Geology topics

Jonathan Delph

Publications and source records attributed to Jonathan Delph.

5 recordsLinked to original sources

USGS2026

Cascadia Subduction Zone science: Call for the next generation community seismic velocity model

The Cascadia subduction zone (CSZ) hosts major seismic and tsunami hazards, yet key questions persist about the relationship between margin structure, fluid distribution, episodic tremor and slip, shallow megathrust behavior, shaking and tsunamigenesis, and the resulting hazard estimates. Addressing these problems requires an empirically grounded, three‐dimensional seismic velocity model to illuminate subsurface structure and properties and to provide a basis for geophysical studies such as earthquake simulations and ground‐motion estimation. In May 2024, the National Science Foundation‐funded Cascadia Region Earthquake Science Center (CRESCENT) community velocity model (CVM) working group, with U.S. Geological Survey and regional partners, convened a workshop to identify priorities for such a model. Participants emphasized the features necessary for addressing key science questions, including implementing findability, accessibility, interoperability, and reusability (FAIR) access, capturing along‐strike and along‐dip structural heterogeneity, resolving shallow offshore–onshore structure, constraining elastic properties and quantifying their uncertainties for numerical wave propagation simulations, their validation benchmarks, and supporting associated accurate earthquake ground‐motion simulations and hazard assessments. This article describes the priorities defined in the workshop, and a description of how, guided by these needs, CRESCENT plans to develop multiple generations of a CVM to advance CSZ science and improve seismic and tsunami hazard modeling across the Pacific Northwest. The CVM will span the CSZ from the surface to ∼100 km depth, offshore and east of the Cascades into Idaho (∼132°–110° W) and the southern and northern tectonic regime transitions (∼36°–52° N) to capture the entire tectonic system as well as its surroundings.

Cascadia Subduction Zone↗

USGS2023

Breaking plates: Creation of the East Anatolian fault, the Anatolian plate, and a tectonic escape system

Lateral movement of lithospheric fragments along strike-slip faults in response to collision (escape tectonics) has characterized convergent settings since the onset of plate tectonics and is a mechanism for the formation of new plates. The Anatolian plate was created by the sequential connection of strike-slip faults following ≥10 m.y. of distributed deformation that ultimately localized into plate-bounding faults. Thermochronology data and seismic images of lithosphere structure near the East Anatolian fault zone (EAFZ) provide insights into the development of the new plate and escape system. Low-temperature thermochronology ages of rocks in and near the EAFZ are significantly younger than in other fault zones in the region, e.g., apatite (U-Th)/He: 11–1 Ma versus 27–13 Ma. Young apatite (U-Th)/He ages and thermal history modeling record thermal resetting along the EAFZ over the past ~5 m.y. and are interpreted to indicate thermal activity triggered by strike-slip faulting in the EAFZ as it formed as a through-going, lithosphere-scale structure. The mechanism for EAFZ formation may be discerned from S-wave velocity images from the Continental Dynamics–Central Anatolian Tectonics (CD-CAT) seismic experiment. These images indicate that thin but strong Arabian lithospheric mantle extends ~50–150 km north beneath Anatolian crust and would have been located near the present surficial location of the Bitlis-Zagros suture zone (co-located with the EAFZ in our study area) at ca. 5 Ma. Underthrusting of strong Arabian lithosphere facilitated localization of the EAFZ and thus was a fundamental control on the formation of the Anatolian plate and escape system.

East Anatolian Fault↗

USGS2020

Age and mantle sources of Quaternary basalts associated with “leaky” transform faults of the migrating Anatolia-Arabia-Africa triple junction

The Anatolia (Eurasia), Arabia, and Africa tectonic plates intersect in southeast Turkey, near the Gulf of İskenderun, forming a tectonically active and unstable triple junction (the A 3 triple junction). The plate boundaries are marked by broad zones of major, dominantly left-lateral transform faults including the East Anatolian fault zone (the Anatolia-Arabia boundary) and the Dead Sea fault zone (the Arabia-Africa boundary). Quaternary basalts occur locally within these “leaky” transform fault zones (similar to those observed within oceanic transform faults), providing evidence that mantle melting, basalt genesis, and eruption are linked to crustal deformation and faulting that extends into the upper mantle. We investigated samples of alkaline basalt (including basanite) from the Toprakkale and Karasu volcanic fields within a broad zone of transtension associated with these plate-boundary faults near the İskenderun and Amik Basins, respectively. Toprakkale basalts and basanites have 40 Ar/ 39 Ar plateau ages ranging from 810 ± 60 ka to 46 ± 13 ka, and Karasu volcanic field basalts have 40 Ar/ 39 Ar plateau ages ranging from 2.63 ± 0.17 Ma to 52 ± 16 ka. Two basanite samples within the Toprakkale volcanic field have isotopic characteristics of a depleted mantle source, with 87 Sr/ 86 Sr of 0.703070 and 0.703136, 143 Nd/ 144 Nd of 0.512931 and 0.512893, 176 Hf/ 177 Hf of 0.283019 and 0.282995, 206 Pb/ 204 Pb of 19.087 and 19.155, and 208 Pb/ 204 Pb of 38.861 and 38.915. The 176 Hf/ 177 Hf ratios of Toprakkale basalts (0.282966–0.283019) are more radiogenic than Karasu basalts (0.282837–0.282965), with some overlap in 143 Nd/ 144 Nd ratios (0.512781–0.512866 vs. 0.512648–0.512806). Toprakkale 206 Pb/ 204 Pb ratios (19.025 ± 0.001) exhibit less variation than that observed for Karasu basalts (18.800–19.324), and 208 Pb/ 204 Pb values for Toprakkale basalts (38.978– 39.103) are slightly lower than values for Karasu basalts (39.100–39.219). Melting depths estimated for the basalts from both volcanic fields generally cluster between 60 and 70 km, whereas the basanites record melting depths of ~90 km. Depth estimates for the basalts largely correspond to the base of a thin lithosphere (~60 km) observed by seismic imaging. We interpret the combined radiogenic isotope data (Sr, Nd, Hf, Pb) from all alkaline basalts to reflect partial melting at the base of the lithospheric mantle. In contrast, seismic imaging indicates a much thicker (>100 km) lithosphere beneath southern Anatolia, a substantial part of which is likely subducted African lithosphere. This thicker lithosphere is adjacent to the surface locations of the basanites. Thus, the greater melting depths inferred for the basanites may include partial melt contributions either from the lithospheric mantle of the attached and subducting African (Cyprean) slab, or from partial melting of detached blocks that foundered due to convective removal of the Anatolian lithosphere and that subsequently melted at ~90 km depth within the asthenosphere. The Quaternary basalts studied here are restricted to a broad zone of transtension formed in response to the development of the A 3 triple junction, with an earliest erupted age of 2.63 Ma. This indicates that the triple junction was well established by this time. While the current position of the A 3 triple junction is near the Amik Basin, faults and topographic expressions indicate that inception of the triple junction began as early as 5 Ma in a position farther to the northeast of the erupted basalts. Therefore, the position of the A 3 triple junction appears to have migrated to the southwest since the beginning of the Pliocene as the Anatolia-Africa plate boundary has adjusted to extrusion (tectonic escape) of the Anatolia plate. Establishment of the triple junction over the past 5 m.y. was synchronous with rollback of the African slab beneath Anatolia and associated trench retreat, consistent with Pliocene uplift in Cyprus and with the current positions of plate boundaries. The A 3 triple junction is considered to be unstable and likely to continue migrating to the southwest for the foreseeable geologic future.

Geosphere↗

USGS2019

Melt equilibration depths as sensors of lithospheric thickness during Eurasia-Arabia collision and the uplift of the Anatolian Plateau

A co-investigation of mantle melting conditions and seismic structure revealed an evolutionary record of mantle dynamics accompanying the transition from subduction to collision along the Africa-Eurasia margin and the >1 km uplift of the Anatolian Plateau. New 40 Ar/ 39 Ar dates of volcanic rocks from the Eastern Taurides (southeast Turkey) considerably expand the known spatial extent of Miocene-aged mafic volcanism following a magmatic lull over much of Anatolia that ended at ca. 20 Ma. Mantle equilibration depths for these chemically diverse basalts are interpreted to indicate that early to middle Miocene lithospheric thickness in the region varied from ∼50 km or less near the Bitlis suture zone to ∼80 km near the Inner Tauride suture zone. This southward-tapering lithospheric base could be a vestige of the former interface between the subducted (and now detached) portion of the Arabian plate and the overriding Eurasian plate, and/or a reflection of mantle weakening associated with greater mantle hydration trenchward prior to collision. Asthenospheric upwelling driven by slab tearing and foundering along this former interface, possibly accompanied by convective removal of the lithosphere, could have led to renewed volcanic activity after 20 Ma. Melt equilibration depths for late Miocene and Pliocene basalts together with seismic imaging of the present lithosphere indicate that relatively invariant lithospheric thicknesses of 60–70 km have persisted since the middle Miocene. Thus, no evidence is found for large-scale (tens of kilometers) Miocene delamination of the lower lithosphere from the overriding plate, which has been proposed elsewhere to account for late Miocene and younger uplift of Anatolia.

Anatolian Plate↗

USGS2016

Seismic imaging of the metamorphism of young sediment into new crystalline crust in the actively rifting Imperial Valley, California

Plate-boundary rifting between transform faults is opening the Imperial Valley of southern California and the rift is rapidly filling with sediment from the Colorado River. Three 65–90 km long seismic refraction profiles across and along the valley, acquired as part of the 2011 Salton Seismic Imaging Project, were analyzed to constrain upper crustal structure and the transition from sediment to underlying crystalline rock. Both first arrival travel-time tomography and frequency-domain full-waveform inversion were applied to provide P-wave velocity models down to ∼7 km depth. The valley margins are fault-bounded, beyond which thinner sediment has been deposited on preexisting crystalline rocks. Within the central basin, seismic velocity increases continuously from ∼1.8 km/s sediment at the surface to >6 km/s crystalline rock with no sharp discontinuity. Borehole data show young sediment is progressively metamorphosed into crystalline rock. The seismic velocity gradient with depth decreases approximately at the 4 km/s contour, which coincides with changes in the porosity and density gradient in borehole core samples. This change occurs at ∼3 km depth in most of the valley, but at only ∼1.5 km depth in the Salton Sea geothermal field. We interpret progressive metamorphism caused by high heat flow to be creating new crystalline crust throughout the valley at a rate comparable to the ≥2 km/Myr sedimentation rate. The newly formed crystalline crust extends to at least 7–8 km depth, and it is shallower and faster where heat flow is higher. Most of the active seismicity occurs within this new crust.

California↗