Faulted and tilted Pliocene olivine-tholeiite lavas near Alturas, NECalifornia, and their bearing on the uplift of the Warner Range

Abstract

Late Miocene-Pliocene (8-3 Ma) olivine basalt lavas, dated in thisstudy by the (super 40) Ar/ (super 39) Ar method, have been faultedand tilted on both the east and west sides of the Warner Range of NECalifornia, which is itself a tilted block rising to 2960 m at itscrest that is composed of Miocene-Oligocene lavas and volcaniclasticrocks. The late Miocene-Pliocene lavas, distinctively poor in K (sub2) O and rich in MgO, are called low-K olivine tholeiites and have adifferent mantle source region than that of the oldersubduction-related lavas of the main Warner Range. Hays Canyon Range(max. elev. 2400 m) lies to the east of the Warner Range, and thebroad Surprise Valley separates the two fault-bounded ranges. MiddleMiocene (ca. 15 Ma) basic lavas, with a small easterly dip, cap theHays Canyon Range and overlie Oligocene silicic ash-flow deposits anda basaltic andesite spatter volcano. Middle Miocene basic lavas alsoform the crest of the Warner Range and its westerly dip slope (approximately 15 degrees ). Nearly horizontal basic lavas of the sameage are also found on both sides of the Warner Range, and it is aplausible conclusion that these middle Miocene basalts were acontiguous group before faulting and uplift of the Warner Range.Derived estimates of uplift rates ( approximately 1 mm/yr) of theWarner Range indicate that uplift could have been initiated at ca. 4Ma, a period of the most voluminous eruption of low-K olivinetholeiite lavas. If the slower Cretaceous exhumation rate of theSierra Nevada (0.5-1.0 mm/yr) is applied to the total offset of theWarner Range (4270 m), and it did not vary with time, then the upliftof the Warner Range was initiated at ca. 8 Ma, which coincides withthe age of the oldest low-K olivine tholeiite lava (8 Ma). Low-Kolivine tholeiites require a hot shallow asthenospheric source, and itis the rise of this hot mantle that is presumed to have caused theuplift of the Warner Range. Whether or not the widespread eruption ofsmall volumes of Pliocene low-K olivine tholeiites in central andeastern Oregon is associated with crustal uplift is unknown.

54 samples from this reference