Cathy J. Busby

Subaqueous Explosive Eruption and Welding of Pyroclastic Deposits

Silicic tuffs infilling an ancient submarine caldera, at Mineral King in California, show microscopic fabrics indicative of welding of glass shards and pumice at temperatures >500�C. The occurrence indicates that subaqueous explosive eruption and emplacement of pyroclastic materials can occur without substantial admixture of the ambient water, which would cause chilling. Intracaldera progressive aggradation of pumice and ash from a thick, fast-moving pyroclastic flow occurred during a short-lived explosive eruption of ∼26 cubic kilometers of magma in water ≥150 meters deep. The thickness, high velocity, and abundant fine material of the erupted gas-solids mixture prevented substantial incorporation of ambient water into the flow. Stripping of pyroclasts from upper surfaces of subaqueous pyroclastic flows in general, both above the vent and along any flow path, may be the main process giving rise to buoyant-convective subaqueous eruption columns and attendant fallout deposits.

Evolutionary model for convergent margins facing large ocean basins: Mesozoic Baja California, Mexico

Mesozoic rocks of the Baja California Peninsula form a convergent-margin complex that is one of the best-preserved and longest-lived convergent-margin complexes in the world. It shows a three-phase evolutionary trend that we propose is typical of arc systems facing large ocean basins. The trend progresses from phase 1, highly extensional intraoceanic-arc systems, to phase 2, a mildly extensional fringing-arc system, to phase 3, a compressional continental-arc system. This trend is largely due to the progressively decreasing age of lithosphere that is sub- ducted. The modern Earth is strongly biased toward long-lived arc-trench systems, which are compressional, and so evolutionary models for convergent margins must be constructed from well-preserved ancient examples like Baja California.

Tectonics of sedimentary basins : recent advances

Investigating the complex interplay between tectonics and sedimentation is a key endeavor in modern earth science. Many of the worlds leading researchers in this field have been brought together in this volume to provide concise overviews of the current state of the subject.

The tectonic significance of high-K2O volcanism in the Sierra Nevada, California

K 2 O contents have long been recognized as a potential indicator of tectonic processes, and in the Sierra Nevada, California, high-K 2 O volcanism has been attributed to lithosphere root delamination. However, new data from the central Sierra suggest a very different control: K 2 O concentrations can be explained by variations in the degree of partial melting in the mantle, where high-K 2 O volcanics are derived from low-degree partial melts of mantle lithosphere. Field evidence in the central Sierra further suggests that the pulse of high-K 2 O volcanism there was synchronous with the development of a pull-apart structure along a series of right-stepping dextral transtensional faults at the onset of Walker Lane transtensional faulting. In our alternative interpretation, high-K, low-degree partial melts were tapped by the inception of transtensional stresses, recording the birth of a plate boundary. We speculate that high-K 2 O lavas in the southern Sierra are similarly related to the onset of transtensional stresses, not delamination. A regional southward increase in incompatible element contents and decrease in erupted volumes are also consistent with a model for crustal thickness controls on magmatism. Depth-integrated density models show that dry mafic magmas beneath thick crust have insufficient buoyancy to erupt, but low-degree partial melts carry sufficient volatiles to allow eruption; as with K 2 O, degree of partial melting, not source-region heterogeneity, controls water contents and buoyancy.

Tectonic history of a Jurassic backarc-basin sequence (the Gran Canon Formation, Cedros Island, Mexico), based on compositional modes of tuffaceous deposits

The Jurassic Gran Canon Formation (Cedros Island, Baja California, Mexico) constitutes an unusually well preserved and exposed example of ancient backarc-basin fill. Petrofacies analysis conducted on tuff- aceous sandstone and tuff samples from this formation complement and reinforce prior lithofacies interpretations, but with some modification. When temporal and spatial trends in petrographic data (detrital modes) are analyzed and compared to mod- els based on data collected from Deep Sea Drilling Project and Ocean Drilling Pro- gram cores, the trends indicate a second, heretofore unrecognized, phase of backarc rifting. Basalt lavas interstratified with da- citic pyroclastic rocks of the primary vol- canic lithofacies, previously interpreted to record the eruption of differentiated mag- mas at the climax of growth of the Gran Canon island arc, are now as a result of this study considered to be the product of arc extension and rifting. Our method of modal analysis uniquely combines the quantification of textural at- tributes of pyroclastic and epiclastic debris that reflect eruption style and magma com- position, as well as the effects of reworking and mixing in marine settings. This study demonstrates that detailed petrographic analysis is useful in the interpretation of ancient volcaniclastic deposits suspected of having formed in backarc-basin settings.

Miocene evolution of the western edge of the Nevadaplano in the central and northern Sierra Nevada: palaeocanyons, magmatism, and structure

The Sierra Nevada of California is the longest and tallest mountain range in the co-terminus USA, and has long been regarded as topographically very young ( < 6 Ma); however, recent work has provided evidence that the range is very old (>80 Ma), and represents the western shoulder of a Tibetan-like plateau (the Nevadaplano) that was centred over Nevada. A great deal of effort has been invested in applying modern laboratory and geophysical techniques to understanding the Sierra Nevada, yet some of the most unambiguous constraints on the Sierran landscape evolution are derived from field studies of dated strata preserved in the palaeochannels/palaeocanyons that crossed the range in Cenozoic time. Our work in the Sierra Nevada suggests that neither end-member model is correct for the debate regarding youth vs. antiquity of the range. Many features of the Cenozoic palaeocanyons and palaeochannels reflect the shape of the Cretaceous orogen, but they were also affected by Miocene tectonic and magmatic events. In the central Sierra Nevada, we infer that the inherited Cretaceous landscape was modified by three Miocene tectonic events, each followed by ∼2–5 Myr of subduction-induced magmatism and sedimentation during a period of relative tectonic quiescence. The first event, at about 16 Ma, corresponds to the westward sweep of the Ancestral Cascades arc front into the Sierra Nevada and adjacent western Nevada. We suggest that this caused thermal uplift and extension. The second event, at about 11–10 Ma, records the birth of the ‘future plate boundary’ by transtensional faulting and voluminous high-K volcanism at the western edge of the Walker Lane belt. The third event, at about 8–7 Ma, is associated with renewed range-front faulting in the central Sierra, and rejuvenation and beheading of the palaeocanyons. Volcanic pulses closely followed all three events, and we tentatively infer that footwall uplift of the Sierra Nevada occurred during all three events. By analogy with the ∼11 Ma event, we speculate that high-K volcanic rocks in the southern part of the range mark the inception of yet a fourth pulse of range-front faulting at 3–3.5 Ma, which resulted in a fourth tilting and crestal uplift event. Cenozoic rocks along the western edge of the Nevadaplano record the following variation, from the central to the northern Sierra: decrease in crustal thickness (and presumably palaeoelevation), decrease in palaeorelief and attendant decrease in coarse-grained fluvial- and mass-wasting deposits, and greater degree of encroachment by Walker Lane-related faults beginning at 10–11 Ma. By mapping and dating Cenozoic strata in detail, we show that what is now the Sierra Nevada was, at least in part, shaped by the Miocene structural and magmatic events.

Evolution of the Guerrero composite terrane along the Mexican margin, from extensional fringing arc to contractional continental arc

The western margin of Mexico is ideally suited for testing two opposing models for the growth of continents along convergent margins: accretion of exotic island arcs by the consumption of entire ocean basins versus accretion of fringing terranes produced by protracted extensional processes in the upper plate of a single subduction zone. We present geologic and detrital zircon evidence that the Zihuatanejo terrane of the Guerrero composite terrane originated from the latter mechanism. The evolution of the Zihuatanejo terrane can be explained by extensional and compressional processes operating entirely within the upper plate of a long-lived subduction zone that dipped east under the Mexican margin. This process controlled crustal growth by continental margin rifting and addition of new igneous and volcaniclastic material during extension, followed by accretion and thickening of the crust during contraction. Prior to this study, all Mesozoic rocks in the western part of the Guerrero composite terrane were considered to be part of a single arc. However, we divide it into four distinctive tectonostratigraphic assemblages: (1) a Triassic–Early Jurassic accretionary complex (Arteaga complex); (2) a Jurassic to earliest Cretaceous extensional volcanic arc assemblage; (3) an Early Cretaceous extensional arc assemblage; and (4) a Santonian–Maastrichtian compressional arc assemblage. (1) The Arteaga subduction complex forms the basement to the Zihuatanejo terrane and includes Grenville, Pan-African, and Permian detrital zircon suites that match the Potosi fan of the Mexican mainland. (2) The Jurassic to earliest Cretaceous extensional volcanic arc assemblage shows a Callovian–Tithonian (ca. 163–145 Ma) peak in magmatism; extensional unroofing began in this time frame and continued into through the next. (3) The Early Cretaceous extensional arc assemblage has two magmatic peaks: one in the Barremian-Aptian (ca. 129–123 Ma), and the other in the Albian (ca. 109 Ma). In some localities, rapid subsidence produced thick, mainly shallow-marine volcano-sedimentary sections, while at other localities, extensional unroofing of all older assemblages resulted in recycling of zircon from all older units (1, 2, 3). (4) For the Santonian–Maastrichtian compressional arc assemblage, our new detrital zircon dates show for the first time that arc volcanic rocks of this age are present in the coastal Zihuatanejo terrane. The contractional arc developed atop assemblages 1–3, which were shortened between Turonian and Santonian time (ca. 93 and 84 Ma). Taken together, the western Zihuatanejo terrane records a more protracted history of arc magmatism than has yet been dated in other terranes of western Mexico, but it closely matches the history of Baja California to the northwest

Carson Pass–Kirkwood paleocanyon system: Paleogeography of the ancestral Cascades arc and implications for landscape evolution of the Sierra Nevada (California)

Tertiary strata of the central Sierra Nevada are dominated by widespread, voluminous volcanic breccias that are largely undivided and undated, the origin of which is poorly understood. These dominantly andesitic strata are interpreted to be eruptive products of the ancestral Cascades arc, deposited and preserved within paleocanyons that crossed the present-day Sierra Nevada before Basin and Range faulting began there. These strata are thus important not only for understanding the paleogeography of the Ancestral Cascades arc, but also for reconstructing the evolution of the Sierra Nevada landscape. Our regional-scale mapping shows that paleocanyon fills of the central Sierra Nevada are dominated by intrusions and vent-proximal facies along the present-day Sierran crest, with more distal facies extending westward down the paleocanyons. Vent-proximal facies consist of lava domes that collapsed to generate block and ash flow tuffs, which in turn were remobilized down-canyon to produce coarse-grained volcanic mudflow and dilute flow (fluvial) deposits. Lava flows are rare; instead, magmas invaded wet volcaniclastic sediment to form in situ peperite piles that were partly remobilized to form debris flows or block and ash flows with peperite domains. Detailed mapping and dating of previously undifferentiated Tertiary strata in the Carson Pass–Kirkwood area of the central Sierra Nevada has allowed us to identify 6 unconformity-bounded sequences preserved within a paleocanyon as deep as 650 m cut into Mesozoic granitic basement. The Carson Pass–Kirkwood paleocanyon trends NE-SW, with a paleo-transport direction roughly parallel to the modern Mokelumne River drainage (toward the SW). Sequence 1 consists of Oligocene silicic ignimbrites sourced from Nevada. Sequences 2 through 6 consist of dominantly andesitic rocks of the Miocene Ancestral Cascades arc, with 40 Ar/ 39 Ar ages ranging from 14.69 ± 0.06 Ma to 6.05 ± 0.12 Ma. These new dates provide constraints on the ages of unconformities. Vertical relief on the unconformities within the paleocanyon ranges from 12 to 303 meters; with paleoslope gradients range from 3° to 48°. No evidence exists for widening or deepening of the paleocanyon into Mesozoic basement during Tertiary time, so we infer that unconformity 1 (the paleocanyon floor and walls) was inherited from Cretaceous time. The deepest unconformities within the paleocanyon fill reincised into granitic basement in the early Miocene (unconformity 2), between 14 and 10 Ma (unconformity 5), and between 10 and 6 Ma (unconformity 6). The paleocanyon was also beheaded (cut off from sources to the east) by ca. 10 Ma. We suggest that the early Miocene reincision records tectonism related to the onset of arc magmatism in the Sierra Nevada. The middle Miocene reincision may record the onset of Basin and Range faulting in the central Sierra. The late Miocene reincision may correspond to uplift attendant with the northward sweep of the triple junction through the latitude of the central Sierra. Paleocanyons of the central Sierra differ from those of the northern and southern Sierra by showing steeper local paleorelief, and the bouldery stream deposits attest to higher axial gradients than envisioned for other parts of the range. This may indicate that the uplift history of the range is not uniform from segment to segment.

Paleogeographic and Tectonic Setting of Axial and Western Metamorphic Framework Rocks of the Southern Sierra Nevada, California

This paper represents an update of our 1978 S.E.P.M. Mesozoic Paleogeography synthesis for the southern Sierra Nevada. We originally postulated that much of the southern Sierra Nevada pre-batholithic metamorphic framework consisted of lower Mesozoic siliciclastic, carbonate and pelitic strata with variable arc volcanic admixtures (Kings sequence). Recent syntheses, however, have attempted to minimize the importance of early Mesozoic strata in the region and to extend coherent Paleozoic terranes into the framework as the predominant protoliths. Neither lithologic correlations nor structural analysis can substantiate such a view, however, and the proposed configuration of the Paleozoic terranes is in conflict with the petrochemical zonation pattern of the Cretaceous batholith. We present stratigraphic relations for the relatively well-preserved lower Mesozoic stratified rocks of the southern Sierra which in general supports our 1978 synthesis. As pointed out by more recent syntheses, however, we now recognize the likelihood of Paleozoic basement rocks occurring in some or many of the Kings sequence pendants. Such rocks are disparate fragments of a highly dismembered polygenetic basement composed of Paleozoic ophiolitic, Shoo Fly, miogeoclinal and possibly Antler belt rocks rather than coherent terranes or crustal blocks. The lower stratal levels of the lower Mesozoic Kings sequence appears to have formed part of a regional post-Sonoman (Triassic) marine overlap sequence above this basement complex. Dismemberment and accretion of the basement complex involved transform truncation of the southwest Cordillera and Foothills ophiolite belt emplacement prior to and coincident with Sonoman thrust tectonics. Following the establishment of a Carnian-Norian carbonate platform as part of the overlap sequence, the region subsided and became part of a regional Early Jurassic forearc to intra-arc extensional basin system with the deposition of Kings sequence turbidites and olistostromes. The basin system was destroyed by Middle and Late Jurassic thrusting. The assertion that much of the Kings sequence is Paleozoic in age is based on the discovery of probable Eocambrian-Cambrian miogeoclinal strata in the Snow Lake pendant of the east-central Sierra Nevada (Lahren and others, 1991). These authors offer a reconstruction of the displacement of these strata as part of a large crustal block from the western Mojave region through the axial Sierra Nevada along a now cryptic fault. The bounds of the hypothetical crustal block, however, are at odds with batholithic petrochemical patterns. We propose a more conservative offset history for the Snow Lake pendant rocks which considers a broader uncertainty in the bounds of the possible source area for the rocks, and satisfies offsets of both batholithic petrochemical patterns and igneous-metamorphic assemblages of the Sierran batholithic complex.

Structural and stratigraphic evolution of extensional oceanic arcs

basin, in contrast, is a fault-bounded or hybrid basin (fault-bounded basin hereafter), which was downthrown into deep water relative to the stratovolcano along a fault zone. This fault zone may have controlled the siting of the stratovolcano, and the boundary of an 8-km-wide caldera that formed at its summit (Fig. 2A). Two distinct evolutionary phases of the Alisitos arc terrane are recognized (Figs. 1 and 2). We predict that these phases will be recognized in most modern and ancient oceanic arc terranes. Phase I is characterized by intermediate to silicic explosive and effusive volcanism, culminating in caldera-forming silicic ignimbrite eruptions (Fig. 2A). Phase II is characterized by mafic effusive and hydroclastic volcanism and injection of dike swarms. We interpret phase I to represent an extensional island arc; the onset of arc rifting being recorded in the climactic calderaforming eruption. Phase II records rifting of the arc. The phase I‐phase II cycle reflects the 10‐15 m.y. episodicity of rifting in arc-backarc systems resulting from progressive migration of the back-arc spreading center away from the trench (Taylor and Karner, 1983). The purpose of this paper is to define a model for extensional (phase I) and rift (phase II) phases in the structural and stratigraphic evolution of extensional oceanic arcs (Fig. 2). This model combines our field data with recently