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1. Volcanic and Topographic Evolution of the Sierra San Francisco, Baja California Sur, Mexico

   https://doi.org/10.1130/abs/2022CD-373847  

   HAUSBACK, Brian; GRANDY, Samuel; DORSEY, Rebecca J.; DARIN, Michael; BENNETT, Scott (April 2022, GSA Cordilleran 2022 Meeting)

   The Sierra San Francisco (SSF) is a Neogene volcanic range along the topographic crest of the Baja California peninsula in northern Baja California Sur, Mexico. The SSF is ~55 km long (NW-SE) and ~30 km wide and its highest peaks exceed 1500 m elevation. The SSF has a long history of volcanism and has been eroded by deep, rugged, radially-draining canyons. The development of SSF topography is intimately associated with the volcanic evolution of the range. The SSF is a large and complex dacitic adakite dome complex largely built of a thick, up to 800 m, stratigraphic succession of dacitic tuff breccias with minor interbedded basaltic andesite lavas. These deposits overlie rare exposures of aeolian sandstone of unknown age. The tuff breccias represent block-and-ash-flows and lahars generated from steep-sided peleean dacite and andesite domes, with three radiometric dates of 11-10 Ma. This intermediate sequence is unconformably capped by widespread bajaite mafic lavas, 5.5-4.5 Ma. SSF topography evolved dramatically since the late Miocene: 1) From 11-10 Ma, adakite domes erupted across the central SSF, locally along NNW faults. Thick sequences of bedded tuff breccias accumulated around the domes and are radially inclined away from source domes. The duration of this volcanism is unknown. 2) From 10-5 Ma, deep erosion of the pyroclastic strata formed a range-wide radial drainage network, with channel depths of up to 130 m or more. 3) From 5.5-4.5 Ma, voluminous bajaite lavas from cinder cones and dike vents flooded the top of the range and flowed down the radial drainages with flow distances up to 12 km. Vents are strongly aligned along steep NNW normal faults. 4) After 4.5 Ma, erosion removed interfluves of tuff breccia not armored by younger mafic lavas. Today, the long, steep-sided, lava-capped ridges are inverted topographically. At Santa Martha, an area in the central SSF with the highest concentration of domes, hydrothermal alteration of the volcanic deposits during and after the dome volcanism caused severe material weakening and slope failure within the volcanic center. The area is now a distinctive erosional basin, partly filled with clay-rich landslide deposits. Comparable volcanic history and topographic development are likely to have occurred in a dome field of similar age and size at Santa Agueda, 60 km SE of Santa Martha. 

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2. Miocene-Pliocene Volcanism in the Sierra San Francisco, Central Baja California Peninsula, Mexico

   https://doi.org/10.1130/abs/2022CD-374066  

   Grandy, Sam; Hausback, Brian; Bennett, Scott; Dorsey, Rebecca; Darin, Michael (April 2022, GSA Cordilleran 2022 Meeting)

   Volcanic rocks of the Sierra San Francisco (SSF), in northern Baja California Sur, Mexico, record post-subduction magmatism related to slab melting and slab window opening. The range is composed of andesitic and dacitic domes, mafic lavas, and volcaniclastic deposits (debris and block-and-ash-flow, lahar, and fluvial) that constitute the proximal to distal facies of a volcanic field with local eruptive ages that postdate the regional transition from subduction to transtension. Lowest observed volcanic units consist of interbedded and hydrothermally altered mafic lavas, tuff breccias, and andesite/dacite domes. These are overlain by volcaniclastic units and dacite domes that erupted between ~11-10 Ma. Volcaniclastic deposits comprise a section up to 800 m thick, locally flank and dip radially away from domes, and are likely associated with dome collapse. These deposits are unconformably overlain by a series of ~5.5-4.5 Ma Mg-enriched basaltic andesites (bajaites) that typically erupted along NNW-trending normal faults. Low interbedded mafic lavas are chemically similar to syn-subduction lavas (>15 Ma) SE of the SSF, suggesting a typical subduction supraslab mantle source during waning, late Miocene Farallon plate subduction. ~11-10 Ma dacite domes and debris flow blocks display an adakitic geochemical signature, implying an origin involving late Miocene foundering and melting of the edges of the subducted Farallon plate during the opening of a slab window after the 12.3 Ma transition from subduction to transtension. Adakitic rocks of the SSF and the Santa Clara volcanic field 60 km to the SW may constrain the E-W extent of the slab window. The ~5.5-4.5 Ma bajaites display enriched REE and trace element patterns, potentially resulting from the rise of enriched subslab mantle through the slab window and interaction with supraslab mantle, previously metasomatized by slab melts. Thermal pulses associated with Gulf of California rifting may have provided the heat to generate Mg-rich magmas which ascended along rift-related faults, precluding significant crustal contamination or fractionation, and allowing magmas to retain their primitive character. Further analysis will elucidate the timing of slab window development and the post-subduction mantle processes that drove the chemical evolution of SSF magmas. 

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3. The Growth and Collapse of a Volcanic Field: Detrital Zircon U/Pb Geochronology and Provenance of the Oligocene Bear Valley Formation, Southwest Utah, USA

   https://doi.org/10.2110/001c.138729  

   Rivera, Tiffany; Malone, David H; Biek, Robert F; Lesmann, Sarah Rose; Stevens, Grace; Hacker, David; Braunagel, Michael J; Griffith, W Ashley; Rowley, Peter D (June 2025, The Sedimentary Record)

   The Bear Valley Formation (Fm.) is a distinctive eolian sandstone interbedded with thick volcanic rocks of the Marysvale volcanic field of southwest Utah, the southern part of which failed during eruptive activity along three mega-scale gravity slides. The formation is as thick as 300 m and extends over an area of >2,500 km²in the Black Mountains and Markagunt Plateau. The Bear Valley Fm. is composed of tuffaceous sandstone interbedded with tuff, conglomerate, and polymict volcanic mudflow breccias. The sandstone beds are lithic arenite and lithic wacke that occur as massive beds with large-scale cross bedding. The Bear Valley Fm. occurs in the upper plate of the Markagunt gravity slide and is in both the upper and lower plates of the Black Mountains gravity slide. We used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to acquire U/Pb dates of detrital zircons (N = 3, n = 346) from the autochthonous Bear Valley Fm. at Kane Spring and Jako Wash in the Black Mountains and the allochthonous Bear Valley at Sandy Wash in the central Markagunt Plateau. All samples are dominated by Oligocene zircons with maximum likelihood ages for deposition ranging from 23.6 to 24.0 Ma. The western-most sample from Jako Wash also preserves a slightly older group of zircons, indicating derivation from either the underlying Wah Wah Springs Fm. or another unit erupted from the Indian Peak caldera complex to the west. Thus, the upper Bear Valley Fm. was deposited within ~400 kyr before the emplacement of the Markagunt gravity slide at 23 Ma, reflecting accelerated uplift of the northern Marysvale complex that ultimately resulted in collapse and slide emplacement. 

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4. Complex staged emplacement of a basaltic lava: The example of the July 1974 flow of Kīlauea

   https://doi.org/10.1007/s00445-025-01817-0  

   Biass, S.; Houghton, B_F; Llewellin, E_W; Curran, K_C; Thordarson, T.; Orr, T_R; Parcheta, C_E; Mouginis-Mark, P. (March 2025, Bulletin of Volcanology)

   Abstract Basaltic lava flows can be highly destructive. Forecasting the future path and/or behavior of an active lava flow is challenging because topography is often poorly constrained and lava has a complex rheology and emplacement history. Preserved lavas are an important source of information which, combined with observations of active flows, underpins conceptual models of lava flow emplacement. However, the value of preserved lavas is limited because pre-eruptive topography and, thus, syn-eruptive lava flow geometry are usually not known. Here, we use tree-mold data to constrain pre-eruptive topography and syn-eruptive lava flow geometry of the July 1974 flow of Kīlauea (USA). Tree molds, which are formed after advancing lava encloses standing trees, preserve the lava inundation height and the final preserved thickness of lava. We used data from 282 tree molds to reconstruct the temporal and spatial evolution of the ~ 2.1 km-long July 1974 flow. The tree mold dataset yields a detailed dynamic picture of staged emplacement, separated by intervals of ponding. In some ponded areas, flow depth during emplacement (~ 5 m) was twice the preserved thickness of the final lava (2–3 m). Drainage of the ponds led to episodic surges in flow advancement, decoupled from fluctuations in vent discharge rate. We infer that the final breakout occurred after the cessation of fountaining. Such complex emplacement histories may be common for pāhoehoe lavas at Kīlauea and elsewhere in situations where the terrain is of variable slope, and/or where lava is temporarily perched and stored. 

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5. Pyroclastic deposits of Ubehebe Crater, Death Valley, California, USA: Ballistics, pyroclastic surges, and dry granular flows

   https://doi.org/10.1130/GES02526.1  

   Valentine, Greg A.; Fierstein, Judy; White, James D.L. (November 2022, Geosphere)

   Abstract We describe and interpret deposits associated with the final Ubehebe Crater-forming, phreatomagmatic explosive phase of the multivent, monogenetic Ubehebe volcanic center. Ubehebe volcano is located in Death Valley, California, USA. Pyroclastic deposits occur in four main facies: (1) lapilli- and blockdominated beds, (2) thinly bedded lapilli tuff, (3) laminated and cross-laminated ash, and (4) massive lapilli ash/tuff. Lapilli- and block-dominated beds are found mostly within several hundred meters of the crater and transition outward into discontinuous lenses of lapilli and blocks; they are interpreted to have been deposited by ballistic processes associated with crater-forming explosions. Thinly bedded lapilli tuff is found mainly within several hundred meters, and laminated and cross-laminated ash extends at least 9 km from the crater center. Dune forms are common within ~2 km of the crater center, while finer-grained, distal deposits tend to exhibit planar lamination. These two facies (thinly bedded lapilli tuff and laminated and cross-laminated ash) are interpreted to record multiple pyroclastic surges (dilute pyroclastic currents). Repeated couplets of coarse layers overlain by finer-grained, laminated horizons suggest that many or most of the surges were transient, likely recording individual explosions, and they traveled over complex topography in some areas. These two factors complicate the application of classical sediment-transport theory to quantify surge properties. However, dune-form data provide possible constraints on the relationships between suspended load sedimentation and bed-load transport that are consistent using two independent approaches. Massive lapilli ash/tuff beds occur in drainages below steep slopes and can extend up to ~1 km onto adjacent valley floors beneath large catchments. Although they are massive in texture, their grain-size characteristics are shared with laminated and cross-laminated ash facies, with which they are locally interbedded. These are interpreted to record concentrated granular flows sourced by remobilized pyroclastic surge deposits, either during surge transport or shortly after, while the surge deposits retained their elevated initial pore-gas pressures. Although similar surge-derived concentrated flows have been described elsewhere (e.g., Mount St. Helens, Washington, USA, and Soufriére Hills, Montserrat, West Indies), to our knowledge Ubehebe is the first case where such processes have been identified at a maar volcano. These concentrated flows followed paths that were independent of the pyroclastic surges and represent a potential hazard at similar maar volcanoes in areas with complex terrain. 

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