Structural consequences of the emplacement of Ubehebe Peak Intrusion, Death Valley, California

The quartz-monzonitic Ubehebe Peak Intrusion is located in Death Valley National Park, California (USA). It belongs to a series of alcalic intrusions, which formed the Hunter Mountain Batholith. The emplacement of the intrusion took place at 173±1 Ma (D2) into a sequence of Palaeozoic carbonates. It intruded at ∼1,5 kbar and produced 675^oC at the contact. Interpretation of deformation is complicated by pre-existing structures and the occurrence of syn-intrusive deformation. The pre-intrusive permian to triassic compressional event of the Sonoma orogeny formed a SW dipping thrust-sheet-system (D1). Deformation in the area is heterogeneous, but D1 related strain clearly increases towards the west, where a foliation is formed in carbonates. Ductile deformation in the west is characterized by mylonites with sinistral movement accompanied by SSW-vergent folds. Lineations plunge 40 degrees, indicating top towards SSE movements. The dip of the fold-axes in the east are 100/60, while in the west they dip with more than 80 degrees towards SSW, and are overturned in places. Syn-intrusive deformation (D2) affected host rocks proximal to the intrusion (∼800m). No new foliation is recognized. It led to a preferred orientation of c-axes of recristallized carbonates and folding of the pre-existing D1 fold-axes. The axial planes of folds outside the contact zone strike N-S, while closer towards the intrusion, fold hinges are deflected towards the edges of the intrusion. In the west they are deflected towards SW and in the east towards SE respectively. South of the intrusion a kilometre scale fold hinge was bend anticlockwise about 90 degrees, to strike E-W. It is now aligned between the Hunter Mountain main mass and its satellite, the Ubehebe Peak Intrusion. Tremolite in the metamorphic aureole locally defines a lineation in the D1 foliation. We interpret this to show that growth was controlled by the pre-existing foliation. Neither tremolite nor delicate tabular forsterite show any signs of post-growth deformation. Hence, there is no evidence of deformation after the emplacement and growth of the contact metamorphic assemblage. The brittle, extensional Basin Range-Tectonic (D3) cut the area into large blocks with little or no tilting.