Geology topics

Research about Mono Basin

Source-linked reports with geographic coverage including Mono Basin.

4 recordsLinked to original sources

USGS2021

No ring fracture in Mono Basin, California

In Mono Basin, California, USA, a near-circular ring fracture 12 km in diameter was proposed by R.W. Kistler in 1966 to have originated as the protoclastic margin of the Cretaceous Aeolian Buttes pluton, to have been reactivated in the middle Pleistocene, and to have influenced the arcuate trend of the chain of 30 young (62−0.7 ka) rhyolite domes called the Mono Craters. In view of the frequency and recency of explosive eruptions along the Mono chain, and because many geophysicists accepted the ring fracture model, we assembled evidence to test its plausibility. The shear zone interpreted as the margin of the Aeolian Buttes pluton by Kistler is 50−400 m wide but is exposed only along a 7-km-long set of four southwesterly outcrops that subtend only a 70° sector of the proposed ring. The southeast end of the exposed shear zone is largely within the older June Lake pluton, and at its northwest end, the contact of the Aeolian Buttes pluton with a much older one crosses the shear zone obliquely. Conflicting attitudes of shear structures are hard to reconcile with intrusive protoclasis. Also inconsistent with the margin of the ovoid intrusion proposed by Kistler, unsheared salients of the pluton extend ∼1 km north of its postulated circular outline at Williams Butte, where there is no fault or other structure to define the northern half of the hypothetical ring. The shear zone may represent regional Cretaceous transpression rather than the margin of a single intrusion. There is no evidence for the Aeolian Buttes pluton along the aqueduct tunnel beneath the Mono chain, nor is there evidence for a fault that could have influenced its vent pattern. The apparently arcuate chain actually consists of three linear segments that reflect Quaternary tectonic influence and not Cretaceous inheritance. A rhyolitic magma reservoir under the central segment of the Mono chain has erupted many times in the late Holocene and as recently as 700 years ago. The ring fracture idea, however, prompted several geophysical investigations that sought a much broader magma body, but none identified a low-density or low-velocity anomaly beneath the purported 12-km-wide ring, which we conclude does not exist.

California↗

USGS2015

Imaging the magmatic system of Mono Basin, California with magnetotellurics in three--dimensions

A three–dimensional (3D) electrical resistivity model of Mono Basin in eastern California unveils a complex subsurface filled with zones of partial melt, fluid–filled fracture networks, cold plutons, and regional faults. In 2013, 62 broadband magnetotelluric (MT) stations were collected in an array around southeastern Mono Basin from which a 3D electrical resistivity model was created with a resolvable depth of 35 km. Multiple robust electrical resistivity features were found that correlate with existing geophysical observations. The most robust features are two 300 ± 50 km3 near-vertical conductive bodies (3–10 Ω·m) that underlie the southeast and north-eastern margin of Mono Craters below 10 km depth. These features are interpreted as magmatic crystal–melt mush zones of 15 ± 5% interstitial melt surrounded by hydrothermal fluids and are likely sources for Holocene eruptions. Two conductive east–dipping structures appear to connect each magma source region to the surface. A conductive arc–like structure (< 0.9 Ω·m) links the northernmost mush column at 10 km depth to just below vents near Panum Crater, where the high conductivity suggests the presence of hydrothermal fluids. The connection from the southernmost mush column at 10 km depth to below South Coulée is less obvious with higher resistivity (200 Ω·m) suggestive of a cooled connection. A third, less constrained conductive feature (4–10 Ω·m) 15 km deep extending to 35 km is located west of Mono Craters near the eastern front of the Sierra Nevada escarpment, and is coincident with a zone of sporadic, long–period earthquakes that are characteristic of a fluid-filled (magmatic or metamorphic) fracture network. A resistive feature (103–105 Ω·m) located under Aeolian Buttes contains a deep root down to 25 km. The eastern edge of this resistor appears to structurally control the arcuate shape of Mono Craters. These observations have been combined to form a new conceptual model of the magmatic system beneath Mono Craters to a depth of 30 km.

California↗

USGS2014

Geochemical fingerprinting of Wilson Creek formation tephra layers (Mono Basin, California) using titanomagnetite compositions

Nineteen tephra layers within the Wilson Creek formation near Mono Lake provide a record of late Pleistocene to early Holocene volcanic activity from the nearby Mono Craters and are important chronostratigraphic markers for paleomagnetic, paleoclimatic, and paleoecologic studies. These stratigraphically important tephra deposits can be geochemically identified using compositions of their titanomagnetite phenocrysts. Titanomagnetite compositions display a broad range (X Usp 0.26–0.39), which allow the tephra layers to be distinguished despite the indistinguishable major-element glass compositions (76–77 wt% SiO 2 ) of their hosts. The concentrations of Ti and Fe in titanomagnetite display geochemical and stratigraphic groupings that allow clear discrimination between older (> 57 ka) and younger (< 41 ka) tephras. Some individual tephra layers can be uniquely identified on the basis of titanomagnetite MgO, MnO, and Al 2 O 3 contents. In addition, a few tephra layers can be correlated to their source vents by their titanomagnetite compositions. The unique geochemical fingerprint of the Mono Craters-sourced titanomagnetites also allows the discrimination of two tephra layers apparently sourced from nearby Mammoth Mountain volcano in Long Valley.

California↗

USGS1968

Seismic evidence for the thickness of Cenozoic deposits in Mono Basin, California

From gravity and limited seismic data obtained in 1957, Pakiser and others (1960) reported a thickness of Cenozoic deposits in the deepest part of Mono Basin, California, of 5.5 ± 1.5 km. Later, in 1962, from a series of chemical explosions in the westernmost part of Mono Basin and outside the limits of the main depressed structure, the thickness of Cenozoic deposits was estimated to be 1.6 km. In 1966, a series of ten 1-ton chemical explosions was detonated in Mono Lake near the deepest part of the Mono Basin structure for the purpose of studying the relative effectiveness of different types of explosives in generating seismic energy. Seismic waves recorded at distances 25.0 to 92.3 km from the explosions were delayed by 1.43 seconds (referred to a shot on bedrock) as they descended through the low-velocity Cenozoic deposits of Mono Basin. By using the velocities of Cenozoic deposits as determined during the 1957 field season, the thickness of Cenozoic deposits required to account for the 1.43-second delay determined in 1966 has been estimated to be about 5 ± 1 km. The delay of seismic waves emerging in Long Valley was less than expected, indicating that they were propagated into Long Valley mainly through high-velocity rocks. From the rate of deposition of Cenozoic rocks in Mono Basin based on the age and depth of burial of the Bishop Tuff, it was estimated that Mono Basin began to subside in early or middle Pliocene time.

California↗