V23C-01
Structural Indicators for the Emplacement of Cretaceous Plutonic Complexes from the Southern Peninsular Ranges Batholith, Mexico
According with the premise that all plutons are nearly circular in plan view in almost any environment, we interpret pluton-related structures (PRS) from LandsatTM images in Baja California, between latitudes 28 and 29N. In this region the NW regional trending of the Peninsular Ranges Batholith changes to be E-W. The presence and shape of the plutons is inferred from the fracture distribution, which parallels compositional zoning, or can be related to the basic stress anisotropy. It is common that nested or concentric discrete bodies form plutonic complexes. It is proposed from a PRS curve-density map that the ridges are zones of higher rates of magma accumulation and their axis can be correlated with magma flow direction. The largest, named Main Ridge, is 30 km long, trending E-W. Because magma moves laterally at shallow depths, we interpret broad areas where lateral movements of magma promote the development of second-order ridges that occupy cortical discontinuities. Three Cretaceous plutonic complexes (U-Pb ages of zircons), showing different structures, lithology and associations with the enclosing rocks were selected for their comparative analyses. From west to east: Punta Prieta (PP; 128.1 +/- 2.1Ma), Nuevo Rosarito (NR; 108 +/- 2.2 Ma) and two from La Rinconada (LR; 113 +/- 2.2 Ma and 102.4 +/- 1.5 Ma). PP is located in the Pacific coast; NR and LR are in the western and southwestern edge of the Main Ridge, respectively. Excepting in PP, vertical fractures (longitudinal) tend to be parallel to the ridges. In PP, the sense of flow is inferred from the 290° trending of vertical fractures, dikes and magmatic foliation, together with conspicuous shearing planes trending 283/73 and 292/78 developed in the plutonic and enclosing rocks, respectively. In NR vertical fractures, magmatic foliation, permeation structures and foliation of the enclosing rock (metabreccia of 139.5 +/- 2 Ma) show a remarkable NNW orientation, parallel to a second order ridge; in the two plutons of LR vertical fractures, magmatic foliation and dikes trend 300° and more than 80 percent of the horizontal fractures average 294/21. For more than 50 km, the complexes located in the crest and western edges of the Main Ridge show a tectonic foliation trending 311/78. This foliation is almost absent in the northern edge of the Main Ridge. It is proposed that the distribution of the plutonic complexes is the result of almost E-W cortical discontinuities.
V23C-02
Evolution of the Late Cretaceous-Paleogene Cordilleran arc magmatism in NW Mexico: a review from updated geochronological studies.
During most of the Mesozoic and Cenozoic, the locus of subduction related arc magmatism in northwestern
Mexico was relatively mobile, probably due to changes in the mechanical conditions of the Farallon-North America
plate convergence. The older Mesozoic events recognized in this region occurred in the Late Triassic and
Jurassic, but the associated rocks are poorly preserved. However, a belt of Late Cretaceous through Paleogene
magmatic rocks is well exposed along Baja California, Sonora and Sinaloa. Since the late 70's, it was noted that
during the Early Cretaceous the igneous activity along this belt remained relatively static in the westernmost part,
but migrated eastward in the Late Cretaceous, penetrating more than 1000 km into the continent. The arc
magmatism reached western Sonora at about 90 Ma, and then it started to move faster inland, presumably due to
flattening of the subducted oceanic slab. Recent U-Pb zircon data revealed unexpected old ages (89-95 Ma) near
the eastern edge of Sonora, which are difficult to explain on the basis of the classic tectonic interpretations. A
model based on two synchronic sites for magma emplacement may explain the age overlapping observed along
the belt; however, a profound re-evaluation a proper geodynamic scenario to support this model is required. Even
if restoration of the large Neogene crustal extension is made, particularly for central and northern Sonora, the
relatively flat-subduction regime commonly accepted for the Laramide event appears unable to explain the
anomalously broad expression of the magmatic belt in northwestern Mexico. An alternative model based on two
synchronic sites of magma emplacement, as suggested by the new age data, may better explain the large
volume of igneous rocks produced during this time in Sonora and most of Chihuahua. This mechanism may
differ southwards in Sinaloa, where the magmatic belt becomes considerably narrower. Moreover, the possible
existence of two spatially distinct sites for magma generation may help understand the post-Laramide volcanism,
commonly interpreted as a result of a fast return of a single magmatic arc to the trench, due to a progressive
steepening of the subducted oceanic slab.
http:www.geologia-son.unam.mx
V23C-03
Reassessing Magmatic Space-Time-Composition Patterns in the Colorado Mineral Belt, USA
The Colorado Mineral Belt (COMB) is a northeast trending zone of Late Cretaceous to Early Tertiary (~75-50Ma) magmatism that accompanied the development of basement cored uplifts of the Laramide orogeny in Colorado. The origin of COMB magmatism remains enigmatic despite decades of study, largely because the magmatism coincided temporally with subduction at the western margin of North America but occurred some 1000 km inboard of the active trench. Many workers have attributed COMB magmatism to progressive shallowing of the oceanic lithosphere of the Farallon plate subducting beneath this region and have suggested that COMB magmatism was generally related to lower crustal melting (Simmons and Hedge, 1978, Stein and Crock, 1990). Others have attributed the COMB magmatism to upwelling of passive hot spots resulting from lithospheric deformation (Mutschler et al., 1987, 1998). Radiogenic isotopic data available for basaltic COMB volcanic clasts found in the Late Cretaceous Windy Gap member of the Middle Park Formation in northern Colorado (Farmer and Larson, unpublished data) support a mantle rather than crustal origin for COMB parental magmas. Further insights into the origin of the COMB clearly require a better understanding of the factors controlling the space-time-composition patterns in the magmatism. However, data compiled in the North American Volcanic and Igneous Rock Database (NAVDAT) illustrate the difficulty in assessing age patterns within the COMB. Little high quality age information is currently available for the COMB igneous rocks, and taken as a whole, little obvious space-time patterns can be discerned (Karlstrom and Humphreys, 1998). But when only the highest quality age determinations are used (including unpublished Ar-Ar ages from graduate theses), there is support for a progressive younging in igneous activity from the central COMB (~65 Ma) to northern COMB (~55 Ma; Cunningham et al., 1994). Cessation of COMB magmatism coincides with the onset of alkaline magmatism in the Black Hills of eastern Wyoming and western South Dakota (58-45 Ma; Duke and Singer, 2002). Recent plate reconstructions (Saleeby, 2003) reveal that the COMB trend is co-parallel with Farallon-North American motion and lies inboard of known area of continent affected by low angle subduction. Based on these observations we suggest an alternative hypothesis in which both the COMB and Black Hills magmatism ultimately represent the product of decompression melting of sublithospheric mantle produced by convective instabilities in the mantle that were induced above and/or along the margins of shallowly subducting oceanic lithosphere. Testing of this model will require additional age and chemical data from COMB igneous rocks, along with numerical modeling of mantle flow induced by the shallowly subducting oceanic lithosphere.
V23C-04
New insights into large volume rhyolite generation at the mid-Tertiary Sierra Madre Occidental Province, Mexico, revealed by U-Pb geochronology
The voluminous (~390,000 km3) and prolonged (~18 myr) explosive silicic volcanism of the mid Tertiary Sierra Madre Occidental of Mexico is generally considered to have formed by fractional crystallisation from crustally contaminated andesitic parental magmas (AFC), with <20% crustal contributions. Evidence for larger crustal contributions has been constrained by the lack of isotopic variation among the lower crustal xenoliths and coeval SMO rhyolite and basaltic andesite to andesite volcanic rocks. Here, we use zircon age populations as probes to assess crustal involvement in Sierra Madre Occidental silicic magmatism. Laser ablation ICP-MS analyses of zircons from rhyolitic ignimbrites located at the northeastern and southwestern sectors of the province yield U-Pb ages that are generally consistent with previously obtained K/Ar ages from these areas. However, zircon xenocrysts with new overgrowths in some of the oldest rhyolite ignimbrites from the northeastern sector provide direct evidence for some involvement of Proterozoic crustal materials, and potentially of more importance, the derivation of zircon from Mesozoic and Eocene age, and isotopically primitive subduction related igneous basement. The youngest rhyolitic ignimbrites from the southwestern sector show even stronger evidence for inheritance in the age spectra but lack old inherited zircon (ie. Eocene or older). Instead, inherited grain ages range between ~20-32 Ma in the southern and youngest Sierra Madre Occidental ignimbrites that have eruptive ages of ~18 and 25 Ma; these inherited zircon ages suggest much of the zircon in the youngest rhyolites was derived by remelting of igneous rocks formed during earlier phases of Sierra Madre Occidental volcanism. The incorporation of zircons derived from earlier phases of rhyolite generation may indicate that the crustal source regions had become overwhelmed by coeval igneous underplate and/or the locus of crustal melting had migrated to shallower crustal levels by the terminal stages of magmatism. The evidence for involvement of young and isotopically primitive crustal materials via the Late Mesozoic to Oligocene age zircon xenocrysts have important implications for how we interpret mantle-like isotopic compositions in rhyolites, as generally observed for the Sierra Madre Occidental. Strong zircon undersaturation, and estimations for very rapid dissolution rates of entrained zircons preclude the coeval mafic magmas as being parental to the rhyolite magmas via assimilation and crystal fractionation (AFC) processes. A greater role for crustal anatexis is indicated such that long-lived basaltic fluxes into the crust may result in the early onset of the recycling of newly formed igneous crustal materials.
V23C-05
Mantle Source Volumes and the Origin of Mid-Tertiary Ignimbrite Flare-up in the Southern Rocky Mountains, Western U.S.
Voluminous intermediate to silicic composition volcanic rocks were generated throughout the southern Rocky Mountains, western U.S., during the mid-Tertiary -ignimbrite flare-up", principally at the San Juan (SJVF) and Mogollon-Datil (MDVF) volcanic fields. At both volcanic centers, existing radiogenic isotope data have been interpreted as evidence that 50% or more of the volcanic rock (by mass) was derived from mantle-derived, mafic parental magmas. However, unlike other portions of western North America affected by major mid-Tertiary magmatism, existing xenolith and seismic data suggest that thick mantle lithosphere (>100 km) was present beneath the southern Rockies in the mid-Tertiary and remains present there today. As a result, basaltic magmas parental to the mid-Tertiary volcanism in this region were unlikely to have been generated by decompression melting of upwelling, normal potential temperature, sublithospheric mantle. The main alternative possibility is that the basaltic magmas were generated by conductive melting of -lithospheric" mantle that had been hydrated and refrigerated by oceanic lithosphere that subducted at a low angle beneath the interior portions of the continent during the Late Cretaceous/Early Tertiary Laramide Orogeny. Melting of this hydrated -lithospheric" mantle was then triggered in the mid-Tertiary by exposure to upwelling sublithospheric mantle during slab roll- back. To test this possibility, we generated first-order estimates of the volumes of basaltic magma, and of the mantle source volume needed to produce those magmas, required to fuel the ignimbrite flare-up in the southern Rockies. Conservative estimates of the volume of mantle that must have partially melted to supply the MDVF and SJVF volcanism are ~2Mkm3 and ~7Mkm3, respectively. If derived by conductive heating of the base of pre-existing mantle lithosphere during the ~20 m.y. duration of volcanic activity at these centers, as a consequence of upwelling of -normal" potential temperature mantle (1290°C), then these estimated mantle source volumes require that the lower ~20 km of the mantle lithosphere beneath the entire southern Rocky Mountains have partially melted during the mid-Tertiary. Such widespread melting of lithospheric mantle implies that this mantle was uniformly fertile and primed for melting and so must have experienced widespread hydration and refrigeration during early Tertiary low angle subduction. However, the mafic magmas must also have been focused laterally within the lithosphere for distances of up to ~300 km into each major volcanic center. Restricting mantle melting to a smaller footprint removes the problem of long lateral magma transport distances to each major volcanic center, but requires that upwelling sublithospheric mantle have contributed a significant proportion of the parental basaltic magmatism, which, in the absence of significant lithospheric thinning, would require that the upwelling mantle have had high (-excess") potential temperatures.
V23C-06 INVITED
Post-Subduction Volcanism in the Baja California Peninsula, Mexico: The Effects of Tectonic Reconfiguration in Volcanic Systems.
Late Cenozoic volcanism in the Baja California Peninsula records the effects of cessation of subduction at a previously convergent plate margin. Prior to 12.5 Ma the predominant volcanic activity had a calcalkaline signature. After 12.5 Ma, during the period of tectonic transition from a convergent to a strike-slip boundary, the style and composition of the magmatic products changed dramatically. It has been proposed that the origin of post-subduction "anomalous" magmas (i.e., adakites, Nb-enriched basalts, Mg-andesites) is linked to regional tectonic events such as the subduction of an active ridge, the break off of the subducted slab or the thermal equilibration of the stalled slab. None of these models, however, included information concerning the spatial- temporal or other physical characteristics of the volcanic products. Here we suggest an alternative model based on the concept of volcanic systems, in which the stress field and the tensile strength of the rock overlying the zones of partial melting plays a much more active role in controlling the occurrence of post-subduction volcanism than previously envisaged. Our model therefore allows us to reconcile the apparent contradictory observations concerning the spatial-temporal distributions of post-subduction volcanism in Baja California, and highlights the role played by tectonic reconfiguration.
V23C-07 INVITED
Miocene to Recent Volcanism in NE Baja California and its Correlation to Adjacent Regions
This paper reviews the status of knowledge of volcanic rocks from part of NW Mexico. Rocks of the Puertecitos Volcanic Province, NE Baja California, span the time period from subduction-related volcanism (the "early to middle Miocene arc" of Gastil et al.) through to modern rift-related volcanism. Ages and major element geochemistry of the principal rock packages were summarized by Martin-Barajas and colleagues in various publications. Additional geochronology and paleomagnetic studies were done by Lewis (1994) and Nagy (1997) to characterize the ages, magnetic polarities, and distributions of the major pyroclastic flow deposits and lavas. The arc-related rocks consist of voluminous epiclastic material of 20-17 Ma and local vents of mafic to intermediate lavas as young as 15.5 Ma. These were eroded prior to deposition of the ca. 12.5 Ma Tuff of San Felipe, a peralkaline high-Si rhyolite pyroclastic flow deposit that is inferred to indicate post-subduction volcanism. Intermediate lavas were erupted in this region ca 9 Ma, followed by a 6.4-6.1 Ma sequence of rhyolite domes and ignimbrites. In the southern Puertecitos Volcanic Province, another pulse of volcanism from 3.3 to 2.7 Ma produced a series of at least 20 thin, high-temperature pyroclastic flow deposits. Volcanism continued with Pliocene and Quaternary andesites and more evolved lavas. This volcanic history is compared to that of surrounding regions. The 20-15 Ma arc rocks are partly younger than similar rocks farther north in the Baja California peninsula and are coeval with volcanic arc rocks farther south. Northward, the centers decrease in frequency in map view; volumes decrease and the relationship of these lavas to coeval lavas near the border (e.g., Alverson Fm in S. California) is not clear. This may be an effect of the northern limit of the corresponding subduction zone. The ca. 6 Ma volcanism is related to rifting of the northern Gulf of California, particularly due to its structurally controlled location (an accommodation zone in the rift system). The ca 3 Ma pulse of volcanism has been related to a "ridge jump" type event (relocation of the plate boundary from the Lower Tiburon basin to the Lower Delfin Basin, within a single spreading segment of the Pacific-North America rift). Both the 6 Ma pulse and the 3 Ma pulse thus seem to be controlled by local processes rather than by regional events. The ca. 12.5 Ma Tuff of San Felipe erupted before the Gulf opened, when Baja California and Sonora were adjacent; the likely vent location is on the modern Sonoran coast north of Bahia de Kino. Work by Oskin (2002), and ongoing studies, allow outcrops of this unit to be correlated over a modern distance of at least 430 km from NE Baja California to east of Hermosillo, Sonora. It has been included by Vidal-Solano and others (2005) as part of a significant episode of post-subduction peralkaline volcanism in Sonora, attributed to regional extension and lithospheric thinning.
V23C-08 INVITED
What Controls Space-Time Patterns of Magmatism in Western North America: Plate Tectonics, Delamination, or Convection?
Mesozoic and Cenozoic magmatism in western North America is commonly explained by shallowing and
steepening of subduction along the west coast of North America, and progressive destruction of the subduction
system by development of the San Andreas transform fault system. This hypothesis makes several specific
predictions about space-time patterns of magmatism, including eastward and westward sweeps, development of
slab-window magmatism, and progressive northward extinction of an ancestral Cascade arc. However, analysis
of space-time patterns using the NAVDAT database indicates that these predicted patterns are curiously obscure
in the magmatic record, although other unexplained patterns are strong.
Animation of about 29,000 Cenozoic U.S. points from NAVDAT (www.navdat.org) demonstrates that: (1) calc-
alkaline, intermediate volcanism is poorly linked to the subduction system; (2) there is little evidence for slab-
window magmatism; (3) there was no ancestral Cascade arc south of Oregon until ca. 10 Ma; (4) magmatism
shifted from primarily silicic to dominantly basaltic throughout the Miocene; and (5) magmatism was clearly
migratory in several directions in ways that cannot be explained by plate-tectonic processes, at length scales
ranging from 1000s to 10s of km.
Space-time patterns that cannot be readily linked to plate-tectonic control include: (1) a silicic sweep from
Montana into Nevada from 50 to 20 Ma; (2) a clockwise sweep around the Colorado Plateau from New Mexico to
southern Nevada from about 30 to 15 Ma; (3) a burst of magmatism at about 16 Ma in northern Nevada, followed
by outward sweeps to Yellowstone, Oregon, and the Sierra Nevada; (4) progressive encroachment of basaltic
magmatism onto the Colorado Plateau, and (5) several local migrations, including from Phoenix north onto the
Colorado Plateau and from the San Francisco Bay area north to the Geysers geothermal field. These migrations
typically occurred at 20-50 mm/yr. Possible origins include convective upwelling related to extension, local,
migrating delamination of the North American lithosphere or subducted Farallon plate, edge-driven convection at
lithospheric discontinuities, and forced convection as the subducted Mendocino fracture zone scraped under
North America.
http:www.navdat.org