Harpp Karen S.

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Karen S.

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  • Article
    Submarine Fernandina : magmatism at the leading edge of the Galapagos hot spot
    (American Geophysical Union, 2006-12-19) Geist, Dennis J. ; Fornari, Daniel J. ; Kurz, Mark D. ; Harpp, Karen S. ; Soule, Samuel A. ; Perfit, Michael R. ; Koleszar, Alison M.
    New multibeam and side-scan sonar surveys of Fernandina volcano and the geochemistry of lavas provide clues to the structural and magmatic development of Galápagos volcanoes. Submarine Fernandina has three well-developed rift zones, whereas the subaerial edifice has circumferential fissures associated with a large summit caldera and diffuse radial fissures on the lower slopes. Rift zone development is controlled by changes in deviatoric stresses with increasing distance from the caldera. Large lava flows are present on the gently sloping and deep seafloor west of Fernandina. Fernandina's submarine lavas are petrographically more diverse than the subaerial suite and include picrites. Most submarine glasses are similar in composition to aphyric subaerially erupted lavas, however. These rocks are termed the “normal” series and are believed to result from cooling and crystallization in the subcaldera magma system, which buffers the magmas both thermally and chemically. These normal-series magmas are extruded laterally through the flanks of the volcano, where they scavenge and disaggregate olivine-gabbro mush to produce picritic lavas. A suite of lavas recovered from the terminus of the SW submarine rift and terraces to the south comprises evolved basalts and icelandites with MgO = 3.1 to 5.0 wt.%. This “evolved series” is believed to form by fractional crystallization at 3 to 5 kb, involving extensive crystallization of clinopyroxene and titanomagnetite in addition to plagioclase. “High-K” lavas were recovered from the southwest rift and are attributed to hybridization between normal-series basalt and evolved-series magma. The geochemical and structural findings are used to develop an evolutionary model for the construction of the Galápagos Platform and better understand the petrogenesis of the erupted lavas. The earliest stage is represented by the deep-water lava flows, which over time construct a broad submarine platform. The deep-water lavas originate from the subcaldera plumbing system of the adjacent volcano. After construction of the platform, eruptions focus to a point source, building an island with rift zones extending away from the adjacent, buttressing volcanoes. Most rift zone magmas intrude laterally from the subcaldera magma chamber, although a few evolve by crystallization in the upper mantle and deep crust.
  • Article
    Construction of the Galapagos platform by large submarine volcanic terraces
    (American Geophysical Union, 2008-03-19) Geist, Dennis J. ; Diefenbach, Bridget A. ; Fornari, Daniel J. ; Kurz, Mark D. ; Harpp, Karen S. ; Blusztajn, Jerzy S.
    New multibeam bathymetric and side-scan sonar data from the southwestern edge of the Galápagos platform reveal the presence of ∼60 large, stepped submarine terraces between depths of 800 m and 3500 m. These terraces are unique features, as none are known from any other archipelago that share this geomorphic form or size. The terraces slope seaward at <2° and are surrounded by escarpments that average ∼300 m in height with average slopes of 24°. The stepped morphology, fine-scale features, and sinuous planform continuity of terrace edges indicate that each terrace results from a sequence of major submarine volcanic eruptions, similar in extent to young deep-water (>3000 m) lava flow fields west of Fernandina and Isabela Islands. The terraces are formed of thick sequences of lava flows that coalesce to form the foundation of the Galápagos platform, on which the subaerial central volcanoes are built. The compositions of basalts dredged from the submarine terraces indicate that most lavas are chemically similar to subaerial lavas erupted from Sierra Negra volcano on southern Isabela Island. There are no regular major element, trace element, or isotopic variations in the submarine lavas as a function of depth, relative stratigraphic position, or geographic location along the southwest margin of the platform. We hypothesize that magma supply at the western edge of the Galápagos hot spot, which is influenced by both plume and mid-ocean ridge magmatic processes, leads to episodic eruption of large lava flows. These large lava flows coalesce to form the archipelagic apron upon which the island volcanoes are built.
  • Article
    Multiple expressions of plume-ridge interaction in the Galapagos : volcanic lineaments and ridge jumps
    (American Geophysical Union, 2012-05-31) Mittelstaedt, Eric ; Soule, Samuel A. ; Harpp, Karen S. ; Fornari, Daniel J. ; McKee, C. ; Tivey, Maurice A. ; Geist, Dennis J. ; Kurz, Mark D. ; Sinton, Christopher ; Mello, C.
    Anomalous volcanism and tectonics between near-ridge mantle plumes and mid-ocean ridges provide important insights into the mechanics of plume-lithosphere interaction. We present new observations and analysis of multibeam, side scan sonar, sub-bottom chirp, and total magnetic field data collected during the R/V Melville FLAMINGO cruise (MV1007; May–June, 2010) to the Northern Galápagos Volcanic Province (NGVP), the region between the Galápagos Archipelago and the Galápagos Spreading Center (GSC) on the Nazca Plate, and to the region east of the Galápagos Transform Fault (GTF) on the Cocos Plate. The NGVP exhibits pervasive off-axis volcanism related to the nearby Galápagos hot spot, which has dominated the tectonic evolution of the region. Observations indicate that ~94% of the excess volcanism in our survey area occurs on the Nazca Plate in three volcanic lineaments. Identified faults in the NGVP are consistent with normal ridge spreading except for those within a ~60 km wide swath of transform-oblique faults centered on the GTF. These transform-oblique faults are sub-parallel to the elongation direction of larger lineament volcanoes, suggesting that lineament formation is influenced by the lithospheric stress field. We evaluate current models for lineament formation using existing and new observations as well as numerical models of mantle upwelling and melting. The data support a model where the lithospheric stress field controls the location of volcanism along the lineaments while several processes likely supply melt to these eruptions. Synthetic magnetic models and an inversion for crustal magnetization are used to determine the tectonic history of the study area. Results are consistent with creation of the GTF by two southward ridge jumps, part of a series of jumps that have maintained a plume-ridge separation distance of 145 km to 215 km since ~5 Ma.
  • Article
    Low-volume magmatism linked to flank deformation on Isla Santa Cruz, Galapagos Archipelago, using cosmogenic He-3 exposure and Ar-40/Ar-39 dating of fault scarps and lavas
    (Springer, 2022-08-04) Schwartz, Darin M. ; Harpp, Karen S. ; Kurz, Mark D. ; Wilson, Emily L. ; Van Kirk, Rita
    Isla Santa Cruz is a volcanic island located in the central Galápagos Archipelago. The island’s northern and southern flanks are deformed by E–W-trending normal faults not observed on the younger Galápagos shields, and Santa Cruz lacks the large summit calderas that characterize those structures. To construct a chronology of volcanism and deformation on Santa Cruz, we employ 40Ar/39Ar geochronology of lavas and 3He exposure dating of fault scarps from across the island. The combination of Ar–Ar dating with in situ-produced cosmogenic exposure age data provides a powerful tool to evaluate fault chronologies. The 40Ar/39Ar ages indicate that the island has been volcanically active since at least 1.62 ± 0.030 Ma (2SD). Volcanism deposited lavas over the entire island until ~ 200 ka, when it became focused along an E–W-trending summit vent system; all dated lavas < 200 ka were emplaced on the southern flank. Structural observations suggest that the island has experienced two major faulting episodes. Crosscutting relationships of lavas indicate that north flank faults formed after 1.16 ± 0.070 Ma, but likely before 416 ± 36 ka, whereas the faults on the southern flank of the island initiated between 201 ± 37 and 32.6 ± 4.6 ka, based on 3He exposure dating of fault surfaces. The data are consistent with a model wherein the northeastern faults are associated with regional extension owing to the young volcano’s location closer to the Galápagos Spreading Center at the time. The second phase of volcanism is contemporaneous with the formation of the southern faults. The expression of this younger, low-volume volcanic phase was likely related to the elongate island morphology established during earlier deformation. The complex feedback between tectonic and volcanic processes responsible for southward spreading along the southern flank likely generated persistent E-W-oriented magmatic intrusions. The formation of the Galápagos Transform Fault and sea-level fluctuations may be the primary causes of eruptive and deformational episodes on Santa Cruz.