Abstract

At least two deformational episodes have been identified in the 1250 Ma Allamoore Formation and the 1120 Ma Hazel Formation in the Millican Hills near Van Horn, Culberson County, Texas. The first phase of deformation was associated with a north-northeast aligned σ₁. This stress orientation, along with previously established east-west-oriented structural zones, uplifted the Allamoore Formation. Erosion and weathering of the Allamoore Formation into alluvial fans led to the development of the Hazel Formation.

The second phase comprises two working hypotheses that involve the Allamoore Formation, the Hazel Formation, and a series of pink rhyolite outcrops that have been assigned by previous workers to the 1380 Ma Carrizo Mountain Group.

The first hypothesis infers a northwest-southeast-oriented σ₁ as well as right-lateral strike-slip movement along the Hillside Fault zone. The Hillside Fault is postulated to have been part of the Grenville Front and the southern edge of the North American craton during Mesoproterozoic time. Right-lateral movement along the Hillside Fault generated two principal orientations of shear, west-northwest Riedel shears and east-west P shears. These directions of shear are represented by distinct outcrops of Allamoore Formation talc. The second hypothesis infers northwest-southeast-oriented σ₁ and left-lateral strike-slip along the Hillside Fault zone. This hypothesis has west-northwest-positioned P shears and east-west-directed Riedel shears. The latter hypothesis is the preferred one based on field observations. The second hypothesis also omits the need for a subsequent third phase of deformation that is required if the first working hypothesis is used.

Regardless of what hypothesis is used, the Riedel and P shears have generated an anastomosing pattern of faults and shears that comprise the Millican Hills shear zone. The shear zone is constrained at its southern boundary by the Hillside Fault zone and to the north by a monoclinal flexure that separates steeply dipping beds of the Hazel Formation from nearly horizontal beds of the Hazel Formation.

The Millican Hills shear zone is 30 km long by 9 km wide. The smallness of this shear zone indicates that deformation was localized along this part of the Grenville Front, possibly because of plate boundary irregularities.

This study offers an alternative interpretation to the deformational history of the Allamoore and Hazel Formations in the Millican Hills area. The long-standing interpretation of others suggests that the Allamoore and Hazel Formations were deformed by the Streeruwitz Thrust Fault. The thrust fault supposedly brought rocks of the Carrizo Mountain Group against rocks of the Allamoore Formation about 1000 Ma.