geosciences Article Eocene Origin of Owens Valley, California Francis J. Sousa College of Earth, Oceans, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA; [email protected] Received: 22 March 2019; Accepted: 26 April 2019; Published: 28 April 2019 Abstract: Bedrock (U-Th)/He data reveal an Eocene exhumation difference greater than four kilometers athwart Owens Valley, California near the Alabama Hills. This difference is localized at the eastern fault-bound edge of the valley between the Owens Valley Fault and the Inyo-White Mountains Fault. Time-temperature modeling of published data reveal a major phase of tectonic activity from 55 to 50 Ma that was of a magnitude equivalent to the total modern bedrock relief of Owens Valley. Exhumation was likely accommodated by one or both of the Owens Valley and Inyo-White Mountains faults, requiring an Eocene structural origin of Owens Valley 30 to 40 million years earlier than previously estimated. This analysis highlights the importance of constraining the initial and boundary conditions of geologic models and exemplifies that this task becomes increasingly difficult deeper in geologic time. Keywords: low-temperature thermochronology; western US tectonics; quantitative thermochronologic modeling 1. Introduction The accuracy of initial and boundary conditions is critical to the development of realistic models of geologic systems. These conditions are often controlled by pre-existing features such as geologic structures and elements of topographic relief. Features can develop under one tectono-climatic regime and persist on geologic time scales, often controlling later geologic evolution by imposing initial and boundary conditions through mechanisms such as the structural reactivation of faults and geomorphic inheritance of landscapes (e.g., [1,2]). Identifying such paleo-features is a prerequisite to the optimization of models of geologic processes and assessing the assumptions that underly them. Low-temperature thermochronometry provides numerous examples of this phenomenon (e.g., [3,4]). A time–temperature analysis of data from Owens Valley, California leads to insights into the Cenozoic geologic evolution of the southern United States (U.S.) Cordillera, placing new constraints on the timing and mechanism of the demise of the Nevadaplano [5]. (U-Th)/He data from several published transects across Owens Valley document a major Eocene exhumation difference between the bedrock to the east and west of the valley (Figure1). This di fference occurs across a discrete fault-bounded segment of Owens Valley, indicating significant Eocene structural activity on these faults. This excludes the possibility of Neogene geographic continuity of the Nevadaplano with the southern Sierra Nevada, which is in direct opposition to inferences made by previous workers (e.g., [6–8]). In shifting the initial and boundary conditions for the regional late Cenozoic tectonic evolution of Owens Valley, this new analysis requires that contiguity of the Nevadaplano with the southern Sierra Nevada did not persist past circa 50 Ma, roughly 30 to 40 million years earlier than previous estimates [6–8]. Geosciences 2019, 9, 194; doi:10.3390/geosciences9050194 www.mdpi.com/journal/geosciences Geosciences 2019, 9, 194 2 of 14 Geosciences 2019, 9, x FOR PEER REVIEW 2 of 15 FigureFigure 1. Overview 1. Overview map map of Owensof Owens Valley, Valley, California California inin thethe vicinity of of the the Alabama Alabama Hills Hills (AH). (AH). Base Base imageryimagery is 10-m is 10-m digital digital elevation elevation model model overlain overlain with with partiallypartially transparen transparentt hillshade. hillshade. Extent Extent and and centercenter line ofline the of areathe area used used for for topographic topographic swath swath profileprofile A–A’ A–A’ (Figure (Figure 2)2 )are are shown shown in black in black lines. lines. (U-Th)(U-Th)/He/He sample sample sites sites are symbolizedare symbolized according according to to data data source.source. Purple Purple solid solid circles: circles: Mount Mount Whitney Whitney data [data9]. Purple[9]. Purple squares squares with with black black dot: dot: Alabama Alabama HillsHills data [10] [10].. Salmon Salmon solid solid triangles: triangles: Inyo Inyo MountainsMountains data data [11]. [11] OVF:. OVF: Owens Owens Valley Valley Fault Fault [[12,13]12,13].. SNFF: SNFF: Sierra Sierra Nevada Nevada Frontal Frontal Fault Fault [11,12] [11. ,12]. IWMF:IWMF: Inyo-White Inyo-White Mountains Mountains Fault Fault [12 [12]]. GB:. GB: Golden Golden BearBear Dike. W: W: Mount Mount Whitney. Whitney. I: Independence, I: Independence, California.California. LP: LoneLP: Lone Pine, Pine, California. California. CA: CA: California. California. NV: Nevada. Nevada. 2. Geologic2. Geologic Context Context OwensOwens Valley Valley coincides coincides with awith crustal-scale a crustal-scale boundary boundary between between the composite the composite Sierra Sierra Nevada Nevada Batholith Batholith (SNB) to the west and older, metamorphosed, and structurally deformed mostly pre- (SNB) to the west and older, metamorphosed, and structurally deformed mostly pre-Cretaceous crust Cretaceous crust to the east. Near the Alabama Hills (Figure 1), the Sierra Nevada Frontal Fault to the east. Near the Alabama Hills (Figure1), the Sierra Nevada Frontal Fault (SNFF) exhibits about (SNFF) exhibits about three kilometers of topographic relief separating the SNB outcrops of the high threesouthern kilometers Sierra of topographic from consanguineous relief separating outcrops the near SNB the outcrops modern of valley the highfloor. southern Two additional Sierra from consanguineouskilometers down outcrops to the near bottom the modernof Owens valley Valley floor. fill ma Tworks a additional total bedrock kilometers relief of over down five to kilometers the bottom of Owens([14]; Valley Figure fill marks2). a total bedrock relief of over five kilometers ([14]; Figure2). SeveralSeveral lines lines of indirect of indirect evidence evidence have have led workersled workers to inferto infer a late a late Cenozoic Cenozoic origin origin of Owensof Owens Valley. TheseValley. include These a lack include of pre-late a lack of Miocene pre-late rocksMiocene in ro Owenscks in Owens Valley, Valley, evidence evidence of late of Cenozoic late Cenozoic activity on theactivity SNFF on (e.g., the [SNFF15,16 ]),(e.g. Owens [15,16]), Valley Owens Fault Valley (OVF; Fault e.g.,(OVF; [13 e.g.,17 [13,17])]) and Inyo-Whiteand Inyo-White Mountains Mountains Fault (IWMF;Fault e.g., (IWMF; [18,19 e.g.]), and[18,19]), coupled and coupled geomorphic geomorphic and volcanicand volcanic stratigraphic stratigraphic resemblances resemblances betweenbetween the the high southern Sierra Nevada and Inyo Mountains [6,8,20]. Greater than 60 km of dextral offset high southern Sierra Nevada and Inyo Mountains [6,8,20]. Greater than 60 km of dextral offset was was measured across Owens Valley post-83 Ma based on correlation of the Golden Bear dike in the measured across Owens Valley post-83 Ma based on correlation of the Golden Bear dike in the study area (Figure1) and the Coso dikes about 60 km to the south [ 21]. However, the structure responsible for the offset is not constrained and no Eocene exhumation difference or relief generation is ascribed to it. Geosciences 2019, 9, x FOR PEER REVIEW 3 of 15 study area (Figure 1) and the Coso dikes about 60 km to the south [21]. However, the structure Geosciencesresponsible2019, 9for, 194 the offset is not constrained and no Eocene exhumation difference or relief generation3 of 14 is ascribed to it. FigureFigure 2. 2.Topographic Topographic swath swath profile profile acrossacross OwensOwens Va Valley,lley, California. The The extent extent of of the the swath swath is shown is shown in Figurein Figure1. Average1. Average elevation elevation across across the the swath swath is is shown shown as as a solida solid black black line. line. Range Range from from minimum minimum to maximumto maximum elevation elevation across across each each segment segment of swath of swat areh shown are shown as dark as dark purple purple range. range. Apatite Apatite (U-Th) (U-/He samplesTh)/He are samples projected are projected onto the onto center the line center of the line swath of the profile. swath profile. Sample Sample average average AHe ageAHe is age plotted is onplotted each sample. on each Geometry sample. Geometry and depth and of Owensdepth of Valley Owens fill Valley (shown fill in (shown beige) isin from beige) [14 is]. from Locations [14]. of SNFF:Locations Sierra of Nevada SNFF: FrontalSierra Nevada Fault, OVF: Frontal Owens Fault, Valley OVF: Fault,Owens and Valley IWMF: Fault, Inyo-White and IWMF: Mountains Inyo-White Fault areMountains from [12]. Fault W: Mount are from Whitney. [12]. W: Mount Whitney. DiDifferentfferent scenarios scenarios for for the the pre-Neogene pre-Neogene paleogeographypaleogeography of of the the Owens Owens Valley Valley region region have have been been posited,posited, including including potentially potentially integrated integrated PaleogenePaleogene drainages, flowing flowing both both north-to-south north-to-south [22] [22 and] and east-to-westeast-to-west [23 [23],], as as well well as as a a continuous continuous low-relief,low-relief, high-elevation landscape landscape spanning spanning from from the the high high peakspeaks of of the the southern southern Sierra Sierra Nevada Nevada eastward eastward across across the the Inyo-White