsts of continental crust, which are relicts of early rifting, are now fixed to the North American plate. Gravity anomalies show that the Tosco-Abreojos Fault Zone may have been much wider and more discontinuous than indicated by shallow seismic-reflection data. The eastern part of the Rivera Fracture Zone and the East Pacific Rise north of lat. 17.5°N are reorienting in a small but very complicated area where the boundaries of five plates and crustal blocks nearly coincide. We identify the seaward extensions of the Colima Graben and the Tepic-Chapala Fault Zone, which bound the Jalisco tectonic block.

Abstract The Gulf of California is one of a group of structures which have resulted from the coincident westward dilation of North America and the dextral motion between the Pacific and North American plates. During the early history of the Gulf, this dilation occurred above a subduction zone. We have subdivided this large province into 22 structural domains, each with a contrasting geologic history and have divided these histories into four intervals. The earliest of these is prior to 23 Ma, when the first nonmarine strata ponded in the initial depression. The period from 23-13 Ma was the premarine interval of arc volcanism; 13-5 Ma was the Protogulf era; and 5 Ma to the Present was the era of the modern Gulf. The earlier intervals, characterized by large elevational changes and dilation unrelated to rhombochasmic opening, contrast with the recent interval characterized by passive strike-slip translation south of the Transverse Ranges.

Abstract Most of the structural patterns revealed by bathymetric and magnetic mapping of the oceanic crust west and south of southern California and Baja California are inherited from the time of crustal accretion on the northern East Pacific Rise. Since the Oligocene this spreading center has separated the Pacific plate from several narrow east-flank plates, and their changing motions are recorded by reorientation of spreading axes, appearance and disappearance of spreading-center offsets, and varying speeds and directions of migration of the offsets. Parts of most of the east-flank plate fragments avoided subduction by being captured by the Pacific plate, and survive along the continental margin, surrounded by fossil spreading centers and transform faults and by a fossil trench alongside the North American continent. Many volcanoes have been superimposed on the mainly fault-produced primary structure. Some of these seamounts are in short rows built close to spreading axes, especially near sites of maximum upwelling to the accreting plate boundary. Other seamounts belong to the trail of a major hotspot, which produced large alkali-basalt cones episodically from 20 to 5 Ma, and may also have built rows of smaller tholeiitic shields aligned with the main alkali-basalt chain. This hotspot may now be affecting continental crust of the peninsula. The revised, more detailed geologic history of the oceanic crust also has implications for Californian continental geology, especially for helping to explain patterns of subaerial volcanism and the temporal and spatial distribution of strike-slip faulting.

Abstract In terms of the distribution of pre-Neogene rocks, the California Continental Borderland and associated coastal areas can be divided into the inner Borderland and the outer Borderland, each of which is floored with juxtaposed Mesozoic to Paleogene sedimentary sequences and a coeval melange resembling the Great Valley Sequence-Franciscan Complex couple of central and northern California. The stratigraphy of the Borderland shows local differences, and only the contemporary basins preserve more complete stratigraphic records that can be compared with each other. Seismic sequence analyses provide a basis for inferring that the Patton, Tanner, Santa Catalina, Santa Monica, San Pedro basins and San Diego Trough are floored with Franciscan-type basement, covered by Miocene to Holocene sediments. The Santa Cruz, San Nicolas, and Santa Barbara basins are underlain by thick Mesozoic to Paleogene sequences draped by Miocene strata and filled with Paleocene to Holocene sediments. Generally speaking, Mesozoic to Paleogene sequences are composed of shallow- to deep-water siliciclastic deposits. The Miocene sequences are dominated by volcaniclastic and biogenic sediments, and the Pliocene to Holocene sequences are dominated by deep-water mass-flow deposits. Based on the available chrono-stratigraphic information, a preliminary interbasinal stratigraphic correlation can be established. The tectono-stratigraphic history of the Borderland can be accounted for by the interactions between the East Pacific Rise and the North American continent. During Mesozoic to early Tertiary times, the Borderland was a part of an Andean-type continental margin characterized by the coeval accumulation of fore-arc basin sequences and trench melange. After the East Pacific Rise impinged on the North American continent in the late Oligocene, transform tectonism gradually replaced subduction as the triple junctions passed along the continent edge. In the middle Miocene, wrench processes transposed a piece of continental margin northwestward to form the outer Borderland, and caused formation of Borderland basins and deposition of widespread Miocene volcaniclastics and San Onofre-type sediments. After the major transform activity jumped to the present San Andreas fault in the late Miocene, tectonism dwindled in the offshore area and the residual transform forces gradually forged the present basin-and-ridge configuration. Available stratigraphic information indicates that the Cretaceous and Eocene apparently were periods of high sea stand, as evidenced by extensive marine incursion in the coastal area, whereas the Paleocene and Oligocene are periods of relative low sea stand. The Miocene is a period of high sea stand marked by extensive biogenic pelagic sedimentation. The Miocene-Pliocene boundary is characterized by a transition from biogenic sedimentation to mass-flow deposition. A period of slow mass-flow sedimentation in the late Pliocene is attributed to reduced terrigenous sediment input during the sea-level rise.

Abstract Detailed marine geophysical surveys of the inner California Continental Borderland west of northern Baja California show that the region is underlain by two major northwest-trending Quaternary dextral wrench fault systems. The San Clemente fault system lies along the western part of the inner borderland and comprises the San Clemente and San Isidro fault zones. Together, these fault zones connect to form a long (>300 km), narrow (<5 to 10 km), continuous zone of shear similar to the longer San Andreas transform system onshore. The Agua Blanca fault system is a complex northwest-trending zone of dextral shear delineated by three or more subparallel wrench fault zones in the eastern part of the inner borderland. The westernmost, San Diego Trough-Bahia Soledad fault zone, consists of relatively long (-50 km), continuous main fault traces which cut the Quaternary sediments of the nearshore basin trough. The Coronado Bank-Agua Blanca fault zone is more complicated, with numerous discontinuous, subparallel, right- and-left-stepping, en echelon and anastomosing fault traces which are associated with substantial structural relief. A nearshore zone of faulting, marked by the Estero-Descanso fault zone in the south and the Newport-Inglewood-Rose Canyon fault zone in the north, parallels the coast and defines the eastern boundary of the California Continental Borderland structural province. All of these eastern fault zones merge into the transpeninsular Agua Blanca fault, and their N30°W trend differs significantly (>20°) from the trend of the major Peninsular Ranges fault zones. The ridge-and-basin physiography of the inner borderland and transtension evident along the Agua Blanca fault system show that dextral oblique rifting has dominated the late Cenozoic tectonic style of the inner California Continental Borderland. Systematic differences in the post-Miocene style of deformation, with extension in the south, right-slip in the center, and convergence in the north, imply that Transverse Ranges convergence is affecting inner borderland tectonism. Historical earthquake activity shows that the inner borderland is an active part of the present-day Pacific-North American plate boundary, with focal mechanisms generally consistent with the tectonism inferred from the geologic structure. Counterclockwise rotation of a semirigid “Southern California Shear Zone,” which is the splintered northern end of the long, narrow, Baja California microplate, explains the observed patterns of deformation within the region. Because significant right-slip may occur to the west of Baja California along the San Clemente fault system, and does not cross the peninsula to the Gulf of California transform system, estimates of Pacific-North American relative plate motion based on sea-floor-spreading rates at the mouth of the Gulf of California may be in error.

Abstract Sea Beam provides a new tool for accurately mapping sea-floor morphology. Recent maps compiled from Sea Beam data collected in the California Continental Borderland show many sea-floor geomorphic features that are similar to tectonic landforms observed along subaerial fault zones. In particular, numerous features associated with recently active strike-slip faults have been mapped along the offshore San Clemente-San Isidro, San Diego Trough-Bahia Soledad, and Coronado Bank-Agua Blanca fault zones. Linear scarps, trenches and ridges, hillside benches and valleys, and small closed depressions are aligned along what are inferred to be active traces of these major offshore fault zones. Other nontectonic sea-floor geomorphic features observed include submarine canyons and channels; these may be truncated, offset, or deviated by tectonic movements in the area. Detailed study of such features may provide data necessary to determine the displacement and slip-rates on submarine faults.

Abstract A new Bouguer gravity anomaly map has been produced for northern Mexico using approximately 15,000 new gravity stations observed over the last 10 years. An analysis of the trend surfaces of elevations and Bouguer gravity anomalies were used to predict regional Bouguer gravity anomalies from topography resulting in residual/isostatic maps. From these maps, no recognizable offset is observed in the location of a proposed Mojave-Sonora megashear, although a crustal boundary is indicated by a change in trend or truncation of various features in northwestern Sonora. Also these maps show possible trends of trench-related features in the Baja region which continue the length of Baja and apparently are not offset by major transverse structures. Three crustal models were generated using previous seismic control. These models indicate that the crust in the Gulf of California region is approximately 10 km thick, whereas the crust in the Sierra Madre Occidental is more than 50 km thick. The lithosphere is approximately 10 km thick over the Baja rift and more than 95 km thick in the area of the Sierra Madre Occidental.

Abstract The geologic evolution of southern California and the spreading in the mouth of the Gulf of California are known well enough to constrain a model for the formation of the Gulf of California. The San Andreas fault has been increasingly active during the past ~17 m.y., culminating at its present slip rate of ~35 mm/yr by 4 to 5 Ma. Another ~15 mm/yr of right-lateral shear is inferred to exist on recently activated faults near the southern California coast. These include the predominantly strike-slip, northwest-trending, transpeninsular Agua Blanca and Elsinore faults and the east-trending zone of convergence within the western Transverse Ranges. The total relative plate rate across the southern California region is modeled at ~53 mm/yr, which is comparable to global kinematic plate model rates. At the Tamayo spreading center in the mouth of the Gulf, magnetic anomaly patterns record a constant spreading rate of ~48 mm/yr for the most recent ~4 m.y., though evidence exists for a proto-gulf prior to 10 Ma. Our present understanding of the deformation in the vicinity of southern California is difficult to reconcile with a plate rate near 48 mm/yr, suggesting 5 to 7 mm/yr of additional deformation near the mouth of the Gulf, probably largely the result of continental extension on the southeastern side of the mouth of the Gulf. We suggest the following model for the formation of the Gulf of California. From 28 to 17 Ma, most of the relative plate motion occurred near the continental rise, although active strike-slip faulting within the southern California Borderland is inferred by the rotation of the crust that is now the western Transverse Ranges. From 17 to 4.5 Ma, the proto-gulf developed through oblique rifting in direct relation to the increasingly active San Andreas system. About 150 km of San Andreas-related shear occurred during this time. Shear across the Borderland and rotation of the western Transverse Ranges continued. From 4.5 to 1 Ma, both the Gulf and the San Andreas fault were fully active. Transpeninsular faults are thought to have supplied Borderland shear related to the continuing rotation of the western Transverse Ranges, initially on faults located south of those currently active. During the past million years, the transpeninsular shear has been confined to the northernmost portion of the peninsula and, as a result, rotation of the western Transverse Ranges greatly diminished in rate or ceased and high rates of contraction initiated there. This history is more protracted than most workers have proposed, and represents a migration of Pacific-North America plate-related activity steadily from the continental rise into the North American continent.

Abstract This chapter summarizes Mesozoic and Cenozoic paleomagnetic data from southern and Baja California that document significant latitudinal and rotational displacements of large-scale blocks within this region. Much of the displacement is associated with two large allochthonous terranes—the Santa Lucia allochthon and the Peninsular Ranges terrane—that accreted to the North American craton during the Tertiary. The Santa Lucia allochthon amalgamated at equatorial paleolatitudes during the Mesozoic, traveled northward relative to North America between 70 and 50 Ma, and accreted to North America in the Eocene. The Peninsular Ranges terrane amalgamated along the western margin of southern Mexico and Central America during the Mesozoic and early Tertiary, traveled northward relative to North America some time after 40 Ma, and accreted to North America and the Santa Lucia allochthon by the early Miocene. The suture zone between the Peninsular Ranges terrane and the Santa Lucia allochthon is marked by 90° clockwise rotations of structural blocks within these terranes. Other structural blocks in southern California have also been rotated, either as part of the tectonic processes associated with terrane accretion, or as part of later regional extension and right-lateral strike-slip motion along southern California faults.

Abstract The western margin of the Sierra Cucapa is defined by the Laguna Salada fault, a complex zone of active oblique-dextral slip faults. The northwestern section of the fault zone consists of a single fault strand which has had an oblique-dextral sense of slip throughout the late Pleistocene and Holocene. The southeastward continuation of this single fault strand has been comparatively inactive during the Holocene. Slip during the late Quaternary has instead been stepped several hundred meters to the southwest onto two distinct oblique-dextral fault strands. The easterly strand extends southeastward and intersects the Cañon Rojo fault, a northwest-dipping normal fault which has been active throughout much of the late Quaternary. This zone of normal slip and other northwest-dipping normal faults have transferred dextral slip from the Laguna Salada fault zone during the late Quaternary to several other oblique-dextral zones successively farther to the southwest. This connected series of northwest-striking oblique-dextral and northwest-dipping normal fault zones collectively forms a series of dilational right steps that are responsible for basin subsidence and pull-apart within Laguna Salada. Geomorphic evidence suggests that pull-apart was not continuous during this period and ceased at least once within the northern part of the Laguna Salada basin.

Abstract In the short historical record of northern Baja California, the largest earthquake has a magnitude 7.1. Most of the earthquakes with M L > 6.0 occur along the Cerro Prieto, Imperial, and San Miguel faults; no significant seismicity is known to have occurred along the Agua Blanca fault. A large part of the microseismicity (M L < 3.0) is sporadic, dominated by swarms, and in good correlation with mapped faults. Seismogenic depths are restricted to the upper part of the crust. Two seismic zones are defined by the microseismicity of the Mexicali-Imperial Valley; this activity is mostly clustered in the basement. Typical focal mechanisms are consistent with a strike-slip, right-lateral motion, striking northwest. This is the case for the last four events of M L > 6.0 in the Salton Trough. Focal mechanisms for the seismic zones and the head of the Gulf of California are mainly of a strike-slip and dip-slip nature.

Abstract Southern California and the northern part of the peninsula of Baja California form a common region affected by a number of regional-scale active faults, all constituting part of the San Andreas-Gulf of California fault system. The Cerro Prieto-Imperial fault is located in the Mexicali-Imperial Valley, and the Cucapa and the Laguna Salada faults constitute the southern extension of the Elsinore fault. South of the Laguna Salada are two lineaments, the El Chinero and the San Felipe. The second extends southward, connecting with an extensional region near Puertecitos. West of the two lineaments, the San Pedro Martír fault and the Sierra de Juarez fault constitute part of the Main Gulf Escarpment. In the peninsula of Baja California, two main active fault systems are recognized—the Agua Blanca fault, which is oriented anomalously counterclockwise to the general trends of the southern San Andreas fault system, and the San Miguel-Vallecitos faults, which extend from the Sierra de Juarez escarpment toward the Tijuana-San Diego area. Both the Agua Blanca and San Miguel-Vallecitos faults extend offshore and connect with active faults in the Continental Borderland. All these faults are seismically active to different degrees as a result of the interaction between the Pacific-North American plates in the Gulf of California region. Although in the northern Gulf the amount of displacement measured between plates is less than in the southern Gulf, the difference in movement (2.5 to 3.0 cm/yr) can be spread along the strike-slip faults on the peninsula and those on the Continental Borderland. A structural relationship should exist between the Guaymas lineament and the Puertecitos extensional region, which is the conduit where movement is induced to the onshore peninsular faults.

Abstract Magmatic events in the Baja peninsula-Gulf of California region are closely related to sequential tectonic regimes of: (1) late Tertiary subduction beneath the continental margin until ~16 to 12.5 Ma; and (2) continental to oceanic rifting that began about 13 Ma. Orogenic calcalkaline volcanic rocks formed subparallel belts of rhyolite ignimbrite of Oligocene age (~34 to 27 Ma) east of the Gulf in the Sierra Madre Occidental and andesite of Miocene age (~24 to 11 Ma) along the eastern Baja peninsula. Shutoff of the Miocene andesitic arc broadly corresponds to migration of the Pacific-Farallon (Guadalupe)-North America triple junction along the Baja peninsula. Orogenic andesitic volcanism ended at ~16 Ma in northern Baja, and at ~11 Ma in southern Baja, within ~1 to 2 Ma of cessation of subduction. Waning orogenic magmatism persisted along the southern Baja peninsula during the initial stages of rifting. Paleogeographic implications of the distribution of circum-Gulf volcanic rocks and their inferred easterly source areas suggest that, from about 13 to 8 Ma, the rift consisted of a narrow seaway along the eastern side of the present Gulf. By about 6 Ma, the Gulf had broadened westward to approximately its present width. The period from 13 to 10 Ma was a time of tectonic transition and magmatic diversity. During this interval, medium-K calcalkaline, high-K calcalkaline, alkalic, and tholeiitic magmas erupted from the central to southern part of the Baja peninsula-Gulf region, and from about 14 to 8 Ma, rhyolite ignimbrite erupted in the northern Gulf region. Since 10 Ma, volcanism on the western Baja peninsula has been dominated by alkalic magmas, the Gulf margins by calcalkaline magmas, and the Gulf by tholeiitic magmas. Postsubduction calcalkaline andesite to rhyolite erupted sporadically and locally along the northern Gulf margins from ~9 Ma through Holocene time. Alkalic magmas associated with rifting in the Gulf of California are unlike alkalic magmas from intraplate settings. They comprise nepheline- to quartz-normative, basaltic to andesitic rocks characterized by diverse trace element ratios. They are broadly similar to intraplate alkalic rocks in having high K, P, Ba, Sr and the light REE (rare earth elements), but have some trace element characteristics typical of orogenic magmas (low Nb and Ta relative to K, Ba, Sr, and La) that distinguish these alkalic rocks from intraplate or cratonic rift alkalic rocks. Rb abundances (mostly <25 ppm, as low as 5 ppm) are unusually low in these alkalic rocks, both in contrast to the high K (~1.7-4.1 wt% K 2 O), Ba (~800-2200 ppm), and Sr (~1000-3700 ppm), and in comparison to Rb abundances in mafic lavas in general; many samples have Rb at levels typically found in low-K tholeiites. The high K/Rb ratios, mostly ~1000 to 5000, are among the highest reported for any lavas. Samples ranging in SiO 2 from 48 to 61% have Mg’ ≥ 65, indicating that a broad range of compositions may represent primary melts. The generally high Mg’, and the low Rb, Th, Nb, and Ta in some lavas, suggest that depleted refractory mantle equivalent to a mid-ocean ridge basalt (MORB) source contributed to these melts. The distinctive trace element enrichments are attributed to the stabilization in the source of amphibole and apatite from metasomatic fluids related to prior subduction beneath the peninsula. In discrimination diagrams, these alkalic rocks typically cluster outside previously defined fields for various magma types. The common tectonic setting among these and geochemically similar alkalic suites is the occurrence in a continental margin that is undergoing extension, located along a recently or currently active convergent plate boundary. Tholeiitic basalts have erupted since the earliest stages of continental rifting through development of oceanic spreading centers. Gulf of California tholeiites exhibit an evolution distinguished particularly by differences in rare-earth-element (REE) abundances. Early-rift magmas are similar in trace element geochemistry to some intraplate tholeiites from ocean islands and continental flood basalts, and have convex-upward REE patterns. The most recent tholeiites from the East Pacific Rise in the mouth of the Gulf are equivalent to N-MORB (normal mid-ocean ridge basalt) from other mid-ocean rifts, and are depleted in the light-REE. The transition in tholeiite geochemistry from intraplate tholeiite to MORB is attributed to progressive fusion of more refractory mantle components. Incipient-rift tholeiites are derived from selective fusion of clinopyroxenite veins, and with continued melting, N-MORB are formed by fusion of peridotite. This tholeiite sequence reflects the evolution of mantle source regions in rifts that sustain an ensialic to oceanic transition.