A strike that would have involved some 400 workers in northwest Iowa appears to have been avoided.Registered nurses at Sioux City’s Mercy Medical Center are voting on a new contract today that was assembled after a ten-hour marathon negotiation session yesterday. The tentative agreement halted a strike set for midnight last night. Union leaders say they -do- recommend the contract though details haven’t been disclosed. The strike threat came last week after Mercy offered a pay raise of nine-and-a-half percent over three years.