David R. Sherrod

New K-Ar ages and the geologic evidence against rejuvenated-stage volcanism at Haleakalā, East Maui, a postshield-stage volcano of the Hawaiian island chain

The postshield and previously inferred rejuvenated-stage history of Haleakalā volcano is reevaluated on the basis of 52 new K-Ar ages, 42 from the postshield Kula Volcanics and 10 from the overlying Hāna Volcanics. Postshield extrusion was robust from 0.93 to 0.76 Ma. A period of low extrusion rate or volcanic quiescence occurred between 0.76 and 0.65 Ma, well within Kula time. A chemical change to increasingly alkalic lava occurred at this time as the volcano changed from broadly hawaiitic to basanitic in its eruptive products and robust extrusion resumed. A slightly longer period of low extrusion rate or quiescence occurred after ca. 0.4 Ma, but only trifling change in geochemical character is observed. Geochemically, the Hāna Volcanics unit, chiefly basanitic, overlaps greatly with the upper part of the Kula Volcanics; there is a weak tendency to slightly more alkaline character among the Hāna Volcanics. The age of the Kula/Hāna boundary is ca. 0.15–0.12 Ma; thus, volcanic quiescence of only ∼0.03 m.y. separates the two formations, much shorter than the previously known limit of 0.25–0.30 m.y. The brevity of this hiatus, coupled with coincident vent loci and broadly similar geochemical characteristics for the Hāna and the upper part of the Kula Volcanics, indicates that the Hāna Volcanics unit comprises deposits of postshield-stage volcanism that has waned substantially since ca. 0.4–0.3 Ma. Haleakalā has not yet begun a classically defined rejuvenated stage. Our findings support recent numerical modeling of plume-lithosphere interactions that predict that Haleakalā is near the end of its postshield growth.

Chronology of the episode 54 eruption at Kilauea Volcano, Hawaii, from GOES-9 satellite data

The free availability of GOES satellite data every 15 minutes makes these data an attractive tool for studying short-term changes on cloud-free volcanoes in the Pacific basin. We use cloud-free GOES-9 data to investigate the chronology of the January 1997, episode 54 eruption of Kilauea Volcano, Hawaii. Seventy-six images for this effusive eruption were collected over a 60-hour period and show the opening and shutdown of active fissures, the draining and refilling of the Pu‘u ‘O‘o lava lake, and the cessation of activity at the ocean entry.

Rejuvenated-stage volcanism after 0.6-m.y. quiescence at West Maui volcano, Hawaii: new evidence from K-Ar ages and chemistry of Lahaina Volcanics

Abstract West Maui’s rejuvenated-stage Lahaina Volcanics were erupted from four discrete sites. New K–Ar ages indicate two pulses of volcanism, the older about 0.6 Ma and the younger about 0.4 Ma. Compositionally the lava flows are entirely basanitic, but each pulse is diverse. The underlying postshield-stage Honolua Volcanics were emplaced by about 1.2 Ma on the basis of previously published ages. Therefore the duration of volcanic quiescence prior to rejuvenation is about 0.6 m.y. at West Maui, much longer than estimated previously.

Geologic setting and tertiary structural evolution of southwestern Arizona and southeastern California

Volcanic and sedimentary rocks and structures record the Tertiary structural evolution of the lower Colorado River region in southwestern Arizona and southeastern California. A late Eocene or early Oligocene (prior to ∼33 Ma) episode of faulting is indicated by medium- to coarse-grained arkosic rocks in the Chocolate and southern Trigo Mountains. Much of the area was relatively quiescent tectonically during extrusion of volcanic rocks from ∼33 to 22 Ma, but the southernmost part was periodically uplifted and eroded into its underlying crystalline rocks. A major episode of extensional deformation and tilting occurred after deposition of welded tuff about 22 Ma and affected the entire area from the Palo Verde Mountains on the west to the Kofa Mountains on the east. Extension-related faulting quickly waned and had largely ceased by about 20 Ma in the Kofa Mountains; elsewhere the timing is poorly constrained. By ∼13 Ma, thick alluvial fans filled many grabens and half grabens among tilted fault blocks throughout the area. Volcanism in the lower Colorado River region may have been coincident with a broad structural depression now oriented east-west. The northern limit of the volcanic terrane defines a tilt-domain boundary. The northern boundary, reaching from the New Water Mountains in Arizona to the little Chuckwalla Mountains in California, ultimately evolved to separate a terrane of relatively untilted crystalline horsts on the north from a series of east or northeast dipping fault blocks on the south. The southern margin is less well defined but is subparallel to the northern boundary and to the Chocolate Mountains anticlinorium.

Exhumation history of the Orocopia Schist and related rocks in the Gavilan Hills area of southeasternmost California

The Gavilan Hills area of southeasternmost California exposes three distinctly different crystalline rock packages in a postmetamorphic, E–W elongated dome. Structurally deepest is the relatively high-pressure, eugeoclinal Orocopia Schist, which underlies the low-angle Chocolate Mountains fault. Above the schist are gneisses derived from midto lower-crustal levels of the Mesozoic Cordilleran magmatic arc. The gneisses, in turn, are separated by the Gatuna fault from low-grade metasedimentary and metavolcanic rocks of the Winterhaven Formation. The Chocolate Mountains fault was originally thought to be a SW-dipping subduction thrust along which an exotic continental sliver was sutured to North America. Recent workers, however, have proposed that it is a late fault responsible for exhumation of the Orocopia Schist and that it places no constraints on burial history. The latter interpretation is supported by the presence in the schist of two distinct structural fabrics, an older one presumably related to underthrusting, and a younger one attributed to exhumation. The older fabric is preserved in schist away from the Chocolate Mountains fault and is associated with a NNE–SSW-trending lineation that formed during prograde metamorphism to lowermost amphibolite facies. The younger fabric is best developed within 100 m structurally of the Chocolate Mountains fault and is characterized by discrete shear zones, greenschist-facies retrogression, and E–W-trending lineations. Lineations with similar orientation also occur in gneiss adjacent to both the Chocolate Mountains and Gatuna faults and in the Winterhaven Formation. This Jacobson, C.E., Grove, M., Stamp, M. M., Vucic, A., Oyarzabal, F.R., Haxel, G.B., Tosdal, R.M., and Sherrod, D.R., 2002, Exhumation history of the Orocopia Schist and related rocks in the Gavilan Hills area of southeasternmost California, in Barth, A., ed., Contributions to Crustal Evolution of the Southwestern United States: Boulder, Colorado, Geological Society of America Special Paper 365, p. 129–154. *E-mail: cejac@iastate.edu C.E. Jacobson et al. 130 observation, combined with interpretation of outcrop patterns, suggests that the Gatuna fault, which was previously considered a steep, shallow-level fault of Miocene age, is a low-angle structure that accommodated relatively high-temperature deformation similar to that recorded by the Chocolate Mountains fault. Both faults may have been synchronously active. However, because the Gatuna fault exhibits more intense brittle overprinting of early mylonitic fabrics and greater structural excision than the Chocolate Mountains fault, it is indicated to have been more recently active and the more important of the two in exhuming the schist and overriding gneiss to shallow crustal levels. Thermal history results based upon Ar/Ar analysis of hornblende, muscovite, biotite, and K-feldspar and previous apatite fission track measurements reveal a twostage exhumation history that we relate to slip along the Chocolate Mountains and Gatuna faults, respectively. An initial phase of rapid cooling occurred from 60 Ma to 44 Ma. Younger and more discordant Ar/Ar ages recorded by hornblendes from the schist (52–57 Ma) relative to the gneiss (59–64 Ma) confirm postpeak metamorphic juxtaposition of the two units along the Chocolate Mountains fault in a manner consistent with normal faulting. Coincidence of muscovite Ar/Ar ages between the Orocopia Schist and gneiss implies that this juxtaposition occurred by 48 2 Ma and indicate that the Chocolate Mountains fault is a Laramide-age structure. Biotite ages ranging from 45 to 31 Ma reveal that the initial exhumation phase was followed by protracted residence of the schist and gneiss in the middle crust at 350 C. Kfeldspars record a second period of rapid exhumation from 28 to 24 Ma, which we correlate with the brittle phase of movement on the Gatuna fault. This second phase of exhumation is considered to reflect an early stage of the middle Tertiary extensional event that is widespread in southeastern California and southwestern Arizona. Localized disruption of the Chocolate Mountains fault that juxtaposed structurally deeper schist against gneiss at the eastern end of the Gavilan Hills probably also occurred at this time.