UI Thread Block

I have created a simple WPF Application and added a button to the default window. When I click on the button, a simulated long working method(simulated using a Thread.Sleep(15000) is called. I am trying to make the button execute asynchronously however despite following online examples, the button and entire window locks as soon as I click and remains so until the Thread.Sleep(...) finishes.

Any ideas why this is happening?

Here is the code:

private void button1_Click(object sender, RoutedEventArgs e)
{
   DoSomeAsyncWork();
}

private void DoSomeAsyncWork()
{
     System.Windows.Threading.Dispatcher.Run();
     Thread thread = new System.Threading.Thread(
         new System.Threading.ThreadStart(
          delegate()
          {
               Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Normal, new Action(() => Thread.Sleep(15000)));
          }
        ));
     thread.Start();
}