docs: def plot_plate_limits_field(axis, rcmb, ridges, trenches):
      """plot arrows designating ridges and trenches in 2D field plots"""
      for trench in trenches:
          xxd = (rcmb + 1.02) * np.cos(trench)  # arrow begin
          yyd = (rcmb + 1.02) * np.sin(trench)  # arrow begin
          xxt = (rcmb + 1.35) * np.cos(trench)  # arrow end
          yyt = (rcmb + 1.35) * np.sin(trench)  # arrow end
          axis.annotate('', xy=(xxd, yyd), xytext=(xxt, yyt),
                        arrowprops=dict(facecolor='red', shrink=0.05))
      for ridge in ridges:
          xxd = (rcmb + 1.02) * np.cos(ridge)
          yyd = (rcmb + 1.02) * np.sin(ridge)
          xxt = (rcmb + 1.35) * np.cos(ridge)
          yyt = (rcmb + 1.35) * np.sin(ridge)
          axis.annotate('', xy=(xxd, yyd), xytext=(xxt, yyt),
                        arrowprops=dict(facecolor='green', shrink=0.05))